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

Cacao (Theobroma cacao L.) seedlings at the onset of a flushcycle were exposed to five different irrigation treatments.The expansion of all leaves in an emerging flush was followed,and estimates of leaf water potential(_w) were made on each daythat leaf areas were measured. The growth in area of the leaveswas fitted with a modified logistic curve of the form y = a/[1+ be ^–(ct+dt2)], and parameters of leaf growth were derivedfrom the fitted constants. A coefficient of stress exposure,S, was derived as the slope of the relation between cumulative_w and time. Three parameters of the logisitc function (a, cand d) were strongly associated with S. By fitting regressionsfor the relations between S and estimates of each parameterfor the five treatments, values of a, b, c and d may be estimatedat any level of S encompassed by these data. The effects ofwater stress on leaf expansion rate and on the final leaf areaattained by the emerging flush can be adequately predicted bythis technique. Theobroma cacao L.cacao, leaf expansion, logistic model, water stress