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.
