4.1. Crop Development and Crop Yield Parameters
Soil warming accelerated OSR development during early crop growth and during spring, which was similar to what we expected. It is known that in temperate climates an elevation in soil temperature can stimulate crop development [5
], which has also been shown for winter wheat [4
]. In the present study, OSR grown under elevated soil temperature was taller compared to ambient soil temperature from early growth stages to the beginning of May 2017 (DC 65). Afterwards, the soil warming effect on canopy height vanished, possibly due to a reduction in soil moisture due to higher air temperatures and less precipitation during summer compared to the period of spring. This is similar to an OSR field study in 2014, which was also performed within the HoCC experiment [34
]. Accordingly, OSR was taller under soil warming until April and afterwards the huge difference in canopy height diminished until final harvest. Therefore, the effect of soil warming on canopy height appeared to decrease with increasing ambient temperatures in air and soil. In addition, soil warming resulted in a greater impact on smaller than on taller plants.
In our study, no change in total aboveground biomass was observed under soil warming at maturity, which was in agreement with a study using winter wheat [4
]. In contrast, Bamminger et al. [34
] found higher OSR aboveground biomass under soil warming at maturity. Increasing ambient air temperatures result in soil warming, which can alter plant–microbe interactions with impacts on the allocation of nutrients belowground in the rhizosphere [35
]. It is possible that the higher mean air (+1 °C) and soil temperatures (+0.4 °C) in the study of Bamminger et al. [34
] in 2014 compared to 2017 changed the nutrient availability for the plants and therefore promoted plant biomass production.
We observed stable seed yield under soil warming as well as under reduced precipitation amount and frequency. The achieved seed yield of OSR planted under control conditions (ambient soil temperature and precipitation) either under roofs or without roofs was 5.0 Mg ha−1
and 4.3 Mg ha−1
, respectively. Hence, seed yield of the control treatment correspond to the average winter OSR seed yield of 3.8 Mg ha−1
in the region Stuttgart in 2017. The average OSR seed yield in Stuttgart was achieved under normal agricultural practice [36
To date, only a few studies investigating soil warming effects have been conducted on crop yield, e.g., of winter wheat [4
] or maize [37
] in temperate climates. However, elevated soil temperature can differ in their impacts on crop yield as compared to elevated air temperature. On the one hand, elevated air temperature can shorten the period of grain filling [38
]. In low latitudes, elevation of air temperatures during the grain filling period are associated with a decrease in crop yield as a consequence of a reduction in plant photosynthesis, degradation of thylakoid components, and lower carbon exchange rate per unit of leaf area [39
]. On the other hand, it has been observed that soil warming can affect the diversity and abundance of soil microorganisms [40
]. Thus, alterations in plant–microbe interactions can occur due to impacts on the nutrient supply from the microbiome to the crop. Soil warming can stimulate the activity of soil microorganisms for a short time, but the microbial community seems to acclimatize to soil warming after a long exposure time [41
]. This corresponds to the observations in the present study. Under permanent soil warming, we have observed stable seed yields of OSR. Thus, our results suggest an adaptation of soil microorganisms to permanent soil warming, assuming fewer alterations in plant–microbe interactions and no or minor impacts on the nutrition supply from the microbiome to the crop.
However, other explanations should also be considered. The detected stable OSR seed yield under soil warming in our study may have been due to sufficient water availability during the seed-filling period from beginning of April 2017 (DC 60) until end of June 2017 (DC 89), which mitigated evaporation consequences resulting from elevated soil temperature. This period included a wet April 2017 and high ambient precipitation amounts at the end of May 2017. Another explanation could be that soil warming by about 2 °C was too low to result in changes in factors such as the activity of soil microorganisms and the distribution of nutrients from the rhizosphere to the crop. In another soil warming experiment, crop yield of winter wheat remained unaffected at 5 °C elevated soil temperature to 100 mm depth [4
In contrast, decreased OSR and cereal yields were observed in several studies with elevated air temperature [42
]. Therefore, stable crop yields of OSR observed in our study under soil warming seemed to be a further indication that elevated temperatures in soil or air can result in different impacts on crop yield.
Besides elevated temperature, water scarcity also impacts seed yield. The time in plant life at which water scarcity appears is associated with effects on seed yield: reduced water amount during the periods of seed set or seed-filling can result in a decrease in seed yield [47
]. Furthermore, Champolivier and Merrien [13
] observed a reduction in seed yield in winter OSR after a period of water shortage which persisted from flowering until maturity. Our study did not detect reduced crop yields in OSR grown under reduced precipitation amount, presumably due to relatively wet conditions in the summer of 2017. Moreover, we hypothesized that seed yield of OSR decreases under reduced precipitation frequency. Accordingly, longer drought periods reduced the seed-filling period in OSR, as demonstrated by Hlavinka et al. [48
] and Istandbulluoglu et al. [7
]. In contrast, we found stable seed yield in OSR under reduced precipitation frequency and no change in seed-filling period, most likely due to high precipitation amounts during this period. Furthermore, our hypothesis that a simultaneous occurrence of soil warming, reduced precipitation amount, and reduced precipitation frequency have additive negative effects on seed yield of OSR could not be confirmed. This was perhaps due to moderate air temperatures below 30 °C from June to August 2017, resulting in moderate mean soil temperatures (24.6 °C) and sufficient water supply during the growing period. In addition, we observed relatively heavy rainfall events of between 20 and 30 mm at the beginning and end of June 2017. They appeared to mitigate negative effects of evapotranspiration and of a dry period in mid-June 2017 on soil water availability.
The SLA of crops is known to decline under elevated temperatures coupled with water shortage by a decrease in final leaf size [49
]. Similarly, the SLA of OSR in the present study decreased under soil warming in combination with reduced precipitation amount.
Significant main effects and their interactions between amount and frequency of precipitation were detected in biomass allocation before the precipitation manipulation in 2017 began. Thus, biomass of flowers and senescent leaves increased, whereas SLA decreased, most likely as a result of plants producing smaller leaves under conditions of limited water availability. These are long-term effects of the precipitation manipulation, which is conducted every year during summer (always from June until August) in the same way at the same subplots since 2008. With regard to other long-term studies, which have been conducted mainly in grasslands, forests, and shrublands, variability in precipitation patterns over several years can lead to changes in, for example, soil respiration, as a consequence of alterations in soil structure or in the composition of the soil microbial community [50
]. In our study, long-term changes in amount and frequency of precipitation may have resulted in an altered composition of the soil microbial community, which in turn affects availability of nutrients for the crops, and in the end, leaf size, leaf senescence, and biomass allocation of OSR.
4.2. Seed Quality
Alterations in seed yield, resulting from environmental stresses (e.g., temperature and precipitation patterns) can stimulate changes in the quantity of the seed oil produced [47
]. Since seed yield did not vary under changes in soil temperature and precipitation in the present study, it appears that the total seed oil content remained unaffected in all treatments. By contrast with what is hypothesized, a reduction in precipitation amount neither increased the protein nor decreased the oil concentration, as had been found previously in OSR seeds when water scarcity was applied during the ripening period [13
]. These contrasting results could be due to the fact that the reduced precipitation amount in this study was based on relatively high ambient precipitation amounts during the growing period of OSR. Thus, the simulated decrease in precipitation amount was too small to effect shifts in protein and oil concentrations.
In the present study, the observed changes in the essential amino acids phenylalanine, isoleucine, and lysine, when exposed to altered precipitation patterns alone or in combination with soil warming, seem to be negligible. In a previous study, an increase in those amino acids in leaves of OSR by 10% to more than 300% after two-day and four-day drought events was found [53
The lipid biosynthesis of oil producing crops can be affected due to global warming, because an elevation in temperature can result in less desirable fatty acid profiles of vegetable oils [47
]. Water availability is a second factor, one which can alter the composition of oilseeds since crops are prone to close their stomata under reduced water supply. This reduces carbon dioxide assimilation as well as sugar uptake by embryos [47
]. Similarly, in the present study, minor changes in the fatty acid composition of OSR seeds were observed. Capric acid concentration decreased under elevated soil temperature combined with reduced precipitation amount and lignoceric acid increased if precipitation amount and precipitation frequency were reduced both. As far as we know, the function of saturated fatty acids in the metabolism of OSR is currently not fully understood. Capric acid, as a medium-chain fatty acid, is a valuable ingredient in OSR seed oil and used as feedstock in the production of biodiesel, cosmetics, lubricants, and surfactants [54
]. Thus, a decrease in capric acid concentration could be unfavorable for the industrial usage of OSR seeds.
OSR is used for the production of edible oil or as a protein source for livestock so, for quality reasons, the concentrations of glucosinolates in mature seeds is restricted to 18 µmol g−1
], which was adhered to in all treatments. In this study soil warming increased glucosinolates in seeds. A positive correlation between increasing temperatures and glucosinolates was also shown in Brassica oleracea
]. Since glucosinolates are part of the plant defense reaction in the Brassicales order [56
], water shortage during late growth stages can increase the glucosinolate concentration of OSR seeds at maturity [17
]. In the present study, no effects of reduced amount and frequency of precipitation on the total glucosinolate concentration were observed. Several studies have reported that lower water availability will lead to a reduction in the number of seeds per plant, with glucosinolates distributed to fewer seeds, resulting in increased concentrations of glucosinolates in seeds [13
]. In the present study, however, a stable seed yield was observed, which constitutes a stable sink capacity for glucosinolates. Most likely for that reason, total glucosinolate concentration did not change under water scarcity. A second explanation for why the precipitation treatments showed no effects on total glucosinolate concentration is that the growing period of OSR was relatively wet in 2017. Thus, the reduction in amount and frequency of precipitation was too mild to significantly increase the production of glucosinolates. Similarly, in a former field study with OSR grown in lysimeters, the glucosinolate concentration in seeds was not affected under mild drought stress conditions during the late developmental stage (pod filling stage) [17
]. When determining the environmental effects on the quality of OSR grown across Victoria, Pritchard et al. [58
] reported that the impact of cultivar on the glucosinolate content was greater than environmental impacts such as air temperature and rainfall amount.