Thermal acclimation of respiration in rice and chickpea

The world population has grown from 3 billion in 1961 to 7.6 billion in 2018, and is expected to reach nearly 10 billion by 2050. Global crop production must at least double by 2050 in order to meet the needs of a growing world population. Achieving high yields depends on increasing biomass production in the lead up to anthesis, which is determined mainly by the balance between respiration and photosynthesis. Knowledge of how fluctuating environments, particularly temperature, affects this carbon balance in rice is of significant importance yet not well understood. The overall aim of my PhD research was to understand thermal acclimation of physiological processes in rice. I first characterised the extent of acclimation of respiration and photosynthesis to varying temperature in rice. The results showed that respiratory acclimation did occur in rice newly-developed leaves that formed subsequent to temperature transfer, although not to the extent that respiration homeostasis was maintained. Contrastingly, light-saturated photosynthesis acclimated strongly in newly-developed leaves with photosynthesis being homeostatic at their growth temperatures. The key findings of the molecular and biochemical analyses were: (1) there was a transient adjustment in transcript abundance in pre-existing leaves within the first 24 h following temperature shift, aligning with respiration and photosynthesis perturbations; and (2) with longer exposure, cytochrome c oxidase (COX) was declined in abundance at hotter temperature. I also explored the extent to which respiration varies throughout the day/night cycle. Independent of growth temperature, I observed acclimation capacity to significantly vary over the diurnal cycle. The strongest respiration acclimation phenotype was observed at the end of the day. Based on metabolic profiling analysis, temperatures and the time of day both drive variations in respiratory metabolite levels, independently of one another. Finally, I scaled from isolated mitochondria to whole leaf tissues of chickpea to better understand the mechanistic basis of thermal acclimation of leaf respiration. Interestingly, there were reductions in the capacity of cytochrome pathway along with a decrease in the abundance of COX protein, and an associated decline in rates of respiration per unit mitochondrial protein. Thus, it seems that a common feature of respiratory acclimation across species is a heat-dependent reduction in respiration and associated decline in cytochrome pathway capacity, likely as a result of reduced COX protein abundance. These insights into the mechanisms of respiratory adjustments to temperature will contribute towards a better understanding of how crops respond to global warming