CHANGES IN DORMANCY LEVEL IN SORGHUM-HALEPENSE SEEDS INDUCED BY WATER-STRESS DURING SEED DEVELOPMENT

1. Dormancy in Sorghum halepense seeds as affected by intermittent water stress imposed on the mother plant during seed development was investigated. The drought treatment was imposed in cycles within the maturation period by withholding water for 5 days, rewatering at the end of each drought cycle and withholding water again. 2. The results showed that two sources of dormancy exist in S. halepense seeds: one is inherent in the caryopsis itself and is not affected by water stress during seed development. The other is imposed by the presence of the glumes which cover the caryopsis completely, and is susceptible to removal by drought during seed development. As a result dormancy levels in seeds that had matured in water-stressed mother plants were lower than those recorded in control seeds. Differences in dormancy level were maintained even after 3 months of moist chilling. 3. An attempt was made to identify the nature of the inhibitory properties found in glumes from spikelets that had ripened in well-watered mother plants, and to explain the lack of glume-imposed dormancy in droughted seeds. No water-soluble germination inhibitor was detected in control glumes, and imbibition of the caryopses was shown not to be prevented by the presence of control glumes. However, results obtained with experiments in which oxygen uptake by different parts of the spikelets was measured, suggested that control glumes might be acting as an effective barrier for the satisfaction of oxygen requirements of the caryopses enclosed within them. It is concluded that water stress during seed development reduces dormancy of S. halepense seeds through modifications in the properties of the glumes that, apparently, result in an enhancement of their permeability to oxygen diffusion. We discuss the ecological implications of the observed responses.