Scope for improvement of proxy traits to better predict CO2 response
Specifically to improve the predictability of genotypicA max gains under e-CO2 we attempted to refine the indices used as proxy traits, particularly as measured under ambient CO2 conditions. Among the individual traits measured in Exp.2, there were none stood out as candidates according to PCA (Fig. 4). However, physiological reasoning suggested that the source term of LSSR should be improved by including the areal leaf chlorophyll content (easily measured with SPAD, thereby factoring in implicitly leaf thickness), and the sink term with the mean filled grain weight (routinely measured by breeders). This increased theA max-ratio prediction from 35% to 50% of the observed variation (Fig. 5). This, however, remained significantly inferior to the predictability using proxy traits measured under elevated [CO2], a condition unavailable to breeders.
Further improvement of the predictability of genotypic e-CO2 response, particularly for grain yield or biomass, may require a more fundamental rethinking of the proxy concept, which does not necessarily have to be based on morphology. Leaf sucrose accumulation, commonly pronounced in the afternoon (Kölling et al. , 2015; Fabre et al. , 2019, 2020), has been described as indicating sink limitation (Lemoine et al. , 2013; White et al. , 2016; Burnett et al. , 2016; Sonnewald and Fernie, 2018) and causing feedback inhibition of photosynthesis (Huber and Huber, 1992; Moore et al. , 1999; Iglesias et al. , 2002; Paul and Pellny, 2003; Fabre et al. , 2019) but it was of limited predictive value in our study. We did not measure here fluorescence-based, diagnostic variables of the photosynthetic system (except the electron transport rate ETR, Fig. 3) which might be incorporated in observable, predictive indices. The disadvantage of such transient variables is their sensitivity to rapid environmental fluctuations, as opposed to morphology. On the sink side, we know of no readily deployable measurements of actual physiological demand for assimilates, and breeders are probably more comfortable with morphological sink proxies. More research on the processes involved in CO2 acclimation of photosynthesis may open other avenues.
Lastly, it should be noted that sink-source relations exist also during vegetative growth and affect pre-floral biomass accumulation under e-CO2 conditions (Dingkuhn et al., 2021). For example, genotypic tillering potential has been shown to contribute positively to crop e-CO2 response in rice (Ziska et al., 2013; Kadam et al., 2019). Genotypic branching capacity in soybean has a similar effect (Kumagai et al., 2015). The search for predictive proxy traits with regards to vegetative biomass production is largely virgin territory.