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Excision of the flower from the peach (Prunus persica (L.) Batsch.) flower bud raised the 50% injury temperature of flowers (cooled at 1C/hour) from -18 and -20C to -10 and -13C on two test dates, 26 Feb. 1988 and 5 Dec. 1990, respectively. Ice inoculation of the excised flowers at -3C further raised the 50% injury temperature to -7 and -8C for the two dates, respectively. These observations suggest that supercooling is a true mechanism for avoiding freezing injury. Low temperature scanning electron microscopy of freeze fractured cells verified that the flower froze intracellularly, whereas the subtending tissue froze extracellularly. Ice inoculation of the flower and the flower bud axis from which the scales were removed demonstrated that a barrier to ice propagation (effective to -11C) from the flower bud axis to the flower was present. This barrier may involve the provascular strands and a cell zone at the flower base (BZ) that were devoid of intercellular spaces. These tissues also had smaller cells, smaller vacuoles, greater ratio of cell wall thickness to cell size than tissue just below the BZ, which may result in greater cell rigidity and restriction of extracellular freezing. The cells outside the provascular strands at the base of the flower were also lacking in intercellular space, were smaller in size, and had a higher ratio of cell wall thickness to cell size compared to cells near the base of the scales. In the intact flower buds in which the flowers supercool below -11C, the presence of the first and second scales was important to full expression of supercooling because their removal raised the supercooling point, whereas the removal of lower scales did not. Sequestration of ice by the first two subtending scales during the early stages of freezing may be important to the creation of a dry region at the flower base that prevents ice propagation into the flower at temperatures below -11C. Supercooling of flowers within dormant buds of several genera and woody plant species appears to be a mechanism for avoiding freezing injury. This supercooling is often referred to as deep supercooling as it occurs below the freezing point of the subtend- ing tissue. Deep supercooling has been observed in flower buds of rhododendron spp. (George et al., 1974; Graham and Mullin, 1976), blackberry (Warmund et al., 1988), blueberry (Biermann et al., 1979), flowering dogwood (Sakai, 1979a), forsythia (Nus et al., 1981), Rubus spp. (Warmund et al., 1988), and several Prunus spp. (Burke and Stushnoff, 1978; Quamme, 1974), some conifers (Sakai 1978, 1979b), and in multiple buds of grape (Pierquet and Stushnoff, 1980). Low temperature injury to buds of these species is associated with sudden freezing of a supercooled fraction of water within the bud that was detected by thermal analysis as a low- temperature exotherm (LTE). The supposition is that the LTEs of buds are produced by intracellular freezing of the flower because of their sudden occur- rence and lethal effects but this has not been verified. Extracellular ice crystals have been seen in the tissue subtending the flower in rhododendron (Ishikawa and Sakai, 1981), peach (Dorsey, 1934; Quamme, 1978) and plum (Dorsey and Strausbaugh, 1923) with aid of a light microscope. However, it has not been possible to determine if intracellular ice is present in the flower with aid of a light microscope (Quamme, 1978).

Anatomical Features Facilitating Supercooling of the Flower within the Dormant Peach Flower Bud