Modelling rice and wheat response to rising carbon dioxide concentration

Modelling rice and wheat response to rising carbon dioxide concentration

Better crop photosynthetic efficiency is important for enhancing field crop production. The improvement in the photosynthetic efficiency of a crop depends on its efficiency in the usage of resources, including CO2, water, nitrogen (N) and radiation. However, prolonged exposure to elevated carbon dioxide concentration (e[CO2]) and, a short supply of other resources may lead to a decline in photosynthesis – a process referred to as ‘acclimation.’ Studies have demonstrated photosynthetic acclimation at the flag leaf level in a variety of crops. However, progress is limited in addressing the gaps in knowledge about the link between leaf-level acclimation phenomena and canopy level performance, which is influenced by different growth and development processes and abiotic factors. Therefore, there is a need for crop models capable of accurately extrapolating the leaf-level response to canopy level, to understand the overall impact of changes in photosynthesis at the biochemical level and its consequence on crop growth, development and productivity. In this regard, the research described in this thesis is founded on the hypotheses, that i) primary plant responses, photosynthesis and stomatal conductance to e[CO2] are regulated by the interaction of different environmental variables ii) photosynthesis acclimation, on prolonged exposure to e[CO2], is associated with a change in the leaf ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) and N concentration and, iii) photosynthetic acclimation can be better captured when biochemical parameters are included in the crop models like APSIM which is based on the concepts of cross-scale modelling, facilitating crop growth and development.

A meta-analysis of the studies reported in the literature was conducted to evaluate the impact of e[CO2] on two major physiological processes, photosynthesis and stomatal conductance in two primary functional groups of plants – C3 and C4. Within C3 and C4 crops, more specific groups including legumes, non-legumes, flowers, trees, shrubs and grasses were examined to evaluate their respective responses to e[CO2] under different abiotic stresses. The abiotic factors like water, N and temperature were found to be critical in determining the photosynthetic efficiency and thus, the biomass of plants. Understanding the role of abiotic factors, particularly N, in the photosynthesis under continuous exposure to e[CO2] is essential to predict the crop response to the possibility of an e[CO2] in the earth’s atmosphere, in the future. In this study, rice response to e[CO2] was estimated using a system dynamics modelling tool, STELLA. An analytical modelling framework embedding leaf-level crop system including RuBisCO and N dynamics and crop growth processes are developed using the STELLA software. The secondary data on rice from a growth chamber experiment was utilised to validate the model. The simulated response strongly supported the occurrence of photosynthetic acclimation at both growth and biochemical levels, under different e[CO2], at different levels of N supply.

Further, this study evaluated photosynthesis, in-depth, in determining e[CO2]-induced acclimation and thus, growth. Two major parameters that were used for estimations are the maximum carboxylation capacity (Vc.max) and the electron transport capacity (Jmax). Data from the Australian Grains Free-Air CO2 Enrichment (AGFACE), Horsham, Victoria, Australia were analyzed and modelled to determine the changes in the photosynthetic response of another C3 crop, wheat, to e[CO2]. The Agriculture Production System Simulator coupled with the diurnal canopy photosynthesisstomatal conductance model (hereafter referred to as APSIMDCP) was used to validate the APSIMDCP model and evaluate the range of parameters associated with photosynthetic acclimation under e[CO2]. It was established that APSIMDCP could adequately link the biochemical and crop level responses, to enable extending the leaf level model to the canopy level. Further, it successfully simulated the photosynthetic acclimation responses to e[CO2] for different wheat cultivars which were characterized by reduction of Vc.max, Jmax and leaf N concentration. However, all cultivars were not equally responsive to the e[CO2], with some showing no response at all and, others showing responses of varying magnitude, illustrating genotypic variation in this trait. In summary, this study investigated the impact of e[CO2] on variation in photosynthesis in rice and wheat at different physiological stages of growth to predict the biomass and yield responses accurately.