Mesophyll conductance response to short-term changes in pCO 2 is related to leaf anatomy and biochemistry in diverse C 4 grasses.

Mesophyll CO ₂ conductance (g _m ) in C ₃ species responds to short-term (minutes) changes in environment potentially due to changes in leaf anatomical and biochemical properties and measurement artefacts. Compared with C ₃ species, there is less information on g _m responses to short-term changes in environmental conditions such as partial pressure of CO ₂ (pCO ₂ ) across diverse C ₄ species and the potential determinants of these responses. Using 16 C ₄ grasses we investigated the response of g _m to short-term changes in pCO ₂ and its relationship with leaf anatomy and biochemistry. In general, g _m increased as pCO ₂ decreased (statistically significant increase in 12 species), with percentage increases in g _m ranging from +13% to +250%. Greater increase in g _m at low pCO ₂ was observed in species exhibiting relatively thinner mesophyll cell walls along with greater mesophyll surface area exposed to intercellular air spaces, leaf N, photosynthetic capacity and activities of phosphoenolpyruvate carboxylase and Rubisco. Species with greater CO ₂ responses of g _m were also able to maintain their leaf water-use efficiencies (TE _i ) under low CO ₂ . Our study advances understanding of CO ₂ response of g _m in diverse C ₄ species, identifies the key leaf traits related to this response and has implications for improving C ₄ photosynthetic models and TE _i through modification of g _m .
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