Study On Variation Regularity Of Magnesium Content And Its Functioning Mechanism In Cucumber (Cucumis Sativus L.) Under Greenhouse

Magnesium (Mg) which is un-substitutable by other cation plays an important role in the physiological process especially in photosynthesis of plants. In case of Mg deficiency, the whole physiological metabolic activity will be largely affected and the growth of plants will be influenced. Cucumber(Cucumis sativus L.) is one of the most important vegetables cultivated in greenhouses. In recent years, cucumber in greenhouse, especially for that with a long growth season, often appears the Mg deficiency symptom. To solve the problem of Mg deficiency in cucumber in greenhouse, and to investigate the impact of Mg stress to the growth of cucumber, the factors which influence the Mg content of cucumber in greenhouse, the regularity of Mg absorption and utilization and the distribution of Mg in various organs of cucumber, and impact of temperature and Mg stress to the breeding of cucumber have been researched, by using perlite cultivation in greenhouse and artificial climatic cabinate. The results are as follow:1.The influence factors of Mg content in the blossom and fruiting period of cucumber in greenhouseBy means of universal rotational combinational design with three-factors and five-levels, the impact of ground temperature, Mg²⁺ concentration in nutrient solution and K+ concentration ratio to Mg content in the blossom and fruiting period of cucumber in greenhouse was studied, and the corresponding mathematic regression models were set up. The models for prophase, metaphase and anaphase were as follow respectively: Y=8.291491+0.25382X₁+0.57983X²+0.14774X₃+0.106438X₁X²+0.052937X₁X₃ -0.45944X²X₃-0.18869X₁²-0.1405X²2-0.24527X₃² Y=8.634707+0.60173X₁+0.94884X²+0.20542X₃+0.231813X₁X²+0.136313X₁X₃ˉ0.66181X₂X₃ˉ0.16578X₁²Ë‰0.1534X²2ˉ0.4772 X₃² Y=8.260308+0.38767X₁+0.92443X²+0.24856X₃+0.208625X₁X²+0.221875X₁X₃ˉ0.57788X²X₃ˉ0.32114X₁²Ë‰0.14618X²2ˉ0.4X₃²Through the analysis of the models, it was shown that: (1) In the prophase, metaphase, anaphase of blossom and fruiting period of cucumber, the sequence of different factors influencing Mg content in leaves of cucumber was: Mg²⁺ concentration > ground temperature > K+ concentration. (2) There were interactions among different factors, ground temperature was positively correlated with Mg²⁺ concentration and K+ concentration, while Mg²⁺ concentration was negatively correlated with K+ concentration. (3) On the basis of equations optimization in computer, in the whole blossom and fruiting period of cucumber, the value of the three factors was: X₁=1, X₂=1.682, X₃=-1. That is, when the ground temperature, Mg²⁺ concentration and K+ concentration in nutrient solution was 29.13℃, 4mmol/L and 4.43 mol/L, respectively, the Mg content in leaves was the highest: 8.9638 mg/g DW in prophase, 10.0256 mg/g DW in metaphase and 8.5703 mg/g DW in anaphase. (4) The models were verified to be accurate and reliable by the subsequent experiment.2. The impacts of Mg on the absorption and distribution of Mg, K and Ca, as well as the yield and quality of cucumberUsing soilless culture method (with expansile perlite as cultivation groundsubstance), the influence of Mg²⁺ concentration in the rhizosphere to the absorption and distribution of Mg, K and Ca during the blossom and fruiting period as well as the yield and quality of greenhouse cucumber was researched. The results showed that: (1) Under identical Mg²⁺ concentration, the diversity of Mg content was slight and the content of K and Ca was gradually degraded in the stems and leaves during the blossom and fruiting period of cucumber. The tolerance of cucumber to Mg stress during the blossom and fruiting period was ranked as: prophase > anaphase > metaphase. (2) The Mg²⁺ concentration of cucumber in the rhizosphere affected not only the content of Mg, K and Ca in plants, but also the distribution in various organs. The Mg content in plants increased with Mg²⁺ concentration in rhizosphere, but the content of K and Ca in plants varied in different phases of blossom and fruiting period; The content of Mg and Ca in various organs was ranked as: leaf > root > stem > melon; The K content of various organs was different with the diversity of Mg²⁺ concentration in rhizosphere, i.e., under suitable Mg²⁺ concentration, the K content of various organs was ranked as: stem > leaf > melon > root, but under the Mg stress, it was ranked as: stem > melon > leaf > root. (3) The Mg²⁺ concentration in rhizosphere affected the growth, yield and quality of cucumber. Mg stress reduced the yield per plant, the yield per unit area and melons per plant; Excessive Mg degraded the nitrate content of cucumber, increased the content of soluble protein, reducing sugar and soluble sugar, enlarged the diameter of stem and leaf area; Mg deficiency stress significantly degraded the content of vitamin C, increased the content of soluble protein, reducing sugar, soluble sugar and nitrate, and inhibited the height of plants, total leaf number and leaf area. Application of Mg fertilizer was beneficial to improve the quality of melon. 3. The effects of Mg stress on growth of cucumber seedlings under low temperatureUsing climatic cabinate,Li-6400 photosynthesis instrument and soilless culture (with expansile perlite as cultivation groundsubstance), The effects of Mg on the growth and photosynthesis characteristics, scavenging system of active oxygen and metabolism of C/N of cucumber seedlings under low temperature have been researched. The results showed that: (1) Under low temperature, Mg stress obviously inhibited the growth of cucumber; while under suitable temperature, it decreased the diameter of cucumber seedlings, total leaves, leaf area, dry weight of above-ground parts, and increased the height of plant and root/shoot ratio, and excessive Mg was beneficial to the growth of under-ground parts. (2) No matter the temperature was high or low, Mg stress increased chlorophyll A/B. The increase extent of ChlA/ChlB was more under Mg deficiency than under Mg overplus. The contents of ChlA, ChB and carotenoids(Cxc) were decreased under Mg deficiency, while they were increased under Mg overplus and suitable temperature, but decreased under Mg overplus and low temperature; The Mg content in cucumber leaves was positively correlated to both Mg2+ concentration in rhizosphere and temperature. Pn, Gs and Tr were decreased under Mg stress, and they were the lowest under Mg overplus and low temperature; The effect of Ci, WUE and Ls were different on the basis of Mg2+ concentration and temperature; The main limiting factor of photosynthesis of cucumber seedlings was non-stomata factor under Mg deficiency, but was stomata factor under Mg overplus. (3) Mg stress caused the increase of MDA content, O2- producing rate and H2O2 content in leaves of cucumber seedlings, and the extent was increased more under Mg deficiency than under Mg overplus, and the membrane lipid peroxidation were enhanced under low temperature. Under low temperature, POD activity were decreased, SOD activity were increased, but CAT activity was not significantly affected; under suitable temperature, Mg stress made POD activity upgraded, CAT activity descended, but SOD activity varied insignificantly. The plants were hurt greatly under low temperature and Mg deficiency, which might be related to the rise of AOS. (4) Under suitable temperature, Mg stress caused the soluble protein of cucumber leaves descented; under low temperature, the NR activity was degraded, Mg overplus made the content of soluble protein decreased, but Mg deficiency made it increased, which might be related to the transform of nitrogen metabolite. (5) Under suitable temperature, the content of reducing sugar and total soluble sugars in leaves of cucumber seedlings were decreased by Mg stress, and the decrease extent was more under Mg overplus than under Mg deficiency. Mg stress enhanced the vigor of root system, which indicated the adaptability of plants to the adversity. Under low temperature, the vigor of root system of cucumber dropped; but the content of reducing sugar increased, the rise extent: excessive Mg > deficient Mg > suitable Mg; while the content of total soluble sugars decreased, the decrease extent: suitable Mg > deficient Mg > excessive Mg. It showed that the adaptability of plant root system to the adversity was selective and limited.4. The effects of ground temperature and Mg stress on growth of cucumber in blossom and fruiting periodThe effect of Mg stress and ground temperature on the growth and photosynthetic characteristics of cucumber in blossom and fruiting period were studied in greenhouse. The results showed that: (1) The growth of cucumber plants was inhibited, the yield of melon was reduced and the contents of ChlA, ChlB and Cxc in leaves of cucumber were decreased under Mg stress and low ground temperature, and the effect of Mg2+ concentration was greater than that of ground temperature; (2) Net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr) of cucumber leaves varied daily in single-apex curve under Mg deficiency, and they were decreased with the descent of Mg supply and ground temperature, while Pn and Tr varied in double-apex curve under Mg overplus; Light saturation point, CO₂ saturation point, apparent quantum efficiency and carboxylation efficiency of leaves under excessive Mg and suitable Mg were greater than under Mg deficiency. The daily variation of intercellular CO₂ concentration (Ci) and the response of Pn to light intensity and CO₂ concentration were affected very little by Mg deficiency, indicating that the main limiting factor of photosynthesis was non-stomata one under Mg deficiency, while both stomata and non-stomata factors existed simultaneity under Mg overplus; (3) The variation of Pn of cucumber leaves under high ground temperature showed the trend that the old leaves kept high photosynthetic rate all the time under suitable Mg, rose slightly under Mg overplus, dropped all along under Mg deficiency; (4) MDA in blossom and fruiting period rose at first, and dropped after a period of time. The active oxygen was mainly eliminated by SOD under low ground temperature, and mainly by CAT under high and suitable ground temperature, keeping balance between the production and elimination of active oxygen; (5) Ground temperature and Mg nutrition affected the contents of Mg, K and Ca in the leaves of cucumber as well as the rhizosphere environment. Mg2+ concentration in the rhizosphere was positively correlated with the contents of Mg and Ca in the leaves and EC value of rhizosphere groundsubstance, but was negatively correlated with the content of K in the leaves and pH value of rhizosphere.