A model-guided holistic review of exploiting natural variation of photosynthesis traits in crop improvement

Abstract

Breeding for improved leaf photosynthesis is considered as a viable approach to increase crop yield. Whether it should be improved in combination with other traits has not been assessed critically. Based on the quantitative crop model GECROS that interconnects various traits to crop productivity, we review natural variation in relevant traits, from biochemical aspects of leaf photosynthesis to morpho-physiological crop characteristics. While large phenotypic variations (sometimes >2-fold) for leaf photosynthesis and its underlying biochemical parameters were reported, few quantitative trait loci (QTL) were identified, accounting for a small percentage of phenotypic variation. More QTL were reported for sink size (that feeds back on photosynthesis) or morpho-physiological traits (that affect canopy productivity and duration), together explaining a much greater percentage of their phenotypic variation. Traits for both photosynthetic rate and sustaining it during grain filling were strongly related to nitrogen-related traits. Much of the molecular basis of known photosynthesis QTL thus resides in genes controlling photosynthesis indirectly. Simulation using GECROS demonstrated the overwhelming importance of electron transport parameters, compared with the maximum Rubisco activity that largely determines the commonly studied light-saturated photosynthetic rate. Exploiting photosynthetic natural variation might significantly improve crop yield if nitrogen uptake, sink capacity, and other morpho-physiological traits are co-selected synergistically.

Keywords: Canopy traits; QTL; crop model; electron transport; source–sink relationships; trait synergy; yield improvement.

Fig. 1.

A simplified qualitative scheme of the quantitative crop model GECROS connecting hierarchical scales from biochemical parameters to crop yield, and covering both photosynthetic (source) and morpho-physiological (sink) traits. Items in rectangles are traits quantified in the model along the hierarchical scales, while those without rectangles are model parameters. Abbreviations and symbols: A_canopy, canopy photosynthesis rate; A_leaf, leaf photosynthesis rate; A_max, maximum rate of light-saturated A_leaf; EV, early vigour; f_cyc, fraction for cyclic electron transport; f_pseudo, fraction for pseudocyclic electron transport; GAI, green surface area index; g_m, mesophyll conductance; g_s, stomatal conductance; HI, harvest index; I_intercept, photosynthetically active radiation intercepted by canopy; J_max, maximum capacity of light-saturated linear electron transport; k_L, light extinction coefficient in canopy; k_N, leaf nitrogen extinction coefficient in canopy; LA, leaf angle; pGS, potential grain size; R_d, leaf day respiration; R_crop, crop respiration; RUE, radiation use efficiency; SG: stay-green; SLA, specific leaf area; SLN, specific leaf nitrogen content; T_p, rate of triose phosphate utilization; V_cmax, maximum carboxylation capacity of Rubisco; β, absorptance by leaf photosynthetic pigments; Φ_2LL, quantum efficiency of electron transport of PSII under limiting light; Φ_CO2, quantum efficiency of CO₂ assimilation under limiting light; κ_2LL, efficiency of converting incident light into linear electron transport under limiting light conditions. Further details of the scheme are described in Box 1 and in the main text.