The inherent variability of water stress indicators in apple, nectarine and pear orchards, and the validity of a leaf-selection procedure for water potential measurements

The following two topics were examined: (1) The variability in the measurement of leaf water potential (LWP), stem water potential (SWP), maximum daily trunk shrinkage (MDS), and soil water tension (SWT) in apple, nectarine and pear orchards; and (2) The validity of a leaf-selection procedure for SWP measurements in commercial apple orchards. 27 trees were selected in an apple orchard, 27 in a nectarine orchard, and 30 in a pear orchard. The trees were close to each other. The measurements comprised of: midday SWP in apple, nectarine and pear; midday LWP in apple; MDS in apple and nectarine; and SWT in pear. The mean and standard errors (SEs) of each water status indicator in each species were calculated for an increasing sample size. The sample sizes required for stable averages were: SWP - 4, 5, and 8 trees for apple, nectarine and pear, respectively; MDS - 17 and 16 trees for apple and nectarine, respectively; SWT - 21 for pear trees. The relative SEs (i.e. percent of population mean) were 2.4, 6.1 and 10.1% in SWP/LWP, MDS and SWT, respectively. Possible explanations for the differing variability of the various water status indicators are discussed. The results show that smaller samples were sufficient to represent SWP and LWP properly than what was required for MDS and SWT. 9 commercial apple plots were selected and about 25 randomly selected leaves were used for midday SWP measurements in each plot (i.e. experimental sets). About 5 leaves on closely adjacent "representative" trees were selected in each of the commercial plots (i.e. commercial sets) and midday SWP was measured. The average difference in SWP between the experimental and the commercial sets was -0.127 MPa. The choice of closely adjacent trees increased the deviation from the experimental sets. The use of a reasonable sample size (n=7) may enable midday SWP to be measured within +/- 0.15 MPa in most commercial orchards.