LEAF PHYSIOLOGY AND DROUGHT STRESS OF TWO <em>PANDANUS</em> SPECIES IN A HUMID LOWLAND TROPICAL CLIMATE

Abstract

Water was withheld for eight weeks from Pandanus dubius and Pandanus tectorius trees to determine their physiological responses and adaptations to water deficits. When light was abruptly reduced as occurs when cumulus clouds block sunlight, stomata responded with a gradual decline in stomatal conductance to water (g_s). This response occurred in 2 to 3 min and increased water use efficiency by reducing transpiration. A slow increase in g_s occurred upon a return to high light, but this brief lag temporarily limited net CO., assimilation (A_co2). Water deficits inline-need this stomatal response by slowing down recovery of gs after the return to high light following several minutes of low light. The dependence of A_co2 on irradiance conformed to an asymptotic exponential model. Trees experiencing water deficits exhibited a greater decrease in A_co2 in the lower levels of irradiance than did well-watered trees. The decrease in A_co2 caused by water deficits was due to direct inhibition of photosynthesis, since internal CO₂ concentration actually increased with the decrease in A_co2 and g_s. Carboxylation efficiency, photochemical efficiency, and relative leaf water content were reduced by drought stress. In contrast, leaf osmotic potential was not influenced by eight weeks of with holding water. Leaf tissue was fully re-hydrated following re-watering, but g_s and A_co2 did not return to pro-stress levels until after 11 days of re-watering.