OSMOTICUM, WATER AND DRY-MATTER DEPOSITION RATES IN THE GROWTH ZONE OF SORGHUM LEAVES UNDER WATER-DEFICIT CONDITIONS

Osmotic adjustment is an important mechanism to maintain turgor in the leaf. By use of the fluid dynamic model, the net local deposition rates for osmotica, water and dry matter content were calculated at 1-cm intervals through the sorghum leaf growth zone. The deposition rates per unit length for the well-watered leaves peaked at about 2 cm from the attachment point and decreased to slower rates at 4 cm. Under a mild water stress the rates in the first 2 cm were approximately the same and then dropped sharply to near zero at 3 cm. To take into consideration the local growth increments in three dimensions, the net deposition rate was calculated on a per unit volume basis. In well-watered leaves, the deposition rate on a per unit volume basis was maximal up to 2 cm and then declined, compared to the continuous decline from the point of attachment to nil at 3 cm for the mildly water-stressed treatment. This changed pattern of deposition reflects the shortening of growth zone and shows that the osmotic adjustment is largely the result of the restriction of growth.