RESUMEN
The species richness of C(4) grasses is strongly correlated with temperature, with C(4) species dominating subtropical ecosystems and C(3) types predominating in cooler climates. Here, the effects of low temperatures on C(4) and C(3) grasses are compared, controlling for phylogenetic effects by using Alloteropsis semialata, a unique species with C(4) and C(3) subspecies. Controlled environment and common garden experiments tested the hypotheses that: (i) photosynthesis and growth are greater in the C(4) than the C(3) subspecies at high temperatures, but this advantage is reversed below 20 degrees C; and (ii) chilling-induced photoinhibition and light-mediated freezing injury of leaves occur at higher temperature thresholds in the C(4) than the C(3) plants. Measurements of leaf growth and photosynthesis showed the expected advantages of the C(4) pathway over the C(3) type at high temperatures. These declined with temperature, but were not completely lost until 15 degrees C, and there was no evidence of a reversal to give a C(3) advantage. Chronic chilling (5-15 degrees C) or acute freezing events induced a comparable degree of photodamage in illuminated leaves of both subspecies. Similarly, freezing caused high rates of mortality in the unhardened leaves of both subtypes. However, a 2-week chilling treatment prior to these freezing events halved injury in the C(3) but not the C(4) subspecies, suggesting that C(4) leaves lacked the capacity for cold acclimation. These results therefore suggest that C(3) members of this subtropical species may gain an advantage over their C(4) counterparts at low temperatures via protection from freezing injury rather than higher photosynthetic rates.