Mesophyll conductance to CO2 transport estimated by two independent methods: effect of variable CO2 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 CO2 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 CO2 concentrations (C-i), a peak at 180-200 mu mol 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 mu mol 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.