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

Developmental and physiological responses of 11 Sorghum bicolor (L.) Moench 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'' (increase 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 between RGR300BAR and the following parameters during the process of adaptation: inhibition of growth, decrease of the shoot:root ratio, and shoot Na+ uptake. There were more pronounced physiological perturbations during adaptation than during the pre-existing resistance response to salinity. During the first 25 days of exposure to NaCl, inhibition of growth and shoot Na+ concentration were not correlated. The much lower value for the intergenotype coefficient of variation (ICV) for Na+ K+ as compared to that for Na+ or K+ suggests that the sum of these ions 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 is concluded that the nature of the response of the plant, rather than Na+ toxicity, was responsible for the effects of salinity on growth.