Determination of the photosynthetic capacity of grapevine leaves

Abstract

At fully irrigated, container-grown vines (cv. Riesling) kept outdoor, measurements of stomatal conductance and photosynthesis of leaves were performed in the morning and in the afternoon. The results revealed reductions of the photosynthetic capacity (maximal net photosynthesis at saturating light conditions and at saturating CO₂ partial pressure) in the afternoon. This observation was associated with a higher sensitivity of stomata to CO₂ in the afternoon: 0.0016-mu-bar-1 in the morning, 0.0046-mu-bar-1 in the afternoon. Due to inhibition of photorespiration determinations of photosynthetic capacity in the morning at 1-2% O₂ revealed maximal values at lower CO₂ partial pressures and, thereby, at maximal stomatal conductance. These values, corrected for photorespiration, were close to those obtained at ambient O₂ and high CO₂ partial pressures. Thus, it is assumed that in our experiments stomata did not limit the rates of photosynthesis which were determined in the morning. To avoid reduction of photosynthesis by photoinhibition it is proposed to minimize the duration of exposure of leaves to high light and to reduce light intensity to values where photosynthesis begins to reach light saturation.