Attempts have been made to determine the water status of vineyards using different methods, with variable results according to the different authors. Therefore, various methods have been used to estimate soil humidity (TDR, tensiometer, neutron probe, etc.) and the water status of the plant (dendrometer, porometer, leaf water potential, xylem water potential, etc.). The leaf water potential measured with leaf and «bagged» leaf, which are subjected to comparison in this report, are two useful and simple methods for finding out the water status of the vines, based on the use of a simple instrument that is acceptably available in economic terms, the pressure chamber.
The trial was performed at an experimental Tempranillo vineyard in which the main source of variation was the watering regime (no irrigation, 20 % ETo irrigation, 40 % ETo irrigation) and in which a change in planting density was also included (low, 2.7x1.4 m; high, 2.2x1.5 m). The resulting experimental treatments were: B00 (low density and no irrigation), B20 (low density and 20%ETo), H20 (high density and 20%ETo) and H40 (high density and 40 % ETo). The total amount of water applied to the treatments with the lesser irrigation rate (0.2 ETo mm) was equal to 61 mm and to the treatment with the higher irrigation rate (0.4 ETo mm) was equal to 122 mm, up to the week of August 29, 2002. The measurements of Yf were performed before dawn (aa) on June 14, at solar noon (12:00 p.m.) on July 19 and 26 and on August 29, 2002, on adult leaves located in the middle zone of the shoot, above the height of the clusters, on the eastern face of the trellised vines. Yx was measured at 6:00 a.m. on June 14 and at solar noon (12:00 p.m.) on July 19 and 26 and on August 29, 2002, also on adult leaves in the middle zone of the shoot, above the zone of the clusters, leaves located on the western face of the trellised vines and therefore shaded, and covered at least one hour prior to taking the measurement.
The relationship between the water potential of the leaf and that of the «bagged» leaf has proven to draw closer as the cycle has advanced and the vines have been more stressed, in such a way that the best relationship between the two indicators was obtained in the measurement performed at the end of the month of August. When the water differences are very notable, both indicators are able to show the water status of the vineyard expected in accordance with the rate of irrigation applied.
The measurement of water potential of the leaf has been easier to take, because it is not necessary to cover the leaves prior to taking the measurement (except in the measurement before dawn, in which case one must be in the vineyard at an unpleasant hour). However, using the potential of the xylem it has been possible to make better observations of the differences between treatments, when these differences are not very important.
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