Phenology and drought affects the relationship between daily trunk shrinkage and midday stem water potential of peach trees

The relationship between maximum daily shrinkage in trunks (MDS), daily trunk growth (DTG), predawn water potential (psi(pd)) and midday stem water potential (psi(stem)) were studied in an irrigation experiment in peach trees. Control trees were irrigated to replace evapotranspiration, with trees receiving regulated deficit irrigation (RDI) watered at 35% of this rate during Stage II of fruit development and after harvest. The RDI trees were watered as controls during Stage III of fruit development. Minimum psi(pd) and psi(stem) fell to -0.6 MPa and -1.2 MPa, respectively in RDI plots compared with -0.2 and -0.6 MPa in the controls. Trunk growth was less in the RDI plots than in the controls during drought. In contrast, MDS was higher when deficit irrigation was applied in the RDI trees. When site differences were considered the correlation between psi(pd) and accumulated trunk growth over an ample period was loose, while maximum daily shrinkage and midday stem water potential remarkably improved such a correlation. However, pooling all available data, the correlation between psi(stem) and MDS was very poor (R-2 = 0.44) and it substantially improved only when using data from specific phenological periods (i.e. R-2 = 0.75). A seasonal drift in MDS values was observed and it was related to the seasonal changes in trunk growth rates, (i.e. highest shrinkage was found when growth rates were lowest). We concluded that phenology in combination with drought reduce the reliability of the water status information obtained from MDS.