PHOTOSYNTHETIC LIGHT RESPONSES IN RELATION TO LEAF TEMPERATURE IN SUN AND SHADE LEAVES OF GRAPEVINES

How light attenuation influences the photosynthetic properties of sun and shade leaves within grapevine canopies was investigated by measuring photosynthesis along the shoots over the growing season, light responses of selected leaves at different temperatures and the specific leaf areas. Differences in photosynthetic rate became apparent in late spring when the canopies start to close; photosynthetic rates of shaded leaves peaked at 6 μmol (CO₂) m^-2 s^-1 while sun leaf rates peaked at 11 μmol (CO₂) m^-2 s^-1, and the differences accentuated as the season progressed. By mid-season (December), light-saturated photosynthetic rates at 12 μmol (CO₂) m^-2 s^-1 were significantly higher in sun compared with shade leaves at 5.5 μmol (CO₂) m^-2 s^-1. Basal and mid-node position leaves gradually become shaded while leaves at the apical shoot end still maintained high photosynthetic rates. Changes in leaf morphology also occurred, with the specific leaf area of the basal leaves shifting from sun to shade type leaves. Photosynthetic light responses showed typical sun-shade contrasts with higher Pmax, lower photon yields and higher light-saturation in sun leaves. However, these responses were highly temperature-dependent and Pmax was highest at 25°C and declined at 20 and 35°C and above. By contrast, light intensities required to saturate photosynthesis increased from about 600 μmol (photons) m^-2 s^-1 at 20°C to over 1100 μmol (photons) m^-2 s^-1 at 35-40°C. We conclude that although shade has a marked impact on photosynthesis and leaf morphology, the temperatures that the vines experience over the growing season also have a marked impact on the photosynthetic responses to light.