Implications of interveinal distance for quantum yield in C4 grasses:: a modeling and meta-analysis

The distance between veins has the potential to affect photosynthesis in C-4 grasses because photon capture and photosynthetic carbon reduction are primarily restricted to vascular bundle sheath cells (BSC). For example, BSC density should increase as interveinal distance (IVD) decreases, and thus IVD may influence photon capture and photosynthesis in C-4 grasses. The objective of this study is to evaluate the potential importance of IVD to the function of C-4 grasses, and a literature survey is conducted to test the hypothesis that quantum yield of photosynthesis (Phi) increases with decreasing IVD. First, a meta-analysis of Phi and IVD values obtained for 12 C-4 grass species supports this hypothesis as Phi and IVD are significantly negatively correlated (r=-0.61). Second, a regression of carbon isotope discrimination (Delta) versus IVD was conducted and the regression equation was used in a simple biochemical model that relates Phi to Delta and leakage of CO2 from the BSC. The modeling analysis also supports the hypothesis that Phi decreases with increasing IVD in C-4 grasses. C-4 grasses are virtually absent from shaded habitats, and the biochemical model is employed to examine the implications of IVD for shade-tolerance in C-4 grasses. The model predicts that only those species with uncommonly small IVD values are able to tolerate prolonged shade.