Evaluation of plant-based water status indicators in mature apple trees under field conditions

The performance of different indicators of plant water status as a tool for irrigation management was evaluated in mature field grown 'Golden Delicious' apple trees during the late summer of 1998. Control (C) and stress (S) treatments were studied. In the C treatment trees were irrigated daily at 100% ETc whereas in the S treatment water was withheld during 31 days (DOY's 236-266). Predawn water potential (Psi(pd)) and midday stem water potential (Psi(stem)) were measured several times a week during the experimental period. Three daily measurements of stomatal conductance (g(s)) and stem water potential were made during five consecutive days in mid-September. Trunk diameter changes (TDC) were recorded by LVDT sensors, and from these measurements, maximum daily shrinkage (MDS), daily growth (DG), and cumulative growth (CG) were calculated. Midday Psi(stem) showed the best ratio between the response to moderate water stress and tree variability ("signal/noise" ratio) among the indicators studied here, followed closely by Psi(pd). On the other hand, the poorest water status indicator was g(s). Due to the low trunk growth rate of the trees, and its high variability, DG and CG were not adequate indicators. MDS showed a lower sensitivity to water stress and a higher variability (CV = 0.19) than midday Psi(stem) (CV = 0.08) and Psi(pd) (CV = 0.10). However, MDS correlated well with ET0 and with midday Psi(stem) (R 2= 0.79) thus, making this parameter an interesting and promising tool for irrigation management in apple orchards. More research needs to be done in order to define reference values for MDS and plant water potential indicators, in relation to evaporative conditions and in different phenological periods, and to quantify the relationship between water status indicators values and apple tree yield and fruit quality.