Osmotic adjustment enables plants to maintain turgor potential under conditions where water is limiting plant growth. Plants with a good ability to adjust osmotically (High OA) are expected to have better growth and yield compared to plants with a low osmotic adjustment or a poor ability to adjust osmotically (Low OA) when grown under a water deficit.
The main objective of the present study was to examine the effect of osmotic adjustment on growth and grain yield of sorghum in a dry environment. Two sorghum sub-populations differing in their ability to
adjust osmotically but similar in all other respects were subjected to water stress for 5 weeks prior to anthesis. The plant were grown in the field with rainfall excluded by the use of a "rain out" shelter. The plant were rewatered after anthesis to maintain growth and yield during the grain filling stage. The predawn leaf osmotic potential was determined 5 times at 21, 41, 48, 57, and 68 days after planting (DAP) and the osmotic adjustment was obtained from the difference between leaf osmotic potential (∏100)DAP and the later measurement occasions.
The highest osmotic adjustment was 0.3 MPa in the High OA group at 57 DAP. However, the difference (up to 0.054 MPa) between two osmotic adjustment groups was not significant at any measurement occasion. The leaf turgor potential values obtained for the High OA group tended to he higher than those for the Low OA group but there was no significant difference in turgor potential between two OA groups at any time of measurement. The reduction in leaf osmotic potential of the Low and High OA groups from 41 to 57 DAP reduced grain yield and total dry matter. A positive relationship was found between turgor potential and grain yield and total dry matter for the Low OA group when grown under moisture stress but there was a negative
relationship for the High OA group.
The Low and High OA groups had similar dry matter allocation to plant parts, but the leaf area index was significantly lower in the High OA than in the Low OA group at 57 DAP in the stress experiment.
At maturity, there was a tendency for plants in the High OA group to have a higher total dry matter, head dry matter, leaf dry matter, leaf area index and grain yield but a lower harvest index and distribution index when compared to the Low OA plants in the stress experiment although the differences were not significant. The non significant difference in almost all characters studied in this experiment was attributed in part to the large genetic variation within the two sub-population groups, and, insufficient stress development. A further study therefore needs to be conducted using material which has a smaller variation in sub-population groups and under an adequate level of stress.