Accumulation of Solutes in Leaves of Sorghum and Sunflower in Response to Water Deficits

The influence of water deficits on the concentrations of major solutes in fully expanded sorghum leaves and fully expanded and partly expanded sunflower leaves was studied in order to assess their contribution to osmotic adjustment. The decreased osmotic potential at full turgor in fully expanded sorghum leaves at a moderate level of stress (predawn leaf water potential of -0.85 MPa) was fully accounted for by increases in sugars, potassium and chloride. The contributions of total inorganic ions and sugars (glucose and sucrose) were approximately equal. In fully expanded sunflower leaves stressed to a predawn leaf water potential of - 1.4 MPa, only half of the decrease in leaf osmotic potential was accounted for by changes in the concentrations of the solutes studied: increases in the concentrations of the inorganic ions, potassium, magnesium, calcium and nitrate, together with free amino acids were approximately equal to the decrease in leaf osmotic potential at full turgor, but the contributions of these solutes were offset by a decrease in the concentration of total carboxylic acids. Sugars did not contribute to the decrease in leaf osmotic potential at full turgor in fully expanded sunflower leaves. The major solutes responsible for the changes in leaf osmotic potential at full turgor in partly expanded sunflower leaves exposed to severe stress treatment (predawn leaf water potential of -2.3 MPa) were the inorganic anions, chloride and nitrate, and to a lesser extent carboxylic acids (principally aconitate). Free amino acids made a significant contribution (18%) to the decrease in leaf osmotic potential at full turgor, but there was a decrease in the level of soluble sugars.