Mid-summer gas exchange and water relations of seven C3 species in a desert wash in Baja California, Mexico

Gas exchange and water relations of two palm trees, four leafy sub-shrubs, and one microphyllous sub-shrub were measured at mid-day in mid-summer 1990 in a desert wash in northern Baja California, Mexico in order to examine, in a preliminary manner, some of the diverse adaptations of these species to this hot, high-insolation environment. Temperatures of the large (approximately 1 m in diameter) leaves of two species of palm tree were at or below air temperatures. Apparent mechanisms underlying leaf cooling were different in the two species. Leaves of Erythea armata were covered with a thick, light blue wax which reflected light, keeping leaf temperatures low without expending large amounts of water, yet also depressing CO2 uptake rates. Leaves of Washingtonia filifera were bright green and glabrous, reflecting less light, allowing higher rates of photosynthesis, but also resulting in high transpiration rates which maintained relatively low temperatures. Ecophysiological parameters of three small shrubs with non-vertical, densely pubescent leaves (Hyptis emoryi, Lupinus excubitus, Salvia apiana) differed little from a shrub with vertical, glabrous leaves (Baccharis glutinosa). All four species exhibited high conductances and transpiration rates. Only one species of the seven, a microphyllous shrub (Bebbia juncea), showed signs of drought stress, as indicated by zero turgor potentials and low photosynthetic rates. The two palms exhibited low water potentials, yet even lower osmotic potentials, which maintained high turgor potentials. Important adaptations to this potentially stressful environment include vertical leaf orientation, reflective wax on leaf surfaces, leaf pubescence, low osmotic potentials, and high transpiration rates.