Sterility in Rice (Oryza Sativa L.) Subject to Drought during the Booting Stage Occurs not because of Lack of Assimilate or of Water Deficit in the Shoot but because of Dehydration of the Root Zone

In rice plants a drought at the booting stage is the event most damaging to grain yield because it drastically increases sterility. Our objective was to establish what phosiological processes caused sterility during drought. We applied several soil-drying treatments to pot-grown plants of three rice cultivars (one lowland and two upland) for 10 days during the booting phase. Flag leaf elongation rate (LER), the rate of dry matter production (DMP) and leaf water potential (LWP) decreased and sterlity increased in all cultivars as the intensity of soil drying increased. Increasing sterility was closely correlated with reduced DMP in all cultivars. It has been assumed previously that suppressed assimilate supply causes sterility. However, when DMP was strongly suppressed by shading under well-watered conditions, sterility was unaffected. Moreover, when part of the root system was partially dried at the booting stage, sterility significantly increased and leaf diffusive conductance decreased when compared with the results for well-watered plants, although there was no significant difference in LWP between the treatments. We suggest that sterility in rice plants subjected to soil drying during the booting stage occurs not by reduced assimilation or because of water deficits in the shoot, but by some chemical signal from roots to shoots.