Field validation of water-use efficiency of the CAM plant Opuntia ellisiana in south Texas

In single plant studies, at time scales less than a growing season, the conversion efficiency of water to dry matter several times has been shown to be greater: for CAM plants than either C-3 or C-4 plants. The objective of this work was to determine if this high conversion efficiency would occur at the held level over an entire growing season. Thus, Opuntia ellisiana water-use efficiency values were obtained under field conditions in the third and fourth year of growth of a plantation. Although O. ellisiana is the slowest growing of all the spineless Opuntias, it was used because it is the only spineless Opuntia that is completely cold hardy in Texas. Above-ground dry matter production was obtained by complete harvest and by estimating above-ground biomass with regression techniques. Evapo-transpiration was estimated as the precipitation minus runoff, deep drainage, change in soil water content and water stored in the plant. In the fourth year, surface runoff was captured and measured in below-ground tanks. Based on the measurements, surface runoff in the third year was estimated using the regression technique. Soil evaporation was directly measured over a 2-week period, four times per year using microlysimeters. Using these direct soil evaporation measurements and companion meteorological data, yearly transpiration was computed using a Radial Basis Function (RBF) neural network. This yearly soil evaporation estimate allowed the separation of transpiration from evapo-transpiration. At the beginning of the fourth years growth Opuntia ellisiana achieved a leaf area index (LAI) of 2.02. In the fourth year, 662 mm rainfall occurred. Soil evaporation (214 mm) and runoff (143 mm) were the major losses. A significant quantity of water (17 mm = 170,000 kg ha(-1)) was stored in the cactus that could be of use for animal drinking water. The cactus transpired 285 mm of water producing 17,670 kg dry matter ha(-1) for a transpiration water-use efficiency of 162 kg H2O kg(-1) dry matter. This is among the greatest water-use efficiency of any plant species (including C-3 and C-4) that has been measured under long-term held conditions. While this species is not particularly useful, other less cold hardy opuntias are widely used for fruit, vegetable and forage. Given water shortages on arid lands, much more development attention needs to be given to useful Opuntia species. (C) 1997 Academic Press Limited.