The Pressure Bomb in Prunes
The California Prune Board and other agencies have supported irrigation research demonstrating that applying deficit irrigation (less than full crop water use) to mature prune trees can have economically beneficial effects on prune production, including improved (lower) fruit drying ratio, reductions in excessive shoot growth, and less money spent on irrigation water.
A reliable way to achieve these benefits is to monitor tree stress using midday stem water potential, and scheduling irrigations accordingly. This guide will provide specific information necessary to use this monitoring method for scheduling irrigations in prune trees. The pressure chamber is used to measure the amount of tree water stress. Irrigation water is only applied when the tree reaches a mild to moderate level of water stress.
Bar: A pressure unit equal to about 14.5 pounds per square inch (psi).
Pressure Chamber: A device that measures the degree of water stress within the plant using air pressure.
Midday Stem Water Potential: Water stress expressed as a tension (negative pressure units).
Baseline Value (of midday stem water potential): The value of midday stem water potential that would be considered normal for a fully irrigated tree.
Target Value: The value of midday stem water potential that is desired for best horticultural production
The number of trees that are required to effectively manage irrigation will depend on a number of factors, but 10 trees spread out over one irrigation block (up to 50 acres), is reasonable. It is best to pick trees for monitoring and then use those trees all season. If there are known problem areas, such as sandy streaks or if there is variation in the soil wetting pattern related to the irrigation system, (e.g., a difference from head to tail down the furrow or high and low pressure areas in a pressurized system), then the grower must decide which trees will be the best indicators of the orchard average. If there is doubt about the number of trees needed, start with a larger number of trees, then, after a few irrigations, see which ones are most indicative of the field average. For a very uniform orchard, as few as three trees may be enough. Weekly monitoring is recommended, but after some experience, growers may find that less monitoring is required.
Technical procedures for making pressure chamber measurements are found in the companion guide and . Begin monitoring in early spring, once the first leaves are mature. Early monitoring can give an advanced warning of water stress problems because the pressure chamber can detect water stress in the tree before it is apparent to the eye. For fully irrigated prune trees (i.e., no limitation in soil water), regardless of tree age, midday stem water potential will depend on the local weather conditions as shown in Table 1.
Air Relative Humidity (RH, %)
This table is based on the following publications: McCutchan and Shackel, 1992. Stem-water potential as a sensitive indicator of water stress in prune trees (Prunus domestica L. cv. French). Journal of the American Society for Horticultural Science 117(4):607-611 and Shackel et al. 1997. Plant water status as an index of irrigation need in deciduous fruit trees. HortTechnology 7(1):23-29. Values measured in early spring can often be less negative than those indicated in the table, but this appears to be normal for prunes.
For young prune trees, where maximum vegetative growth may be desirable, the average value of midday stem water potential should match the baseline. If the average is more negative than the baseline value at the temperature and relative humidity conditions when the measurements are made, then the trees are under water stress and vegetative growth may be compromised. If trees remain more negative than the baseline, and the soil is wet, then there may be a problem with root health.
Target Values for Mature Trees
Based on research, the desired (target) level of stress for mature trees at different times in the season, regardless of weather conditions, is given in Table 2.
For full coverage systems such as flood or sprinkler, these values should be used as irrigation thresholds: wait until the average of the monitored trees reaches these targets before irrigating. For higher frequency systems such as drip or micro-sprinkler, irrigation should be started once the trees reach the target, but either the frequency or duration (or both) should be adjusted during the season to keep the trees close to the target value for that time of year. For either type of irrigation system, allowing the trees to reach a stress of -15 bars or more during the crack sensitive period (late June/early July) can be dangerous because fruit cracking can occur when tree water stress is suddenly reduced following an irrigation. Sudden reductions in water stress near harvest have also been associated with increased pre-harvest fruit drop. Since each orchard is unique, only experience will tell the grower what it takes to achieve these targets. The pressure chamber should be used to manage irrigation by fine tuning current irrigation practices. For instance, if trees are showing no stress just after a flood irrigation, and show only slight stress just before the next, then it should be possible to lengthen the irrigation interval. In addition to having horticultural benefits on fruit quality, reducing the amount of applied irrigation water is also likely to have a beneficial impact on the environment.