Impact of cuticle on calculations of the CO2 concentration inside leaves.

Water vapor over-estimates the CO 2 entering leaves during photosynthesis because the cuticle and epidermis transmit more water vapor than CO 2 . Direct measurements of internal CO 2 concentrations may be preferred. The CO2 concentration inside leaves (c i) is typically calculated from the relationship between water vapor diffusing out while CO2 diffuses in. Diffusion through the cuticle/epidermis is usually not considered. This study was undertaken to determine how much the calculations would be affected by including cuticle properties. Previous studies indicate that measurable amounts of CO2 and water vapor move through the cuticle, although much less CO2 than water vapor. The present experiments were conducted with sunflower (Helianthus annuus L) leaves in a gas exchange apparatus designed to directly measure c i, while simultaneously calculating c i. Results showed that, in normal air, calculated c i were always higher than directly measured ones, especially when abscisic acid was fed to the leaves to close the stomata and cause gas exchange to be dominated by the cuticle. The effect was attributed mostly to the reliance on the gas phase for the calculations without taking cuticle properties into account. Because cuticle properties are usually unknown and vary with the turgor of the leaf, which can stretch the waxes, it is difficult to include cuticle properties in the calculation. It was concluded that direct measurement of c i may be preferable to the calculations.