Physiological advantages of C(4) grasses in the field: a comparative experiment demonstrating the importance of drought

Global climate change is expected to shift regional rainfall patterns, influencing species distributions where they depend on water availability. Comparative studies have demonstrated that C(4) grasses inhabit drier habitats than C(3) relatives, but that both C(3) and C(4) photosynthesis are susceptible to drought. However, C(4) plants may show advantages in hydraulic performance in dry environments. We investigated the effects of seasonal variation in water availability on leaf physiology, using a common garden experiment in the Eastern Cape of South Africa to compare 12 locally occurring grass species from C(4) and C(3) sister lineages. Photosynthesis was always higher in the C(4) than C(3) grasses across every month, but the difference was not statistically significant during the wettest months. Surprisingly, stomatal conductance was typically lower in the C(3) than C(4) grasses, with the peak monthly average for C(3) species being similar to that of C(4) leaves. In water-limited, rain-fed plots, the photosynthesis of C(4) leaves was between 2.0 and 7.4 μmol m(−2) s(−1) higher, stomatal conductance almost double, and transpiration 60% higher than for C(3) plants. Although C(4) average instantaneous water-use efficiencies were higher (2.4–8.1 mmol mol(−1)) than C(3) averages (0.7–6.8 mmol mol(−1)), differences were not as great as we expected and were statistically significant only as drought became established. Photosynthesis declined earlier during drought among C(3) than C(4) species, coincident with decreases in stomatal conductance and transpiration. Eventual decreases in photosynthesis among C(4) plants were linked with declining midday leaf water potentials. However, during the same phase of drought, C(3) species showed significant decreases in hydrodynamic gradients that suggested hydraulic failure. Thus, our results indicate that stomatal and hydraulic behaviour during drought enhances the differences in photosynthesis between C(4) and C(3) species. We suggest that these drought responses are important for understanding the advantages of C(4) photosynthesis under field conditions.