Supercooling Characteristics of Isolated Peach Flower Bud Primordia1

The amount of unfrozen water in dormant peach (Prunus persica [L.] Batsch, cv Redhaven) flower buds, isolated primordia, and bud axes was determined during freezing using pulse nuclear magnetic resonance methods. Differential thermal analysis studies were conducted on whole buds and isolated primordia in the presence of ice nucleation. The results showed that some of the water in isolated primordia remained supercooled in the presence of ice nucleation. Although most tissue water froze (57.5%) following ice nucleation at -2.50C, a considerable amount of water was found to supercool. In the presence of ice nucleation, increased hydration of isolated primordia resulted in the elimination of the supercooling characteristic. The structural integrity of isolated primordia appeared to be essential for supercooling. Dormant peach flower bud primordia avoid ice formation by deep supercooling (8, 9). Since freezing of supercooled primordia is lethal, this characteristic is often considered a limitation to winter survival of peach buds (5). Primordia remain supercooled in spite of ice being present in bud scales and axes (1, 9). It has been suggested that a barrier exists for not only dehydration of primordia but also ice propagation from surrounding tissues into the primordia. Studies on structural characteristics and dye movement in buds have shown that continuous xylem vessels were absent in dormant flower buds of peach and other Prunus species. This lack of full xylem development in dormant buds may exclude ice from primordia. Development of xylem vessels in deacclimating buds resulted in the disappearance of the supercooling characteristic (1, 2). Studies on peach buds by Quamme (9, 10) showed that dehydration of bud axes and vascular elements below the primordia occurred as a result of freezing in bud scales. This led to the suggestion that dry regions in bud axes, formed at the expense of freezing in bud scales, may prevent ice growth into primordia. However, very little is known about the freezing behavior of primordia in the event of ice nucleation. The objective of this study was to characterize the freezing behavior of isolated primordia and specifically to determine if isolated primordia can supercool in the presence of ice. l Supported partially by the U.S. Department of Agriculture grant 86-CRCR1-2065 and Kansas Agricultural Experiment Station (Contribution No. 88-500-J) MATERIALS AND METHODS Sections of dormant peach (Prunus persica [L.] Batsch, cv Redhaven) twigs were collected from the Horticultural Research Center at Wichita on January 7, 1988. Samples were stored in closed polyethylene bags at -2°C until use. Studies were conducted on whole flower buds, isolated primordia, and bud axes. Differential Thermal Analysis DTA2 studies on whole buds and isolated primordia were conducted using a cooling rate of 4°C/h. The DTA apparatus consisted of six thermoelectric modules, which were made from N and P elements of semiconductor, bismuth telluride (Melcor, Trenton, NJ). Each thermoelectric module was housed in a sample chamber and cooled in a bench top freezer (Tenney Engineering, Union, NJ). The cooling process and collection of data were accomplished by a microcomputer (Intel 8088 microprocessor) with a 12 bits analog/digital converter (Adalab, Interactive Microwave, State College, PA). Temperature data were collected on a slow integrating channel at 1 sample/second. Single buds and isolated primordia were placed on the ceramic surface of the thermoelectric modules at 0°C and cooled at 4°C/h to about -45°C. The primordia were nucleated with a small amount of ice at -2.5°C. Sample temperature was monitored by a 24 gauge copper-constantan thermocouple.