Genotypic response to water deficit applied at reproductive stage in rice: is the response unique across two contrasted climates?

Background: Rice crop is known as very sensitive to water deficit, especially during the reproductive phase when growth of vegetative organs and formation of spikelets are concomitant. The present study questioned whether the maintenance of vegetative organ expansion during a water deficit at the reproductive phase affects the reproductive structure organogenesis, and if relevant traits for adaptation can be identified. To answer these queries, the response to a same reproductive water deficit of six contrasted japonica genotypes has been analyzed under two contrasted climates differing in incident radiation, air hygrometry and temperature (greenhouse in Brazil, Sitis, and growth chamber in France, GC).Results: Under irrigation, plant growth rate was reduced in GC while crop duration was extended: the trade-offs resulted in similar plant height and biomass. From a method able to determine a posteriori the date of panicle initiation (PI), elementary processes were positioned in time, allowing to evaluate how much each process was affected, despite the diversity in phenology across genotypes. Elongation rate decreased with water deficit and was highly associated with an increase in soluble sugars in stem and flag leaf in both experiments, while starch was reduced in Sitis and negligible in GC. This unique response, however, induced a different effect across experiments on branch and spikelet formation. In short, in both experiments, maintenance of spikelet number was highly associated with maintenance of flag leaf and internode width. All variables, including branch and spikelet number, were highly affected with IAC 25 in both experiments, while variations were non-significant with Cirad 409. In addition, some other genotypes expressed a differential response between Sitis and GC, conveying a specific sensitivity to low radiation or to high air temperature. Conclusions: This study highlighted the importance of finely mapping in time elementary growth processes, thanks to PI date determination, in order to detect key traits of adaptation to abiotic stress through phenotyping across genotypes of diverse phenology. The maintenance of flag leaf width or internode diameter under a mild water deficit was highlighted here as a trait associated with the maintenance of yield components.