Limited internal hydraulic conductance is preventing excessive water use of minimally-pruned vines even under conditions of good water supply

Abstract

The objective of the study was to test the feasibility of a conversion of vineyards with vertical shoot positioning (espalier type) to minimal pruning with 50 % of the original plant density with regard to their water relations under non-irrigated conditions. The experiment took place in a south-exposed steep slope vineyard of the Rheingau region (Germany (50 °N, 8 °E) with an average annual rainfall of 526 mm and a potential evapotranspiration of 795 mm (Penman-Monteith). The tested cultivar was ‘White Riesling’ grafted onto ‘5C’ rootstocks.

Hydraulic conductance was assessed by combining diurnals of leaf and stem water potential with xylem flow measurements (by means of custom-made Granier-type sensors). Daily water use per vine was significantly higher in minimal pruning, yet maximal water flow rates were similar in both systems. Maximum values of hydraulic conductance turned out to be similar as well, with minimally-pruned vines operating closer and more often at this limit due to differences in leaf area and canopy structure. Thus, while water use of conventionally-pruned grapevines was highly correlated with evaporative demand, water use of minimally-pruned vines was already beginning to saturate at potential evapotranspiration rates of about 3 mm per day. Overall, water use per unit soil surface in minimal pruning during summer was not higher than in the control treatment despite its at least similar energy absorption throughout the growing season. This hydraulic limitation already occurred under conditions of good water supply as e.g. in spring.

This finding is of practical importance in all viticultural situations (partially) dependent on soil water reserves during the vegetation period. Since the rapid and voluminous development of its canopy is a key feature for minimal pruning, this limitation prevents excessive water use early in the season which otherwise might result in severe water shortage during the ripening period.