Osmoregulation in Higher Plants: Effect of Maintaining a Constant Na:Ca Ratio on the Growth, Ion Balance and Organic Solute Status of NaCl Stressed Cowpea (Vigna sinensis L.)

Abstract

Plants of V. sinensis tolerated NaCl concentrations as high as 150 mM in their growth media and showed a tendency to halophility by exhibiting a higher yield up to 50 mM NaClext. A constant Na/Ca ratio of 5.0 with increasing NaClext helped to alleviate the adverse effect of high salinity. This was evidenced by an improvement in all the growth parameters. The presence of higher Ca+2 helped to prevent passive entry of Na+ and in no case did the endogenous K/Na ratio fall below 1.0. The status of all the other cations and anions was also modified. In contrast, the ratio of endogenous K/Na was lower than 1.0 when Na/Caext was not held constant with increasing NaClext concentration. In both situations Cl- accumulated to a very high extent. The accumulation of some organic solutes related to stress resistance and/or metabolism was different under the two different conditions of culture. Measurement of RWC showed that a water deficit induced by salinity was not the principal cause of growth reduction under salt stress. Of the inorganic ions, K+ and NO3- played the key role in osmoregulation. However, at high salinities the function of Na+, tolerated to some extent by the plants, must also be considered. The amount of NO3- accumulated indicated that this plant is probably a nitrophile. Among the organic solutes, soluble sugars and polyols responded positively to salinity indicating their role as the principal organic osmotica. Free proline and quarternary ammonium compounds behaved as stress indicators.