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.