Ecophysiology of Yucca brevifolia, an arborescent monocot of the Mojave sesert.

Photosynthetic characteristics and transpiration of Yucca brevifolia, an evergreen tree endemic to the Mojave Desert of California and Nevada, were examined in the field and the laboratory. Yucca brevifolia was confirmed to be a C3 plant, with no CAM tendencies observed for its semi-succulent leaves. The species exhibited a maximum net CO2 uptake of 12 μmol m-2 s-1 at 22°C when grown at day/night air temperatures of 31°C/17°C (data expressed on a total area basis for these opaque leaves). The optimum temperature for CO2 uptake shifted 4.5°C per 10°C change in daytime growth temperature, so that observed leaf temperatures in the field were near optimum temperatures throughout the midday period in all but the hottest months of the year. Leaves also acclimated to low and high temperature extremes, tolerances ranging to-11°C and to 59°C, respectively, suggesting that low temperatures limit the distribution of Y. brevifolia but high temperatures do not. Light saturation of photosynthesis occurred at a relatively low PAR of about 500 μmol m-2 s-1, similar to the actual PAR within a rosette. Diurnal patterns of leaf conductance shifted from a broad midday peak in wet seasons to a reduced, narrow, early morning peak in the dry season, indicating effective stomatal control of water use. The approximately 5-month-long winter-spring growth season accounted for 80% of the yearly CO2 uptake, with a predicted annual uptake of about 22 mol m-2 y-1 and a transpiration ratio of 700.