Mary C. Acock

Modeling the effects of nitrogen on components of coca yield

Estimating yields (leaf dry weights) of Erythroxylum coca var. coca Lam. (coca) in regions where there is no access requires a method that can be used remotely. A simulation model that characterizes coca growth under varying weather conditions offers such a method. To develop this model, crop response to its environment must be quantified. The current experiment examined how a range in nitrogen (N) availability affected N status of various parts of the coca plant and components of yield. Minimal and maximal values for N concentration in leaves, stems, and roots were determined and N status calculated. Young leaves (a small yield component) had the highest N concentration, the greatest demand for N, and showed the greatest N stress. Old leaves had the next highest N concentration and a high demand for N, but showed the lowest N stress. Nitrogen status in old leaves was most affected by N availability. Dry weights of each leaf type increased with increased N availability, but leafcocaine concentration was not affected. Stems and taproots had the lowest N concentrations and these were not affected by N availability. Lateral roots, closer to the source ofN, had N concentrations that reflected N availability, however lateral root dry weights were not affected by N supply. Increased N availability increased (a) the number of growing points on the plant, (b) the number of leaves per growing point, and (c) the size of old leaves. Increases in these components translate into increased yields. Leaf yields correlated with the N status of leaves (r=0.58, p≤0.001), but variability in leaf yields for a given N status was too high to have strong predictive power.