Whole-plant gas-exchange measurements in grapevine to estimate water-use efficiency

Research was conducted in two different vineyards: one in central Italy (Brunello di Montalcino production area) with upwards vertically trellised vines; and one in southern Italy (table grape) with horizontal trellising (tendone). Four plants were included in a plastic chamber provided with an air circulation system to measure plant-atmosphere gas exchange (CO2 and water vapour). Additionally, light interception by the canopies was measured. In the vertical trellising site, grapevine photosynthetic potential was measured using the "tree enclosure" system, demonstrating variability among the sampled plants with the greatest photosynthesis values measured during the mid-morning and, only in the second day, also in the mid-afternoon. These data are correlated to canopy light interception with low values of light. An increase in light interception always led to a higher photosynthesis, while, after a threshold of about 300 pmol/m(2)/s, the increase in light interception was not linked to an increase in photosynthesis. This implies that at this light level there is a saturation point. It must be considered that measures have been obtained in a conditioned environment where a plastic chamber can modify some plant responses-there is an increase in air temperature and humidity as well as in wind turbulence. Measurements show higher transpiration during the period of both highest air temperature and light interception. In the horizontal trellising site, the different canopy architecture strongly modified the CO2 assimilation model as different from the vertical trellising system. Photosynthesis and transpiration showed greatest values around midday with some variation among the different chambers. Absolute values were quite high compared with the vertical trellis. It must be considered that in this case the vineyard is irrigated, with high water availability during the measurements. Moreover, the "tendone" training system, with a sparse canopy, causes a high percentage of leaves to be in the full sun with few shaded inside the canopy and, thus, with high efficiency. Light interception is correlated strongly to water-use efficiency (calculated as the ratio of photosynthesis to transpiration). With an increase in light interception, there is a decrease in water-use efficiency.