Abstract
Although improving photosynthetic efficiency is widely recognized as an underutilized strategy to increase crop yields, research in this area is strongly biased towards species with C3 photosynthesis relative to C4 species. Here, we outline potential strategies for improving C4 photosynthesis to increase yields in crops by reviewing the major bottlenecks limiting the C4 NADP-malic enzyme pathway under optimal and suboptimal conditions. Recent experimental results demonstrate that steady-state C4 photosynthesis under non-stressed conditions can be enhanced by increasing Rubisco content or electron transport capacity, both of which may also stimulate CO2 assimilation at supraoptimal temperatures. Several additional putative bottlenecks for photosynthetic performance under drought, heat, or chilling stress or during photosynthetic induction await further experimental verification. Based on source-sink interactions in maize, sugarcane, and sorghum, alleviating these photosynthetic bottlenecks during establishment and growth of the harvestable parts are likely to improve yield. The expected benefits are also shown to be augmented by the increasing trend in planting density, which increases the impact of photosynthetic source limitation on crop yields.
Highlights
The global human population has seen a staggering increase over the last century, and is currently estimated to be 7.8 billion people (United Nations, 2019).This signifies a tripling of the population compared with 2.6 billion people in 1950 and, this explosive growth is projected to gradually taper off, substantial further population growth is still predicted (UN Population Division projections)
Improvements in crop yields can be achieved via development of new higher yielding varieties or via closure of the so-called ‘yield-gap’ between attainable and realized yield (Foley et al, 2011).With regards to the former, the plant breeding strategies of the green revolution have led to spectacular improvements in yield potential via increases in yield components such as harvest index and light capture efficiency, but less so for the conversion efficiency of captured solar energy to energy contained in plant biomass.Theoretical analyses of the maximum conversion efficiency (Zhu et al, 2010) have provided estimates for an upper limit of 4.6–6%
The model simulations show that for the observed increase in average maize plant density from five to eight plants m−2 between 1987 and 2016 (Assefa et al, 2018), productivity and yield per unit area marginally increase (Fig. 6A, B), but productivity and yield per plant decline (Fig. 6C, D) while source– sink balance approximately halves (Fig. 6E). These results are only weakly affected by more upright leaf angles, which favours canopy photosynthetic activity in modern maize hybrids via more uniform vertical light distribution across the canopy (Ort et al, 2015).crop yield becomes more strongly source limited with increasing plant density, and the general trend of increasing plant density is likely to enhance the importance of photosynthetic efficiency for yield, especially in the grain crops sorghum and maize
Summary
The global human population has seen a staggering increase over the last century, and is currently estimated to be 7.8 billion people (United Nations, 2019).This signifies a tripling of the population compared with 2.6 billion people in 1950 and, this explosive growth is projected to gradually taper off, substantial further population growth is still predicted (UN Population Division projections). There is significant carbon exchange between the CBB and C4 cycle (Arrivault et al, 2017), which probably helps to maintain flexibility to respond to variable environmental conditions.To account for this complexity, metabolic models which capture the kinetics of all the major reaction and diffusion steps in C4 photosynthesis can be used to identify the relative control exerted by any of the modelled factors over the rate of CO2 assimilation, by computing control coefficients, defined as the relative change in net CO2 assimilation rate (An), as a result of a relative change in the control factor Using their model for NADP-ME photosynthesis (Fig. 3) to simulate the control of individual factors over the rate of assimilation,Wang et al (2021) computed that under high light, control over steady-state An is shared between Rubisco in the CBB cycle (dAn/dRubisco=0.46) and Jmax, namely the capacity for chloroplastic electron transport (dAn/ dJmax=0.38).
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