Study On The Response And Adaptation Mechanism Of Greenhouse Cucumber (Cucumis Sativus L.) To Drought Stresses And Elevated CO₂ Concentration

Drought stress is a prevalent problem for agriculture in the world. The elevation of CO₂ concentration around the world has obtained great attention in recent years, and CO₂ can be used as fertilizer in greenhouse. Based on the two points, this study was deployed with cucumber (JinYou No.1) in two adjacent identical test greenhouses located at Northwest A & F University during 2006-2007. Under both elevated CO₂ concentration and ambient CO₂ concentration, this study focused on several aspects, such as the growth and development of cucumber seedlings, microstructure and ultrastructure of cucumber leaves, photosynthetic gas exchange, photochemistry efficiency, metabolism of nonstructural carbon hydrate and the activity of involved enzymes, reactive oxygen species (ROS) and protective system of anti-oxide and so on. The major results are as follows:1. The effects of drought stresses and elevated CO₂ concentration on the growth and development of cucumber seedlingsUnder drought stress, the plant height, leaf area, leaf area ratio and relative growth rate of cucumber seedlings were declined significantly, whereas the root/shoot ratio, growth index were raised. The growth and development of seedlings were impaired by severe drought stress, and the accumulation of dry matter was changed to a large extent. The rise of root/shoot ratio was caused by the decrease of shoot dry mass, instead of the greater accumulation of dry matter for root. Under elevated CO₂ environment, the height of seedlings, leaf area, shoot dry mass and relative growth rate (RGR) increased significantly, although the diameter of stem, root dry mass, root/shoot ratio was not changed significantly.2. The effects of drought stresses and elevated CO₂ concentration on the microstructure and ultrastructure of cucumber leavesThe thickness of cucumber leaf declined under drought stressed, but increased under elevated CO₂ concentration. The number of chloroplast decreased but the size of chloroplast increased significantly under drought stresses. The number of chloroplast increased significantly and the size had no changes under elevated CO₂. The starch grain accumulated and the size of which were increased by both drought stresses and elevated CO₂ concentration. Under drought stresses conditions, choloroplast swelled, stroma became thin, the thickness and number of grana decreased significantly, thylakoid swelled, lamella of grana and stroma became blurry, some of membrane broke and disorganized, many big plastoglobules appeared. Elevated CO₂ stimulated thylakoid development through increasing the thickness of grana and the number lamella to some extent. The density of stomata was increased, but the ratio of aperture stomata, stomata aperture were lowered by drought stresses. However, elevated CO₂ not only decreased the density of stomata, but also decreased the ratio of aperture stomata and stomata aperture. At the same time, the density of stomata and the ratio of aperture stomata of abaxial surface were higher significantly than adaxial surface.3. The effects of drought stresses and elevated CO₂ concentration on photosynthetic gas exchange of cucumber seedlingsThe photosynthetic rate, transpiration rate, stomata conductance and intercellular CO₂ concentration of cucumber seedlings decreased with treatment of drought stresses. However, the water use efficiency was raised by the cooperation between the increase of photosynthetic rate and decrease of transpiration rate. Under drought stress, apparent quanta efficiency, CO₂ assimilation maximum depressed significantly. Light compensatory points and CO₂ compensatory points were enhanced. At the same time, carboxylation efficiency, respiration rate, CO₂ assimilation maximum, light saturated electron transport rate [1], triose phosphate utilization (TPU) and mesophyll conductance (gm) decrease significantly. Contrarily, the parameters about photosynthesis went the opposite direction under elevated CO₂ environment, i.e. elevated CO₂ promoted photosynthesis of cucumber seedlings. Therefore, the factor of stomata and non-stomata resulted in the decrease of photosynthesis when cucumber seedlings subjected to drought stress. Nevertheless, under elevated CO₂ environment, photosynthesis of cucumber seedlings was promoted through the adjustment of stomata and non-stomata factor, in order to keep metabolism and energy circulation running well, and alleviate or avoid the impair of drought stress. 4. The effects of drought stresses and elevated CO₂ concentration on photochemical efficiency of cucumber seedlingsThe photosynthetic ability and electron transport rate decreased when cucumber seedlings subjected to drought stresses. However, elevated [CO₂] alleviated the negative effects of drought stresses to a certain extent mainly by increasing Fv/Fm, qP and PPFDsat, and decreasing NPQ. Furthermore, the above four parameters could be used to evaluate the interactive effects of drought stresses and elevated [CO₂] on photochemistry efficiency of cucumber seedlings.5. The effects of drought stresses and elevated CO₂ concentration on metabolism of nonstructural carbonhydrate and involved activity of enzymes of cucumber seedlingsUnder elevated CO₂ environment, nonstructural carbonhydrate of cucumber leaves increased, including glucose, fructose, sucrose, stachyose. Hence, the osmotic potential decreased subsequently, avoiding the severe dehydration of cells, maintaining turgor pressure of cells, and the ability of seedlings resisting drought stress was improved. During drought stress of 7 days, sucrose synthase (SS), soluble acid invertase and alkaline invertase of leaves climbed up firstly, and then declined; soluble acid invertase and alkaline invertase of root system increased gradually, whereas sucrose phosphate synthase increased firstly and then decreased. The treatment of elevated CO₂ increased activity of sucrose synthase, but decrease activity sucrose phosphate synthase. These two enzymes cooperated with invertase in order to accelerate the decomposition of sucrose and impair the synthesis of sucrose, so that the hexose accumulated which lowered the osmotic potential of cells, maintained the ability of water uptake for cells, and then cucumber seedlings could keep growing and developing when subjected to drought stresses.6. The effects of drought stresses and elevated CO₂ concentration on osmotic adjustment, metabolism of reactive oxygen species and their scavenge system of cucumber seedlingsWith the aggravation of drought stresses and prolong of stress time, plentiful proline and soluble protein accumulated. During the treatment of drought stresses, the content of these two matters increased sharply in the first several days, and then the accumulation rate of osmolates lowered. After several days'treatment, soluble sugar content decreased although the cucumber seedlings still subjected to drought stresses. It indicates the finitude and provisionality of osmotic adjustment. At the same time, the permeability of cell membrane increased, the content of MDA climbed up, reactive oxygen accumulated with the aggravation of drought stresses and prolong of stress time. It means that cell membrane was damaged to same extent because of the accumulation of reactive oxygen species, which was beyond of the ability of protective antioxide system. Therefore, the physiological function of cucumber seedlings was inhibited. However, elevated CO₂ could alleviate the accumulation of reactive oxygen species and MDA to some extent when cucumber seedlings subjected to drought stresses, and the damage for cell membrane was prevented partially.The protective enzyme (SOD, POD, CAT, APX and GR) activity increased, and AsA and GSH content climbed up with the aggravation of drought stresses and prolong of stress time. It indicated that the accumulation of reactive oxygen species (ROS) could induce the elevation of activity of protective enzyme, in order to eliminate excess ROS effectively, and further to protect seedlings from attack of excess ROS. Elevated CO₂ could enhance the ability of enzyme and non-enzyme system to eliminate ROS, which alleviate or avoid the negative effects of ROS on cucumber seedlings to some extent.