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Generally, water stress reduces yield in annual crops. However, for mature fruit trees, this relationship may not hold in many situations, thus providing the opportunity for saving water without losing production. Indeed, even an increase in productivity may be achieved as we better learn how to manipulate processes within the tree through moderate water stress. Several areas of research have shown promising results. The reduction of irrigation after harvest of early maturing peaches and plums has demonstrated substantial savings of water with no loss of production. Peaches can suffer fruit quality problems such as doubling and deep suturing, but these can be overcome with well-timed irrigations in the previous late summer. Water stress imposed before harvest has also shown some promise. Reports from Australia have demonstrated significant increases in yield and fruit size in peach and pear, although researchers in other locations have generally been unable to replicate these results. The timing and/or rate of stress development appear to be critical factors. Under the right conditions, stress can alter the allocation of resources between vegetative and fruit growth. Before implementation of these practices can be achieved, further research will need to focus on developing good tools for measuring stress in the trees, obtaining a better understanding of adaptation of trees to rapidand slow-developing stress, documenting the effects of stress on vegetative and fruit growth during different times of the season, and understanding the interaction of stress with other factors such as fruit load.
The heat balance method was used to estimate transpirational sap flow through 60- to 75-mm-diameter stems (trunks) of 3-year-old peach [Prunus persica (L.) Batsch. cv. O'Henry] trees under field conditions. On rare occasions, heat balance estimates agreed well with independent lysimetric measurements, but on most occasions, heat balance estimates of sap flow were unrealistic in both direction and magnitude. In some cases, the errors in sap flow approached two orders of magnitude and were always the result of a calculation involving division by a very small and sometimes negative temperature differential between the stem surface temperature above and below the gauge heater. The occurrence of negative temperature differentials under positive transpiration conditions may be inconsistent with a fundamental assumption in the heat balance model, namely that temperature differentials are solely a consequence of the dissipation of energy supplied to the gauge heater. In the absence of heating power applied to the gauge, temperature differentials exceeding - 1C were correlated with the rate of change in stem temperature, indicating that ambient conditions themselves can impose a bias in gauge signals and, hence, influence gauge accuracy. Our results suggest that the effect of ambient conditions on gauge signals should be critically evaluated before considering heat balance estimates of sap flow as reliable under any given conditions.