Mild preflowering drought priming improves stress defences, assimilation and sink strength in rice under severe terminal drought.

Drought stress is a prominent and persisting constraint for sustaining global rice (Oryza sativa L.) production. Priming with mild drought can be effective in reducing the impact of severe terminal drought stress affecting seed set and grain filling in rice. The cultivars N22 (drought tolerant), NSIC Rc222 and IR64 (high yielding, drought sensitive) were tested for short-term mild drought priming before flowering and subsequently exposed to severe drought stress either at the highly sensitive flowering or at the early grain filling stage under greenhouse conditions. Drought stress increased oxidative damage and reduced photosynthesis and sink enzymatic activity, ultimately reducing seed set (20-46%) and grain yield (22-68%) across cultivars. However, priming with mild drought significantly reduced oxidative damage, and increased photosynthesis, stomatal conductance and enzymatic activity, contributing to improved sink strength, thereby significantly reducing seed set (7-18%) and grain yield (12-59%) losses. The higher activity of key enzymes associated with sink strength such as cell wall invertase and sucrose synthase in primed plants probably reduced drought-induced losses at the grain filling stage. The findings support mild drought priming before flowering as a promising strategy for reducing yield penalty by providing partial protection against subsequent severe terminal drought stress. However, application of mild drought priming at the field level would need further investigation.

Physiological and proteomic approaches to address heat tolerance during anthesis in rice (Oryza sativa L.).

Episodes of high temperature at anthesis, which in rice is the most sensitive stage to temperature, are expected to occur more frequently in future climates. The morphology of the reproductive organs and pollen number, and changes in anther protein expression, were studied in response to high temperature at anthesis in three rice (Oryza sativa L.) genotypes. Plants were exposed to 6 h of high (38 degrees C) and control (29 degrees C) temperature at anthesis and spikelets collected for morphological and proteomic analysis. Moroberekan was the most heat-sensitive genotype (18% spikelet fertility at 38 degrees C), while IR64 (48%) and N22 (71%) were moderately and highly heat tolerant, respectively. There were significant differences among the genotypes in anther length and width, apical and basal pore lengths, apical pore area, and stigma and pistil length. Temperature also affected some of these traits, increasing anther pore size and reducing stigma length. Nonetheless, variation in the number of pollen on the stigma could not be related to measured morphological traits. Variation in spikelet fertility was highly correlated (r=0.97, n=6) with the proportion of spikelets with > or = 20 germinated pollen grains on the stigma. A 2D-gel electrophoresis showed 46 protein spots changing in abundance, of which 13 differentially expressed protein spots were analysed by MS/MALDI-TOF. A cold and a heat shock protein were found significantly up-regulated in N22, and this may have contributed to the greater heat tolerance of N22. The role of differentially expressed proteins and morphology during anther dehiscence and pollination in shaping heat tolerance and susceptibility is discussed.