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.