Mesophyll conductance response to short-term changes in CO2 is related to leaf anatomy and biochemistry in diverse C4 grasses

Summary Mesophyll CO 2 conductance (g m ) in C 3 species responds to short-term (minutes) changes in environment potentially due to changes in some leaf anatomical and biochemical properties and due to measurement artifacts. Compared to C 3 species, there is less information about g m responses to short-term changes in environment conditions like p CO 2 across diverse C 4 species and the potential determinants of these responses. Using 16 diverse C 4 grasses we investigated the response of g m to short-term changes in CO 2 and how this response related to the leaf anatomical and biochemical traits. For all the measured C 4 -grasses g m increased as CO 2 decreased; however, the percent change in g m varied (+13% to +250%) and significantly related to percent changes in leaf transpiration efficiency (TE i ). The percent increase in g m was highest in grasses with thinner mesophyll cell walls and greater leaf nitrogen, activities of phosphoenolpyruvate carboxylase (PEPC), Rubisco and carbonic anhydrase, and a higher affinity of PEPC for bicarbonate. Our study demonstrates that CO 2 response of g m varies greatly across diverse C 4 grasses and identifies the key leaf anatomical and biochemical traits related to this variation. These findings have implications for improving C 4 photosynthetic models, and in attempts to improve TE i through manipulation of g m .