Determinants of leaf water use efficiency in the C4 crop sorghum bicolor

Abstract

Intrinsic water use efficiency (iWUE) is an important leaf trait that can influence crop productivity. This PhD project aimed to identify leaf morphological and functional traits that correlate with high iWUE in a key C4 crop. In the first two experimental chapters, a small number (6-10) Sorghum bicolor genotypes were grown under different growth temperatures (Chapter 2) or under changing diurnal light conditions (Chapter 3), and leaf gas exchange was correlated with leaf anatomy and stomatal kinetic responses. In the third experimental chapter (Chapter 4), a large number (89) of Sorghum genotypes with different aquaporin alleles were grown under wet and dry conditions. All experiments were carried out in potted plants grown either in the glasshouse (Chapters 2 and 4) or growth cabinets (Chapter 3). In chapter 2, the importance of leaf width in determining stomatal conductance and iWUE was elucidated. Narrow leaves were generally thinner, with smaller guard, mesophyll and bundle sheath cells and airspace compartments. This compact arrangement likely allowed for more efficient gas exchange and thermoregulation under high temperatures. In chapter 3, the regulation of morning and midday iWUE was correlated to morning and midday stomatal conductance, while afternoon conductance had little effect on afternoon iWUE as well as integrated diurnal iWUE. Tight control on the stomatal aperture was the key factor in reducing conductance and increase diurnal iWUE, not strictly stomatal structural features. High diurnal iWUE was associated with speedy stomatal closure and water conservation under photosynthetically unfavourable conditions. In chapter 4, the onset of water stress in a large set of genotypes revealed the changing relationship between stomatal conductance and carbon assimilation, exposing the differential contributions of both components to iWUE under different conditions. The extent of genetic variation in gas exchange and hydraulic traits was assessed. In chapter 5, I discuss those different findings and attempt to integrate them via exploring how carbon assimilation and stomatal conductance vary under different conditions and explaining the impact of vapour pressure deficit in determining anatomical control on stomatal conductance. The findings of this thesis are put into the context of global change and the need for improved agricultural productivity. Suggestions are made on the possible agronomic impact of traits found beneficial in this thesis, their possible trade-offs, and how these findings can be taken further in future research.

Date of Award	2021
Original language	English