Mesophyll conductance to CO(2) transport estimated by two independent methods: effect of variable CO(2) concentration and abscisic acid.

Mesophyll conductance (g(m)) and stomatal conductance (g(s)) are two crucial components of the diffusive limitation of photosynthesis. Variation of g(m) in response to CO(2) concentration was evaluated by using two independent methods based on measurements of variable electron transport rate (J) and instantaneous carbon isotope discrimination, respectively. Both methods of g(m) estimation showed a very similar shape of the g(m)/C(i) relationship, with an initial increase at low substomatal CO(2) concentrations (C(i)), a peak at 180-200 micromol mol(-1) C(i), and a subsequent decrease at higher C(i). A good correlation was observed between values of g(m) estimated from the two methods, except when C(i) <200 micromol mol(-1), suggesting that the initial increase of g(m) at low C(i) was probably due to unreliable estimates over that range of C(i). Plants were also treated with abscisic acid (ABA), which induced a reduction in g(s) without significantly affecting the rate of photosynthesis, g(m) or the photosynthetic capacity. The present results confirm, using two independent methods, that g(m) is strongly sensitive to C(i), and that the relationship between g(s) and g(m) is not conservative, differing between control and ABA-treated plants.