Ice nuclei active at approximately -2°C and intrinsic to woody tissues of Prunus spp. were shown to have properties distinct from bacterial ice nuclei. Soaking 5-centimeter peach stem sections in water for 4 hours lowered the mean ice nucleation temperature to below -4°C, nearly 2°C lower than stems inoculated with ice nucleation-active Pseudomonas syringae strain B301D. Ice nucleation activity in peach was fully restored by air-drying woody stem sections for a few hours. The ice nuclei in woody tissue were inactivated between 40 and 50°C, but unaffected by treatment with bacterial ice nucleation inhibitors (i.e. NaOCI, tartaric acid, Triton XQS-20), sulfhydryl reagents (i.e. p-hydroxymercuribenzoate and iodine) and Pronase. Ice nuclei could not be dislodged from stems by sonication and were shown to be equally distributed in peach bud and internodal stem tissue on a per unit mass basis; outer and inner stem tissues were also indistinguishable in ice nucleation activity. Development of ice nuclei in immature peach and sweet cherry stems did not occur until midsummer and their formation was essentially complete by late August. Once formed the ice nuclei intrinsic to woody stems were stable and unaffected by seasonal changes in growth. The apparent physiological function of the ice nuclei is discussed in relation to supercooling and mechanisms of cold hardiness in Prunus spp.Frost tender plants avoid intracellular ice formation and concomitant lethal injury because of the capacity of water in plant cells to supercool. Although pure water supercools to approximately -40°C before homogeneous ice nucleation occurs, ice generally forms at higher temperatures because exogenous ice nuclei order water molecules into a configuration conducive to growth of ice crystals (23, 29). An ice nucleus is active at a discrete threshold temperature, and only a few substances, primarily crystalline forms of organic compounds, show ice nucleation activity at temperatures above -5°C (14). Accordingly, water within the tissues of annual plants, such as beans, tomatoes and maize, has the capacity to supercool to temperatures below -5°C because these tissues lack intrinsic ice nuclei active at high freezing temperatures (23). Appreciable supercooling, however, does not occur in most woody plants such as Prunus spp., which begin freezing around -2°C apparently due to one or more types of constitutive ice nuclei within woody tissue (9, 17). Accordingly, water contained in dormant flower primordia supercools 'This report is based, in part, on research conducted and supported as a part of SAES Western

`Delicious' apple (Malus domestica Borkh.) trees received regulated deficit irrigation (RDI) early in the growing season to determine if fruit quality and storage life would he altered compared to well-watered trees. Soil moisture and leaf water potential were lower in RDI trees than in those that did not receive RDI most of the season. Internal ethylene concentration increased logarithmically earlier in RDI apples. At harvest, RDI fruit were smaller and had a higher soluble solids concentration (SSC) and lower titratable acidity. Starch degradation was delayed in RDI fruit, and their color was not affected. Firmness was not affected when the effect of size on firmness was removed. The SSC of RDI apples remained higher during storage, but starch content, titratable acidity, firmness, and color were similar.

Additional index words. Malus domestica, croploadAbstract. A standard fruit growth curve, used commercially as an aid to hand thinning, was compared to periodic volume measurements of apple fruit (Malus domestica Borkh. 'Delicious') subjected to early season regulated deficit irrigation (RDI) to determine when to end RDI, which is used to control vegetative growth and save water. RDI suppressed stem water potential, stomatal conductance, and fruit growth rate compared to the trickleand furrow-irrigated controls, which wetted about one-half and the entire soil volume, respectively. Full irrigation was restored to RDI trees by trickle and microsprinklers, which wetted about one-half and the entire soil volume, respectively, after terminal buds set. Stem water potential, stomatal conductance, and fruit growth rate of RDI trees increased to that of the controls, except for RDI/trickle trees, which had 80% the stomatal conductance of the other treatments. Fruit weight at harvest was affected by an interaction of irrigation treatment and cropload. RDI trees had similar or less vegetative growth and similar or higher yield efficiency than the controls. We recommend ending RDI before fruit growth declines below the standard curve.

Deciduous fruit tree orchards located in the Pacific Northwest were surveyed over a 3-year period for the presence of ice nucleation-active (INA) bacteria. In the Yakima Valley, only about 30% of the fruit tree orchards contained INA bacteria (median population ca. 3 x 102 CFU/g [fresh weight]) in contrast to nearly 75% of the orchards in the Hood River Valley (median population ca. 5 x 103 CFU/g [fresh weight]). These INA populations ranged from less than 10 to over 106 CFU/g (fresh weight) of blossoms and, in Hood River Valley orchards, generally comprised over 10% of the total bacterial population. Populations of INA bacteria fluctuated during the year with highest levels developing on buds and flowers during the cool, wet spring, followed by a drop in populations during the warmer, drier, summer months and finally a gradual increase in the autumn. The INA bacteria persisted on dormant buds from which they again colonized young developing vegetative tissues. All INA bacteria were identified as Pseudomonas syringae. The frequency of ice nucleation at-5°C for these strains ranged from nearly every cell being INA to less than 1 in 107 cells. The median frequency of ice nucleation at-5°C was 104 cells per ice nucleus. The INA P. syringae strains from individual orchards were diverse with respect to bacteriocin typing and in ice nucleation frequency. The consistent absence of detectable INA bacteria or presence of low populations in most of the orchards surveyed during periods when critical temperatures (i.e.,-2 to-5°C) were common indicated a limited role for INA bacteria in frost susceptibility of most Pacific Northwest orchards.

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