Salinity Stress and the Content of Proline in Roots of Pisum sativum and Tamarix tetragyna

Abstract

Amino acid composition of the free amino acid pool and the TCA-insoluble protein fraction were investigated in root tips of pea and Tamarix tetragyna plants grown at various levels of NaCl salinity. Salinity stress induced an increase of proline content, mainly in the free amino acid pool in both plants, and of proline or hydroxyproline content in the protein. Externally-supplied proline was absorbed and incorporated into protein, by pea roots, more effectively than by Tamarix roots. Salinity stress, apparently, stimulated the metabolism of externally-supplied labelled proline. Pea roots have a very large pool of free glutamic acid; however, 70 per cent of the ¹⁴C from externally-supplied ¹⁴C-U-glutamic acid was released as CO₂. Very small amounts of it were incorporated into protein. No measurable amount of radioactivity could be detected in any one of the individual amino acids, either of protein hydrolysate or the free amino acid pool. Proline very effectively counteracted the inhibitory effect of NaCl on pea seed germination and root growth. A similar effect but to a lesser degree was achieved with phenylalanine and aspartic acid. The feasibility of proline being a cytoplasmic osmoticum is discussed.