Compensatory Photosynthesis of Sporobolus flexuosus (Thurb.) Rydb. Following Simulated Herbivory in the Northern Chihuahuan Desert

Abstract

Leaf net photosynthesis (Pn), conductance to water vapor, water potential, solar radiation penetration within the canopy, and biomass production of Sporobolus flexuosus plants in the northern Chihuahuan Desert were measured 4, 18, and 34 d following removal of one-half of all shoots. Regrowth in partially defoliated plants was limited because of a delay in any substantial precipitation until relatively late in the growing season and the mature stage of plant development at the time of defoliation. There were no differences in Pn between partially defoliated and control plants 4 d after defoliation but on days 18 and 34, Pn rates of defoliated plants were 35% higher than for leaves of similar age on control plants. However, significant differences between treatments did not occur until midday when incident solar radiation was greatest. Daily integrated solar radiation penetration within the canopy of defoliated and control plants averaged 66% and 32%, respectively, of the total irradiance received in the open. The simultaneous direct effects of defoliation and indirect effects of increased irradiance levels within the canopy of partially defoliated plants may have contributed concurrently to the compensatory photosynthesis. Compensatory photosynthesis evident in this study was not due to changes in conductance or improvements in plant water status. There were no differences between treatments in leaf conductance but, on all three sample days, leaf water potentials of partially defoliated plants were 20%-30% more negative than those of control plants. Results of this study indicate that compensatory photosynthesis may be a common response in defoliated range grasses despite the suboptimal environmental conditions common in the field.