Stomatal conductance does not correlate with photosynthetic capacity in transgenic tobacco with reduced amounts of Rubisco

High-resolution imaging of chlorophyll a fluorescence from intact tobacco leaves was used to compare the quantum yield of PSII electron transport in the chloroplasts of guard cells with that in the underlying mesophyll cells. Transgenic tobacco plants with reduced amounts of Rubisco (anti-Rubisco plants) were compared with wild-type tobacco plants. The quantum yield of PSII in both guard cells and underlying mesophyll cells was less in anti-Rubisco plants than in wild-type plants, but closely matched between the two cell types regardless of genotype. CO₂ assimilation rates of anti-Rubisco plants were 4.4 μmol m^-2 s^-1 compared with 17.3 μmol m^-2 s^-1 for the wild type, when measured at a photon irradiance of 1000 μmol m^-2 s^-1 and ambient CO₂ of 380 μmol mol^-1. Despite the large difference in photosynthetic capacity between the anti-Rubisco and wild-type plants, there was no discernible difference in the rate of stomatal opening, steady-state stomatal conductance or response of stomatal conductance to ambient CO₂ concentration. These data demonstrate clearly that the commonly observed correlation between photosynthetic capacity and stomatal conductance can be disrupted in the long term by manipulation of photosynthetic capacity via antisense RNA technology. It was concluded that stomafal conductance is not directly determined by the photosynthetic capacity of guard cells or the leaf mesophyll.