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 CO 2 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 compensati on concentration Gamma. Therefore, we applied modified indices in whic h A is replaced by estimates for the rate of carboxylation. Such estim ates, P-1 and P-2, were determined by adding to A the quotient of Gamm a and the sum of gas phase resistance r(g) and intracellular resistanc e 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 plu mbaginifolia, the C-3-C-4 intermediate species Diplotaxis tenuifolia, and the C-4 species Zea mays. The data were collected at widely varyin g levels of irradiance and CO2 concentration. For all species uniform relationships between g(L) and the new indices were found for the whol e range of CO2 concentrations below and above Gamma. Correlations betw een 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) an d the ratios P-1/c(a) and P-2/c(i).