For the leaf photosynthesis in the C3 plants, CO2 diffuses from ambient air, through stomata, to chloroplasts. The CO2 partial pressure decreases significantly from the ambient air to chloroplast; the CO2 conductance through stomata (gs) and mesophyll (gm) are the two components determining CO2 in the chloroplast. The CO2 drawdown by gm is in the same order, or even larger, as that by gs, and thus, strongly limits leaf photosynthesis in the C3 plants. Additionally, gm may affect plant water use efficiency and plant production through the regulation of photosynthetic rate. To compare the effect of gm on plant water use efficiency and plant production with that of gs, we estimated gm for the plants of different life forms and transgenic plants overexpressing plasma membrane aquaporin (water channel protein) using the simultaneous measurement of leaf gas exchange and carbon isotope discrimination during the leaf gas exchange. In the many of the cases, gm limits leaf photosynthesis more strongly than gs. The plants having large gm show enhanced plant growth compared to those having low gm, and in some cases, show enhanced leaf water use efficiency. The interaction between gm and gs in relation to the regulation of leaf water use efficiency and production will be discussed.