Selection of methods for evaluation on cold tolerance of six pomegranate varieties

Abstract:【Objective】Pomegranate, originating in central Asia, is an important economic fruit tree, but it is vulnerable to frost damage in cultivation areas in central and northern China, which seriously restricts the development of pomegranate industry in China.Based on the comprehensive evaluation on cold tolerance of six pomegranate cultivars ('Tunisiruanzi''Israel 2''Qingpiruanzi''Mengzitianlüzi''Sanbai'and'Shandazi') , we screened a reliable method to select pomegranate resources with better tolerance to low temperatures as suitable parents for breeding new varieties with cold hardiness by crossing.【Methods】At present, there are many methods for the evaluating cold tolerance of fruit trees, and each method has its advantages and disadvantages.Therefore, we have to comprehensively consider the effects of various indicators when evaluating cold hardiness of pomegranates.This research took annual dormant branches of six pomegranate cultivars as materials, which were even in size and free from diseases and pests.After they were rinsed in de-ionized water, their water content including free water and bound water measured.The branches were then put into incubators and exposed to -4, -8, -12, -16 or -20 ℃ for 24 hours, and various physiological indexes were measured including the relative conductivity (REC) , the half-lethal temperature (LT50) , superoxide dismutase (SOD) activity and the contents of proline (Pro) , soluble sugars and malondialdehyde (MDA) .According to the change of each index in response to the low temperature stress-es, we analyzed the cold resistance of the six varieties.Comprehensive evaluation on cold tolerance of the six pomegranate cultivars was conducted using membership function.【Results】According to the ratio of bound water and free water in the branches of the six pomegranate varieties, the order of their cold tolerance was'Sanbai'>'Shandazi'>'Mengzitianlüzi'>'Qingpiruanzi'>'Israel 2'>'Tunisiruanzi'.The ratio was positively correlated with cold resistance.REC of the six varieties increased with the decrease in temperature, which indicated that loss of cell membrane integrity became more severe.The increase in REC was negatively correlated with cold resistance.Cold-hardiness according to LT50 was in the order of'Sanbai'>'Shandazi'>'Mengzitianlüzi'>'Qingpiruanzi'>'Tunisiruanzi'>'Israel 2'.Proline and soluble sugars serve as the osmotic regulation substances in plants, and their accumulation increases cell osmotic potential, delays cell dehydration, and protects the cytoplasm colloid from solidification when the cells are exposed to cold.Proline content in some varieties constantly increased as cold stress became more severe, while the others varieties rose first and decreased later.The order of cold hardiness based on proline content was'Shandazi'>'Sanbai'>'Qingpiruanzi''Mengzitianlüzi'>'Tunisiruanzi'>'Israel 2'.Soluble sugar content in the six varieties rose first and then decreased as temperature became lower.The order of cold hardiness based on soluble sugar content was'Sanbai'>'Shandazi'>'Mengzitianlüzi'>'Tunisiruanzi'>'Israel 2'>'Qingpiruanzi'.Active oxygen in plants is stable under the suitable growth environment, but increases dramatically when plants are exposed to stresses.Active oxygen species are removed by protective enzyme system in plant, such as SOD.The order of cold hardiness based on SOD activity was'Shandazi'>'Mengzitianlüzi'>'Sanbai'>'Tunisiruanzi'>'Israel 2'>'Qingpiruanzi'.MDA is one of the end products from membrane lipid peroxidation, and its content indicates the degree of membrane damage.Except for'Shandazi', MDA content in other varieties increased initially but then declined.The cause of decline might be that the low temperature was already beyond the range of MDA regulation.The cold-hardiness order based on MDA content was'Sanbai'>'Qingpiruanzi'>'Shandazi'>'Israel 2'>'Mengzitianlüzi'>'Tunisiruanzi'.The increase of MDA was negatively correlated with cold resistance of the plant.Because different methods generated different results, we further analyzed cold hardiness of the pomegranate cultivars using membership function.【Conclusion】Cold tolerance of the six varieties were comprehensively evaluated using membership function, and its order was'Sanbai'>'Shandazi'>'Mengzitianlüzi'>'Qingpiruanzi'>'Tunisiruanzi'>'Israel 2', which agreed with the order based on LT50, showing that LT50 is a feasible method for quick identification of cold tolerance of pomegranate varieties.