Leaf gas exchange responses to abrupt changes in light intensity for two invasive and two non-invasive C4 grass species.
Transient and steady state responses of leaf gas exchange (photosynthesis (A) and stomatal conductance to water vapor (gs)) to marked changes in photosynthetic photon flux density (PPFD) were studied for two invasive [Cynodon dactylon (L.) Pers. and Sorghum halepense (L.) Pers.] and two non-invasive, native [Bothriochloa ischaemum (L.) Keng and Chrysopogon gryllus (Torn.) Trin.] perennial C4 grass species from semiarid temperate grasslands or croplands. Following an abrupt drop in PPFD from 1300 to 270 µmol photon m-2 s-1, the two invasive species reduced gs to a greater extent than A, resulting in higher intrinsic photosynthetic water use efficiency (PWUE=A/gs) at low, compared to high-light conditions. For non-invasives, a comparable drop in gs and A led to invariant PWUE, which was lower than that for the invasive group under low light. The duration and speed of stomatal closure was similar for the four species. However, unlike the other grasses, the noxious weed S. halepense exhibited a negligible net loss in PWUE during the high-to-low light transition. Responses of the native B. ischaemum were mostly intermediate between those of the two invasive species and the non-invasive C. gryllus, which is in agreement with the species' ecological intermediacy: non-invasive but often reaches local dominance following a disturbance. With a sudden reverse change in PPFD photosynthetic light induction was not faster for invasives than for non-invasives. These results indicate more efficient water use under variable light for invasive compared to non-invasive perennial C4 grasses which may contribute to their success in semiarid temperate habitats with a heterogeneous light regime. Yet, rapid photosynthetic light induction appears to be of less importance in such environments.