Accurate photosynthetic parameter estimation at low stomatal conductance: effects of cuticular conductance and instrumental noise

Accurate estimation of photosynthetic parameters is essential for understanding plant physiological limitations and responses to environmental factors from the leaf to the global scale. Gas exchange is a useful tool to measure responses of net CO2 assimilation (A) to internal CO2 concentration (C-i), a necessary step in estimating photosynthetic parameters including the maximum rate of carboxylation (V-cmax) and the electron transport rate (J(max)). However, species and environmental conditions of low stomatal conductance (g(sw)) reduce the signal-to-noise ratio of gas exchange, challenging estimations of C-i. Previous works showed that not considering cuticular conductance to water (g(cw)) can lead to significant errors in estimating C-i, because it has a different effect on total conductance to CO2 (g(tc)) than does g(sw). Here we present a systematic assessment of the need for incorporating g(cw) into C-i estimates. In this study we modeled the effect of g(cw) and of instrumental noise and quantified these effects on photosynthetic parameters in the cases of four species with varying g(sw) and g(cw), measured using steady-state and constant ramping techniques, like the rapid A/C-i response method. We show that not accounting for g(cw) quantitatively influences C-i and the resulting V-cmax and J(max), particularly when g(cw) exceeds 7% of the total conductance to water. The influence of g(cw) was not limited to low g(sw) species, highlighting the importance of species-specific knowledge before assessing A/C-i curves. Furthermore, at low g(sw) instrumental noise can affect C-i estimation, but the effect of instrumental noise can be minimized using constant-ramping rather than steady-state techniques. By incorporating these considerations, more precise measurements and interpretations of photosynthetic parameters can be obtained in a broader range of species and environmental conditions.