Functional aspects of root and leaf development in dryland crop water use under elevated CO2

© 2019 Dr Shihab UddinAtmospheric CO2 concentration ([CO2]) is rising due to anthropogenic activities and is expected to reach ~550 μmol mol-1 by 2050 and exceed ~700 μmol mol-1 by the end of this century. As the main substrate of photosynthesis, this rising [CO2] has direct implications for plant metabolism, such as stimulating net photosynthetic CO2 assimilation rate (Anet) in C3 crops and leading to greater biomass production and yield through the so-called ‘CO2 fertilisation effect’. In addition, elevated [CO2] (e[CO2]) lowers stomatal conductance (gs), and thus may reduce transpiration rate. Increased assimilation and lower transpiration result in higher leaf-level water use efficiency, which lead to the assumption that crop water use will be lower under e[CO2]. On the other hand, e[CO2] increases leaf area, which tends to increase transpiration and therefore canopy water use. Therefore, the net response of crop water use to e[CO2] is dependent on the balance between e[CO2]-induced reduction of gs and e[CO2]-induced stimulation of transpiring leaf area. These responses under e[CO2] are further complicated by other environmental variables and growing conditions. The response of crop water use to e[CO2] will be of particular interest for dryland agriculture, where water is nearly always the most limiting factor for crop production. This project investigated the functional aspects of root and leaf development on water use of dryland wheat (Triticum aestivum L.) and canola (Brassica napus L.) under a future e[CO2] using experiments with different water and nitrogen regimes, soil types and cultivars. Free Air CO2 Enrichment (FACE) technology was used to simulate future growing conditions in the field with a target atmospheric [CO2] expected by the middle of this century. This was supplemented by glasshouse studies to investigate crop physiological response to e[CO2] under more controlled conditions. Increased leaf-level water use efficiency under e[CO2] stimulated biomass and yield per unit water used, but this commonly resulted in little change in seasonal water use in this dryland, terminal drought environment. However, the dynamics of crop water use during the growing season varied depending on [CO2], whereby early in the season greater stimulation of leaf growth counteracted the increased leaf-level water use efficiency and resulted in greater water use under e[CO2] relative to a[CO2]. Under field conditions, the accumulated water use at the end of the season was then similar both under a[CO2] and e[CO2], pointing to the overriding effect of the seasonal conditions. Under water-limited conditions, e[CO2]-induced stimulation of root growth especially in the deeper soil layers maintained plant physiological processes by improving access to deeper soil water. This greater assimilation rate later in the season ensured better assimilate supply to the developing grains, which resulted in better yield benefits from the ‘CO2 fertilisation effect’. In addition, this thesis shows that interactions between growing conditions (experimental water and N regimes) and expression of genotypic traits (cultivars contrasting in vigour, transpiration efficiency and N use efficiency) play a decisive role in determining potential biomass and yield benefits from rising [CO2]. Observed genotypic variability in response to e[CO2] suggests a potential breeding opportunity to maximise the benefit from ‘CO2 fertilisation effect’