Assessment of discretely measured indicators and maximum daily trunk shrinkage for detecting water stress in pomegranate trees

Measurements obtained by the continuous monitoring of trunk diameter fluctuations were compared with discrete measurements of midday stem water potential (Ψstem) and midday leaf conductance (gl) in adult pomegranate trees (Punica granatum L. cv. Mollar de Elche). Control plants (T0) were irrigated daily above their crop water requirements in order to attain non-limiting soil water conditions in 2009 and 2010, while T1 plants were subjected to water stress by depriving them of irrigation water for 34 days in 2010, after which time irrigation was restored and plant recovery was studied for 7 days. T1 plants showed a substantial degree of water stress, which developed slowly. Maximum daily trunk shrinkage (MDS) was identified to be the most suitable plant-based indicator for irrigation scheduling in adult pomegranate trees, because its signal:noise ((T1/T0):coefficient of variation) ratio was higher than that for Ψstem ((T1/T0):coefficient of variation) and gl ((T0/T1):coefficient of variation). MDS increased in response to water stress, but when the Ψstem fell below −1.67 MPa, the MDS values decreased. For non-limiting water conditions, MDS could be predicted from mean daily air temperature (Tm) through exponential equations fitted to pooled data across several seasons. First-order equations were also obtained by pooling data across several seasons to predict MDS from crop reference evapotranspiration (ETo), mean daily air vapour pressure deficit (VPDm), Tm and solar radiation (Rs), but these should be used only within a certain range of values (ETo, 2.1-7.4 mm; VPDm, 0.64-2.96 kPa; Tm, 12.1-28.3 °C; Rs, 119.4-331.3 Wm−2). Hence, automated MDS measurements have the potential to be used in irrigation scheduling of pomegranate, and these values can be normalized to non-limiting water conditions by locally derived empirical relationships with meteorological variables.