Physiological Characteristics And Key Redox Couples In Ziziphus Jujuba Mill.var.spinosa Adapted To Drought

Loess Plateau of Northern Shaanxi is a typical arid/semi-arid area. Severe soil erosion and ecological degradation restrict the local economic development. The vegetation construction is the key measure to control soil erosion and promote ecological restoration in this area. However, water shortage is the key limiter for vegetation restoration. Native species play an important role in ecological restoration due to their strong adaptabilities to the local environment. Sour jujube(Ziziphus jujuba Mill. var. Spinosa(Bunge) Hu ex H. F. Chow) is one of the major native species widely distributed in the Loess Plateau. Planting Sour jujube not only meets the requirements of ecological restoration and water conservation, but also promotes local economic development due to its great medicinal value. Therefore, it is significant to study Sour jujube at the aspect of response mechanism and physico-ecological adaptabilities to drought.In our study, two populations of Sour jujube individually from dry-climate(Yan An, abbreviated to YA) and wet-climate(Yang Ling, abbreviated to YL) were selected as research materials. Different response parameters to drought were compared and analyzed between YA and YL. The patameters involved morphology, growth, photosynthesis, ROS(Reactive oxygen species) metabolism, antioxidant mechanism and redox regulation. This study is significant to reveal the p hysio-ecological mechanism and strategy of drought adaptabilty of Sour jujube. Main conclusions are as follows:(1) The parameters of Leaf morphology and anatomy were compared between YL sour jujube and YA sour jujube planted in the same condition. It showed that YA sour jujube has smaller leaf area, larger leaf thickness and higher ratio of palisade tissue and tighter epidermic cuticular. These characteristics are related to drought adaptability. In addition, the leaves of YA sour jujube showed bigger stomatal density and index than those of YL sour jujube, which are common characteristics of plants in arid region. Soil drought made greater impact on YL sour jujube compared to YA sour jujube, such as plant growth, biomass, relative water content of leaves, water consumption and water-use efficiency YA sour jujube perfomed lower growth rate, higher water-use efficiency. It indicated that YA sour jujube had stronger adaptability to drought than YL sour jujube.(2) YA sour jujube exhibited relatively higher LSP(Light saturation point) than the YL sour jujube, which is the adaptability to high light radiation on the Loess region. Drought stress had no significant effects on photosynthetic response parameters AQY(Apparent quantum efficiency) and LCP(Light compensation point) for YA sour jujube, smaller influence was made on LSP and Pn max(Maximum net photosynthetic rate) for YA than those for YL, Rd(Dark respiration) dropped sharply under drought condition for YA. It indicated that photosynthetic system of YA has stronger adaptability to drought than that of YL. During the drought treatment, the leaf photosynthetic rate decreased in both populations mainly resulting from the stomatal limitation, however, the influence of non-stomatal limitation on leaf photosynthesis increased during the lateral drought period. C hlorophyll fluorescence analysis showed that the leaves of YA has higher NPQ(Non-photochemical quenching) value under drought condition. It indicated that the photosynthetic system of YA leaves has afficient self-protection mechanism to avoid damage, thus YA leaves showed higher effic iency both in photochemical reaction and water using than YL leaves. Summarily, compared with YL sour jujube, YA sour jujube showed lower water consumption, lower photosynthesis rate, higher water use efficiency and better performance in photochemical reaction system. The results of photosynthetic parameters in single- leaf level conform to the growth parameters mentioned above.(3) ROS level of YA was less influenced by drought than that of YL. The activities SOD(Superoxide dismutase), CAT(Catalase) and APX(Ascorbate peroxidase) were motivated by drought at initial stage in both populations, but no significant difference was found, YA showed higher POD(Phenolic peroxidase) and GR(Glutathione reductase) activities than YL under drought conditions. Except to Car(Carotenoids), the contents of ASC(Ascorbate acid), GSH(Reduced glutathione), total flavonoids and proline gradually increased during 1st-12 th days of progressive soil drying in both populations. After 12 days of sustained water supply, The contents of all the non-enzyme antioxidants in both YA and YL recovered to the control level. The leaves of YA had higher antioxides accumulation such as GSH, Car, total flavonoids and proline. These antioxides exert an important role to maintain low level of ROS.(4) The ASC pool in both sour jujubes was depleted with drought stress, whereas the extent of depletion in YL was significantly larger than that of YA. The change of DHA(Dehydroascorbate) and ASC content indicated that ASC comes to homeostasis among oxidative state and reductive state. The ratio of ASC/DHA indicated that tissue redox response is earlier in YA than in YL. The change of GSH and GSSG(O xidized glutathione) content indicated that the cell of YA stays at reductive state at the initial stress time, whereas the cell of YL stays at oxidative state; as the stress progressed, The change of GSH and GSSG content indicated that YA has stronger ability to sustain GSH pool than YL; at the later stress time, the cellular total thiol level decreased in both populations, and more decrease was found in YL than in YA. NAD(P)H(Coenzyme I(II)) pools in both populations increased firstly, then decreased with drought stress. At the final stress period, NAD(H) and NADP(H) in YA came to recover, whereas NADP(H) pool in YL decreased. The reason may be attributed to further reduction or supplement inhibition, which related to the extent of oxidation stress.(5) We obtained the gene fragments and reference gene of the key enzymes involved in the metabolism of ASC and GSH through using degenerate primer designed according to the conserved sequence of the closely related species. The Gen Bank accession numbers of above gene fragments are as follows: GPX(KP663413), GST(KP663414), ?-GCS(KP663415), MDHAR(KP663416), GAPDH(KP663417), GR(KP663418), APX(KP663419), Gal LDH(KP663420), DHAR(KP663421).(6) Drought induced up regulation of APX gene expression earlier in YA than in YL, but transcription response of MDHAR and DHAR were earlier in YL than in YA. GR, GST, GPX, Gal LDH and ?-GCS were induced by drought stress. At the 4th day of stress, these gene showed higher m RN A levels in YL than that in YA, except for APX. The changes of majority enzymes’ activities were not consistent with the changes of m RNA levels. In addition, YA exhibited higher activities of MDHAR(Monodehydroascorbate reductase), DHAR(Dehydroascorbate reductase) and GR than the YL under drought condition, which indicated that ASC-GSH cycle was more efficiency in YA to maintain cell redox state.(7) Under drought stress, the young leaves and roots had larger ROS response than the mature leaves, and ROS response of roots was earlier than that of leaves. Roots had higher activities of SOD, APX, DHAR, MDHAR and GR than the leaves under drought stress. It was closely related to the lower H2O2 concentration in root. The mature leaves showed high CAT and POD activities. After 48 hours drought stress, the activities of SOD, CAT, DHAR, MDHAR and GR were motivated among all organs. The responses of NAD(P) pool to drought stress were earlier than the ASC pool, and they were both earlier than the GSH pool among all organs. The NAD(P) pool in roots had bigger changes than that in leaves, but ASC pool in leaves had bigger changes than that in root. The GSH pool responed significantly after 24 h of drought stress. In summary, there were different response of ROS, antioxidative enzyme activities and redox components to drought stress between leaves and root, and between mature tissue and young tissue.(8) The redox status of tissue culture environment was changed through adding the reduced substances(ASC, GSH, DTT), o xidized substances(DHA, GSSG) and GSH synthesis inhibitors(BSO, L-Buthionine-sulfoximine) into sour jujube suspension cell culture system, respectively. The result showed that oxidized substances addition, especially BSO, had bigger influence on cell growth and metabolites accumulation than the reduced substances addition. Total free amino acids, including proline, ?-aminobutyric acid(GABA), and polyamine were accumulated under oxidized substances treatment; Soluble sugars, sugar alcohols, catechins, limonenes and hydroxy methyl glutaric acids were also accumulated under oxidized substances treatment, but their relative contents varied widely under DHA, GSSG and BSO treatments. The results indicated that different oxidized substances addition lead to different response of metabolites.