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

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 m M ). When exposed to 200 m M NaCl, micromolar levels of external Ca 2+ elevated survival of both varieties. The Ca 2+ 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 Ca 2+ ‐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.