Photosynthetic responses of Microstegium vimineum (Trin.)

Microstegium vimineum (Trin.) A. Camus, a shade-tolerant C4 grass, has spread throughout the eastern United States since its introduction in 1919. This species invades disturbed understory habitats along streambanks and surrounding mesic forests, and has become a major pest in areas such as Great Smoky Mountains National Park. The focus of this study was to characterize the photosynthetic induction responses of M. vimineum, speci®cally its ability to utilize low light and sun ̄ecks, two factors that may be critical to invasive abilities and survival in the understory. In addition, we were curious about the ability of a grass with the C4 photosynthetic pathway to respond to sun ̄ecks. Plants were grown under 25% and 50% ambient sunlight, and photosynthetic responses to both steady-state and variable light were determined. Plants grown in both 25%and 50% ambient sun became 90% light saturated between 750±850 lmol m s; however, plants grown in 50% ambient sun had signi®cantly higher maximum steadystate photosynthetic rates (16.09 ‹ 1.37 lmol m s vs. 12.71 ‹ 1.18 lmol m s). Both groups of plants induced to 50% of the steady-state rate in 3±5 min, while it took 10±13 min to reach 90% of maximum rates, under both ̄ashing and steady-state light. For both groups of plants, stomatal conductance during induction reached maximum rates in 6±7 min, after which rates decreased slightly. Upon return to low light, rates of induction loss and stomatal closure were very rapid in both groups of plants, but were more rapid in those grown in high light. Rapid induction and the ability to induce under ̄ashing light may enable this species to invade and dominate mesic understory habitats, while rapid induction loss due to stomatal closure may prevent excess water loss when low light constrains photosynthesis. TheC4 pathway itself does not appear to present an insurmountable barrier to the ability of this grass species to respond to sun ̄ecks in an understory environment.