An Empirical Model for Leaf Expansion in Cacao in Relation to Plant Water Deficit

Cacao (Theobroma cacao L.) seedlings at the onset of a flush cycle were exposed to five different irrigation treatments. The expansion of all leaves in an emerging flush was followed, and estimates of leaf water potential(ψw) were made on each day that leaf areas were measured. The growth in area of the leaves was fitted with a modified logistic curve of the form y = a/[1 + be−(ct+dt2)], and parameters of leaf growth were derived from the fitted constants. A coefficient of stress exposure, S, was derived as the slope of the relation between cumulative ψw and time. Three parameters of the logisitc function (a, c and d) were strongly associated with S. By fitting regressions for the relations between S and estimates of each parameter for the five treatments, values of a, b, c and d may be estimated at any level of S encompassed by these data. The effects of water stress on leaf expansion rate and on the final leaf area attained by the emerging flush can be adequately predicted by this technique.