INFLUENCE OF SALINITY ON PLANT METABOLISM: SOME PHYSIOLOGICAL AND BIOCHEMICAL CHANGES IN PEANUT SEEDLINGS (ARACHIS HYPOGEA L.) WITH PARTICULAR REFERENCE TO CELL WALL PROTEINS

Abstract

There are vast stretches of wasteland all over the world due to high concentration of salts in the soil. The saline soil is wide spread in various parts of India as well. Salinity creates problems due to its effects on crop species which are predominantly salt-sensitive. Peanut seedlings were used in present investigations to determine the effect of saline stress on cell wall proteins and certain other biochemical parameters. Young peanut seedlings do not tolerate salt concentration higher than 100 mM NaCl which confirms its salt sensitive nature. There is marked reduction in seedling growth under salt stress. Decrease in length and fresh weight of all parts of seedlings as well as whole seedling is observed on exposure to salt. In contrast, dry weight of whole seedling and its various parts is increased. The poor growth in salts may be ascribed either to low water uptake or to high internal salt concentration. Further marked difference were also noted in anatomical features under salt stress. For example, in roots, while the number of cortical cell layers increase, their diameter decreases with increase in NaCl concentration. Reduction in cell size could be due to low water availability and increase in cell layer could be for accommodating higher ion concentration.However, in shoot anatomy no significant change Was observed under salt stress. Salt treatment leads to accumulation of Na and Cl~ ions in various plant parts which could be due to large passive leakage in membranes due to which ion entry will rapidly increase the needs of osmotic adjustment and direct toxicity will result from metabolic interference leading to growth reduction and a positive feedback cycle. Organwise distribution shows that Na+ and Cl" ions accumulated more in roots than the other organs of the seedlings. The reason for this type of organ wise distribution is that majority of glycophytes are leaf excluders and accumulate high levels of ions mainly in their roots. With increase in concentration of salt, decrease in both plasma membrane ATPase and tonoplast ATPase activity is also observed. The reduction in plasma membrane activity may be one of the physiological factors involved in the delay of the normal plant development described under saline conditions. While decrease in tonoplast activity due to the damage of proton pump of the tonoplast by a toxic level of salts can be fatal to the plant cells. Activity of various cell wall glycosidases and acid phosphatase decreases under salt stress. In contrast, there is about two fold increase in cell wall peroxidase activity at 100 mM salt concentration. The decreased activity of various hydrolases and increased activity of peroxidases under salt stress prevents the cell wall damage and contribute rigidity to the wall. Significant increase in cytoplasmic protein content under salt stress is observed. On exposure to salinity, change in ii A cytoplasmic protein profile is also observed. At 75 and 100 mM salt concentrations 13 new polypeptides are formed, of which 3 also appear at 50 mM salt concentration. The functional importance of these stress responsive proteins is not yet established. Under salt stress, increase in cell wall protein content is also observed. There is alteration in cell wall protein profile also under salt stress. At 50 mM salt concentration, a new polypeptide of 19 kDa is appeared. It is not a glycoprotein and is rich in proline and tyrosine. Besides these two, phenylalanine, glycine and lysine are also present in significant amount. This proline and tyrosine rich protein may be specific for the adaptation of the cell wall to salt stress and may cause changes in the cell wall that allow cells to tolerate salt stress.