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

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 mu mol 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 O-2 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.