Limitações na determinação da condutância estomática e para eliminar a camada limite foliar

Abstract

The widespread adoption of portable photosynthesis measurement systems has led to disregarding the fundamentals of stomatal conductance to water vapor. The aim is to provide experimental data to accumulate evidence that there is a need to review the values of the molecular diffusion coefficient of water vapor through stomata and that the conductance of the leaf boundary layer cannot be disregarded in the determinations of plant gas exchange. A portable photosynthesis measurement system (model LI-6400XT) was used to determine stomatal conductance and anatomical variables were used to calculate this conductance with different values of the molecular diffusion coefficient of water vapor. The results showed that the relationship between measured stomatal conductance and anatomical variables fluctuated randomly, with no significant correlation. Stomatal conductances calculated from different diffusion coefficients varied significantly. The critical value of leaf boundary layer conductance for water vapor can only be obtained with a thin boundary layer, which requires a very high wind speed, unfeasible in natural and experimental conditions. The method used to obtain anatomical variables was inadequate, compromising the analysis of the effects of the diffusion coefficient on stomatal conductance. Stomatal conductances calculated from these variables and the molecular diffusion coefficients of water vapor varied significantly, indicating the need to review these values. The wind speed required to equal the leaf boundary layer conductance to the total is impractical but can be disregarded because the wind speed required to exceed stomatal conductance is low, and the error in estimating the transpiration rate is small.