Abstract
Photosynthetic assimilation is remarkably altered by heat and drought, and this depends on the individual or combined occurrence of stressors and their respective intensities and durations. Abiotic stressors may also alter the nutritional quality and economic value of crops. In this controlled greenhouse study, we evaluated the response of Brassica napus L., from flowering to seed development, to two temperature and water treatments and a combination of these treatments. The diffusional limitations of stomatal conductance and mesophyll conductance on photosynthesis, as well as resource-use efficiency (particularly water and nitrogen), were assessed. In addition, the effects of stressors on the seed fatty acid content and composition and the total protein content were examined. The results showed that the reduction in the net photosynthetic assimilation rate was caused by combinations of heat and drought (heat + drought) treatments, by drought alone, and, to a lesser extent, by heat alone. The stomatal conductance decreased under drought and heat + drought treatments but not under heat. Conversely, the mesophyll conductance was reduced significantly in the plants exposed to heat and heat + drought but not in the plants exposed to drought alone. The carboxylation efficiency rate and the electron transport rate were reduced under the heat treatment. The seed yield was reduced by 85.3% under the heat treatment and, to a lesser extent, under the drought treatment (31%). This emphasizes the devastating effects of hotter weather on seed formation and development. Seed oil content decreased by 52% in the plants exposed to heat, the protein content increased under all the stress treatments. Heat treatment had a more deleterious effect than drought on the seed oil composition, leading to enhanced levels of saturated fatty oils and, consequently, desaturation efficiency, a measure of oil frying ability. Overall, this study showed that except for the photosynthetic assimilation rate and stomatal conductance, heat, rather than drought, negatively affected the photosynthetic capacity, yield, and oil quality attributes when imposed during the flowering and silique-filling stages. This result highlights the necessity for a better understanding of heat tolerance mechanisms in crops to help to create germplasms that are adapted to rapid climate warming.
Highlights
Alexander et al (2006) and Yuan et al (2016) noted the effects of anthropogenic climate change on fluctuating temperature and precipitation patterns in many biomes, including a direct effect on food security
One of the mechanisms by which plants sense drought is through their roots, when the soil water potential falls below a certain threshold, and abscisic acid (ABA)-driven hormonal signaling is transduced to their leaves, triggering stomatal closure to minimize water loss by transpiration (Flexas and Medrano, 2002)
By the end of this century, the average temperature might reach as high as 3.7◦C based on HadGEM2-ES derived from CMIP5 Earth System Models (ESMs) outputs, and yields could decrease from 3.1 to 7.4% for each degree (Celsius) increase in the global mean temperature (Zhao et al, 2017)
Summary
Alexander et al (2006) and Yuan et al (2016) noted the effects of anthropogenic climate change on fluctuating temperature and precipitation patterns in many biomes, including a direct effect on food security. One of the mechanisms by which plants sense drought is through their roots, when the soil water potential falls below a certain threshold, and abscisic acid (ABA)-driven hormonal signaling is transduced to their leaves, triggering stomatal closure to minimize water loss by transpiration (Flexas and Medrano, 2002). This response is concomitant with a decrease in the stomatal conductance (gs) to carbon dioxide (CO2) uptake and the production of carbohydrates to sustain growth and development. This is true for cool-climate crops, such as canola (Prasad et al, 2017; Rieu et al, 2017)
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