C₄ photosynthesis has evolved in at least eight independent clades of grasses. These C₄ species are a major component of savanna and warm temperate grasslands, but they and their C₃ relatives inhabit a range of habitats throughout the tropics and beyond. Experimental evidence has demonstrated leaf-level improvements in the efficiency of water and nitrogen use in C₄ compared to C₃ plants. It has been hypothesized that these contrasts may allow differences in biomass allocation between C₃ and C₄ species. However, recent studies have highlighted phylogeny as a critical factor for understanding the physiological and ecological differences between C₃ and C₄ species. We therefore measured leaf ecophysiological traits and biomass allocation in multiple sister lineages of C₃ and C₄ grasses, sampling NADPme and NADme subtypes of C₄ photosynthesis. Our glasshouse experiment addressed two questions. 1) Which functional traits are consistently modified by photosynthetic pathway in independent clades of grasses (convergent evolution)? 2) Which traits show strong phylogenetic dependence (divergence between clades)?

Results/Conclusions

Photosynthetic pathway had significant overall effects on the ecophysiological traits of leaves. Rates of photosynthesis were higher, stomatal conductance was lower, and the difference between pre-dawn and midday leaf water potential was smaller in the C₄ than C₃ clades. However, the effect of photosynthetic pathway on leaf nitrogen content was not statistically significant. Despite this, leaf photosynthetic nitrogen- and water-use efficiencies were higher in C₄ than C₃ grasses. Biomass allocation to roots was higher in the NADPme clades of C₄ grasses than either NADme or C₃ clades, and tended to trade off against allocation to leaves. Significant phylogenetic effects were detected for leaf nitrogen content, leaf water deficit and biomass allocation. Our data therefore show strong convergence in leaf ecophysiological traits among C₄ grass clades. The influence of photosynthetic pathway on biomass allocation may depend upon the C₄ subtype, but it is also modified by divergence between grass clades.