Water relations, photosynthesis, and leaf anatomy in two species of Calotropis genus

The Calotropis genus is a common weed native from Asia present in arid ecosystems. In Venezuela, the genus is represented by two shrub species, C. procera and C. gigantea, both living in xerophytic habitats and characterized by a marked resistance to drought. In order to define the ecophysiologycal characteristics of these species, the water status, gas exchange and leaf anatomy of plants growing under natural conditions were measured during rainy and dry seasons in a littoral area of the Vargas state, Venezuela. Water potential was not significantly different between species. However, water potential was 50% lower for both species under drought. Photosynthetic rate (A) and stomatal conductance (g sub(s)) were higher during the rainy season in both species. A and g sub(s) were 40 and 48% higher in C. procera than in C. gigantea. g sub(s) decreased by 60% during drought in C. procera, while it did not change in C. gigantea. The highest values in water use efficiency were obtained in C. procera. Leaves of both species showed amphystomatic, isolateral mesophyll structure with high pubescence, and both species differed significantly in foliar thickness and specific leaf area. The results suggest that C. procera had higher photosynthetic capacity than C. gigantea.