GAS-EXCHANGE AND RESOURCE-USE EFFICIENCY OF LEYMUS-CINEREUS (POACEAE)- DIURNAL AND SEASONAL RESPONSES TO NATURALLY DECLINING SOIL-MOISTURE

We examined factors that limit diurnal and seasonal photosynthesis in Leymus, a robust tussock grass from shrub-steppes of western North Ame rica. Data from plants in a natural stand and in experimental field pl ots indicate that this bunchgrass has 1) a high photosynthetic capacit y, 2) high leaf nitrogen content and high nitrogen-use efficiency, 3) a steep leaf-to-air diffusion gradient for carbon dioxide, which enhan ces intrinsic water-use efficiency, and 4) photosynthetic tissues that tolerate severe water stress and recover quickly from moderate water stress. Midday depressions of CO2 assimilation (A) and stomatal conduc tance were slight in plants with plentiful water, but marked in plants subject to moderate water stress. Midday stomatal closure in moderate ly stressed plants reduced intercellular carbon dioxide concentration (c(i)) by approximate to 40 mu l liter(-1). The maximum rate of A achi eved during the day for severely stressed plants (predawn water potent ial = -4 MPa) was one-third and daily carbon gain per unit leaf area w as about one-fourth that of well-watered plants. For plants in the nat ural stand, CO2-saturated photosynthesis declined almost linearly with decreasing soil water availability over the growing season, whereas t here was little effect on A at ambient CO2 levels or on carboxylation efficiency until predawn water potentials reached -1.8 MPa. Nitrogen-u se efficiency declined with diminishing soil moisture, but there was n o seasonal change in stomatal limitation or instantaneous water-use ef ficiency as estimated from A vs. c(i) curves at optimal leaf temperatu re and moderate atmospheric evaporative demand. Thus, reduced stomatal conductance in response to increased evaporative demand may increase stomatal limitation diurnally, but over the growing season, stomatal l imitation estimated from A vs. c(i) curves is relatively constant beca use maximum stomatal conductance is closely tuned to the CO2 assimilat ory capacity of the mesophyll.