Stomatal behaviour, photosynthesis and photorespiration of <i>in vitro</i>-grown grapevines: Effects of light and CO₂

Abstract

To improve photosynthesis and growth of grapevines cultivated in vitro (Seyval blanc and SO 4) effects of light intensity, spectral irradiance and CO₂ concentration on stomatal behaviour, CO₂ fixation and photorespiration were studied. Stomata were shown to respond to changes of light intensity but, unlike photosynthesis, their reactions were delayed and stomatal closure was incomplete in the dark. In contrast, alterations of the CO₂ concentration in the headspace (50-2200 ppm) did not cause stomatal reactions. Photosynthesis vs, light intensity relationships indicated lower light compensation points, higher quantum yield and higher rates of light-saturated photosynthesis with ''Fluora'' lamps (maximal spectral irradiance al 460 and 680 nm) compared to ''projector'' lamps (maximal spectral irradiance at 620 nm). Photosynthesis vs. intercellular CO₂ concentration relationships indicated varietal differences, the carboxylation efficiency and rates of photosynthesis at CO₂ saturation being distinctly higher in the more vigorous variety SO 4 compared to Seyval blanc. Under the usual light conditions of our in vitro culture (50-60 µmol quanta · m^-2 s^-1, Fluora) the headspace CO₂ concentration ranged from 145 to 155 ppm while at the end of a 10-hour dark period it increased to values >3000 ppm. Rates of photorespiration were high (>50 % of photosynthesis) due to the relative low CO₂ concentrations and, presumably, due to elevated O₂ concentrations in the headspace. It is concluded that the often observed low rates of photosynthesis of in vitro plantlets are mainly due to low light intensity and CO₂ concentration in the headspace, the latter depending on the low rates of gas diffusion between ambient air and headspace.