Whole-canopy gas exchange and light interception of vertically trained vitis vinifera L. under direct and diffuse light

A portable, open gas exchange system was developed that enabled the simultaneous measurement of net carbon dioxide exchange (NCE) and transpiration of eight mature Sauvignon blanc grapevines in the field. On clear and cloudy days NCE and transpiration were closely related to the modeled interception of photosynthetically active radiation (PAR). On clear days, the relationship between photosynthesis and incident PAR was poor and NCE peaked in the vertically trained, north-south orientated rows three hours before and after solar noon, resulting in a distinct bimodal pattern to vine gas exchange, which was not evident on cloudy days. Under diffuse light (cloudy) conditions, the vines appeared to be more efficient, photosynthesizing at a higher rate per calculated unit of light intercepted. This efficiency was probably due to the saturation of the exterior and shading of interior leaves under clear conditions, while under diffuse conditions the increased scattered light enhanced the irradiance received by the leaves in the interior of the canopy. Despite the improved light use efficiency under diffuse conditions, lower light intensities meant that CO₂ exchange was normally lower than that recorded during clear days. The effect of the chambers on the sap flux velocity up the vine stems was minor, which suggests that their effect on vine microclimate and resulting transpiration was minimal. There was a strong relationship between concurrent transpiration and sap flux velocity, which was not improved by the use of a time delay between transpiration and sap flux, and suggests that the vines do not rely greatly on water stored in the trunk and other vegetative organs above the ground (capacitance) for transpiration under these conditions.