Rapid variations of mesophyll conductance in response to changes in CO2 concentration around leaves

The effects of short-term (minutes) variations of CO2 concentration on mesophyll conductance to CO2 (g(m)) were evaluated in six different C-3 species by simultaneous measurements of gas exchange, chlorophyll fluorescence, online carbon isotope discrimination and a novel curve-fitting method. Depending on the species, g(m) varied from five- to ninefold, along the range of sub-stomatal CO2 concentrations typically used in photosynthesis CO2-response curves (A(N)-C-i curves; where A(N) is the net photosynthetic flux and C-i is the CO2 concentrations in the sub-stomatal cavity), that is, 50 to 1500 mu mol CO2 mol(-1) air. Although the pattern was species-dependent, g(m) strongly declined at high C-i, where photosynthesis was not limited by CO2, but by regeneration of ribulose-1,5-bisphosphate or triose phosphate utilization. Moreover, these changes on g(m) were found to be totally independent of the velocity and direction of the C-i changes. The response of g(m) to C-i resembled that of stomatal conductance (g(s)), but kinetic experiments suggested that the response of g(m) was actually faster than that of g(s). Transgenic tobacco plants differing in the amounts of aquaporin NtAQP1 showed different slopes of the g(m)-C-i response, suggesting a possible role for aquaporins in mediating CO2 responsiveness of g(m). The importance of these findings is discussed in terms of their effects on parameterization of A(N)-C-i curves.