The Impact of Heat Stress on Canola (Brassica napus L.) Yield, Oil, and Fatty Acid Profile

Abstract

Canola (Brassica napus L.) is an oilseed crop that is currently being impacted by climate change. Heat stress risks production by impacting yield, oil, protein, and fatty acid profile. The purpose of this literature review was to assess the impact of heat stress on canola while briefly evaluating other abiotic stresses, and to address the following research questions: (1) What is the impact of heat stress on canola yield?, (2) What is the impact of heat stress on canola oil and protein content?, and (3) What is the impact of heat stress on the fatty acid profile of canola? Forty papers were selected in relation to B. napus heat stress and impact on yield, oil content, or fatty acid profile, from 1978 to 2025. Key findings revealed that heat stress negatively impacted yield and oil, while significant variation was observed within the fatty acid profile. Genotype, heat stress condition, and growth stage significantly impacted results. Certain genotypes were identified as having potential heat-tolerant traits, providing a basis for future breeding programs. Future field studies with controlled irrigation may better explain variations between controlled environment and field studies when water stress is not a concern. A better understanding of the impact of combined stresses, particularly heat and drought, is also required for breeding tolerant lines in regions with minimal irrigation.

1. Introduction

Canola (Brassica napus L.) is an oilseed crop that is primarily grown for edible oil, and it is also used to produce biofuel, cosmetics, and other industrial products [1]. Canola meal, a byproduct of oil production, is incorporated into animal feed [1]. Canola is grown worldwide, with the largest production areas being Asia, Europe, North America, and Oceania [2]. The timing of canola production systems varies by location and is generally classified into three growing environments [1]. Winter canola is sown in autumn and harvested in early summer, predominantly in the Northern Hemisphere. Spring canola, grown in Canada, is sown in spring and harvested in summer. In the Southern Hemisphere (e.g., Australia), spring canola is sown in late autumn and harvested in spring. Canola was initially bred for its low erucic acid content but is also favoured over other edible oils due to its low saturated fatty acid (SFA) levels (~7%), and higher monounsaturated fatty acid (MUFA) levels (~60%) [3]. High oleic, low linoleic (HOLL) canola has also been bred, desiring higher MUFA over polyunsaturated fatty acid (PUFA), primarily for nutritional benefits and increased shelf stability [4,5]. Other species within the Brassica genus are also used in oil production, including B. juncea and B. rapa. Both species are more common in warmer climates and, thus, are of some interest to oilseed production, particularly in developing stress tolerance [6].

Global canola production has steadily increased over the last two decades [2], with oilseed demand predicted to increase even further [7]. Already, the increased demand for production has expanded the environments of canola cropping into hotter, dryer climates [1]. Regions in the southern hemisphere have already had to alter production systems to avoid the worst heat stress in these environments [1]. This is of particular concern for the future of canola production in the context of climate change. Climate change has already increased the frequency and severity of hot extremes, and a predicted 1.5 °C increase within the next decade will only exacerbate this issue [8]. Multiple climate change models have predicted deleterious impacts on canola yield, mostly through increased heat and drought stress [9,10,11]. Heat stress is more detrimental to canola production than other abiotic stresses, reinforcing heat stress as a primary concern for the future of canola production [12,13,14]. Physiological processes, including photosynthetic activity and chlorophyll biosynthesis, are affected by heat stress, along with changes in proteomic and metabolomic [15]. Heat stress generally decreases the number of fertile siliques per plant, along with the seeds per silique, and the final seed weight [13,16,17,18,19,20,21,22], all of which suggest potential impacts on canola yield, oil content, quality, and fatty acid profile [16,23]. Understanding exactly how heat stress influences these parameters, across different heat stress conditions and different genotypes, is crucial for the future development of heat-tolerant canola.

This review aimed to assess the impact of heat stress on global canola production, investigating yield and quality parameters, including oil content, quality, and fatty acid profile. A focus on more recent studies (within the last 20 years) was emphasised to establish the impacts on the current cultivars and identify potential heat-tolerant genotypes. The literature review sought to answer the following questions: (1) What is the impact of heat stress on canola yield?, (2) What is the impact of heat stress on canola oil and protein content?, and (3) What is the impact of heat stress on the fatty acid profile of canola oil?

The primary goal of this review was to investigate the impact of heat stress on canola, while briefly evaluating other abiotic stresses. The initial literature search for this review was conducted on Web of Science and Scopus between August and October of 2024. The following keywords were used to find articles: “canola” OR “rape seed” OR “rapeseed” OR “oilseed rape” OR “Brassica napus” OR “B. napus”, AND “heat toleran*” OR “heat” OR “heat stress” OR “temperature”, AND “yield” OR “oil” OR “oil quality” OR “fatty acid”. Further articles were found through backward citation searching, resulting in a total of 39 articles. Scopus AI (2024) was used to obtain background information on canola production and abiotic stress. All information was validated by reading the source materials. The review sought to qualitatively and quantitatively assess the impacts of heat stress on yield, oil content, and quality and the fatty acid profile, outlining where possible explanations for contradicting data.

Current reviews on heat stress in canola focus on the physiological impacts and genetic controls [15,24,25]. However, there is a lack of analysis that integrates the impacts on yield parameters and oil profile to determine heat-tolerance. This review fills this gap by consolidating the findings on the impact of heat stress on canola yield and chemical quality, to understand the general effects of heat stress and reasons for conflicting results, as well as identifying more tolerant genotypes. A thorough understanding of the effects of heat stress will provide the foundation for further development of heat-tolerant cultivars that will allow production systems to continue to meet the increasing demand under increasing climate stress.

2. The Impact of Heat Stress on Yield

The impact on yield is outlined in Table 1. Generally, heat stress has a negative impact on canola yield. However, the degree of severity is highly dependent on genotype, heat stress conditions, and the growth stage exposed.

2.1. Genotype

Brassica species respond differently to heat stress; therefore, B. juncea and B. rapa have been used in studies as comparisons to B. napus. Generally, B. juncea has better heat tolerance than B. napus but lower yield to start with [13,17,19,26]. Among the three species, the yield of B. rapa was found to be most impacted by heat stress [17,27]. Thus, B. juncea may be relevant for developing heat-tolerant lines of B. napus if the genes that confer heat tolerance can be identified and are separate from genes that determine yield. B. campestris and B. hirta were also measured in two studies, both of which had significantly lower yield than B. napus [28,29]. However, germplasm and genes for heat stress tolerance could be found in B. rapa [30]. Within B. napus, yield losses vary significantly among cultivars under the same stress conditions [16,18,20,23,31,32,33,34]. Across the literature, genotypes identified to have the least impact on yield were spring types Oscar [16]; Insignia and Emblem [23]; Lumen [32]; HAU02 [18]; Hyola50, Agamax, and Zabol9 [34]; and semi-winter type Tanami [18]. A preliminary controlled environment study investigating over 300 B. napus genotypes found that a small number of genotypes saw an increase in yield under heat stress conditions [35]. This research aims to identify the genes that confer heat tolerance so that they can be used in future breeding programs. Zhang, et al. [33] reported that the interaction of genotype and environment influenced yield more than either factor alone, and that generally early flowering genotypes had higher yield due to completing seed development before the highest temperatures were experienced.

2.2. Heat Stress Condition (Duration and Intensity)

Heat stress conditions vary within the literature, encompassing natural exposure through field studies, replications in greenhouse and growth chamber studies, as well as more extreme and specific stress conditions. In a growth chamber study, sudden heat stress exposure of 40/16 °C (day/night) for 5 days had a more significant effect on yield than a gradual system (20–40/15–35 °C, over 5 days) [16]. The gradual system was designed to emulate conditions seen in the field, suggesting that the impacts of heat stress on production may not be as severe as in other controlled environment studies. While canola cultivar Oscar experienced a yield loss of 43% under sudden heat stress, there was no significant change to yield under the gradual system, thus, it is a prime candidate for heat tolerance with respect to yield. In a further study on three cultivars, only one of the three had significant yield losses when exposed to a short high heat stress event (38/23 °C, 4 days) [23]. Exposure to a longer lower heat stress (28/23 °C, 9 days) had no significant impact on yield for any of the cultivars. In combination, these studies suggest the ability to adapt to the general increase in temperature that climate change will bring, however, indicates little capability to adapt in real-time to extreme climate episodes. Conversely, another study found that yield was more impacted as heat stress was extended, finding most impact from a three-day heat stress, and some further reductions in yield from five- and seven-day treatments [18]. These contradicting data suggest genotypic variation.

Testing a range of heat stress severities on canola, one study found that at the highest temperature stress (32 °C), flowering and pod set were not achieved, thus no yield was obtained [29]. These extreme results were likely a combination of high day and night temperatures (32/27 °C), as most other high day temperature studies allow the night temperature to drop more significantly [17,23,36]. Under moderate heat stress, cooler night temperatures allow for better recovery and reduce the negative impact on yield [18]. High night temperatures alone have also reduced yields [22,37]. Increased night temperatures may be of concern, particularly if they are in combination with high daytime temperatures, as impacts are magnified [22]. Different heat stress patterns result in significant differences in yield reductions, with different growth stages being more or less susceptible to different stress patterns [38]. Of three stress patterns implemented in one growth chamber study, an early mild stress sequence had the most impact on yield, followed by a repeated stress sequence and then a late, heat peak stress sequence. The study suggests that rather than repeated stress episodes causing more damage, it is the timing of these more crucial stresses.

2.3. Growth Stage

Canola is an indeterminate species, meaning heat stress at one time point will expose multiple different growth stages in a single plant. The critical period for canola is around 300-degree days (calculated by average daily temperature minus base temperature of 0 °C) post-flowering, when both new flowers are opening and pods are developing [39]. Both controlled environment and field studies have found that heat stress around flowering causes a significant reduction in yield [13,17,18,20,36,40,41]. Heat stress increases flowering and the number of pods but the number of fertile pods decreases, leading to an overall reduced yield [17,20,36].

Multiple studies have shown that heat stress at early flowering is more detrimental to yield than at early pod development [17,18]. One study reported heat stress to be most detrimental during pod development (main stem); however, further analysis reveals that this was mostly due to a reduction in yield from lateral branches, which are not in sync with the mainstem growth stages [13]. This, supported by the mainstem yield being most impacted by heat stress at flowering, further solidifies that heat stress is more detrimental during flowering than pod development. Contradicting this, another study found that smaller pods (<5 cm) had a greater impact from heat stress than that of flowers or larger pods (>5 cm) [38]. Under all stresses, the yield of larger pods (>5 cm) were least impacted, and their fatty acid profiles were not significantly affected by any stress model, suggesting that pods over 5 cm in size have passed the stage previously proposed where heat stress is detrimental to yield and oil quality [17].

The indeterminate nature of canola allows plants to recover post stress by producing more lateral branches [13,18,36]; however, one study found that pods formed post heat stress were malformed [17]. Despite similar stress conditions, another study reported no adverse effects on pod formation, suggesting that genotype plays a role in recovery [18]. Young, et al. [36] found that plants exposed to two weeks of heat stress followed by normal temperatures responded faster than plants exposed to one week of heat stress, which the researchers proposed may be due to acclimation to stress.

Delayed planting has also been shown to reduce yield and oil content, likely through exposure to higher temperatures during reproductive stages [19,28,34,41,42,43]. However, in the majority of these studies, genotype had a significant impact on yield and oil reduction [19,34,41,42,43]. Due to the indeterminate growth pattern, an earlier planting date would only put some pods past the threshold while branches with younger pods would still be impacted, causing significant reductions in yield, therefore adjusting the planting date alone will not be sufficient to mediate the effects of heat stress on yield. Zhang, et al. [33] found that the earlier flowering genotypes faced less impact on yield, supporting the notion that flowering is the most sensitive stage to heat stress, and that selecting for early flowering genotypes may be another way to mitigate yield losses.

Table 1. (a) The effect of heat stress on yield of oilseed brassicas in controlled environment studies. (b) The effect of heat stress on yield oilseed brassicas in field studies.

(a)
Species	Cultivar/Genotype	Growth Stage	Stress/Duration	Impact on Yield	Reference
napus juncea	Quantum PC98-44, PC98-45, Cutlass	During bud formation, flowering, and pod development	High (35/18 °C) and moderate (28/18 °C)/10 days	Yield was most impacted by heat stress during flowering. Yield was reduced in all cultivars, least in Quantum and most in PC98-44.	[13]
napus	Monty, Range, Oscar	29 DAF	Gradual treatment to max 40 °C/45-degree days (DD) more than control, sudden treatment 40 °C/5 days, 15 DD more than control	Sudden heat stress yield loss of 43% (Oscar), 60% (Monty), and 89% (Range). Gradual heat stress yield loss was not significant (Oscar), 22% (Monty), and 63% (Range).	[16]
napus juncea rapa	Quantum Cutlass Maverick and Parkland	BBCH61, BBCH71	28 or 35 °C/7 days	Yield is most impacted by high heat stress at early flower (−53%) than pod development (−18%). Highest yield was in B. napus, lowest in B. rapa.	[17]
napus	Tanami, RR001, HAU02	BBCH53, BBCH60, 8 DAF, 15 DAF, 22 DAF and 29 DAF	25, 32, or 35 °C/3, 5, or 7 days	Seed yield reduced at 32 and 35°C, most reduced by 5- and 7-day treatments. Tanami and HAU02 had less impact at moderate heat stress. Lower night temperatures lessened negative effects for moderate heat stress.	[18]
napus juncea	16 genotypes 45J10	BBCH53	31 °C/14 days	Heat stress increased the number of flowers but decreased the number of pods and seeds/pod, reducing overall yield. Maintainer lines had higher yields at winter planting while restorer lines were higher for autumn planting.	[20]
napus	Edimax_cl, Mercedes, Popular, 46w94, dkw44-10, dkw46-15CL, Hekip	BBCH60	34/15, 23/20, or 34/20 °C/14 days	Yield losses were more significant when high day temperature was combined with high night temperature.	[22]
napus	Insignia, Emblem, Surpass400	20 DAF 25 DAF	28 °C/9 days (MH10) 38 °C/4 days (VH5)	4-day treatment had bigger impact on yield; loss of 40% (Surpass400).	[23]
napus hirta	Norin, Westar (low erucic acid) Dagan	Bolting	17, 22, 27, or 32 °C/until maturity	At 32 °C, no fruit was obtained, at 27 °C, 50–70% decline in yield parameters across cultivars.	[29]
napus	Solar, Lumen	Flowering	5 °C above environmental temperature/15 days	Yield reduction of 9.2% (Lumen) 27.8% (Solar).	[32]
napus	Susceptible: DKW44-10, DKW46-15, HyCLASS225W, Riley, Wichita Tolerant: 46W94, Edimax CL, Hekip, Mercedes, Popular	7 DAF	High night temperature 20 °C/until maturity	Yield components varied significantly, total seed weight of susceptible cultivars was 36% lower than control, while tolerant was 8–20% lower than control.	[37]
napus	Aviso	GS72 (20% pods reached maximum size)	RSS (repeated stress sequence) EMS (early mild stress) 4LHP (4 late heat peaks)	EMS had biggest impact on yield reduction (−55%), followed by RSS (−35%). Stress during seed set impacted yield the most.	[38]
(b)
Species	Cultivar/Genotype	Location	Condition	Impact on Yield	References
napus juncea	Monty, Oscar 887.1.6.1, JM 25, JM 33, Muscon, 82 No 22–98	WA, Australia	Average daily temperature reached a maximum of ~35 °C around maturity	Yield decreased as sowing was delayed. B. juncea had more pods but less seeds than B. napus, so overall yield was higher in B. napus.	[19]
napus	30+ cultivars	9 sites across Australia	Heat, drought, and frost occurred during critical periods at some sites	Delayed planting reduced yield through exposure to heat and drought stress at flowering. Variety was significant but not reported.	[41]
napus juncea	10 genotypes 1 genotype	WA, Australia	Average daily temperature ranged from 13 to 27 °C across locations in last 3 months of growth	Yield decreased with delayed sowing. B. juncea was more tolerant to heat stress, but overall had a lower yield than B. napus.	[26]
napus campestris	Zephyr Span	NSW, Australia	Not reported	Yield was higher in earlier plantings, B. napus had a higher yield than B. campestris.	[28]
napus	28 cultivars	WA, Australia	Average daily temperature ranged from 15 to 19 °C across locations in last 3 months of growth	Significant environment x genotype interactions were found to impact yield. Early flowering cultivars produced a higher yield in hotter environments.	[33]
napus	Agamax, Hyola4815, Hyola50, Hyola401, Safi6, Zabol9, and Zabol13	Dezful, Iran	Six sowing dates to expose canola to different levels of heat stress	Yield declined as planting date was delayed. Yield was highest in Agamax, Hyola50, and Zabol9	[34]
napus	Tower, Rafal and Global	Dezful, Iran	Hot climate (35.5 °C) with a dry, hot summer, irrigation as needed	Yield declined as planting date was delayed. Yield highest in Tower, lowest in Global.	[43]

Table 1. (a) The effect of heat stress on yield of oilseed brassicas in controlled environment studies. (b) The effect of heat stress on yield oilseed brassicas in field studies.

(a)
Species	Cultivar/Genotype	Growth Stage	Stress/Duration	Impact on Yield	Reference
napus juncea	Quantum PC98-44, PC98-45, Cutlass	During bud formation, flowering, and pod development	High (35/18 °C) and moderate (28/18 °C)/10 days	Yield was most impacted by heat stress during flowering. Yield was reduced in all cultivars, least in Quantum and most in PC98-44.	[13]
napus	Monty, Range, Oscar	29 DAF	Gradual treatment to max 40 °C/45-degree days (DD) more than control, sudden treatment 40 °C/5 days, 15 DD more than control	Sudden heat stress yield loss of 43% (Oscar), 60% (Monty), and 89% (Range). Gradual heat stress yield loss was not significant (Oscar), 22% (Monty), and 63% (Range).	[16]
napus juncea rapa	Quantum Cutlass Maverick and Parkland	BBCH61, BBCH71	28 or 35 °C/7 days	Yield is most impacted by high heat stress at early flower (−53%) than pod development (−18%). Highest yield was in B. napus, lowest in B. rapa.	[17]
napus	Tanami, RR001, HAU02	BBCH53, BBCH60, 8 DAF, 15 DAF, 22 DAF and 29 DAF	25, 32, or 35 °C/3, 5, or 7 days	Seed yield reduced at 32 and 35°C, most reduced by 5- and 7-day treatments. Tanami and HAU02 had less impact at moderate heat stress. Lower night temperatures lessened negative effects for moderate heat stress.	[18]
napus juncea	16 genotypes 45J10	BBCH53	31 °C/14 days	Heat stress increased the number of flowers but decreased the number of pods and seeds/pod, reducing overall yield. Maintainer lines had higher yields at winter planting while restorer lines were higher for autumn planting.	[20]
napus	Edimax_cl, Mercedes, Popular, 46w94, dkw44-10, dkw46-15CL, Hekip	BBCH60	34/15, 23/20, or 34/20 °C/14 days	Yield losses were more significant when high day temperature was combined with high night temperature.	[22]
napus	Insignia, Emblem, Surpass400	20 DAF 25 DAF	28 °C/9 days (MH10) 38 °C/4 days (VH5)	4-day treatment had bigger impact on yield; loss of 40% (Surpass400).	[23]
napus hirta	Norin, Westar (low erucic acid) Dagan	Bolting	17, 22, 27, or 32 °C/until maturity	At 32 °C, no fruit was obtained, at 27 °C, 50–70% decline in yield parameters across cultivars.	[29]
napus	Solar, Lumen	Flowering	5 °C above environmental temperature/15 days	Yield reduction of 9.2% (Lumen) 27.8% (Solar).	[32]
napus	Susceptible: DKW44-10, DKW46-15, HyCLASS225W, Riley, Wichita Tolerant: 46W94, Edimax CL, Hekip, Mercedes, Popular	7 DAF	High night temperature 20 °C/until maturity	Yield components varied significantly, total seed weight of susceptible cultivars was 36% lower than control, while tolerant was 8–20% lower than control.	[37]
napus	Aviso	GS72 (20% pods reached maximum size)	RSS (repeated stress sequence) EMS (early mild stress) 4LHP (4 late heat peaks)	EMS had biggest impact on yield reduction (−55%), followed by RSS (−35%). Stress during seed set impacted yield the most.	[38]
(b)
Species	Cultivar/Genotype	Location	Condition	Impact on Yield	References
napus juncea	Monty, Oscar 887.1.6.1, JM 25, JM 33, Muscon, 82 No 22–98	WA, Australia	Average daily temperature reached a maximum of ~35 °C around maturity	Yield decreased as sowing was delayed. B. juncea had more pods but less seeds than B. napus, so overall yield was higher in B. napus.	[19]
napus	30+ cultivars	9 sites across Australia	Heat, drought, and frost occurred during critical periods at some sites	Delayed planting reduced yield through exposure to heat and drought stress at flowering. Variety was significant but not reported.	[41]
napus juncea	10 genotypes 1 genotype	WA, Australia	Average daily temperature ranged from 13 to 27 °C across locations in last 3 months of growth	Yield decreased with delayed sowing. B. juncea was more tolerant to heat stress, but overall had a lower yield than B. napus.	[26]
napus campestris	Zephyr Span	NSW, Australia	Not reported	Yield was higher in earlier plantings, B. napus had a higher yield than B. campestris.	[28]
napus	28 cultivars	WA, Australia	Average daily temperature ranged from 15 to 19 °C across locations in last 3 months of growth	Significant environment x genotype interactions were found to impact yield. Early flowering cultivars produced a higher yield in hotter environments.	[33]
napus	Agamax, Hyola4815, Hyola50, Hyola401, Safi6, Zabol9, and Zabol13	Dezful, Iran	Six sowing dates to expose canola to different levels of heat stress	Yield declined as planting date was delayed. Yield was highest in Agamax, Hyola50, and Zabol9	[34]
napus	Tower, Rafal and Global	Dezful, Iran	Hot climate (35.5 °C) with a dry, hot summer, irrigation as needed	Yield declined as planting date was delayed. Yield highest in Tower, lowest in Global.	[43]

3. The Impact of Heat Stress on Oil and Protein

The impact of heat stress on oil and protein is outlined in Table 2. Generally, oil and protein content are negatively correlated when exposed to heat stress; that is, oil content decreases while protein content increases [16,23,28,32,37,44]. The relationship between oil and protein content is weaker under combined heat and drought stress [12,40,45,46].

3.1. Genotype

Across the literature, only three genotypes were explicitly identified to increase in oil content, which were observed under controlled environment heat stress conditions. Genotype 46W94 increased oil content by 10% when exposed to high night temperatures (20 °C) [37]. The same study found that tolerant cultivars had a significantly smaller oil reduction than susceptible cultivars. Thus, they are potential candidates for daytime heat stress tolerance. Under prolonged moderate heat stress (28/23 °C, 9 days), inbred spring cultivars (Insignia, Emblem, and Surpass400) exhibited an increase in oil content, with the latter showing the greatest increase in oil content [23]. However, under a shorter, higher heat stress (38/23 °C, 4 days), oil content decreased only in Surpass400. Thus, Surpass400 may be better suited to warm climates, while Insignia and Emblem may cope better with heat stress episodes. Canales, et al. [32] found that of two spring hybrid cultivars, Lumen was less impacted by heat stress than Solar, but still saw a reduction in oil content. Larger field studies significant interaction between genotype and environment when analysing oil content [34,41,43], with one study identifying Hyola50 as having the least impact on oil content [34]. Although research shows B. juncea may exhibit greater heat tolerance, B. napus varieties typically produce higher oil content. Therefore, the advantage in oil yield may not justify substituting B. napus with B. juncea under heat stress [42]. Similar results were reported for B. napus and B. campestris [28]. One study on molecular mechanisms in canola reported that lipid biosynthesis was inhibited when exposed to heat stress by downregulation of BnWRI1, which regulates the de novo fatty acid biosynthesis pathway, suggesting a future genetic target for increasing oil production [47]. Yu, et al. [48] also identified several genes associated with lipid metabolism that were downregulated in response to heat stress. Further understanding of the molecular mechanisms may aid a more targeted approach to heat tolerance through upregulating these genes.

3.2. Heat Stress Condition (Duration and Intensity)

Both gradual and sudden heat stress reduce oil content, with sudden heat stress having a greater impact [16]. Following this, while a shorter, higher heat stress produced the expected decrease in oil and increase in protein, a longer, moderately high heat stress increased oil content while not impacting the protein fraction [23]. Similar to the impact on yield, these studies suggest an ability to adapt to prolonged, less extreme heat stress. However, while current cultivars show a capacity to adapt to the gradual climate warming, they are not as responsive towards extreme climate episodes. Cultivar Garnet saw no decrease in oil content; however, the short duration of heat stress (12 h) may be the cause [21]. Further investigation with longer duration treatment may elucidate if it is an appropriate cultivar. A study investigating combined stress found that, although all cultivars were negatively affected by heat stress, the addition of elevated [CO₂] and [O₃] resulted in increased oil content in Bolero and Tanto [14]. Therefore, the impacts of heat stress may not be as severe when other climatic changes are considered. Oil content is controlled more by genotype than environment, and the strong correlation between oil and protein is not observed in all genotypes, suggesting that it is possible to select for and breed genotypes with both high oil and protein content for hotter environments [33,49].

Table 2. (a) The effect of heat stress on oil content of canola (Brassica napus) in controlled environment studies. (b) The effect of heat stress on oil content of oilseed brassicas in field studies.

(a)
Cultivar/Genotype		Growth STAGE	Stress/Duration	Impact on Oil	Reference
N99-508		38 days after sowing	23 or 29 °C 30% or 90% Water holding capacity. Stress applied until harvest	Oil only decreased under heat stress, but protein increased under all stresses.	[12]
Bolero, Mary, Mozart, Tanto		From seed	19 or 24 °C, ambient and elevated [CO2} and [O3]/until maturity	Heat stress alone reduced oil in all cultivars, and combination stress increased oil in Bolero and Tanto.	[14]
Monty, Range, Oscar		29 DAF	Gradual treatment to max 40 °C/45-degree days (DD) more than control, sudden treatment 40 °C/5 days, 15 DD more than control	Oil decreased and protein increased in Monty and Range in sudden treatment only. No change to Oscar.	[16]
Garnet		14 DAF	34 and 40 °C/12 h	At 34 °C, there was no significant decline in total oil content; however, at 40 °C, it declined by ~41%.	[21]
Insignia, Emblem, Surpass400		20 DAF 25 DAF	28 °C/9 days (MH10) 38 °C/4 days (VH5)	MH10 increased oil in all cultivars, most in Surpass400 (+10%), VH5 decreased oil only in Surpass400 (−7%).	[23]
Solar, Lumen		Flowering	5 °C increase daily/15 days	Oil to protein ratio was more impacted in Solar.	[32]
Susceptible: DKW44-10, DKW46-15, HyCLASS225W, Riley, Wichita Tolerant: 46W94, Edimax CL, Hekip, Mercedes, Popular		7 DAF	High night temperature 20 °C/until maturity	Oil reduction of tolerant cultivars 3%, susceptible cultivars 12%. Among tolerant, 46W94 increased by 10% followed by Mercedes which decreased by 2%.	[37]
Aviso		GS72 (20% pods reached maximum size)	RSS (repeated stress sequence) EMS (early mild stress) 4LHP (4 late heat peaks)	Oil content was on average higher in seeds exposed to heat stress at flowering, followed by pods L < 5 cm and then pods L ≥ 5 cm.	[38]
(b)
Species	Cultivar/Genotype	Location	Condition	Impact on Oil	Reference
napus	30+ cultivars	9 sites across Australia	Heat, drought and frost occurred during critical periods at some sites	Delayed planting reduced yield and oil concentration, variety was significant but not reported.	[41]
napus	40+ cultivars	Victoria, Australia	Average daily temperature ranged from 19 to 23 °C across locations during flowering	Low oil content was associated with warm temperature during seed maturation. Strong inverse relationship between oil and protein.	[44]
napus juncea	Monty and Oscar 887.1.6.1, JM 25, JM 33, Muscon, 82 No 22–98	WA, Australia	Average daily temperature reached a maximum of ~35 °C around maturity	Oil and protein were inversely proportional in both species. Oil decreased and protein increased as sowing was delayed. Monty produced the highest oil concentration across environments. 887.1.6.1 produced the highest oil concentration for B. juncea, similar to or greater than Oscar across environments	[42]
napus	Roundup Ready	Washington, USA	Between 3 and 14 days above 28 °C during flowering across locations	Increasing temperature decreased oil content and increased protein. The same relationship was not seen with combined drought stress.	[40]
napus campestris	Zephyr Span	NSW, Australia	Not reported	Oil concentration was higher in earlier plantings, oil and protein were negatively correlated. Oil content was higher in Zephyr.	[28]
napus	Agamax, Hyola4815, Hyola50, Hyola401, Safi6, Zabol9, and Zabol13	Dezful, Iran	Six sowing dates to expose canola to different levels of heat stress	Delayed planting reduced oil content, with the least impact in Hyola50.	[34]
napus	Tower, Rafal and Global	Dezful, Iran	Hot climate (35.5 °C) with a dry, hot summer, irrigation as needed	Oil was only reduced at the latest planting date, suggesting that oil content is more controlled by genotype, but did not report differences between genotypes.	[43]

Table 2. (a) The effect of heat stress on oil content of canola (Brassica napus) in controlled environment studies. (b) The effect of heat stress on oil content of oilseed brassicas in field studies.

(a)
Cultivar/Genotype		Growth STAGE	Stress/Duration	Impact on Oil	Reference
N99-508		38 days after sowing	23 or 29 °C 30% or 90% Water holding capacity. Stress applied until harvest	Oil only decreased under heat stress, but protein increased under all stresses.	[12]
Bolero, Mary, Mozart, Tanto		From seed	19 or 24 °C, ambient and elevated [CO2} and [O3]/until maturity	Heat stress alone reduced oil in all cultivars, and combination stress increased oil in Bolero and Tanto.	[14]
Monty, Range, Oscar		29 DAF	Gradual treatment to max 40 °C/45-degree days (DD) more than control, sudden treatment 40 °C/5 days, 15 DD more than control	Oil decreased and protein increased in Monty and Range in sudden treatment only. No change to Oscar.	[16]
Garnet		14 DAF	34 and 40 °C/12 h	At 34 °C, there was no significant decline in total oil content; however, at 40 °C, it declined by ~41%.	[21]
Insignia, Emblem, Surpass400		20 DAF 25 DAF	28 °C/9 days (MH10) 38 °C/4 days (VH5)	MH10 increased oil in all cultivars, most in Surpass400 (+10%), VH5 decreased oil only in Surpass400 (−7%).	[23]
Solar, Lumen		Flowering	5 °C increase daily/15 days	Oil to protein ratio was more impacted in Solar.	[32]
Susceptible: DKW44-10, DKW46-15, HyCLASS225W, Riley, Wichita Tolerant: 46W94, Edimax CL, Hekip, Mercedes, Popular		7 DAF	High night temperature 20 °C/until maturity	Oil reduction of tolerant cultivars 3%, susceptible cultivars 12%. Among tolerant, 46W94 increased by 10% followed by Mercedes which decreased by 2%.	[37]
Aviso		GS72 (20% pods reached maximum size)	RSS (repeated stress sequence) EMS (early mild stress) 4LHP (4 late heat peaks)	Oil content was on average higher in seeds exposed to heat stress at flowering, followed by pods L < 5 cm and then pods L ≥ 5 cm.	[38]
(b)
Species	Cultivar/Genotype	Location	Condition	Impact on Oil	Reference
napus	30+ cultivars	9 sites across Australia	Heat, drought and frost occurred during critical periods at some sites	Delayed planting reduced yield and oil concentration, variety was significant but not reported.	[41]
napus	40+ cultivars	Victoria, Australia	Average daily temperature ranged from 19 to 23 °C across locations during flowering	Low oil content was associated with warm temperature during seed maturation. Strong inverse relationship between oil and protein.	[44]
napus juncea	Monty and Oscar 887.1.6.1, JM 25, JM 33, Muscon, 82 No 22–98	WA, Australia	Average daily temperature reached a maximum of ~35 °C around maturity	Oil and protein were inversely proportional in both species. Oil decreased and protein increased as sowing was delayed. Monty produced the highest oil concentration across environments. 887.1.6.1 produced the highest oil concentration for B. juncea, similar to or greater than Oscar across environments	[42]
napus	Roundup Ready	Washington, USA	Between 3 and 14 days above 28 °C during flowering across locations	Increasing temperature decreased oil content and increased protein. The same relationship was not seen with combined drought stress.	[40]
napus campestris	Zephyr Span	NSW, Australia	Not reported	Oil concentration was higher in earlier plantings, oil and protein were negatively correlated. Oil content was higher in Zephyr.	[28]
napus	Agamax, Hyola4815, Hyola50, Hyola401, Safi6, Zabol9, and Zabol13	Dezful, Iran	Six sowing dates to expose canola to different levels of heat stress	Delayed planting reduced oil content, with the least impact in Hyola50.	[34]
napus	Tower, Rafal and Global	Dezful, Iran	Hot climate (35.5 °C) with a dry, hot summer, irrigation as needed	Oil was only reduced at the latest planting date, suggesting that oil content is more controlled by genotype, but did not report differences between genotypes.	[43]

3.3. Growth Stage

The stage of exposure significantly impacts canola oil content, with flowers exposed to heat stress producing the highest oil content, followed by small pods (<5 cm) and lowest in larger pods (>5 cm) [38]. Therefore, oil accumulation is at the most risk when heat stress occurs during seed filling rather than flowering, as observed in yield.

4. The Impact of Heat Stress on the Fatty Acid Profile

The impact of heat stress on the fatty acid profile is outlined in Table 3. The fatty acid profile of canola can be defined by the saturated fatty acids (SFAs), and the unsaturated fatty acids (UFAs), which can be further divided into monounsaturated (MUFAs) and polyunsaturated fatty acids (PUFAs). The MUFAs of interest are oleic and erucic acid and the PUFAs of interest are linoleic and linolenic acid. When cultivated for human consumption, high oleic acid levels are favoured, with lower linoleic acid levels for enhanced nutrition and shelf stability [4,5]. Low SFA levels are also desired for nutritional benefit [3]. Erucic acid, a fatty acid toxic to humans, must remain below 2% to be fit for consumption [1,3]. There is no clear consensus in the literature on the exact impact of heat stress on the fatty acid profile.

4.1. Monounsaturated Fatty Acids (MUFAs)

A collection of studies reports an increase in oleic acid when exposed to heat stress [23,29,50,51]. Field studies have associated warmer climates and varying rainfall with increased oleic acid content [31,34,44,51,52]. However, other studies report a decrease in oleic acid [12,14,16,21]. One study comparing combined stresses found significant variation in oleic acid content across four cultivars exposed to heat stress alone, reporting cultivars Mozart, Bolero and Tanto to decrease, while one cultivar, Mary, had no change [14]. The combined stress treatments (24 °C + [CO₂] and 24 °C + [CO₂] + [O₃]) either returned oleic acid levels to control (Mary and Mozart) or caused a significant increase (Bolero and Tanto). Along with genotype, other climate parameters may explain some of the variation in oleic acid levels across studies.

Erucic acid is another MUFA of interest in canola. However, there is much less research on the impact of heat stress on erucic acid. In the few studies that have reported the effect of heat stress on erucic acid levels, either no change or a decrease in content was observed, suggesting that the strong genotypic control that was bred into canola is stable under heat stress [16,23,29,34].

4.2. Polyunsaturated Fatty Acids (PUFAs)

Within the studies reporting increased oleic acid, the majority report a concomitant decrease in both linoleic and linolenic acid [23,31,34,44,50,51]. One study found high oleic acid and low linoleic acid content to be associated with temperature extremes, while linolenic acid increased with the maximum growing temperature up to 24 °C, not ruling out the possibility of a reversal at higher temperatures [52]. Deng and Scarth [51] found similar results for extreme temperatures and interestingly found field results to be more significant than when the environment was replicated in a growth chamber, contradicting the opinions of other studies.

Among studies reporting decreased oleic acid levels, there is less consensus regarding the impact on PUFAs. One study found linolenic acid decreased under heat stress, while linoleic concentrations remained unchanged [16]. Conversely, Elferjani and Soolanayakanahally [12] reported increased linoleic acid and minimal change to linolenic acid. Lohani, et al. [21] found linoleic acid increased while linolenic acid decreased.

Table 3. (a) The effect of heat stress on fatty acid profile of canola (Brassica napus) in controlled environment studies. (b) The effect of heat stress on fatty acid profile of oilseed brassicas in field studies.

(a)
Cultivar/Genotype	Growth Stage	Stress/Duration		Impact on Fatty Acid Profile	Reference
N99-508	38 days after sowing	23 or 29 °C, 30% or 90% water holding capacity (WHC)/applied until harvest		At 90% WHC, oleic acid decreased, linoleic acid and SFA increased, and linolenic acid had a non-significant decrease. Combined stress increased linoleic acid and SFA, decreased linolenic acid. No comment on oleic acid.	[12]
Bolero, Mary, Mozart, Tanto	From seed	19 or 24 °C, ambient and elevated [CO2] and [O3]/until maturity		All stresses impacted to some degree; however, heat stress had the biggest impact on fatty acid ratios. Oleic acid decreased in Mozart, Bolero, and Tanto, while no change in Mary. Linolenic acid decreased in all cultivars. Combinations of stresses result in a fatty acid profile that was unexpected, heat stress in combination with [CO2] and ozone [O3] increased oleic acid but linolenic still decreased.	[14]
Monty, Range, Oscar	29 DAF	Gradual treatment to max 40 °C/45-degree days more than control sudden treatment 40 °C/5 days, 15DD more than control		Neither treatment caused significant changes to fatty acid profile for Oscar. For Monty and Range, oleic and linolenic acid decreased, with no change for linoleic acid, erucic non-sig, SFAs palmitic and stearic increased.	[16]
Garnet	14 DAF	34 and 40 °C/12 h		No significant change to oleic acid or SFA at 34 °C. At 40 °C, oleic and linolenic acid decreased, while linoleic acid and SFA increased.	[21]
Insignia, Emblem, Surpass400	20 DAF 25 DAF	28 °C/9 days (MH10) 38 °C/4 days (VH5)		MH10 had a bigger impact on fatty acid ratio: oleic increased (7.5% VH5 vs. 11% MH10, average across cultivars), linoleic and linolenic decreased, erucic unchanged, palmitic and stearic increased. Surpass400 and Emblem had higher oleic acid values (initial and after heat stress) than Insignia.	[23]
Regent, Stella (HOLL cultivar)	First flower	12, 25 or 30 °C/10, 20, 30, or 40 days		Oleic acid increased, linoleic and linolenic acid decreased in both cultivars. Linolenic acid decreased significantly with longer duration heat stress in both cultivars (oleic not reported). Heat stress only increased SFA in Stellar. Overall, Regent was less impacted by heat stress.	[51]
Primor	End of flowering	12, 17, 22, 27 °C/4–8 weeks		In the 22 and 27 °C treatment, oleic acid increased (60% after 8 weeks of heat stress), linoleic and linolenic acid decreased.	[50]
Norin, Westar (low erucic acid)	Bolting	17, 22, 27 or 32 °C/until maturity		At 32 °C no fruit was obtained, and at 27 °C insufficient seed number for fatty acid analysis. Oleic acid increased to a maximum of 67% in Westar, along with a decrease in linolenic acid. Erucic acid decreased in Norin under heat stress, along with increased oleic and linoleic acid, and no impact on linolenic acid.	[29]
Open pollinated (susceptible): DKW44-10, DKW46-15 Hybrids (tolerant): Edimax CL, Mercedes	7 DAF	High night temperature 20 °C/until maturity		SFA increased in susceptible cultivars, but no significant change on unsaturated fatty acids (UFAs). No change to fatty acid profile for tolerant cultivars.	[37]
Zheyou-50, Jiuer-13	Flowering	High night temperature (HNT) 25 °C/until maturity		HNT decreased total fatty acid content by 13.7% (Zheyou-50) and 18.9% (Jiuer-13). HNT reduced oleic acid, increased linoleic and linolenic acid in both cultivars.	[53]
(b)
Species	Cultivar/Genotype	Location	Condition	Impact on Fatty Acid Profile	Reference
napus	ATR-Beacon, ATR-Eyre, Karoo, Surpass300TT, Surpass501TT	14 cropping environments across Australia	Average maximum temperature ranged from 16 to 25 °C across locations throughout growing season	Higher oleic acid, and lower linoleic acid at temperature extremes (high or low), while linolenic increased with temperature. ATR-Beacon had the highest oleic acid content at all temperatures (62% oleic acid averaged across all locations). SFA decreased with increased maximum temperature.	[52]
napus	Regent, Stella	9 cropping environments across Canada	Average temperature ranged from 10 to 25 °C across locations throughout growing season	High temperature locations associated with increased saturated fatty acid (SFA), increased oleic acid and decreased linolenic acid for both cultivars.	[51]
napus	40+ cultivars	7 cropping environments across Victoria, Australia	Average daily temperature ranged from 19 to 23 °C across locations during flowering	Warmer climates increased oleic acid content and reduced linoleic or linolenic content.	[44]
napus juncea	6 conventional, 4 Clearfield, and 7 triazine-tolerant genotypes 11 genotypes	11 cropping environments across Australia	Average maximum temperature ranged from 17 to 30 °C across locations in last 3 months of growth	FA changes from genotype significant in conventional but not Clearfield or triazine-tolerant breeding lines in B. napus. Locations with increased temperature and reduced rainfall resulted in lower oleic acid and higher linolenic acid.	[31]
napus	Agamax, Hyola4815, Hyola50, Hyola401, Safi6, Zabol9 and Zabol13	Dezful, Iran	Six sowing dates to expose canola to different levels of heat stress	Delayed planting reduced palmitic, linoleic and linolenic acid, while increasing stearic and oleic acid. Hyola50 maintained the highest levels of oleic acid under heat stress.	[34]
napus	Baldur, Californium, Candice, Caracas, Castille, Explus, Siska, Tenno	17 cropping environments across Austria	Average daily temperature ranged from 20 to 22 °C across locations in last month of growth	Increased temperature at maturation reduced linolenic acid, with no impact on linoleic acid.	[54]
napus	16 genotypes	14 cropping environments across Australia	Early season had more days above 25 °C than mid-season	SFAs were higher in early planting than mid-season. Mid-season had fewer temperature peaks and higher rainfall than early season.	[55]

Table 3. (a) The effect of heat stress on fatty acid profile of canola (Brassica napus) in controlled environment studies. (b) The effect of heat stress on fatty acid profile of oilseed brassicas in field studies.

(a)
Cultivar/Genotype	Growth Stage	Stress/Duration		Impact on Fatty Acid Profile	Reference
N99-508	38 days after sowing	23 or 29 °C, 30% or 90% water holding capacity (WHC)/applied until harvest		At 90% WHC, oleic acid decreased, linoleic acid and SFA increased, and linolenic acid had a non-significant decrease. Combined stress increased linoleic acid and SFA, decreased linolenic acid. No comment on oleic acid.	[12]
Bolero, Mary, Mozart, Tanto	From seed	19 or 24 °C, ambient and elevated [CO2] and [O3]/until maturity		All stresses impacted to some degree; however, heat stress had the biggest impact on fatty acid ratios. Oleic acid decreased in Mozart, Bolero, and Tanto, while no change in Mary. Linolenic acid decreased in all cultivars. Combinations of stresses result in a fatty acid profile that was unexpected, heat stress in combination with [CO2] and ozone [O3] increased oleic acid but linolenic still decreased.	[14]
Monty, Range, Oscar	29 DAF	Gradual treatment to max 40 °C/45-degree days more than control sudden treatment 40 °C/5 days, 15DD more than control		Neither treatment caused significant changes to fatty acid profile for Oscar. For Monty and Range, oleic and linolenic acid decreased, with no change for linoleic acid, erucic non-sig, SFAs palmitic and stearic increased.	[16]
Garnet	14 DAF	34 and 40 °C/12 h		No significant change to oleic acid or SFA at 34 °C. At 40 °C, oleic and linolenic acid decreased, while linoleic acid and SFA increased.	[21]
Insignia, Emblem, Surpass400	20 DAF 25 DAF	28 °C/9 days (MH10) 38 °C/4 days (VH5)		MH10 had a bigger impact on fatty acid ratio: oleic increased (7.5% VH5 vs. 11% MH10, average across cultivars), linoleic and linolenic decreased, erucic unchanged, palmitic and stearic increased. Surpass400 and Emblem had higher oleic acid values (initial and after heat stress) than Insignia.	[23]
Regent, Stella (HOLL cultivar)	First flower	12, 25 or 30 °C/10, 20, 30, or 40 days		Oleic acid increased, linoleic and linolenic acid decreased in both cultivars. Linolenic acid decreased significantly with longer duration heat stress in both cultivars (oleic not reported). Heat stress only increased SFA in Stellar. Overall, Regent was less impacted by heat stress.	[51]
Primor	End of flowering	12, 17, 22, 27 °C/4–8 weeks		In the 22 and 27 °C treatment, oleic acid increased (60% after 8 weeks of heat stress), linoleic and linolenic acid decreased.	[50]
Norin, Westar (low erucic acid)	Bolting	17, 22, 27 or 32 °C/until maturity		At 32 °C no fruit was obtained, and at 27 °C insufficient seed number for fatty acid analysis. Oleic acid increased to a maximum of 67% in Westar, along with a decrease in linolenic acid. Erucic acid decreased in Norin under heat stress, along with increased oleic and linoleic acid, and no impact on linolenic acid.	[29]
Open pollinated (susceptible): DKW44-10, DKW46-15 Hybrids (tolerant): Edimax CL, Mercedes	7 DAF	High night temperature 20 °C/until maturity		SFA increased in susceptible cultivars, but no significant change on unsaturated fatty acids (UFAs). No change to fatty acid profile for tolerant cultivars.	[37]
Zheyou-50, Jiuer-13	Flowering	High night temperature (HNT) 25 °C/until maturity		HNT decreased total fatty acid content by 13.7% (Zheyou-50) and 18.9% (Jiuer-13). HNT reduced oleic acid, increased linoleic and linolenic acid in both cultivars.	[53]
(b)
Species	Cultivar/Genotype	Location	Condition	Impact on Fatty Acid Profile	Reference
napus	ATR-Beacon, ATR-Eyre, Karoo, Surpass300TT, Surpass501TT	14 cropping environments across Australia	Average maximum temperature ranged from 16 to 25 °C across locations throughout growing season	Higher oleic acid, and lower linoleic acid at temperature extremes (high or low), while linolenic increased with temperature. ATR-Beacon had the highest oleic acid content at all temperatures (62% oleic acid averaged across all locations). SFA decreased with increased maximum temperature.	[52]
napus	Regent, Stella	9 cropping environments across Canada	Average temperature ranged from 10 to 25 °C across locations throughout growing season	High temperature locations associated with increased saturated fatty acid (SFA), increased oleic acid and decreased linolenic acid for both cultivars.	[51]
napus	40+ cultivars	7 cropping environments across Victoria, Australia	Average daily temperature ranged from 19 to 23 °C across locations during flowering	Warmer climates increased oleic acid content and reduced linoleic or linolenic content.	[44]
napus juncea	6 conventional, 4 Clearfield, and 7 triazine-tolerant genotypes 11 genotypes	11 cropping environments across Australia	Average maximum temperature ranged from 17 to 30 °C across locations in last 3 months of growth	FA changes from genotype significant in conventional but not Clearfield or triazine-tolerant breeding lines in B. napus. Locations with increased temperature and reduced rainfall resulted in lower oleic acid and higher linolenic acid.	[31]
napus	Agamax, Hyola4815, Hyola50, Hyola401, Safi6, Zabol9 and Zabol13	Dezful, Iran	Six sowing dates to expose canola to different levels of heat stress	Delayed planting reduced palmitic, linoleic and linolenic acid, while increasing stearic and oleic acid. Hyola50 maintained the highest levels of oleic acid under heat stress.	[34]
napus	Baldur, Californium, Candice, Caracas, Castille, Explus, Siska, Tenno	17 cropping environments across Austria	Average daily temperature ranged from 20 to 22 °C across locations in last month of growth	Increased temperature at maturation reduced linolenic acid, with no impact on linoleic acid.	[54]
napus	16 genotypes	14 cropping environments across Australia	Early season had more days above 25 °C than mid-season	SFAs were higher in early planting than mid-season. Mid-season had fewer temperature peaks and higher rainfall than early season.	[55]

A combined stress study reported a decrease in both oleic and linolenic levels under heat stress; however, unlike oleic acid, linolenic acid levels were not remediated by combined stress treatments (24°C + [CO₂] and 24°C + [CO₂] + [O₃]) [14]. These differences are likely influenced by genotype; therefore, selecting for genotypes with reduced linolenic acid may benefit oil stability.

Focused solely on PUFAs, Werteker, et al. [54] found that increased temperature in the last 30 days of maturation resulted in reduced linolenic acid while having no impact on linoleic acid. While temperatures increased, the maximum temperatures were only moderate, which the researchers suggested was the reason for no impact on linoleic acid concentrations.

4.3. Saturated Fatty Acids (SFAs)

Saturated fatty acids have been widely reported to increase under heat stress regardless of the oleic acid response [12,16,21,23,29,51,55]. One study reported a decrease in SFA under increased temperature in field studies; however, the maximum temperature reached (25 °C) was not high enough for heat stress, thus not ruling out the possibility of an increase at higher temperatures [52]. A field study observed a mixed response of SFAs under delayed planting, with stearic acid increasing, but palmitic acid decreasing [34].

4.4. Genotype

Across the literature, genotypes identified to have the best heat tolerance in terms of fatty acid profile are those with either no change or an increase in oleic acid. Cultivar Oscar showed no shift in oleic acid in either a gradual or sudden treatment, while Monty and Range both decreased, suggesting that Oscar has a robust genotype that is well suited to different heat stress conditions [16]. Genotypes that conferred an increase in oleic acid under controlled environment conditions included cultivars Insignia, Emblem, Surpass400, Reagent, Stellar, and Primor. Within these, Aksouh-Harradj, et al. [23] found Emblem and Surpass400 to have higher original values of oleic acid than Insignia; however, they all increased under moderate, prolonged heat stress (28 °C, 9 days). Deng and Scarth [51] reported increases in both Reagent and Stellar; however, SFA increased in Stellar leading Reagent to be more heat tolerant. These trends were also reflected in the field: under even longer heat stress (27 °C, 8 weeks), oleic acid levels increased by 60% in Primor; therefore, this cultivar may be more suitable for environments dealing with prolonged temperature extremes [50]. Yaniv, et al. [29] found oleic acid to increase up to 67% in Westar when mild heat stress (22 °C) was applied from flowering to maturity; however, further investigation of higher heat stresses applied at later growth stages is needed to establish if the cultivar could perform under higher temperatures. In field studies, an increased oleic acid level was reported in several cultivars. One study reported an increase in all cultivars grown, with ATR-Beacon producing the highest oleic acid content under heat stress (62%) [52]. Cultivar Hyola50 was identified in a delayed planting study to have the highest increase in oleic acid in response to heat stress [34]. Another study reported changes to the fatty acid profile that were only significant in traditional cultivars and not Clearfield or triazine-tolerant cultivars, suggesting that while they were not bred intentionally for high-temperature tolerance, they do confer heat tolerance [31]. Further investigations on the unique genetic characteristics of these cultivars may elucidate the conferred heat tolerance.

4.5. Heat Stress Condition (Duration and Intensity)

Sudden heat stress had a greater impact than a gradual system on the fatty acid ratio, despite the gradual system overall exposing the plants to a greater heat load, suggesting the ability of canola to adapt to heat stress when gradually exposed [16]. In a later study, longer, moderately high heat stress had a greater impact than short, high heat stress, whereas the opposite was observed for yield and oil content [23]. The fatty acid profiles of these studies were contradictory despite the almost identical conditions of the sudden heat stress and short high heat stress. The researchers did not provide reasons for why these results differed so drastically; however, the use of different genotypes was likely to be the cause. In the later study, desaturation activity was reduced more in the longer moderate heat stress, suggesting desaturases responsible for changes in the fatty acid profile may be more affected by the longevity of stress rather than intensity [23]. Following the pattern of yield and oil response, this suggests that under gradual exposure, canola can adapt to stress, but under continuous moderate stress, that same adaptation does not occur. One study reported no change in fatty acid profile for one cultivar (Mary). However, the heat stress temperature (24 °C) was lower than in other studies. Thus, further investigation is required to determine suitability under higher heat stresses [14]. Similarly, no change in the fatty acid profile was found for Garnet. However, this was under a short duration heat stress (12 h), meaning results may be different under a longer duration and thus need to be investigated [21].

High night temperatures have also been associated with changes to the fatty acid profile. However, results varied, likely due to genotypic differences [37,53]. One study reported no change in UFA for both susceptible and tolerant cultivars, but increased SFA in susceptible cultivars, suggesting possible heat-tolerant candidates [37]. In contrast, another study reported decreased oleic acid and increased linoleic and linolenic acids [53].

4.6. Growth Stage

A study comparing multiple different heat stress modalities found the general response to heat stress was a reduction in UFA [38]. The fatty acid profile was only significantly impacted in pods < 5 cm, while flowers and larger pods were not significantly affected. While stress during flowering had a more significant impact on yield, stress during seed filling had a greater impact on the fatty acid profile, as observed for oil yield. One study found that the impact on the fatty acid profile was greater in the mainstem than in the general bulk when heat stress occurred during flowering, with the desaturation activity of oleic acid being highest in the earliest formed seeds compared to the bulk [23]. Therefore, stress at flowering on the mainstem may be less detrimental to the overall fatty acid profile than later heat stresses, as the progression of lateral branches is delayed.

5. Gaps and Limitations of the Literature

There appears to be a substantial difference in the effects of heat stress on yield, oil, and protein content and fatty acid profile when comparing controlled environment studies with field studies. While Schulte, et al. [56] were able to create a model based on previous literature that accurately predicted the oil and fatty acid profile of field-grown canola in Manhattan, another group found the fatty acid profiles of two B. napus cultivars varied significantly between field and controlled environment treatments [53]. Controlled environment studies may be able to show the direction of the fatty acid profile under heat stress but cannot quantify the degree of change that will be seen in the field. Field studies with controlled irrigation may provide a better understanding of in-field heat stress patterns to elucidate if the differences between current field and controlled environment studies are due to water availability.

Further investigations into the variation of fatty acid profiles in response to heat stress are needed to elucidate why particular cultivars, such as Clearfield and triazine tolerant cultivars, perform better than conventional cultivars [31].

Limited studies have been conducted on the effects of other abiotic stresses combined with heat stress on canola. Singer, et al. [57] and Raza [58] summarised the impacts of other abiotic stresses on a variety of oilseeds, but there is limited assessment of combined stresses. Heat stress, in conjunction with high solar radiation, can exacerbate its effects, reducing canola oil content [59]. Conversely, elevated [CO₂] and ozone [O₃] in conjunction with high temperatures impact seed yield and oil content to a degree not predicted by combining isolated effects, showing complexity in climate models that requires further study [14].

In a meta-analysis by Secchi, et al. [60], drought stress had an overall yield reduction of 21%, impacting oil yield. However, a significant discrepancy was observed between field studies and controlled environment studies. These confounding data are likely explained by other environmental stresses, as field studies assessing drought stress must also account for the confounding impact of high temperature [19,26,31,40,42,44,45,46,49,52]. Combined rainfall and temperatures can impact canola yield, oil, and protein content, and alter the fatty acid profile [52]. Cooler, wetter climates increase oil content while reducing protein content, whereas drought and temperature stress can reduce yield, oil, and oleic acid contents and increase linoleic and linolenic acid [26,31,44]. However, the opposite impact on the fatty acid profile has also been reported [40], which may be due to differing high-temperature stress or genotypes used. Controlled environment studies compared the effects of drought, heat, and combined stress and found that combined stress had the greatest impact on yield, followed by heat and then drought, suggesting that drought with heat is the more concerning issue for the future of canola production [12,27]. This is further supported by research that heat stress decreases the water use efficiency of canola, minimising the positive effects of irrigation [61]. As heat stress is likely to occur in combination with drought stress, a better understanding of combined-stress controlled environment studies based on future climate models may be required for breeding more tolerant lines in regions where irrigation is not common.

An integration of disciplines to understand the molecular mechanisms behind heat stress response, as explored by Ahmad, et al. [62], will also enable more targeted breeding strategies in the future.

6. Future Perspectives

Extreme heat events are occurring more frequently worldwide, resulting in significant declines in both yield and quality, threatening global food security. A number of studies have investigated the canola response to heat stress in controlled environments as well as field conditions, and some potential heat-tolerant materials have been identified from various germplasm resources [16,18,23,32,34]. In Australia, for example, 323 genotypes from a genetically diverse B. napus population were screened for heat stress tolerance using a prototype heat screening facility over multiple years [63].

In order to elucidate the genetic and molecular basis of these tolerant traits, multi-omics approaches have been applied in canola heat tolerance research. At the genomics level, a genome-wide association study identified some quantitative trait loci (QTL) for heat stress tolerance in spring-type B. napus [64] and B. rapa. QTL for heat stress tolerance were found to be distributed across the genome and occur in diverse genetic groups, flowering phenologies, and morphotypes [30]. Genome-wide association studies identified 34 QTLs, from which 334 candidate genes are potentially related to heat stress tolerance [63]. A comprehensive set of heat shock–responsive genes were identified across 32 Brassicaceae species [65]. Among stress-responsive genes, heat shock proteins and transcription factors are functionally stimulated by heat. Together, they maintain protein stability and mitigate heat damage. Three B. napus heat shock-responsive genes co-located with heat tolerance QTLs and highly expressed under stress, and these are strong candidates for functional studies and marker-assisted breeding.

At the transcriptomics level, RNA sequencing (RNA-seq) and quantitative RT-PCR (qRT-PCR) have been applied to provide an in-depth understanding of the cellular and molecular responses underlying plant adaptation to environmental stresses. Through transcriptional profiling, heat responsive genes were identified in B. napus siliques at the seed-filling stage [48]. Heat stress suppressed canola seed oil accumulation by inhibiting photosynthesis and the BnWRI1 pathway [47]. Molecular events associated with impaired pollen–pistil interactions were identified under heat stress in B. napus [66].

At the proteomics level, proteomic changes under heat stress were investigated at the early seedling stage in B. napus [67]. A recent proteomic analysis revealed a substantial number of differentially abundant proteins (DAPs) involved in post-pollination responses to heat stress in B. napus [68]. Three candidate proteins (0A078I8F7, A0A078JBL3, and A0A078JJT8) were found to be consistently upregulated in all cultivars and at all time points. Pathway enrichment analysis highlighted the significant roles of apoplast and ER proteins across all cultivars tested. The identified candidate proteins and pathways may serve as valuable resources for future breeding programs developing heat-tolerant canola.

At both transcriptomic and metabolomic levels, the canola’s responses to heat stress were investigated [69]. Heat stress not only reduced yield and altered oil composition, but also increased the content of carbohydrates such as glucose, fructose, and sucrose as well as aliphatic glucosinolates in the leaves, whilst it decreased the content of the indolic glucosinolate. RNA-Seq analysis of flower buds revealed a large number of differentially expressed genes (DEGs) at 0, 1, and 2 days after treatment, and at 1 and 7 days of recovery. Heat treatment resulted in downregulation of genes involved in respiratory metabolism, sugar transporters, nitrogen transport and storage, cell wall modification, and methylation. In contrast, upregulated genes mapped to small heat shock proteins (sHSP20) and other heat shock factors that play important roles in thermotolerance [69].

Interestingly, an operative natural gene on–off system for the molecular mechanism behind heat stress–induced deterioration of rice grain quality was reported recently [70]. A natural QT12 locus in rice was controlled by upstream Nuclear Factor Y transcriptive factors, leading to its insensitive response to heat stress, thereby improving grain quality and yield by balancing endosperm storage substances in rice. Low QT12 expression confers superior grain quality and increases elite rice up to 1.31–1.93 times under high temperature trials [70]. Whether the result is positive or negative, the regulators play a critical role in plant response to various environmental factors. It is worth exploring similar gene on–off systems in canola.

In summary, the global genomics, transcriptomics, and proteomics profiling, integrated with the metabolic analysis, shed light on key genes and metabolic pathways impacting and responding to heat stress at the reproductive stage. These DEGs, DAPs, and metabolites, once their functions are validated, could serve as important biomarkers for marker-assisted selection and genomic selection in the development of climate-resilient canola to face climate change challenges.

7. Conclusions

The consensus from the literature is that heat stress is a concern in canola production systems because of its major impact on yield, oil content, oil quality, and fatty acid profile compared to other abiotic stresses. The combination of heat and drought stress, however, is vital as these stresses are likely to occur simultaneously as the impacts of climate change intensify. A substantial difference in findings was identified between field and greenhouse studies.

Yield is reduced under heat stress, and is impacted by genotype, heat stress condition, and growth stage exposed. Genotype differences provide the opportunity to develop more heat-tolerant lines through selective breeding, possibly incorporating traits from B. juncea. Yield responds differently to heat stress types, such that there is a possibility of adaptation to slower onset heat stresses, while sudden and intense heat stresses are more detrimental to yield. Canola is most impacted by heat stress during the flowering and early pod stages for all three parameters, and because of its indeterminate growth pattern, there is a larger window for when heat stress will be detrimental to production.

Oil and protein content are negatively correlated under heat stress, with oil content decreasing; however, certain genotypes are able to avoid this relationship and may be used for breeding.

The high variation in fatty acid profile response suggests that there are already genotypes available that have desirable responses to heat stress. As canola is favoured for its high oleic and low linolenic content, genotypes that result in higher oleic acid and lower linolenic acid under heat stress are likely to be favourable for breeding programs; however, impacts on yield will also need to be taken into account. The general increase in saturated fatty acids is of concern as no tolerant genotypes have yet been identified. Heat stress condition impacts the fatty acid profile similarly to yield, again suggesting the ability to adapt to slower onset heat stresses, while an inability to respond to sudden heat stress.

Further field studies controlling water stress may elucidate the differences between field and controlled environment studies. Further combined heat and drought stress studies may provide a better understanding of the impact of climate change and provide better guidance in breeding heat-tolerant lines.