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

Background and Aims 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 Panieum ecklonii, and C-4 (NADP-ME) species Heteropogon conforms, 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, 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, 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.