| Temperature stress, including heat and frozen stress, has beening one of the limited environmental factors in citrus industry in the world, especially in China. To improve the capacity of citrus to withstand temperature stress,it is very important to research the physiological responses of citrus in the stress condition.Three varieties of citrus {Citrus reticulata Blanco cv. Guoqing No.1, C. sinensis Osbeck cv. Skagges Bonnaza, C. ichangensis Swingle) were used under cold stress, and other citrus varieties (C. reticulata Blanco cv. Guoqing No.l, C. sinensis Osbeck cv. Skagges Bonnaza, C. paradisi Macq) were studied under heat stress. Citrus leaves were subjected to cold and heat stress at -5C and 50C respectively, after Ca2+ was applied with different concentration 5 days later. Physiological and biochemical methods have been carried out to study the mechanism of Ca2+ effect on citrus under high and low temperature. To know more about Ca2+ on the regulation of stress resistance, four different media contenting different Ca2+ were used to cultivate Limon (Citrus lemon) , and then the seedlings were treated in different low temperature stress. The main results obtained in this peper were as follows:1. Under low temperature stress, the content of soluble sugar was in correlation with ability of cold-resistance of citrus varieties. The more cold-resistance citrus variety is the higher content of the soluble sugar. The content of soluble sugar tended to rise with the increasing of external Ca2+ concentration. The increasing rate of the soluble sugar showed reverse ratio with cold-resistance. Colder could variety resist, lower the increasing rate was. The correlative coefficient was -0.9999. The content of soluble protein was similar with that of soluble sugar except that the content of soluble protein tended to rise first, then drop with the increasing of the external Ca2+ concentration. And the correlative coefficient between cold-resistance and soluble protein increasing rate was -0.9459.2. Under low temperature stress, SOD and POD activity was higher when using variety with stronger cold-resistance characteristics. SOD and POD activity of three varieties uniformly tended to rise first, and then fall. When external Ca2+ concentration was 10mmol.L-1, SOD and POD activity were the highest compared with other treatments using different Ca2+ concentration. The increasing rate of both indexes was lower when citrus variety with stronger cold-resistance was used.3. Under low temperature stress, the content of M.DA was distinctly different amongvarieties; the more cold-resistance citrus varieties had higher content of MDA. And its content tended to drop first, and then rise with the increasing of Ca2+ concentration. When Ca2+ concentration was 10mmol.L-1, MDA content reached the minimum. This showed proper Ca2+ concentration may reduce the content of MDA and the harm done to the cell membrane. More cold-resistant citrus varieties had the higher MDA content falling rate.4. Under high temperature stress, the content of soluble sugar was distinctly different among varieties, which tended to rise with the increasing of Ca2+ concentration. The content of soluble protein and the activity of SOD tended to rise first, and then drop. When external Ca2+ concentration was 10 m mol.L-1, SOD activity reached the maximum. And protein content reached the maximum when external Ca2+ concentration was 20mmol.L-1. When leaves of the citrus were sprayed with external Ca2+, The sensitivity of different varieties to heat stress was clearly different using SOD as parameter. The higher the activity of SOD, the senser citrus responded to it and the smaller increasing rate. The correlative coefficient was -0.9966. Under high temperature stress, the content of MDA was distinctly different among varieties. It dropped first, and then rise with the increasing of Ca2+ concentration. When Ca2+ concentration was 20mmol.L-1, MDA content reached the minimum. These results indicated proper Ca2+ might strengthen citrus heat-resistance.5. Limon c... |
