Regulation of the uptake and distribution of Na+ in shoots of rice (Oryza sativa) variety Pokkali: role of Ca2+ in salt tolerance response

Abstract

Soil salinity is a major factor affecting crop productivity worldwide. This study explores mechanisms that contribute to salt tolerance in rice (Oryza sativa L.). Hydroponically grown, 2‐week‐old salt tolerant and sensitive indica rice varieties, Pokkali and Jaya, respectively, were exposed to a 48‐h stress period with NaCl (0–250 mM). When exposed to 200 mM NaCl, micromolar levels of external Ca2+ elevated survival of both varieties. The Ca2+ levels required were lower for Pokkali than for Jaya, but resulted in significantly higher survival. Estimates of Na+ and K+ in root and shoot compartments were made by flame photometry, while X‐ray microanalysis was used to localize Na+ in the extracellular matrix of the shoot. Transpirational bypass flow was estimated using the apoplastic tracer, 8‐hydroxypyrene‐1,3,6‐trisulphonic acid, trisodium salt. Our data demonstrate a Ca2+‐dependent reduction in Na+ transport to shoots, which correlated with a decline in bypass flow and of Na+ in the transpirational stream. In addition, the Na+ that enters the shoot is partitioned among several distinct compartments. Survival is inversely correlated with Na+ levels in the shoot apoplastic fluid, which surrounds the cell and influences cytosolic composition. Pokkali maintained lower Na+ in its apoplast compared with the salt sensitive Jaya at the same total shoot Na+. Na+ in the apoplast appears to be regulated by sequestration into intracellular compartments. This sink supplements the primary response of reducing Na+ influx into the shoot and effectively buffers the apoplastic fluid in Pokkali. All of these mechanisms are operational in Jaya as well but are deployed less effectively.