Low temperature effects on leaf physiology and survivorship in the C3 and C4 subspecies of Alloteropsis semialata

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 photo-damage 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 C3 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.