Drought limitation of photosynthesis differs between C-3 and C-4 grass species in a comparative experiment

Phylogenetic analyses show that C-4 grasses typically occupy drier habitats than their C-3 relatives, but recent experiments comparing the physiology of closely related C-3 and C-4 species have shown that advantages of C-4 photosynthesis can be lost under drought. We tested the generality of these paradoxical findings in grass species representing the known evolutionary diversity of C-4 NADP-me and C-3 photosynthetic types. Our experiment investigated the effects of drought on leaf photosynthesis, water potential, nitrogen, chlorophyll content and mortality. C-4 grasses in control treatments were characterized by higher CO2 assimilation rates and water potential, but lower stomatal conductance and nitrogen content. Under drought, stomatal conductance declined more dramatically in C-3 than C-4 species, and photosynthetic water-use and nitrogen-use efficiency advantages held by C-4 species under control conditions were each diminished by 40%. Leaf mortality was slightly higher in C-4 than C-3 grasses, but leaf condition under drought otherwise showed no dependence on photosynthetic-type. This phylogenetically controlled experiment suggested that a drought-induced reduction in the photosynthetic performance advantages of C-4 NADP-me relative to C-3 grasses is a general phenomenon.