Shoot growth in Coffea arabica, 2: Growth flushing stimulated by irrigation

The growth activity of axillary and terminal vegetative apices of Coffea arabica trees was compared when growing in the field in Kenya under carefully controlled soil moisture regimes in which the trees were irrigated weekly, or when irrigation and rainfall were with held for different periods and ar different times during the hot dry season; the effect of the treatments on internal water stress was studied using a stomatal infiltration technique. Weekly irrigation to maintain the soil beneath trees near to field capacity failed to relieve water stress measured during the day, indicating that the evaporative demand of the atmosphere plays a larger part than soil moisture in determining the water status of the tree. However, trees irrigated after eight weeks' enforced drought flushed immediately, producing leaves faster and opening their stomata during the day to a greater extent than trees which had been regularly irrigated. Trees which experienced twelve weeks enforced drought produced approximately 70 per cent more new laterals when the trees were irrigated again than did trees which experienced shorter periods of drought stress. Water stress appears to stimulate the trees into compensatory physiological activity when the stress is eventually relieved, possibly by decreasing root resistance to water uptake