Research On Tolerance Evaluation And Physiological Mechanism Of Kiwifruit Seedling To Root Zone Hypoxia

Molecular oxygen is the terminal electron acceptor in the mitochondrial electron transport chain and is also the prerequisite conditions that are required by higher plants during their growth and development. However, shortage of oxygen or anoxia is a common environmental challenge which plants have to face throughout their life.Floods and excess of rainfall are examples of nature conditions that lead to root hypoxia or anoxia.Low oxygen concentration can also be a normal attribute of a plant natural environment. The kiwifruit (Actinidia deliciosa) is a large, woody, deciduous vine that was native to the Yangtze Valley of China. Now it is widely planted as a burgeoning commercial planting fruit tree around the world because of its abundant nutrient value. Nevertheless, kiwifruit plants'rootstocks are very sensitive to water flooding or excess of rainfall, which can easily produce root-zone hypoxia stress and reducing conditions, which can damage the plants metabolism. Therefore, hypoxia stress is incidental to the kiwifruit plants during growing season.In this thesis, 10 kinds of kiwifruit seedlings were treated with hydroponics hypoxia or waterlogging to study their root-zone hypoxia tolerance. The experiment seedlings were subjected to hypoxia by ?ushing nutrient solution with N2 gas (99.99% N2) or waterlogging for 11days. The nutrient solution of control plants was continuously ?ushed with air under an air pump. Oxygen concentration in the vessels was monitored with an oxygen meter. Hypoxia-tolerance kiwifruit seedling'Qinmei'(Actinidia deliciosa) and hypoxia-sensitive seedling'Hongyang'(Actinidia chinensis) were selected as experimental materials to study their physiological response differences. The physiology and biochemical mechanism of hypoxia stress on the seedlings were discussed, all above provided the theoretic base and technical guideline for kiwifruit cultivation, soil management and resistive breeding. The results were as follows:1. Growth of all kiwifruit seedlings were inhibited to various degrees under root-zone hypoxia stress. After 11 days waterlogging hypoxia treatment,the max length of new root, plant height, plant biomass about leaves and roots, root activity, relative growth rate of leaves, content of chlorophyll and chlorophyll a/b in leaves under root-zone hypoxia stress obviously declined comparing with control. MDA content, relative conductance in the leaves and roots all increased in 16 kiwifruits. But MDA content and relative conductance in roots was much higher than those of leaves. The sensitivities of 10 kiwifruit seedlings to hypoxia stress were obviously different.2. With the method of subordinate function and cluster analysis, the adversity resistance coefficient (including 6 physiological and 5 growing indexes) of 10 kiwifruit seedlings, were comprehensively evaluated in order to appraise their hypoxia-tolerance abilities. According to the results,'Hayward','Qinmei','Jinxiang','Kuoye','Huayou'kiwifruit seedlings held higher tolerance to root-zone hypoxia stress, while'Hongyang'kiwifruit seedlings was sensitive to root-zone hypoxia stress.The others including'Xixuan','Maohua','Jinhua','Shanli'kiwifruit seedlings kept moderate resistant intensity to root-zone hypoxia stress.3. Under root-zone hypoxia stress, the fresh and dry weights of seedlings treated under hypoxia were less than those of control. Effects of hypoxia stress on A. deliciosa were much slighter than on A. chinensis, which made A. deliciosa perform a better resistance. The ABA contents in A. deliciosa, A. chinensis leaves and roots were enhanced obviously. The IAA contents in A. deliciosa, A. chinensis leaves were raised to the maximum on the 7th day, then reduced but still higher than those of control. Same tendencies happened in the roots of both species, but the peak appeared on the 5th day and differed from that of the leaves. The GA3 and ZR contents in A. deliciosa, A. chinensis leaves and roots were decreased. The ratios of IAA/ABA, ZR/ABA and GA3/ABA were also studied and the results of them decreased, which showed that kiwifruit seedlings were obviously inhibited from growing under hypoxia and the resistant ability had an intimate relationship with endogenous hormones.4. Under Under waterlogging stress, daily net photosynthetic variation rate in leaves of A. deliciosa kiwifruit seedlings was restrained. Compared with control, only one peak was observed at 10:00 and then reduced evidently. At about 13:00, the quantitative data of Pn presented less than zero, which showed the leaves'capability of photosynthesis was destroyed badly and could not keep the capability of photosynthesis.The stomatal conductance was reduced, too.Transpiration rate of kiwifruit seedlings raised to the maximum at 10:00, then reduced but lower than that of control in daily time. Intercellular CO2 rate in daily time presented a'W'shape and raised evidently. With the prolonged waterlogging stress, F0,F0′,qN,Hd of the leaves raised gradually and the Fm,Fv,Fv/Fm,Fv/F0,Fv′/ Fm′,qP,Pc,ETR dropped evidently. Ex raised firstly and then reduced evidently. The percentages of the antenna heat dissipation raised. The percentages of the photochemistry kept stably in the first 3 days and after 5 days, decreased.5. Under hypoxia stress, the activities of SOD, POD, and CAT were all stimulated and increased greatly in leaves and roots. The raise rate and activity peaks of SOD, POD, CAT in leaves and roots of A. deliciosa were higher than those of A. chinensis. The contents of H₂O₂, O₂^·|- , MDA, RMP in leaves and roots under hypoxia stress increased significantly. However, the increase ratio and peak contents of H₂O₂, O₂^·|- , MDA in leaves and roots of A. deliciosa were much lower than those of A. chinensis. On the tendency and peak time of SOD, POD, CAT, H₂O₂, O₂^·|- , MDA, there were also striking differences between them. So, much resistant kiwifruit species had advantageous anti-active oxygen systems, which could scavenge free radicals effectively to avoid injuring the membrane lipid. When facing hypoxic stress, leaves and roots had also different responses and adjusting mechanisms.6. The results showed that root-zone hypoxia accelerated the anaerobic respiration of kiwifruit seedlings'roots. Under root-zone hypoxia stress, the increment of LDH activity and aldehyde, lactate, pyruvate contents were lesser in A. deliciosa seedlings roots than in A. chinensis, but conversely, the increment of PDC, ADH, G-6-PDH activities and alcohol content in A. deliciosa seedlings roots were higher than those of A. chinensis. It was concluded that the acceleration of alcohol fermentation were in favor of the enhancement of root-zone hypoxia tolerance of kiwifruit seedlings'roots. In all process of the stress, MDH activities reduced at all times, which showed that it was not the key enzyme to adjust the resistance.7. Under root-zone hypoxia stress, macronutrients contents N, P, K, Mg, Ca and micronutrients contents Fe, Cu, Zn in leaves and roots of kiwifruit seedlings reduced significantly. However content of Mn increased and the increase in roots was much higher than that of leaves. Reduce ranges of P, K, Ca upon A. deliciosa were much lower than those of A. chinensis seedlings and reduce ranges of N, Mg had no difference between two kinds of kiwifruit seedlings. Maybe P, K, Ca nutrients were the dominant factors that caused the different resistance between A. deliciosa and A. chinensis. But reduce ranges of Zn, Fe, Cu upon A. chinensis were much lower than those of A. deliciosa seedlings. So far as the same nutrient was concerned, there were also much significant differences between leaves and roots. Reduce ranges of P, K, Mg, Zn upon roots were much higher than those of leaves. However, reduce ranges of Ca,Fe,Cu upon roots were much lower than those of leaves.