Unsaturation and Approximate Isotopic Homogeneity in Leaf Air Spaces

- We consider two assumptions of leaf isotope gas exchange measurements: that leaf air spaces are saturated with water vapour, and that this vapour is of a homogeneous isotopic composition. In particular, we consider whether these assumptions can concurrently hold and, if not, which assumption is preferable to retain. - We present two methods using independent measurements of both leaf surfaces to consider these assumptions. The first method determines the isotopic inhomogeneity between the abaxial and adaxial evaporative sites when saturation is assumed. The second method determines the unsaturation in the abaxial and adaxial substomatal cavities when isotopic homogeneity is assumed. The methods are applied on Gossypium hirsutum (cotton) under benign atmospheric demand conditions (1.0 kPa air saturation deficit). - We find evidence that assuming saturation contradicts isotopic homogeneity and vice-versa. We compare each assumption to pre-existing data and find that it is reasonable to assume isotopic homogeneity, but not leaf vapour saturation. Thus, we find that leaves experience unsaturation even under benign atmospheric demand conditions and have a spatial variation in their unsaturation, with lower humidities associated with the surface of least stomatal resistance. - We conclude that leaves cannot be considered to be both saturated and of a homogeneous vapour isotopic composition. They are best approximated as isotopically homogeneous.