Effect Of Water-nitrogen Coupling On Growth And Physiological Characteristics Of Cucumber Under CO₂ Enrichment

In order to investigate the mechanism of water-nitrogen coupling on greenhouse cucumber and to play a maximal yield-increasing effects of water and nutrients by “water by fertilizer” and “water promotes fertilizer”. ‘JinYou 35’ cucumber was used as test material to study the effect of water-nitrogen coupling on growth and physiological characteristics in cucumber of CO2 enrichment through the hydroponic and pot culture conditions respectively. Hydroponic taked split plot and then-split plot design and the main plot setted up two CO2 concentrations: ambient CO2 concentration(400ppm, A) and doubled CO2 concentration(800±20ppm, E). Each CO2 concentration contained two water treatments(control, C and drought stress, D) and two nitrogen treatments(control, C and low nitrogen, N). The common nutrient solution was as the control(C). Drought stress(C) was simulated by PEG 6000. CO2 concentration of pot culture treatments was the same as that of the hydroponic experiment. The split plot setted up two treatments: no drought stress(95% of field capacity, represented by W) and drought stress(75% of field capacity, represented by D).The then-split plot contained low nitrogen treatment(450 kg·hm-2, N1) and high nitrogen treatment(900 kg·hm-2, N2). The results of the present research were as follows:1.CO2 enrichment and high nitrogen treatment promoted the plant height, stem diameter and growth of cucumber leaves area as well as stimulated the compensation growth of cucumber roots cooperating with the drought stress which was conducive to cucumber root elongation growth.2. Doubled CO2 increased chlorophyll a, chlorophyll b and chlorophyll a/b while reduced the carotenoid content. Chlorophyll a, chlorophyll b, and carotenoid contents increased with the increase of nitrogen amount, and the contents of the chlorophyll under drought treatment were higher than those of the normal irrigation treatment. Under high CO2 concentration, ΦPSⅡ, Fv ’/ Fm’, ETR were the highest, and NPQ was the lowest in EWN2 treatment. The results indicated high nitrogen improved the energy conversion efficiency and PSⅡ of electron transport activity, while inhibited the non-photochemical energy dissipation which was beneficial to photosynthetic electron transport to ensure the normal conduct photosynthesis under high CO2 concentration condition.3. Under the drought condition, MDA content increased while the increase extent of MDA content was lower at high CO2 concentration. At ambient CO2 concentration and low nitrogen content condition(450 kg·hm-2), the activities of CAT, POD, SOD, APX and GR were higher, but the results were opposite at high CO2 concentration and high nitrogen content condition(900kg·hm-2) which enhanced the adaptability of cucumber to drought stress.4. Doubled CO2 improved the activities of SS and SPS and the content of sucrose, reducing sugar and starch; The high amount of nitrogen increased the activities of SS and SPS, and the sucrose content, but reduced the starch content; The plants had a higher activity of SS and the content of sucrose and starch were increased while a lower activity of SPS under the drought stress.5. Doubled CO2 increased the activity of GDH and NR, but reduced the GS,GOGAT activity. The high nitrogen treatment increased all four enzymes activitits. Elevated CO2 increased the soluble protein content in leaves through reducing the accumulation of non-protein nitrogen and the nitrogen content in cucumber leaves compared to those of cucumber leaves in ambition CO2 concentration condition.6. Drought and high nitrogen treatment decreased stomatal density of cucumber leaves; high nitrogen fertilizer increased pore size within the mesophyll cells and chloroplasts size, but reduced the size and number of starch granules. While CO2 doubling increased the size and number of chloroplasts, starch granule cell, grana thickness and slice layers within the cucumber mesophyll cells, at the same time, reduced the injury of chloroplast ultra structure caused by the drought and high nitrogen coupling.7. Doubled CO2 increased the cucumber quality through the increase of the soluble protein and soluble sugar content and decrease of nitrate content. Under the condition of high nitrogen, increasing the amount of irrigation can also improve part quality indexes.8. Doubled CO2 increased production of cucumber, water use efficiency and nitrogen use efficiency. Under doubled CO2, regardless of water status, the production of cucumber increased with the increase of nitrogen amount; while at the CO2 concentration in the atmosphere, under normal irrigation, the production increased with the increase of nitrogen amount, while under drought stress, the yield of high nitrogen treatment was lower than that of low nitrogen treatment. In the doubled CO2 treatment, the yield of EWN1、EWN2 than AWN1、AWN2 at ambition CO2 concentration increased by5%,17.5% and water use efficiency increased by 5.1%, 17.7% respectively. Therefore, under CO2 enrichment should increase the amount of nitrogen to meet the nutritional needs of cucumber.