COS 155-2 - Factors affecting stomatal sensitivity to VPD: the role of intercellular CO2 concentration

Abstract: Plant stomata control the gas exchange of the leaf and respond to both environmental and plant cues such as photosynthetic capacity and atmospheric water status. Despite the significant roles of stomata in plant physiological functions like photosynthesis, debate remains regarding how the stomata balances and integrates signals from the external environment like vapor pressure deficit (VPD) and the internal physiology like intercellular CO₂ concentration (C_i). Past studies linked higher stomatal sensitivity to VPD with greater initial stomatal conductance (g). Thus, we hypothesize that stomatal sensitivity to VPD is determined by the C_i value at the initial g and the shape of the net CO₂ assimilation rate (A) to C_i response of the plant. To test the effect of C_i on g sensitivity to VPD, we used a Li-Cor 6800 gas exchange system to measure the A to C_i response and g to VPD response at different initial g and by proxy, C_i values by manipulating the ambient CO₂ concentration (C_a) to three different levels: low (200 ppm), ambient (400 ppm), and high (600 ppm). Usually, greater g corresponds to greater C_i, however, by changing the C_a, we achieved the lowest C_i value at the highest initial g and the highest C_i value at the lowest initial g. We used three C₄ species due to the strong segmentation of their A vs C_i relationship that may lead to a more pronounced C_i response. If the C_i signal affects stomatal sensitivity, we would expect to see the lowest g sensitivity in the low CO₂ treatment despite having the highest initial g. Preliminary results show that stomatal sensitivity was on average 30% lower in the low CO₂ treatment group with the highest initial g (lowest C_i) than at the ambient CO₂ group, even though the highest initial g values were 76% greater than the ambient initial g values. There was no observed difference between g sensitivity of the ambient CO₂ group and at the high CO₂ treatment group, but ambient initial g values were 45% greater than the lowest initial g values. These results support our hypothesis that C_i affects the sensitivity of g to VPD in plants. Since stomata control transpiration in plants, understanding its response to environmental factors like VPD in the context of its photosynthetic physiology is crucial for predicting future changes at the ecosystem and global scale that manifest from physiological forcing.