A study on the relationship between leaf conductance, CO2 concentration and carboxylation rate in various species

Leaf conductance g(L) is strongly influenced by environmental factors like CO2, irradiance and air humidity. According to Ball et al. (1987), g(L) is correlated with an index calculated as the product of net CO2 exchange rate A and ambient water vapour concentration W-a, divided by ambient CO2 concentration c(a). However, this empirical model does not apply to high values of g(L) observed at c(a) below CO2 compensation concentration Gamma. Therefore, we applied modified indices in which A is replaced by estimates for the rate of carboxylation. Such estimates, P-1 and P-2, were determined by adding to A the quotient of Gamma and the sum of gas phase resistance r(g) and intracellular resistance for CO2 exchange r(i), P-1 = A+Gamma/(r(g) + r(i)). or the quotient of Gamma and r(i), P-2 = A + Gamma/r(i). If P-2 is chosen, c(a) in the Ball index has to be replaced by the intercellular CO2 concentration c(i). By using the modified indices P-1.W-a/c(a) and P-2.W-a/c(i), we analysed data from the C-3 species Nicotiana tabacum and Nicotiana plumbaginifolia, the C-3-C-4 intermediate species Diplotaxis tenuifolia, and the C-4 species Zea mays. The data were collected at widely varying levels of irradiance and CO2 concentration. For all species uniform relationships between g(L) and the new indices were found for the whole range of CO2 concentrations below and above Gamma. Correlations between g(L) and P-1.W-a/c(a) were closer than those between g(L) and P-2.W-a/c(i) because P-1/c(a) implicitly contains g(L). Highly significant correlations were also obtained for the relationships between g(L) and the ratios P-1/c(a) and P-2/c(i).