The effects of drought on growth, pigments and 14CO2 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. 14CO2 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 14CO2 photoassimilation, to a certain extent, as compared with drought alone.

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Summary The dormant phase of the flush cycle of leaf growth in cocoa is known to be correlated with high abscisic acid (ABA) levels in the mature leaves of the new flush (NF) and previous flush (PF) leaves. Defoliation of either the NF leaves or PF leaves of cocoa seedlings reduced the length of the dormant phase of the flush cycle, thus showing that the mature leaves were a source of growth inhibitors which could affect shoot apical activity. The application of ABA to the NF and PF leaves led to an extension of the dormant phase, whereas application of zeatin or gibberellic acid decreased it. The distribution of [14C]ABA following its application to NF and PF leaves at different stages throughout the growth cycle showed that [14C] ABA was accumulated by the bud in relatively larger amounts during the final stages of bud dormancy (I‐1 and I‐2) than in the earlier stage (F‐2). The results suggest that internal competition for nutrients may be responsible for the inhibition of growth at the F‐2 stage but that ABA translocated from the mature leaves causes the buds to remain dormant during the subsequent stages of I‐1 and I‐2.

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SUMMARYThe flush leaves of seedling cocoa plants were assessed at each stage of the flush cycle from leaf unfolding to maturity for differences in leaf lamina presentation angle, surface hairs, stomatal number and development and leaf cuticle thickness. Changes in these parameters were then correlated with cuticular and stomatal diffusive resistance of the leaves. It is suggested that perpendicular positioning of the young flush leaves, the presence of surface hairs, the rapid synthesis of the cuticle and the limited stomatal development during the early stages of leaf expansion, restrict water loss from the developing leaves. Maximum rate of water loss from the leaves of a new flush was only attained after leaves were fully expanded, stomata fully developed and the leaves horizontally positioned. It is suggested that the large increase in water loss from the flush leaves following these changes might lead to an internal water deflcit in the plant and hence to increased synthesis of abscisic acid in the shoot which, in turn, will maintain dormancy of the shoot apex in at least part of the interflush period.