N. Conceição

Crop and stress coefficients in rainfed and deficit irrigation vineyards using sap flow techniques

Projected climate changes and expansion of viticulture to drier regions justify the installation and management of deficit irrigation (DI) strategies. Contradictory results on the effect of DI on crops may be ascribed to the incorrect application of these techniques. The lack of discrimination between basal crop (Kcb) and stress coefficients (Ks) can be an obstacle to proper irrigation management. A sap flow (SF) technique associated with microlysimeters and eddy covariance (EC) methods was applied to five commercial vineyards, aiming to discriminate those coefficients, during the driest period of the vegetative cycle. A comparative analysis of the coefficients, in relation to measured vegetation parameters (for Kcb) and plant water status (for Ks) is presented. Kcb, ranging from about 0.35 to 0.75, was highly correlated with leaf area index at stand level. Ks, which decreased till 0.2 in the most stressed vineyard, was well correlated to plant water status (Ks function), represented by predawn leaf water potential. Ks functions for the different experiments exhibited falling slopes with decreasing water status, with variable trends depending on the rates of maximal crop transpiration (Tm). These experimental results show that specific parameters for Ks functions, necessary to estimate water use and irrigation depths, in order to control the stress levels in DI scheduling, are also dependent on Tm.

Root Function: In Situ Studies Through Sap Flow Research

Sap flow measured by the Heat Field Deformation technique, HFD, is sensitive to flow responses to small changes in water potential gradients within the tree hydraulic systems. When these changes occur abruptly, under experimental treatments (severing, localized irrigation, heavy loading), sap flow movement can be used as a marker to study root functionality, for example root ability to redistribute water and withstand heavy machinery pressure. Experiments also showed that a compensation mechanism may operate in trees, with a temporary increase in the absorbed water due to a preferential use of one part of the root system when another part is damaged or when a water source is lost. Long-term measurements of root sap flow allow distinguishing between water uptake from shallow and deep rooted trees, at different exposures at a forest edge and from healthy and infected trees. Root sap flow can be used as an indicator of tree stress or of the prevailing mechanisms used by trees to survive drought, under irrigation or rain-fed conditions.