ADAPTATION VERSUS PREEXISTING RESISTANCE - AN INTERGENOTYPE ANALYSIS OF THE RESPONSE OF SORGHUM-BICOLOR TO SALINITY

Developmental and physiological responses of 11 Sorghum bicolor (L.) M oench genotypes exposed for 3 weeks to a 150 mM NaCl-pretreatment were studied. Following the pretreatment, exposure to 300 mM NaCl resulted in a gradient of response between ''adaptation to salinity'' (increas e in salinity tolerance) and ''pre-existing resistance'' (maintenance of original salinity tolerance). Level of adaptation of each genotype was qualified by determination of the mean relative growth rate of the shoot at 300 mM NaCl (RGR300BAR). There was a positive correlation be tween RGR300BAR and the following parameters during the process of ada ptation: inhibition of growth, decrease of the shoot:root ratio, and s hoot Na+ uptake. There were more pronounced physiological perturbation s during adaptation than during the pre-existing resistance response t o salinity. During the first 25 days of exposure to NaCl, inhibition o f growth and shoot Na+ concentration were not correlated. The much low er value for the intergenotype coefficient of variation (ICV) for NaK+ as compared to that for Na+ or K+ suggests that the sum of these io ns is a parameter of physiological importance, and that these two ions were interdependent and partially interchangeable. It seems that Na+ K+ + Cl- were the main osmotica in the shoot of most genotypes. It i s concluded that the nature of the response of the plant, rather than Na+ toxicity, was responsible for the effects of salinity on growth.