GROWTH AND YIELD OF SORGHUM LINES EXTRACTED FROM A POPULATION FOR DIFFERENCES IN OSMOTIC ADJUSTMENT

From 47 S2 lines which had been extracted from a random mated populati on of sorghum, eight lines for a glasshouse experiment and four lines for a field experiment were divergently selected for variation in osmo tic adjustment, and were grouped into two, High and Low osmotic adjust ment (OA). Both the glasshouse and field experiments examined whether osmotic adjustment modified the plants' response to soil water deficit and also whether grain sink demand for assimilates, varied by removal of 50% spikelets, affected osmotic adjustment. In each experiment, th ere were well-watered control and water stress treatments. In both exp eriments, the dawn osmotic potential in the High OA group was always l ower than in the Low OA group under water limiting conditions, and the difference was significant after anthesis. The difference in osmotic potential was about 0.1 MPa in the field and up to 0.25 MPa in the gla sshouse. In the glasshouse experiment, removal of 50% spikelets at ant hesis significantly decreased osmotic potential during grain filling, suggesting that osmotic adjustment is influenced by the availability o f assimilates in the leaves. Under well-watered conditions, the two gr oups behaved very similarly in terms of maximum leaf area, green leaf area retention during grain filling, total dry matter production, grai n yield and grain number in both experiments. Under water-limiting con ditions, the High OA group produced larger maximum leaf area and had b etter leaf retention during grain filling. Despite similar water use, total dry matter was also significantly higher in the High OA group th ough the difference was small. Grain number was also greater in this g roup in both experiments, whereas grain yield was significantly higher in the High OA group in the field, but not in the glasshouse where se vere water stress developed more rapidly. It is concluded that the adv erse effect of water stress can be reduced by adopting sorghum genotyp es with high osmotic adjustment. However, selection for high osmotic a djustment needs to ensure that osmotic adjustment is not solely due to small head size.