Abstract
Rising atmospheric [CO2] causes global warming but
may also benefit photosynthesis and yield of C3 crops such as rice.
Previous research showed that positive effects depend on a cultivar’s
sink-source ratio as sink limitation incurs acclimation of
photosynthesis to elevated [CO2]
(e-CO2). To enable breeding for e-CO2response, predictive, easily measurable proxy traits under ambient
[CO2] are needed. The local source-sink ratio (LSSR:
flag leaf/panicle size) is a potential proxy trait, proposed by a
previous study. We evaluated this and similar trait indices for two
diverse rice cultivar samples under e-CO2 vs ambient
level in controlled environments. The significant negative effect of
genotypic LSSR on maximum photosynthesis (A max)
under e-CO2, and a similar but weaker effect on the
grain yield response, was confirmed. However, LSSR observed was more
predictive under e-CO2 than ambient, rendering this
proxy trait impractical for field-based selection. This difference was
due to the phenotypic plasticity of LSSR between
[CO2] levels in our populations. Variants of LSSR
incorporating SPAD leaf chlorophyll content and panicle sink capacity
improved LSSR predictive power under ambient [CO2]
for A max. We conclude that genotypic sink-source
ratio is an important physiological determinant of
[CO2] response, but proxy traits need to be further
refined and field-validated to become useful selection or phenotyping
tools for improved e-CO2 response of rice.