Loss of cold temperature tolerance and reacclimation following warm temperatures in peach

Cold hardiness was measured to -30 degrees C in peach (Prunus persica L.) shoots previously exposed to 4 days of cold (-6.7 degrees C) or warm (16.5 degrees C) temperatures. Four days of warm exposure caused a 5 to 6 degrees C loss in flower bud hardiness in 'Harrow Beauty', 'Starfire', and 'Bailey' peach trees. The lethal temperature for 50% (LT50) of the flower buds, estimated from the inflection point, ranged from -23.6 to -25.6 degrees C with cold exposure and from -17.9 to -20.5 degrees C with warm exposure. Xylem parenchyma hardiness ranged from -25.5 to -25.8 degrees C with little difference due to warm exposure or to cultivar. Phloem was the hardiest tissue and showed only partial injury at-16.7 degrees C after warm exposure in 'Starfire' and 'Harrow Beauty' and almost no injury in 'Bailey'. The cambium in 'Bailey' and 'Starfire' was partially injured at -30 degrees C in cold-exposed shoots but was hardy to -22.8 to -25.8 degrees C in warm-exposed shoots. Cambium in 'Harrow Beauty' was hardy to -19.7 to -22.6 degrees C and was severely injured at -30 degrees C with no difference between temperature treatments. In a separate experiment, 'Reliance' flower buds exposed to warm temperature for 6 days (20 degrees C) lost 8.3 degrees C in hardiness compared with 6-day exposure to cold (1 degrees C). When a 3-day cold exposure was followed by a 3-day warm exposure, buds lost 4.2 degrees C hardiness. However, a 3-day warm exposure followed by 3-day cold exposure caused 1.2 degrees C loss indicating that reacclimation had occurred. A similar pattern occurred in the shoot cambium and phloem but not in the xylem, which showed only minor differences due to the different temperature exposures.