Regulation of the uptake and distribution of Na super(+) in shoots of rice (Oryza sativa) variety Pokkali: role of Ca super(2+) in salt tolerance response

• Published in: Physiologia plantarum 2005-08, Vol.124 (4), p.451-464
• Main Authors: Anil, Veena S, Krishnamurthy, Pannaga, Kuruvilla, Sam, Sucharitha, K, Thomas, George, Mathew, M K
• Format: Article
• Language: English
• Subjects: Oryza sativa
• ISSN: 0031-9317; 1399-3054
• DOI: 10.1111/j.1399-3054.2005.00529.x

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 Ca super(2+) elevated survival of both varieties. The Ca super(2+) levels required were lower for Pokkali than for Jaya, but resulted in significantly higher survival. Estimates of Na super(+) and K super(+) in root and shoot compartments were made by flame photometry, while X-ray microanalysis was used to localize Na super(+) 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 super(2+)-dependent reduction in Na super(+) transport to shoots, which correlated with a decline in bypass flow and of Na super(+) in the transpirational stream. In addition, the Na super(+) that enters the shoot is partitioned among several distinct compartments. Survival is inversely correlated with Na super(+) levels in the shoot apoplastic fluid, which surrounds the cell and influences cytosolic composition. Pokkali maintained lower Na super(+) in its apoplast compared with the salt sensitive Jaya at the same total shoot Na super(+). Na super(+) in the apoplast appears to be regulated by sequestration into intracellular compartments. This sink supplements the primary response of reducing Na super(+) 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.