Links between vine water stress indicators: measured versus visual

Abstract

There are a range of methods to quantify water stress effects on grapevine physiology. Some studies aim to evaluate plant water status in the short or long term, while others aim to evaluate the impact of water stress on different plant components. The objective of this research was to evaluate the physiological effects of reducing irrigation by about 50% in a commercial vineyard of Pinot noir in Waipara, New Zealand. During the 2013-2014 season, stem water potential (ψ_s), leaf proline content, leaf osmotic potential (ψ_π), leaf carbon isotope ratio (δ¹³C), stomatal conductance (g_s), chlorophyll content estimated by SPAD, and primary and lateral leaf area were assessed to determine differences between fully irrigated vines (354 mm of rainfall + 26.4 mm of irrigation/season) and those with reduced irrigation (354 mm of rainfall + 14.3 mm of irrigation/season). No differences between water treatments were observed for stem water potential, leaf proline content, leaf osmotic potential, and leaf carbon isotope ratio during the season. However, stomatal conductance decreased under reduced irrigation. Despite little or no impacts on physiological plant water status, from veraison onwards the visual effect of reduced irrigation became evident through abscission of leaves in the cluster zone and change of colour of mid-shoot leaves from green to yellow. Fully irrigated plants showed a larger primary leaf area than those under reduced irrigation, while the lateral leaf area was not affected. SPAD meter evaluations from veraison to harvest showed differences in leaf greenness, with the leaves of the vines under reduced irrigation turning yellow earlier than those being fully irrigated. These results demonstrate that visual indicators of water stress are not linked to results of methods commonly used to evaluate vine water status. The authors conclude that the changes in leaf area and vine physiology act to compensate for water scarcity in order to maintain normal water status.