The hypothesis was confirmed
The two experiments we reported leave no doubt that flag leafA max of rice under elevated [CO2], under the irrigated experimental conditions, is partly controlled by genotypic source-sink relationships. This was initially reported by Fabre et al. (2020) for 5 genotypes and here in Exp.1 for 14 genotypes and in Exp.2 for 17 genotypes. A similar response to e-CO2 of grain yield was reported by Fabre et al. (2020) and was also found here in Exp.2. The LSSR as a proxy trait for source-sink ratio is thus indicative of genotypic variation of acclimation of photosynthesis under e-CO2. This acclimation, where present, consists of a downregulation of the maximum leaf photosynthesis capacity level (A max), which affects grain yield (White et al. , 2016; Fabre et al. , 2020; Dingkuhn et al. , 2020; Gao et al. , 2021).
Importantly, the two constituent traits of the LSSR, flag leaf area and panicle spikelet number, had either no or only a weak correlation with the e-CO2 response of A max or grain yield (Fig. 3). Only the ratio of the two traits was predictive, indicating that the underlying mechanism was indeed related to sink-source relationships, and not to a single morphological trait that might be correlated with acclimation for different reasons. On this basis, we suggest it to be likely that the “true” sink-source ratio of the plant, which would take into account the sums of all its source and sink activities (as opposed to a single leaf’s area and panicle’s spikelet number) would be more predictive than LSSR. Thus, hypothetically, better-performing proxy traits for e-CO2response should exist but may be more difficult to measure.
We gave a greater emphasis to flag leaf A max than to yield or biomass response because i) according to this study’s hypothesis, photosynthesis is directly affected by e-CO2 acclimation whereas biomass and grain production are indirectly affected; and ii) our two samples of genotypes represented genetic diversity for LSSR and included very low-yielding accessions. It can be expected that in tall-traditional cultivars which partition comparatively little photosynthate to grains, yield is only loosely determined by leaf photosynthetic rates. A subsequent, similar study should address a panel of high-yielding genetic materials.