Wheat (Triticum aestivum) is a crop crucial for global food security, as it produces nearly 800 million metric tons of grain, holds the largest area harvested (220 million hectares) and contributes to 20% of calories and protein in the human diet worldwide. Yet, the escalating global warming as well as the increasing frequency and intensity of extreme climate events (e.g., heat waves and drought spells), in the context of climate change, are and will be threatening global wheat production.
Therefore, understanding and quantifying the responses of physiological processes in wheat to various environmental variables, could be of importance for predicting the performance of wheat under future climate change, and for designing adaptive strategies to mitigate negative impacts of climate change on crop yield. Nevertheless, currently, quantitative information about to what extent several key physiological processes (e.g., photosynthesis, respiration, and grain filling) are affected by (non)environmental factors in the context of climate change is generally lacking. The objectives of this thesis were: (i) to quantitatively assess to what extent leaf respiration and photosynthesis of wheat can acclimate to contrasting growth temperatures and water regimes; (ii) to comprehensively assess to what extent wheat canopy carbon and water fluxes can be affected by various heat and drought regimes during the post-anthesis period; (iii) to systematically assess the impacts of multiple (non)environmental factors (i.e., temperature, soil moisture, nitrogen supply and genotypic variation) on post-anthesis source-sink relationships for wheat.
In Chapter 1, I first justified the importance and urgency of understanding and quantifying physiological responses to heat and drought (two major stressors under climate change) for wheat production. Then I introduced the current knowledge for the responses of several key physiological processes (e.g., leaf photosynthesis, leaf day respiration, canopy carbon and water fluxes and source-sink relationships) to environmental factors. Knowledge gaps on understanding and quantifying these physiological processes and research objectives were therefore identified. Leaf day respiration (Rd) plays an essential role in maintaining primary metabolic and physiological functions of plants, yet Rd is difficult to be directly measured as it co-occurs with photosynthetic and photorespiratory processes.
In Chapter 2, I evaluated if a simple method (the so-called NRH method) can estimate Rd for photorespiratory conditions by using gas exchange and chlorophyll fluorescence data, and compared the Rd estimates by this method with those by the previous commonly used methods (the Kok and the Yin methods). Results demonstrated that the Kok and the Yin methods underestimate Rd and thus overestimate light inhibition of leaf respiration under ambient photorespiratory conditions. Besides, I also estimated Rd under contrasting growth temperature and water regimes to explore how Rd acclimates to environmental variables, and found that drought exerted a greater impact on the temperature response of Rd than growth temperature. Plants have a strong ability to acclimate to environmental cues.
In Chapter 3, by using an extended biochemical C3-photosynthesis model, I investigated how photosynthesis and its components (including CO2 diffusional and biochemical parameters) acclimate to drought stress and contrasting growth temperatures in wheat, and examined to what extent ignoring these acclimations would lead to errors in predicting photosynthesis. I found that both CO2 diffusional and biochemical parameters of photosynthesis and their thermal sensitivity acclimate little to growth temperature, but considerably to drought and the combination of drought and growth temperature.
The findings highlighted that the common modeling approaches, which typically consider the response of diffusional parameters, but ignore acclimation responses of biochemical parameters to drought and growth temperature, strongly over-estimate leaf photosynthesis under variable temperature and drought. Quantifying climate change impacts on carbon (i.e., photosynthesis and respiration) and water (i.e., evapotranspiration) fluxes for crop plants is challenging, especially at canopy scale.
In Chapter 4, I analyzed a data set of continuously measured canopy gas exchange for wheat grown under various heat and drought regimes. Results showed that CUE (carbon-use efficiency, defined as the ratio of net photosynthesis to gross photosynthesis) varied under different growth temperature and water regimes, and differed in contrasting genotypes. WUE (water-use efficiency, defined as the ratio of photosynthesis to evapotranspiration) in response to rising growth temperature was predominately regulated by carbon fluxes rather than water fluxes, while the relative contribution of water fluxes to the regulation of WUE increased under elevated CO2 and water stress conditions. Moreover, in line with the variation of CUE, WUE based on net photosynthesis and WUE based on gross photosynthesis showed different response patterns to environmental changes. These findings draw cautions in assuming a conserved CUE for modeling practices, in particularly when assessing the impacts of extreme climatic events on crop plants and agroecosystem functions. Wheat final grain/nitrogen yield is co-determined by source (the availability of photo-assimilates/nitrogen) and sink (the capacity of grains to utilize available assimilates/nitrogen).
In Chapter 5, I analyzed a data set of biomass and nitrogen dynamics during the post-anthesis period in six wheat genotypes. Source and sink were simultaneously considered and compared by a quantitative method to assess post-anthesis source-sink relationships for wheat grown under different (adverse) environments. Results showed that post-anthesis biomass source-sink differences and ratios were only altered by drought and nitrogen deficit, but not by temperature changes. On the other hand, post-anthesis nitrogen source-sink relationships were hardly affected by heat and drought, due to the large remobilization of pre-anthesis stored nitrogen.
Further, source and sink traits for both biomass and nitrogen, and their responses to environments were genotype-dependent. This quantitative analysis provides an overview of the post-anthesis source-sink relationships under various environmental conditions in wheat. Chapter 6 summarized the findings of Chapters 2–5 and broadened the discussion of these findings to the overall achievements of this thesis.
Moreover, based on the findings, potential areas for future research were identified. Overall, I highlighted the significance of systematically assessing physiological processes at different scales, the interlinkages of physiological processes across different scales, and the necessity of well considering the complexity and variability of (non)environmental factors under climate change.