Effects of low temperature stress on photosynthetic parameters and physiological characteristics for seedlings of two pomegranate varieties

Abstract:【Objective】The aim of this study was to investigate the effects of the low temperature stress on photosynthetic and physiological characteristics of Kashi acid pomegranate and Tunisia soft seed pomegranate in Xinjiang. Meanwhile, the cold resistance and suitable growth conditions of the two varieties were evaluated, so as to provide scientific basis for the introduction, breeding, and cultivation management.【Methods】One year cuttings of the two varieties with different cold tolerance types (Kashi acid pomegranate: high cold resistance and Tunisia soft seed: cold resistance) were used as experimental materials and placed in an artificial climate chamber with gradient cooling. Five temperatures (CK: 25 ℃, T1: 6 ℃ mild stress, T2: 3 ℃ moderate stress, T3: 0 ℃ severe stress, T4: -3 ℃ very severe stress) and three durations (2, 4 and 6 d) were set. During this period, the photosynthetic parameters, chlorophyll content, chlorophyll fluorescence characteristics, relative electrical conductivity, malondialdehyde and osmotic regulatory substance (proline, soluble sugar and soluble protein) contents of pomegranates of the two varieties were measured.【Results】With the decrease of stress temperature, the contents of the chlorophyll a, carotenoids, total chlorophyll, net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), water use efficiency (WUE) and fluorescence parameters [Fv/Fm, qP, Y(Ⅱ)] showed a downward trend. Besides, the photosynthesis was inhibited by the decrease of temperature and the extending of stress time. The proline content of these two pomegranate varieties showed an increasing trend with the extension of stress time and the intensification of stress degree. The Kashi acid pomegranate had no significant change under mild stress, while the Tunisia soft seed pomegranate was sensitive to temperature and showed significant change. The increase of proline content was positively correlated with the degree of stress. The soluble sugar content of the Kashi acid pomegranate increased significantly after 2 d and 4 d of stress, then it decreased after 6 d of stress. The Tunisia soft seed pomegranate increased firstly and it decreased after 2 d and 6 d of stress, while it increased firstly and decreased after 4 d of stress. Under the same stress temperature, with the increase of stress days, the two varieties showed an increasing trend except for mild stress. And the other treatments showed a trend of increasing first and then decreasing. Under the same stress time, the soluble protein content was significantly increased with the decreasing of temperature. However, under the same temperature, the soluble protein content increased firstly and then decreased with the extending of stress time. Furthermore, the soluble protein content of theTunisia soft seed pomegranate were higher than that of Kashi. The level of increasing content was negatively correlated with anti- cold type varieties, and positively correlated with the stress levels. The relative conductivity of the Kashi acid pomegranate did not change significantly under the mild low temperature treatment, but decreased under the moderate low temperature treatment, while that of the Tunisia soft seed pomegranate decreased under severe treatment. Under other low temperature stresses, the relative conductivity of the two varieties showed a significant trend of increase. And the increase of relative conductivity of Tunisia soft seed pomegranate was greater than that of the Kashi acid pomegranate, the largest increase reached the highest value after 6 days of extreme stress. With the decrease of stress temperature, the malondialdehyde content of the Kashi acid pomegranate showed a gradual increase trend, while the malondialdehyde content of the Tunisia soft seed pomegranate showed a stable trend at first and then a sharp increase trend. The malondialdehyde content of two pomegranate varieties increased significantly only under severe stress and extremely severe stress after 2 days of the low temperature treatment, and increased with the decrease of temperature after 4 and 6 days of treatment. The malondialdehyde content of the Tunisia soft seed pomegranate was generally higher than that of the Kashi acid pomegranate.【Conclusion】The cold resistance of the Kashi acid pomegranate was significantly higher than that of the Tunisia soft seed pomegranate. The mechanism of cold resistance could be that the leaves of the Kashi acid pomegranate were thick and heavy under the low temperature stress, with strong anti-dehydration ability. Thus, the photosynthetic apparatus had low damage. Besides, the leaves had strong ability to maintain chlorophyll content and high photosynthetic efficiency. Meanwhile, they also had strong ability to maintain cell osmotic adjustment and membrane lipid peroxidation. When the extreme stress lasted for 6 days or 4 days, the photosynthetic system of the Tunisia soft seed pomegranate would be seriously damaged, leading to the inhabition of the leaf growth the cold damage would be cause.【Objective】The aim of this study was to investigate the effects of the low temperature stress on photosynthetic and physiological characteristics of Kashi acid pomegranate and Tunisia soft seed pomegranate in Xinjiang. Meanwhile, the cold resistance and suitable growth conditions of the two varieties were evaluated, so as to provide scientific basis for the introduction, breeding, and cultivation management.【Methods】One year cuttings of the two varieties with different cold tolerance types (Kashi acid pomegranate: high cold resistance and Tunisia soft seed: cold resistance) were used as experimental materials and placed in an artificial climate chamber with gradient cooling. Five temperatures (CK: 25 ℃, T1: 6 ℃ mild stress, T2: 3 ℃ moderate stress, T3: 0 ℃ severe stress, T4: -3 ℃ very severe