Alleviation of Chilling Injury in Postharvest Sweet Basil (Ocimum basilicum L.) with Silicon and Abscisic Acid Applications

Abstract

Sweet basil (Ocimum basilicum L.) is highly susceptible to chilling injury (CI), resulting in the development of CI symptoms during cold storage that reduce postharvest quality and shelf life. This study evaluated whether silicon (Si) and abscisic acid (ABA) applications can mitigate these symptoms. In Trial 1, basil plants had a Si solution (189 mg/L Si from potassium silicate) or deionised water (control) applied during cultivation via rootzone irrigation or foliar spray. Some plants were also foliar sprayed with ABA (1000 mg/L) before harvest. In Trial 2, wollastonite was added to the growing media (0, 1, 2, 3, 4, 5 mL/L) as the Si source. Applying the Si solution using either method reduced leaf necrosis, fresh weight loss, and electrolyte leakage, extending shelf life to at least 14 days. There were also no negative impacts on plant performance during cultivation (chlorophyll content, shoot height, and canopy width). The ABA solution, alone or in combination with Si solution, reduced symptoms but less effectively, extending shelf life up to 8 days. Wollastonite had no positive effects. These findings suggest that Si solution applications are a promising strategy to alleviate CI during postharvest cold storage of basil at 3.5 °C.

1. Introduction

Sweet basil (Ocimum basilicum L.), referred hereafter as basil, is a popular culinary herb known to be highly chilling-sensitive and susceptible to chilling injury (CI) when exposed to temperatures below 12 °C [1]. CI refers to the physiological damage that plants incur from exposure to low, non-freezing temperatures below their tolerable range [2]. Some common visible symptoms of CI include surface lesions (spotting), discolouration, and wilting [1]. The onset and severity of CI symptoms contributes to shorter shelf lives and lower marketability due to accelerated decay and reduced commercial quality (i.e., poorer appearance, taste, smell, nutritional value, etc.), causing significant postharvest losses and impacting the potential economic value of affected commodities. Since basil can often be transported and stored alongside other fresh produce, the majority of which is optimally maintained when kept between 0 and 10 °C [3], it is not always possible to avoid cold exposure. Basil develops moderate to severe CI symptoms at those temperatures, resulting in a shelf life of up to 9 days when kept at 10 °C, 5 days at 7.5 °C, 3 days at 5 °C, and 1 day at 2.5 or 0 °C [1,4]. Being able to use low storage temperatures can help inhibit the growth of fungi and bacteria on basil [5,6], and would allow for easier handling by growers, distributors, processors, and retailers in the supply chain.

Several strategies have been observed to alleviate the severity of CI symptoms in postharvest basil kept at or below 10 °C. Some of these methods include controlling the amount of CO₂ in the atmosphere [7], chill hardening [8], heat treatments [5], electrical lighting treatments [9], nano-curcumin and nano-rosemarinic acid applications [10], abscisic acid (ABA) applications [11], lactic acid and hydrogen peroxide applications [12], nitric oxide applications [13], and selenium amendments [14]. However, no methods to our knowledge have been widely adopted by the industry, possibly due to limited effectiveness or feasibility. This indicates a need for a more practical and effective method to preserve the postharvest quality and shelf life of basil.

Silicon (Si) applications have been shown to confer crops with greater resistance to chilling stress. Li et al. [15] found that Si amendments (75 ppm) to the nutrient solution of hydroponically grown basil could reduce the rate of injuries and death during an accidental cold shock event. This shows the potential of Si applications as a CI alleviation strategy for basil; however, the study was not conducted during postharvest and did not measure any specific CI symptoms. Positive effects were also observed in other cold-stressed plants, such as barley plants [16], citrus trees [17], Chinese cabbage plants [18], cucumber plants and fruits [19,20], grapevine plants [21], maize plants [22], tomato plants [23,24], and turfgrass [25]. However, aside from the study on cucumber fruits, these studies did not investigate postharvest CI alleviation or shelf life since they examined live plants.

CI is suggested to be caused by the overproduction and accumulation of reactive oxygen species (ROS), resulting in oxidative damage to the cell membrane and other cell components through lipid peroxidation, disrupting the structural integrity of the membrane [2]. This leads to the loss of water and solutes from cells, accelerating tissue senescence [2]. The accumulation of ROS is also suggested to trigger stress signalling pathways that promote programmed cell death, resulting in necrosis [25]. The role of silicon in CI alleviation is still under investigation and may involve several mechanisms. However, several studies have shown that silicon increases the expression and activities of antioxidant enzymes which scavenge and reduce ROS, corresponding to observed improvements in the cold tolerance of various plants [16,19,23,26].

Despite growing interest in the role of silicon in cold tolerance and CI alleviation, there is currently no research that specifically investigates its efficacy on postharvest basil during cold storage. Of the CI alleviation strategies that have been tested on postharvest basil, the study conducted by Satpute et al. [11] using ABA foliar applications before harvest tested the lowest storage temperature (3.5 °C) while observing a significant reduction in visible CI symptoms at day 9, the end of the test. Therefore, the objectives of this study were to (1) determine whether Si treatments applied during cultivation via rootzone irrigations, foliar sprays, or growing media amendments could alleviate CI symptoms and extend the shelf life of postharvest basil stored at 3.5 °C, and (2) compare the efficacy of Si and ABA applications, both individually and in combination.

2. Materials and Methods

2.1. Plant Materials and Growing Conditions

Sweet basil (Ocimum basilicum L.) seeds of the cultivar “Genovese” (High Mowing Organic Seeds, Wolcott, VT, USA) were sown in 0.5 L fibre pots (~20 seeds/pot) containing organically certified growing medium (Greenbelt Greenhouse Ltd., Hamilton, ON, Canada) that consisted of 20–30% peat/perlite, 70–80% “compost”, and a fertility pack that included 7–3–2 granular, calcitic limestone, feather meal, fulvic acid, humic acid, Acadian seaweed, and yucca. The “compost” refers to composted material from used growing media (peat-based) with plant residues (e.g., leftover roots and older leaves and stems of baby greens) as feed stock.

During cultivation, the pots were kept on a bench top in a greenhouse located at the University of Guelph Edmund C. Bovey Building. The greenhouse conditions were set at average temperatures of 21/18 °C (day/night), 62% relative humidity, and natural light conditions over a photoperiod of 15 h, supplemented as necessary with high pressure sodium lights at a plant canopy PPFD of 40 µmol/m²⋅s. The plants were thinned to 10 plants/pot once established (~3 weeks after seeding). The pots were then randomly arranged and rotated every 3–4 days. The plants were irrigated as needed to keep the growing media moist, typically every 1–2 days. Organic growing practices were followed, other than the Si and ABA solutions applied in trial 1. Since organic fertilisers were incorporated in the growing medium, only water without fertiliser was used for irrigation.

2.2. Treatments

Two trials were conducted to evaluate the effectiveness of Si and ABA applications at alleviating CI symptoms and extending the shelf life of postharvest basil during cold storage at 3.5 °C.

2.2.1. Trial 1: Silicon and Abscisic Acid

This trial was seeded on 3 April 2024. A Si solution, applied once plants were established (~3 weeks after seeding), was made with deionised (DI) water and a commercial liquid potassium silicate (K₂SiO₃) source that is 2% Si (Pro-Tekt 0–0–3; SUPERthrive, Los Angeles, CA, USA). The Si solution used 3.75 mL of K₂SiO₃/L of DI water, which had a Si concentration of 189 mg/L measured by a commercial laboratory (SGS Canada Inc., Guelph, ON, Canada). The Si solution, or DI water as a control, was applied via rootzone irrigation as needed to keep the growing media moist (every 1–2 days) or by foliar spray until runoff once a week. To compare the efficacy of Si and ABA applications, individually or together, some plants were also foliar sprayed with a 1000 mg/L ABA solution one day before harvest until runoff. The ABA solution was made with DI water and a commercial ABA source that is 20.0% (w/w) ABA (ProTone SG Plant Growth Regulator; Valent BioSciences, Libertyville, IL, USA), in combination with 0.05% (v/v) of Tween 20 as an adjuvant.

There were 7 replicate pots prepared for each of the 9 treatments (63 pots total, each with 10 plants). The treatments were as follows:

• CK (control) = rootzone irrigation with DI water as needed;
• CK+DI = rootzone irrigation with DI water as needed + foliar spray application with DI water a day before harvest;
• Si-irrigation+DI = rootzone irrigation with Si solution as needed + foliar spray application with DI water a day before harvest;
• DI-spray+DI = foliar spray application with DI water once a week, rootzone irrigation with DI water as needed + foliar spray application with DI water a day before harvest;
• Si-spray+DI = foliar spray application with Si solution once a week, rootzone irrigation with DI water as needed + foliar spray application with DI water a day before harvest;
• CK+ABA = rootzone irrigation with DI water as needed + foliar spray application with ABA solution a day before harvest;
• Si-irrigation+ABA = rootzone irrigation with Si solution as needed + foliar spray application with ABA solution a day before harvest;
• DI-spray+ABA = foliar spray application with DI water once a week, rootzone irrigation with DI water as needed + foliar spray application with ABA solution a day before harvest;
• Si-spray+ABA = foliar spray application with Si solution once a week, rootzone irrigation with DI water as needed + foliar spray application with ABA solution a day before harvest.

2.2.2. Trial 2: Wollastonite

This trial was seeded on 21 March 2024. The Si treatments were incorporated before seeding through amendments to the growing media using wollastonite, which consists of rock dust from calcium silicate deposits containing 27% Si (Black Swallow Living Soils, Brantford, ON, Canada). Wollastonite was added to growing media and mixed thoroughly at application rates of 0 (control), 1, 2, 3, 4, or 5 mL of wollastonite/L of growing media. There were 10 replicate pots prepared for each of the 6 treatments (60 pots in total, each with 10 plants).

2.3. Harvest and Cold Storage Test

The plants were harvested 8 weeks after seeding and packaged into polyethylene terephthalate clamshells (19.5 × 17.0 × 7.0 cm) that were divided into quarters. Each quarter contained a replicate consisting of the aboveground tissues (leaves on stem) of all the basil plants from a pot, excluding any damaged leaves which were removed before packaging. The clam shells were stored at a temperature of 3.56 ± 0.2 °C and a relative humidity (RH) of 83 ± 0.8% for 14 days (or earlier if samples all reached the end of their shelf life before then). A data logger recorded the temperature and RH throughout the storage period.

2.4. Treatment Effects

Treatment effects on plant performance during cultivation were evaluated by plant growth (shoot height and canopy width) and leaf chlorophyll content index (CCI). Treatment effects on CI symptoms during cold storage were evaluated by chilling injury index (CII) scores, fresh weight loss (FWL), and leaf electrolyte leakage (LEL).

2.4.1. Plant Growth

Plant growth was evaluated for each replicate before harvest by measuring shoot height and canopy width with a ruler while the plants were in their pots. Shoot height was measured starting from above the growing media and the canopy width using the widest side.

2.4.2. Leaf Chlorophyll Content Index (CCI)

The relative chlorophyll content (greenness) of fully expanded leaves was measured before harvest using a chlorophyll content meter (CCM-200, Opti-Sciences, Hudson, NH, USA). Three CCI measurements were obtained from visually representative leaves and averaged to represent each replicate.

2.4.3. Chilling Injury Index (CII) Scores

The relative severity of leaf necrosis caused by CI was scored using a visually representative leaf from each replicate every two days during cold storage. The CII scores were used to calculate the percentage of plants from each treatment group that remained marketable on each observation day, as well as to determine the rate at which plants surpassed the limit of marketability. Scoring was based on the following scale (Figure 1) adapted from Wongsheree et al. [27]:

• 0 = no visible damage;
• 1 = spots covering <10% of leaf surface;
• 2 = spots covering 10–30% of leaf surface;
• 3 = spots covering 30–50% of leaf surface;
• 4 = spots covering >50% of leaf surface.

A score of 2 was considered the limit of marketability (end of shelf life). Photographs were taken of the representative leaf from each treatment after scoring.

2.4.4. Fresh Weight Loss (FWL)

The initial weight was measured for each replicate after harvest and then reweighed every two days during cold storage to get the current weight using a precision balance (EG 2200-2NM, Kern & Sohn, Balingen, BW, Germany). FWL was calculated according to Equation (1):

FWL (%) = [(initial weight − current weight)/initial weight] × 100

(1)

2.4.5. Leaf Electrolyte Leakage (LEL)

LEL was measured for five randomly selected replicates/treatments according to the method described by Campos et al. [28], with minor adjustments if there were significant visual differences observed among treatments during postharvest cold storage.

Ten leaf disks (~0.6 cm in diameter or 0.28 cm² each) were cut from fully expanded leaves that were visually representative of the replicate. The leaf disks were floated on 25 mL of DI water in Erlenmeyer flasks at room temperature. After 22–24 h, a conductivity meter (PC 300, Oakton Instruments, Charleston, SC, USA) was used to measure the initial electrical conductivity (EC1) of the water. The flasks were then put into an oven set at 90 °C for 2 h to obtain the total conductivity (EC2) after allowing the water to cool to room temperature. LEL was calculated according to Equation (2):

LEL (%) = (EC1/EC2) × 100

(2)

2.4.6. Statistical Analysis

Both trials were conducted using a complete randomised design (CRD).

The data from all measurements except CII scores were analysed using Prism 10 Statistics (GraphPad, version 10.4.1). Analysis of variance (ANOVA) followed by Tukey’s test were conducted to compare the significant differences among treatments for shoot height, canopy width, leaf CCI, FWL, and LEL. All differences were considered significant when p < 0.05.

Analysis of CII scores followed methods described by Nesi et al. [29] and were performed using the survival package in R (version 4.4.1). Kaplan–Meier curves were used to evaluate differences among treatments in their rates of time-to-event. Time-to-event was how many days it took samples to reach a CII score of 2 or greater (the limit of marketability and end of shelf life). Significant differences among treatments were determined using log-rank test. All differences were considered significant when p < 0.05.

3. Results

3.1. Trial 1: Silicon and Abscisic Acid

3.1.1. Plant Growth and Leaf Chlorophyll Content Index (CCI)

The application of the Si solution, via rootzone irrigation or foliar spray (Si-irrigation+DI and Si-spray+DI), did not have any treatment effects on canopy width (18 ± 0.2 cm) and CCI (5.5 ± 0.11) during the cultivation of basil. The Si-spray+DI treatment had 10% shorter shoot heights compared to the Si-irrigation+DI treatment (10 ± 0.3 cm). There was no difference in shoot height among the other treatments.

3.1.2. Cold Injury Index (CII) Scores

The applications of the Si solution on its own during cultivation, via rootzone irrigation or foliar spray (Si-irrigation+DI and Si-spray+DI), were the most effective treatments at delaying the development and severity of visible cold injury symptoms during postharvest cold storage (Figure 2). The rates at which the plants from these treatments surpassed the limit of marketability were significantly slower than the other treatments (Figure 3), with 86% of the plants treated with Si-irrigation+DI and 71% of the plants treated with Si-spray+DI still maintaining a marketable appearance at D14 (

Table 1. Effect of silicon and abscisic acid solutions on the percentage of sweet basil plants that remain marketable at each observation day during cold storage at 3.5 °C.

Treatment	Marketable Plants (%)
	D2	D4	D6	D8	D10	D12	D14	Sig. Differences
CK	71	14	14	14	0	0	0	a
CK+DI	86	29	29	29	14	14	14	ab
Si-irrigation+DI	100	100	100	86	86	86	86	d
DI-spray+DI	100	57	43	29	29	29	14	abc
Si-spray+DI	100	100	100	100	100	100	71	d
CK+ABA	100	86	71	71	43	0	0	bc
Si-irrigation+ABA	100	100	100	86	29	14	0	c
DI-spray+ABA	100	86	57	43	29	0	0	abc
Si-spray+ABA	100	100	100	86	14	14	0	c

Data are percentage of replicates per treatment (n = 7) that had CII scores considered marketable (CII score < 2) at each observation day during cold storage at 3.5 °C. Treatment effects were determined using the Kaplan–Meier method to estimate the rates and probabilities of treatment groups surpassing the limit of marketability on each day. Treatments in the Sig. Differences column not sharing any letters are significantly different according to log-rank test (p < 0.05). Treatment abbreviations are the same as described in Figure 2.

). In comparison, only 14% of CK plants were considered marketable at D4 and 0% of the plants by D10.

Applications of ABA on its own (CK+ABA and DI-spray+ABA) or in combination with the Si solution (Si-irrigation+ABA and Si-spray+ABA) were able to delay the development of visible cold injury symptoms on basil plants, but less effectively compared to applications of the Si solution on its own. The rate at which the plants from these treatments surpassed the limit of marketability was significantly slower than the CK treatment but significantly faster than the applications of the silicon solution on its own, with most plants (>50%) from these treatments considered to have a marketable appearance only up to D8 at most. By D14, only 0–14% of the plants from these treatments were still considered marketable. These treated plants were also observed to have most of their leaves detached from their stems more often and more quickly compared to the other treatments.

3.1.3. Fresh Weight Loss (FWL)

The application of the Si solution on its own during cultivation, via rootzone irrigation or foliar spray (Si-irrigation+DI and Si-spray+DI), was effective at reducing FWL compared to CK during cold storage between D4 and D14 (

Table 2. Effect of silicon and abscisic acid solutions on the fresh weight loss (FWL) of sweet basil plants measured at each observation day during cold storage at 3.5 °C.

Treatment	FWL (%)
	D2	D4	D6	D8	D10	D12	D14
CK	1.5 ± 0.1 a	5.6 ± 0.4 ab	9.5 ± 0.4 a	14.1 ± 0.6 a	18.6 ± 0.7 a	23.8 ± 1.0 a	28.6 ± 1.2 a
CK+DI	1.5 ± 0.1 a	4.3 ± 0.4 abc	7.9 ± 0.9 ab	11.1 ± 1.0 ab	14.3 ± 1.0 abc	18.6 ± 1.2 ab	21.3 ± 1.4 bc
Si-irrigation+DI	1.6 ± 0.1 a	3.9 ± 0.3 bc	6.8 ± 0.2 b	10.7 ± 0.4 b	13.8 ± 0.4 c	17.3 ± 0.4 b	20.1 ± 0.4 c
DI-spray+DI	1.5 ± 0.1 a	3.8 ± 0.3 bc	6.4 ± 0.4 b	9.6 ± 0.4 b	13.1 ± 0.4 c	17.1 ± 0.9 b	20.1 ± 1.1 c
Si-spray+DI	1.6 ± 0.2 a	3.5 ± 0.2 c	6.6 ± 0.3 b	10.9 ± 0.7 ab	14.1 ± 0.8 bc	17.8 ± 0.8 b	20.6 ± 0.8 c
CK+ABA	1.7 ± 0.1 a	5.6 ± 0.2 a	9.7 ± 0.3 a	13.5 ± 0.3 a	16.8 ± 0.4 ab	21.1 ± 0.9 ab	25.1 ± 1.1 abc
Si-irrigation+ABA	2.0 ± 0.2 a	6.0 ± 0.3 a	8.9 ± 0.6 ab	13.3 ± 0.9 ab	16.9 ± 1.1 abc	21.6 ± 1.1 ab	26.4 ± 1.2 ab
DI-spray+ABA	1.7 ± 0.1 a	5.5 ± 0.5 abc	10.0 ± 0.6 a	13.4 ± 0.5 a	16.6 ± 0.5 ab	20.5 ± 0.8 ab	24.4 ± 0.9 abc
Si-spray+ABA	1.5 ± 0.1 a	4.4 ± 0.4 abc	8.3 ± 0.9 ab	11.5 ± 0.8 ab	14.2 ± 0.8 bc	18.0 ± 0.9 b	21.8 ± 1.0 bc

Data are means ± SE (n = 7). Values in the same column (observation day) not sharing any letters are significantly different according to Tukey’s test (p < 0.05). Treatments abbreviations are the same as described in Figure 2.

). However, these Si treatments were often not significantly lower than the other treatments consistently throughout cold storage, including the other control treatments (CK+DI and DI-spray+DI). There were no significant differences among treatments at D2.

The Si-irrigation+DI treated plants had 30–40% lower FWL on D6 compared to the CK (10 ± 0.4%), CK+ABA (10 ± 0.3%), and DI-spray+ABA (10 ± 0.6%) treated plants. On D8, it had 15–21% lower FWL than the CK (14 ± 0.6%), CK+ABA (14 ± 0.4%), and DI-spray+ABA (13 ± 0.5%) treated plants. On D10 it had 18–26% lower FWL than the CK (19 ± 0.7%), CK+ABA (17 ± 0.4%), and DI-spray+ABA (17 ± 0.5%) treated plants. On D12 it had 29% lower FWL than the CK (24 ± 1.0%) plants. On D14 it had 23–31% lower FWL than the CK (29 ± 1.2%) and Si-irrigation+ABA (26 ± 1.2%) treated plants.

The Si-spray+DI treated plants had 30–40% lower FWL than the CK (10 ± 0.5%), CK+ABA (10 ± 0.3%), and DI-spray+ABA (10 ± 0.6%) treated plants on D6. On D10, it had 26% lower FWL than the CK plants. On D12, it had 25% lower FWL than the CK plants. On D14, it had 19–28% lower FWL than the CK and Si-irrigation+ABA treated plants.

3.1.4. Leaf Electrolyte Leakage (LEL)

Application of the silicon solution on its own during cultivation, via rootzone irrigation or foliar spray (Si-irrigation+DI and Si-spray+DI), was the most effective at reducing LEL during cold storage (Figure 4). The Si-spray+DI treatment had the lowest LELs compared to all the other treatments, but not significantly lower than the Si+DI treatment, with 33–56% lower LELs compared to the other treatments. The Si-irrigation+DI treatment had lower LELs compared to all the other treatments except Si spray+DI, but not significantly lower than the Si spray+DI and DI-spray+DI treatments, with 45–53% lower LELs compared to the other treatments

The DI spray+DI treatment also had lower LELs compared to the other treatments except for the Si-spray+DI and Si-irrigation+DI treatments, but not significantly higher than the Si-irrigation+DI treatment or significantly lower than the Si-spray+ABA treatment, with 26–34% lower LELs compared to the other treatments. There was no significant difference among the other treatments.

3.2. Trial 2: Wollastonite

3.2.1. Plant Growth and Leaf Chlorophyll Content Index (CCI)

Adding wollastonite to growing media, regardless of the rate, did not have any treatment effects on shoot height (9 ± 0.1 cm) or canopy width (15 ± 0.2 cm) of the basil plants during cultivation. The 1 mL/L application rate had 23% lower chlorophyll content indexes (CCIs) compared to the control (5 ± 0.3). There was no difference in CCIs among the other treatments.

3.2.2. Cold Injury Index (CII) Scores

Adding wollastonite to growing media when cultivating basil, regardless of the rate, did not have any treatment effect on the rate at which the plants surpassed the limit of marketability (CII score > 2), with only 40% of the plants across all treatments still considered marketable at D2 and 0% of plants by D4. The cold storage test concluded at D6 because all samples had CII scores that surpassed the limit of marketability.

3.2.3. Fresh Weight Loss (FWL)

Adding wollastonite to growing media when cultivating basil, regardless of the rate, did not have any treatment effect on the FWL of the basil plants measured during postharvest cold storage at D2 (3 ± 0.2%) and D6 (8 ± 0.3%). At D4, the 5 mL/L treatment had 29% lower FWL compared to the control (7 ± 0.5%) and the 2 mL/L treatment (7 ± 0.3%). There was no difference among the other treatments at D4.

3.2.4. Leaf Electrolyte Leakage (LEL)

Leaf electrolyte leakage (LEL) was only measured if the visible CI symptoms (CII scores for leaf necrosis) showed significant differences among treatments. Since the treatments exceeded the acceptable CII score of 2 by day 4 and had no significant differences among them, it was decided that LEL would not be measured since there was observed alleviation in CI.

4. Discussion

Two different Si-containing compounds, K₂SiO₃ and wollastonite, were applied in different ways to basil in this study to test whether Si applications during cultivation can alleviate CI symptoms and improve shelf life during postharvest storage under low temperature. A secondary objective was to compare the efficacy of the Si and ABA solutions, individually and in combination. The Si solution made with K₂SiO₃, regardless of the application method, was the most effective treatment at reducing the severity of CI symptoms, increasing the duration that basil remains marketable (its shelf life) while under chilling stress to at least 14 days. The application of ABA, both on its own or in combination with the Si solution, was also effective to a lesser degree, keeping the basil marketable for up to 8 days at most. However, incorporating wollastonite into growing media did not have any positive effect.

Leaf necrosis (spotting) is a common visible symptom of CI and an important determinant of commercial quality. Applications of Si during cultivation using the Si solution, regardless of the method, had the lowest CII scores (least amount of leaf necrosis) throughout cold storage. This is consistent with the results from Habibi et al. [21] and Vu et al. [24], which both observed less leaf necrosis in chill-stressed grapevine plants and tomato seedlings, respectively, using Si applications. However, these studies were conducted on growing plants during cultivation while our study was focused on plants during postharvest.

Increased LEL and FWL can be indicative of CI because chilling stress is believed to damage cell membranes, causing the leakage of water and electrolytes from cells [16,26]. Applying the Si solution made with K₂SiO₃, regardless of the method, had the lowest LELs out of all the treatments. These lower LEL results align with the lower leaf necrosis (CII scores) for the same treatments and could suggest that Si applications alleviate CI symptoms by being involved in maintaining cell membrane integrity. Although using the Si solution also significantly reduced FWL compared to the CK treatment, there was no consistent significant difference compared to the other treatments, including with the other control treatments (CK+DI and DI-spray+DI). Thus, we cannot conclude whether Si applications have an impact on FWL in postharvest basil. It is possible there is another factor influencing these results that our study did not determine. Other studies have observed reduced LEL and FWL in Si-treated plants exposed to chilling stress, such as in cabbage plants [18], tomato plants [23,24], and turfgrass [26]. However, these studies did not test during postharvest cold storage.

The Si solution applications also had no effect during cultivation on the growth or leaf chlorophyll content of basil plants. This is contrary to results from Vu et al. [24] in which tomato seedlings had significant increases in shoot height and chlorophyll content using Si fertiliser applications, as well as contrary to the results from Li et al. [15] in which basil plants had significant increases in shoot height using Si amendments to the nutrient solution. A study by Habibi [22] found that Si applications on maize seedlings under chilling stress accumulated biomass whereas the ones not under chilling stress saw no effect. The same study also found no significant difference in chlorophyll a and b content, regardless of whether seedlings were under chilling stress or not [22]. A potential explanation for the differing results may be that Si does not induce beneficial effects such as improved growth in the absence of chilling stress, which our plants did not experience during cultivation. The effect of Si on chlorophyll content may need more research to better understand since there are varied responses in studies.

Silicon applications using wollastonite amendments at any rate to growing media had no effect, or slightly worse effects, on the growth, leaf chlorophyll content, CII score, and FWL of basil. These results are contrary to Habibi [21] which found that Si applications to soils increased plant growth and resistance to chilling stress in grapevine plants. A potential explanation for the differing results may be that the rates used for the wollastonite amendments were too low or the Si uptake was limited. A preliminary trial prior to starting our study tested the solubility of K₂SiO₃ and found that a solid, scientific-grade product had much lower solubility in water than the commercial liquid formulation used to make the Si solution. We also found that the solid product dissolved better, meaning no precipitate formed, at a higher pH (~8) than a lower pH (~5), whereas the liquid product dissolved better at the lower pH. It may be that the amount of plant-available Si, silicic acid, released by wollastonite was too low due to being a solid form of a Si-containing compound, or from the pH conditions in the rootzone not being optimal. Finer wollastonite was found to release an average of 4 times more silicic acid than a coarser product [30], so using micronized wollastonite instead of the coarser grade used in this study may also provide better results. It is also suggested that calcium silicates should be applied at least 3 months prior to planting [30], whereas this study incorporated wollastonite into growing media immediately before seeding the basil.

ABA applications were demonstrated to alleviate CI in postharvest sweet basil in a study conducted by Satpute et al. [11], which is consistent with our results using the same treatment. Other studies have also observed improvements to CI symptoms using ABA applications, such as for zucchini fruit [31], pepper plants [32], rice plants [33], chickpea plants [34], and orange trees [35]. However, our study showed that applying ABA to basil plants, whether on its own or in combination with the Si solution applications, was not as effective as the Si solution applications. The lower efficacy of the ABA treatments may be due to being applied only once, whereas the Si solutions were applied multiple times during cultivation. Basil plants treated with ABA were also observed to have more leaf abscission occur throughout storage, making it an unsuitable method for basil sold as bundles of stems and leaves. This is consistent with a study that found that ABA induced leaf senescence and abscission in Arabidopsis and rice plants during cultivation [36]. The potential induction of leaf senescence by ABA is a conflicting effect that may limit its CI alleviation effects, resulting in the lower efficacy compared to Si.

In conclusion, the results of our study demonstrate that Si applications to sweet basil during cultivation are an effective method to alleviate CI symptoms and improve shelf life during postharvest storage at 3.5 °C. Rootzone irrigations or foliar spray applications with a 189 mg/L Si solution were found to reduce leaf necrosis and keep basil marketable even after 14 days. LEL measurements also supported these findings. Although ABA applications can also reduce leaf necrosis to keep basil marketable up to 6–8 days, they are not as effective as Si applications on their own. Future studies need to determine the optimal Si concentration to use in solutions made with different sources of Si and using different application methods, allowing basil growers more flexibility in deciding what would work best for their own productions. Future studies can also determine the efficacy of Si applications on the shelf life of postharvest sweet basil under a wider range of storage temperatures, including non-chilling conditions.

5. Patents

A patent, “Method for Improving the Shelf Life of Sweet Basil During Transportation, Storage and Display Under Low Temperature”, is pending (Application N° 3,260,138).