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.