Effects of osmotic shock and calcium on growth and solute composition of Phaseolus vulgaris plants

Abstract

Identification of the primary response to salinity is of great importance in order to develop salt tolerant species. In this work the effect of a NaCl osmotic shock on leaf elongation of Phaseolus vulgaris L. cv. Contender has been studied. After establishing an osmotic shock by adding NaCl to the root medium, three consecutive events could be distinguished. First, there was a sudden interruption of leaf growth, followed by a period of retardation of growth, and then by a restoration of growth to reach the steady‐state growth rate. High calcium (5 mM) in the growth medium diminished the initial loss of water in the roots and kept the water content higher upon restoration of leaf growth, as compared to low calcium levels (0.5 mM). During the low calcium treatment, leaf malate started to increase already 3 h after the start of the osmotic treatment, showing a maximum concentration at around 9 h, then decreasing and approaching the value of control plants. At high calcium, malate concentration remained unchanged with time. Plants grown in the presence of low calcium showed an increase in the concentration of total amino acids upon NaCl shock. It is estimated that organic metabolites, if confined to the cell cytoplasm, contribute significantly to the osmotic adjustment, together with inorganic ions. Our results support the hypothesis that water shortage in the roots is responsible for the initial inhibition of leaf elongation. The beneficial effect of calcium on restoration of growth after the NaCl shock is a consequence of short‐term, energetically expensive osmotic adjustment, in which mainly organic metabolites are involved. Under steady state conditions, high calcium treatment results in a faster growth rate than low calcium. In a process of osmotic adjustment, in which inorganic ions are principally involved, this is the result of a decrease in leaf Na+ concentration in expanding leaves, together with an increase in K+ and Ca2+.