DROUGHT ACCLIMATION OF AN UNDERSTORY SHRUB (PSYCHOTRIA LIMONENSIS; RUBIACEAE) IN A SEASONALLY DRY TROPICAL FOREST IN PANAMA

Physiological responses to seasonal drought were explored for Psychotria limonensis (Rubiaceae), an abundant understory shrub in a seasonally dry tropical forest in Panama. Control and irrigated plants were compared at the beginning and again at the end of the 4-mo dry season. Stomatal conductance remained high throughout for irrigated plants, but fell to very low levels for control plants late in the dry season. Net assimilation rates under both saturating and ambient light were unaffected by irrigation. As a consequence, instantaneous water-use efficiency (assimilation ÷ evapotranspiration), derived from gas exchange measures, and long-term water-use efficiency, estimated from stable carbon isotope ratios of leaf tissue, were similar for both treatments. The maintenance of high assimilation rates despite drought may be related to osmotic adjustment. Control plants had more negative osmotic potentials at full turgor and higher moduli of elasticity in the late dry season.