XVI International Botanical Congess

Water relations and gas exchange data suggest that the dune species Arctotheca populifolia, Scaevola plumeri, and Ipomoea pes-caprae are not limited by the supply of water. Water budgets constructed for S. plumeri show that in low rainfall years plants transpire more water than is supplied by rainfall, suggesting a dependence on ground or long-term stored water. Comparison of d?18 values of water distilled from underground stems of S. plumeri with ground and rainwater values indicates that plants utilize ground water except after substantial rainfall. High transpiration rates may be advantageous in obtaining nutrients in a low nutrient environment. Nutrient concentrations in soil and ground water are such that, the volume of water transpired would be adequate to supply the nutrients assimilated during growth if the water source was ground water, but inadequate if soil water was the sole source.