Contribution of osmotic adjustment to grain yield in Sorghum bicolor (L.) Moench under water-limited conditions. I. Water stress before anthesis

The contribution of osmotic adjustment to grain yield in Sorghum bicolor (L.) subjected to water stress before anthesis was studied using six entries. Three of the entries (Goldrush, E57, and DK470) were selected for high osmotic adjustment and the other three (Texas 6 1 OSR, Texas 67 1, and SC 219-9-1 9-1) for low osmotic adjustment, and divided into early, intermediate and late maturity groups. Entries were either well watered, or subjected to a 41-day period of water shortage prior to anthesis and well watered for the remainder of their growth. Entries selected for higher osmotic adjustment developed higher levels of osmotic adjustment during the pre-anthesis stress period than those selected for low osmotic adjustment in intermediate and late maturity groups, but not in the early group. However, the level of osmotic adjustment was not related to the maturity group, when water stress was imposed at the same developmental stage. Entries with high osmotic adjustment produced higher grain yields than those with low osmotic adjustment. The response varied from 15% for the mean of all maturity groups to 34% for the mean of intermediate and late groups, where there were significant differences in osmotic adjustment. The higher mean grain yield was due mainly to a larger grain number (19%). Even though entries with high osmotic adjustment bad a greater root length, soil water extraction and dry matter production during the pre-anthesis stress period, there was no significant difference in dry matter yield at physiological maturity between low and high osmotic adjustment groups. Consequently, the higher mean grain yield was related solely to a higher harvest index (27%), which was associated with a higher distribution index (25%) and a higher grain number (19%). A detailed analysis is given of the mechanisms by which osmotic adjustment contributed to grain yield in plants subjected to a pre-anthesis stress.