Leaf development in Ricinus communis during drought stress: dynamics of growth processes, of cellular structure and of sink-source transition

Dicot leaf growth is characterized by partly transient tip-to-base gradient s of growth processes, structure and function. These gradients develop dyna mically and interact with dynamically developing stress conditions like dro ught, In Ricinus communis plants growing under well-watered and drought con ditions growth rates peaked during the late night and minimal Values occurr ed in the late afternoon, During this diurnal course the leaf base always s howed much higher rates than the leaf tip. The amplitude of this diurnal co urse decreased when leaves approached maturity and during drought stress wi thout any significant alteration of the diurnal pattern and it increased du ring the first days after rewatering. Unique relationships between leaf siz e and cytological structure were observed, This provided the framework for the analysis of changes in assimilation, transpiration and dark respiration , chlorophyll, protein, carbohydrate, and amino acid concentrations, and of activities of sink-source-related enzymes at the leaf tip and base during leaf development in well-watered and drought-stressed plants, Gas exchange was dominated by physiological rather than by anatomical properties (stomat al density),Tip-to-base gradients in carbohydrate concentrations per dry we ight and sink-source-related enzymes were absent, whereas significant gradi ents were found in amino acid concentrations per dry weight, During drought stress, growing leaves developed source function at smaller leaf size, bef ore specific physiological adaptations to drought occurred, The relevance o f the developmental status of individual leaves for the drought-stress resp onse and of the structural changes for the biochemical composition changes is discussed.