Effects of HgCl2 on CO2 dependence of leaf photosynthesis:: Evidence indicating involvement of aquaporins in CO2 diffusion across the plasma membrane

Experiments were conducted to examine whether mercury-sensitive aquaporins facilitate photosynthetic CO2 diffusion across the plasma membrane of leaf mesophyll cells. Discs without abaxial epidermes from Vicia faba leaflets were treated with HgCl2, an inhibitor of aquaporins. Hydraulic conductivity of the plasma membrane of these discs, measured as the weight loss of the discs in the 1 M sorbitol solution. was inhibited by sub-mM concentrations of HgCl2 by 70 to 80%. Photosynthetic CO2 fixation was also inhibited by the HgCl, treatment in a similar concentration range. When 0.3 mM HgCl2 solution was fed to the V. faba leaflets with intact epidermes via the transpiration stream, the rate of photosynthesis on leaf area basis (A) measured at photosynthetically active photon flux density of 700 mumol m(-2) s(-1) and at leaf temperature of 25degreesC, decreased by about 20 to 30% at any CO2 concentration in the intercellular spaces (C-i). However, when CO2 concentration in the chloroplast stroma (C-c). was calculated from fluorescence and gas exchange data and A was plotted against C-c, A at low C-c concentrations did not differ before and after the treatment. The conductance for CO2 diffusion from the intercellular spaces to the chloroplast stroma (g) decreased to 40 and 30% of the control value, when the leaflets were fed with 0.3 mM and 1.2 mM HgCl2, respectively. Similar results were obtained with leaves of Phaseolus vulgaris. Although effects of HgCl2 were not specific, the present results showed that HgCl2 consistently lowered g(i). It is, thus, probable that the photosynthetic CO2 uptake across the plasma membrane of the mesophyll cells is facilitated by mercury-sensitive aquaporins.