Response Difference And Its Physiological Mechanism Of 'shiranuhi' And 'ponkan' Grafted On Trifoliate Orange To Zinc Stress

Zn is an important and essential micronutrient for the plant growth, it is also one of the commonly lacking mineral nutrients in production.'shiranuhi'(Citrus reticulata×(C. reticulata×C. sinenesis)), with its excellent fruit quality, is currently a perfect citrus cultivar vigorously promoted around China. However, in recent years, distinct symptom of chlorisis was presented on new leaves of'shiranuhi'in Three Gorges Reservoir,but there was no manifested symptom on'ponkan'(Citrus reticulata Blanco) and other citrus cultivars. Some existing researches in field showed that this symptom was caused by Zn deficiency. There can be various kinds of influential factors due to the complexity of the field background. So changes of some physiological indexes, ultrastructure in leaves, and Zn contents in different parts of'shiranuhi'and'ponkan'grafted on trifoliate orange seedlings were investigated under Zn2+ of 0, 0.05 and 0.5 mg·L-1 (0, 0.05 and 0.5 mg·L-1 were considered as Zn deficiency treatment, control and Zn excess treatment, respectively) treatments with sand culture in this study. 1. Effects of zinc stress on antioxidant enzyme activity and zinc distribution of'shiranuhi'and'ponkan'were studied. The results showed that: 1)The contents of chloroplast pigments and leaf area of'shiranuhi'were significantly lower under Zn deficiency treatment than under control and Zn excess treatment, while those of'ponkan'decreased significantly under excess Zn treatments compared to the control. 2)Leaf POD activity of'shiranuhi'and CAT of'ponkan'were not significantly affected under the three different Zn treatments, the activities of CAT and SOD in'shiranuhi'leaves increased with the increase of Zn concentration, under Zn deficiency treatment, the activities of POD, CAT and SOD in'ponkan'leaves were notably higher than those of'shiranuhi', and SOD activity was significantly lower in leaves of'ponkan'under excess Zn treatment than in leaves of'shiranuhi', MDA contents in leaves of'shiranuhi'and'ponkan'notably increased under Zn stress compared to the controls, and under Zn excess treatment, MDA contents were significantly higher in'ponkan'leaves than in'shiranuhi'leaves. 3)In addition, Zn contents in different parts of both cultivars increased with the increase of Zn concentration, under the same Zn treatment, Zn contents were significantly higher in leaves of'ponkan'than in other parts of'ponkan'and in leaves of'shiranuhi', furthermore, Zn contents in the upper leaves of'shiranuhi'were notably higher than those in the basal leaves. 2. Effect of Zn stress on photosynthetic characteristic and their correlation of the two varieties of citrus were studied. The results showed that: the net photosynthetic rate(Pn) of'shiranuhi'presented a one-peak curve under Zn stress, but double-peak curve was found under control, as to'ponkan', similar double-peak curves were observed under control and Zn stress treatments, furthermore, the average Pn of the two citrus were significantly lower under Zn stress than control. Simple correlation analysis showed that the Pn of'shiranuhi'under Zn stress was positively correlated with photosynthetic active radiation (PAR) and transpiration(Tr), while the Pn of control had no significant correlation with relative factors, whereas the Pn of'ponkan'under Zn stress and control were all positively corelated with stomatal conductance(Gs) . There existed a few differences between stepwise multiple regression analysis and simple correlation analysis results. The effects of Zn stress on the photosynthetic characteristics and their correlations of the two varieties of citrus were discussed in this study.3. Effects of zinc stress on leaf ultrastructure of'shiranuhi'and'ponkan'were studied. The results showed that: Under zinc deficiency treatment, distortion or vacuolization was observed in leaf chloroplast of both, as to'ponkan', starch grains and platoglobuli increased, nucleus and mitochondria were normal, whereas starch grains were rare in chloroplast of'shiranuhi', and many small black deposits were found in nucleus. Under zinc excess treatment, vacuolization or blurred chloroplast membrane was detected in chloroplast of both, mitochondria disintegrated, in stroma of chloroplast of'ponkan', platoglobuli increased, and few starch grains were found, while starch grains obviously increased and swelled, and swollen thylakoid lamella were found in chloroplast of'shiranuhi'. It's suggested that there exist obvious effects of zinc stress on leaf chloroplast pigments contents, net photosynthetic rate and ultrastructure of both, the effects were different for different cultivars.These results showed that there were significant difference in physiological response of'shiranuhi'and'ponkan'grafted on trifoliate orange to zinc stress, these differences mainly reflected in relevant enzyme activity, photosynthesis, ultrastructure and zinc distribution in different parts, the effects were different for different cultivars.