Signal intensity based on maximum daily stem shrinkage can reflect the water status of apple trees under alternate partial root-zone irrigation

Signal intensity (SI) and maximum daily stem shrinkage (MDS) are indicators of the water status and irrigation schedule of fruit trees under conventional irrigation (CI). However, whether SI can reflect the water status of fruit trees under alternate partial root-zone irrigation (APRI) has rarely been reported. Field experiments were conducted on apple trees over two years with two irrigation methods (CI and APRI) and two irrigation amounts (400 mm and 500 mm) in an arid area. The followings were measured over the whole growth season: MDS, sap flow (SF), air temperature, net radiation, vapor pressure deficit, reference evapotranspiration, soil water content, midday stem water potential and predawn leaf water potential. The signal intensities based on MDS (SIMDS) and sap flow (SISF) were calculated. The results show: first, MDS was significantly higher under CI at 400 mm than under APRI at 400 mm, while no difference was found between the two irrigation methods at 500 mm. MDS was significantly positively correlated with meteorological factors, while SIMDS and SISF were not. Second, SIMDS was significantly lower under APRI than under CI, while no difference was found in SISF between the two irrigation methods. Third, in contrast to SISF, SIMDS was significantly correlated with soil water content as well as with midday stem water potential and predawn leaf water potential under APRI. These results show that the signal intensity based on maximum daily stem shrinkage accurately indicates the water status of apple trees under alternate partial root-zone irrigation in an arid apple production area.