Water Vapor Conductance and CO"2 Uptake for Leaves of a C"4 Desert Grass, Hilaria Rigida

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Abstract
Availability of soil water was the major influence on seasonal stomatal activity of Hilaria rigida (Thurb.) Benth. ex Scribn. over a 2—yr study period in the Colorado desert section of the Sonoran desert. Major stomatal opening (water vapor conductance >2mm/s) occurred for 4.6 mo in the relatively cool winter—spring (maximum daytime temperatures averaging 17°C) and for 1.7 mo in the late summer—early fall (maximum daytime temperatures averaging 31°C). The temperature optimum for CO2 uptake was generally above daytime temperatures, particularly in the winter. When the daytime growth temperature was raised from 16° to 49°C in laboratory experiments, the temperature optimum for CO2 uptake shifted from 29° to 43°C. Besides the rather high temperature optimum for CO2 uptake, H. rigoda displayed other typical C4 characteristics including Kranz anatomy, a low CO2 compensation point (12 μl/1), and a lack of light saturation of CO2 uptake at full sunlight. Under optimal field conditions, the CO2 uptake rate can be 67μmol°m—2°x—1 (106mg CO2°dm—2 h—2), higher than has been reported for any other species. The accompanying high water use efficiency (mass CO2 taken up by leaf blades/mass H2O lost) may help explain the success of this hardy grass in both the Mojave and Sonoran deserts.