Effects of water stress on growth, pigments and (CO2)-C-14 assimilation in three sorghum cultivars

The effects of drought on growth, pigments and (CO2)-C-14 assimilation were studied in three sorghum cultivars. Water stress applied either at the vegetative or at the reproductive stage was found to reduce relative growth and net assimilation rates. Root growth was less affected by water stress and in certain cases it was increased; consequently, the root/shoot ratio was improved. The sensitivity to drought stress was greater at the reproductive than at the vegetative stage. Dorado was the most drought-tolerant and Giza 15 the least drought-tolerant cultivar, as determined by calculation of the drought susceptibility index for total green leaf area and shoot dry weight. Short-term water stress in the vegetative phase (7 days) improved the chlorophyll content in leaves, and long-term stress in the vegetative and reproductive phases reduced chlorophyll content. Carotenoid content, in general, was not changed by drought stress. (CO2)-C-14 photoassimilation indicated that soluble, insoluble and consequently total photosynthates were reduced at the end of the stress period at both stages. Drought plus defoliation appeared to increase both chlorophyll content and (CO2)-C-14 photoassimilation, to a certain extent, as compared with drought alone.