Low conductances for CO2 diffusion from stomata to the sites of carboxylation in leaves of woody species

ABSTRACT Concurrent measurements of leaf gas exchange and on‐line 13 C discrimination were used to evaluate the CO 2 conductance to diffusion from the stomatal cavity to the sites of carboxylation within the chloroplast (internal conductance; g i ). When photon irradiance was varied it appeared that g i and/or the discrimination accompanying carboxylation also varied. Despite this problem, g i , was estimated for leaves of peach ( Prunus persica) , grapefruit ( Citrus paradisi) , lemon ( C. limon) and macadamia (Macadamia integrifolia) at saturating photon irradiance. Estimates for leaves of C. paradisi, C. limon and M. integrifolia were considerably lower than those previously reported for well‐nourished herbaceous plants and ranged from 1.1 to2.2μmol CO 2 m −2 s −1 Pa −1 , whilst P. persica had a mean value of 3.5 μmol CO 2 m −2 s −1 Pa −1 . At an ambient CO 2 partial pressure of 33Pa, estimates of chloroplastic partial pressure of CO 2 ( C c ) using measurements of CO 2 assimilation rate (A) and calculated values of g i , and of partial pressure of CO 2 in the stomatal cavity ( C st ) were as low as 11.2 Pa for C. limon and as high as 17.8Pa for peach. In vivo maximum rubisco activities ( V max ) were also determined from estimates of C c . This calculation showed that for a given leaf nitrogen concentration (area basis) C. paradisi and C. limon leaves had a lower V max than P. persica , with C. paradisi and C. limon estimated to have only 10% of leaf nitrogen present as rubisco. Therefore, low CO 2 assimilation rates despite high leaf nitrogen concentrations in leaves of the evergreen species examined were explained not only by a low C c but also by a relatively low proportion of leaf nitrogen being used for photosynthesis. We also show that simple one‐dimensional equations describing the relationship between leaf internal conductance from stomatal cavities to the sites of carboxylation and carbon isotope discrimination (Δ) can lead to errors in the estimate of g i . Potential effects of heterogeneity in stomatal aperture on carbon isotope discrimination may be particularly important and may lead to a dependence of g i upon CO 2 assimilation rate. It is shown that for any concurrent measurement of A and Δ, the estimate of C c is an overestimate of the correct photosynthetic capacity‐weighted value, but this error is probably less than 1.0 Pa.