Stomatal opening tends to be regulated such that photosynthesis is approximately co-limited by CO2 diffusion through stomata and light-driven electron transport. This is seen in Fig. 6 as the intersection between the line describing the leaf's conductance for CO2 transport (supply function) and the A-Cc curve (demand function). A higher conductance and higher Cc would only marginally increase CO2 assimilation, but would significantly increase transpiration, since transpiration increases linearly with gs, as a result of the constant difference in water vapor concentration between the leaf and the air (wi-wa) (Sect. 2.2.2, Fig. 28; Sect. 5.4.3 of Chapter 3 on plant water relations). At lower conductance, water loss declines again linearly with gs; however, Cc also declines, because the demand for CO2 remains the same, and the difference with Ca increases. This increased CO2 concentration gradient across the stomata counteracts the decrease in gs. Hence, photosynthesis declines less than does transpiration with decreasing Ccand Ci. The result is an increasing water-use efficiency (WUE) (carbon gain per water lost) with decreasing gs. Less of the total photosynthetic capacity is used at a low Ccand Ci, however, leading to a reduced photosynthetic N-use efficiency (PNUE) (carbon gain per unit leaf N; Sect. 6.1).
Plants tend to reduce stomatal opening under water stress so that WUE is maximized at the expense of PNUE. Under limited availability of N, stomata may open further, increasing PNUE at the
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