Differences in drought sensitivities and photosynthetic limitations between co-occurring C3 and C4 (NADP-ME) Panicoid grasses

The success of C 4 plants lies in their ability to attain greater efficiencies of light, water and nitrogen use under high temperature, providing an advantage in arid, hot environments. However, C 4 grasses are not necessarily less sensitive to drought than C 3 grasses and are proposed to respond with greater metabolic limitations, while the C 3 response is predominantly stomatal. The aims of this study were to compare the drought and recovery responses of co-occurring C 3 and C 4 NADP-ME grasses from the subfamily Panicoideae and to determine stomatal and metabolic contributions to the observed response. Methods Six species of locally co-occurring grasses, C 3 species Alloteropsis semialata subsp. eckloniana, Panicum aequinerve and Panicum ecklonii, and C 4 (NADP-ME) species Heteropogon contortus, Themeda triandra and Tristachya leucothrix, were established in pots then subjected to a controlled drought followed by re-watering. Water potentials, leaf gas exchange and the response of photosynthetic rate to internal CO 2 concentrations were determined on selected occasions during the drought and re-watering treatments and compared between species and photosynthetic types. Key Results Leaves of C 4 species of grasses maintained their photosynthetic advantage until water deficits became severe, but lost their water-use advantage even under conditions of mild drought. Declining C 4 photosynthesis with water deficit was mainly a consequence of metabolic limitations to CO 2 assimilation, whereas, in the C 3 species, stomatal limitations had a prevailing role in the drought-induced decrease in photosynthesis. The drought-sensitive metabolism of the C 4 plants could explain the observed slower recovery of photosynthesis on re-watering, in comparison with C 3 plants which recovered a greater proportion of photosynthesis through increased stomatal conductance. Conclusions Within the Panicoid grasses, C 4 (NADP-ME) species are metabolically more sensitive to drought than C 3 species and recover more slowly from drought.