Study on the mechanisms of frost injury occurrence and the preventing techniques in Japanese pear

Most deciduous fruit trees require winter chilling temperatures to break dormancy of wintering flower buds. The buds sprout and bloom as the weather warms in the spring. These buds and/or flowers are prone to spring frost injury while they suffer no frost damage in mid winter. In fruit trees including the Japanese pear, frost injury can be particularly harmful. Fruit production is severely damaged during the periods of blooming and young fruit stage by spring frost. The objective of this study was to examine the mechanism of freezing and spring frost injury and seasonal changes in the cold resistance of the Japanese pear, in addition, to determine if n-propyl dihydrojasmonate (PDJ), a synthetic Jasmonic acid (JA) derivative can be applicable for preventing spring frost damage to the Japanese pear. 1. Cold hardiness of flower buds changed in correspondence with changed carbohydrate levels in Japanese pear 'Kousui'. During cold acclimation, cold hardiness of flower buds increased as endogenous ABA levels were increasing from autumn to winter. It is suggested that intracellular freezing can be avoided by supercooling of flower bud or the solutions might protect the plasma membranes and proteins from freezing and dehydration in the Japanese pear. 2. Seasonal changes in the ice nucleation activity of various tissues of Japanese pear 'Kousui' were investigated with the aim of studying the manner of freezing, which is an important mechanism for cold hardiness. The ice nucleating temperatures of outer scales and inner scales were maintained at between -7 deg C and -5 deg C throughout the winter months, while those of florets and bracts markedly increased from-12.6 deg C to-6.0 deg C and from -10.4 deg C to-5.6 deg C, respectively from late October to February. In winter flower buds exposed to subfreezing temperatures showed ice crystals located in the space between florets and bracts and within scales where ice nucleation activity was fairly high. This relationship implies that cold hardiness may be controlled by the balance of supercooling and ice nucleation in various tissues. The perennial tree has both easy freezing and supercooling tissues where they are localized in the right place according to circumstances to protect tissues against cold stress. 3. Ice formation and propagation in the flower and young fruit of Japanese pear were investigated with infrared (IR) video thermography imaging system. The changes of thermal response (exotherm) in freezing events could be detected using the infrared thermography camera with exotherm rise accompanied by a marked color change in the tissues. With first bloom flowers, the temperature of peduncles decreased faster than that of the receptacle: the initial ice nucleation occurred primarily in petals, sepals and/or receptacle. Subsequently, ice was propagated along the peduncle. The temperature of receptacles was above that of peduncles during the freezing test at full bloom; the sepals and/or receptacle were initially frozen, then ice nucleation was extended to the petals along the peduncle. After blooming, high ice nucleation temperatures were observed in the petal, receptacle, sepal and pedicel while those of both the stamen and pistil remained very low. The more sensitive tissues to spring frosts were different from the sites where ice nucleation activity is high. These results suggest that the sepals and/or receptacle where the initial nucleation occurred may be an intrinsic or extrinsic source of high ice nucleation activity in flowers of the Japanese pear. 4. Effects of n-propyl dihydrojasmonate (PDJ) treatment on flowers of Japanese pear during spring frost were investigated to study mechanisms for avoiding spring frost injury. PDJ applied during the flowering period resulted in a lower injury index for the ovules and pistils after freezing tests. Average ion leakage in control flowers was 37.9% during the balloon stage at -5 deg C, while the flowers treated with PDJ displayed a 16.6% ion leakage. Similarly, at the full bloom stage, PDJ treatment reduced ion leakage at -5 deg C from 73.1% to 47.8% in the control. The organs of the flower more sensitive to low temperature stress were the ovule, pistil and ovary, but stamens were more resistant Sugar content in the flower at the balloon stage was increased by PDJ when treated at the pink stage. Moreover, free amino acids, especially proline, and ABA increased similarly with PDJ treatment. It is suggested that increasing endogenous ABA levels accompany sugar accumulation and controls osmotic pressure in cells stimulated by PDJ treatment to the flower of Japanese pear. These results show that PDJ affects supercooling capacity of a flower by changing solute content and protects organs from freezing. From a practical viewpoint, the present results may be useful for designing frost protection programs for horticultural crops.