Fluid Uptake Pathways Are Differentially Modified in Rice (Oryza sativa L.) in Response to Drought and Salinity

Salinity and drought adversely affect rice production globally. Here we have examined physiological responses to drought and salinity across four rice cultivars with varying sensitivity to these stresses. The salt tolerant Pokkali restricts fluid entry to limit Na+ uptake under saline stress, while the drought-tolerant ARB6 needs to enhance fluid uptake under drought. Surprisingly, Pokkali does reasonably well when subjected to drought as does ARB6 under saline stress - in contrast to the stress-sensitive but high yielding varieties IR-20 and Jaya. Both tolerant varieties use long roots to mine water under deficit conditions, increasing aerenchyma and suberization of the exodermis to provide oxygen to deep-reaching roots. Major alterations in patterns of suberization in both exodermis and endodermis are undertaken, the patterns being dramatically different under the two stresses. Genes implicated in suberin biosynthesis also showed variation in transcript levels under stress, corresponding with the observed suberization patterns. Osmolyte accumulation drives uptake of water under deficit conditions, while restricting fluid flow to symplastic routes minimizes Na+ entry. Overall, the morphological and physiological responses of the tolerant varieties ensure adequate fluid flow through the transpiration stream without excessive salt uptake, thereby promoting growth under both drought and salinity.