Effects Of NO₃～- Stress On Photosynthesis And Light Allocation In Cucumber (Cucumis Sativus L.) Seedling

A lot of progresses have been made on the photosynthetic functions of plants response to salt stress, but most of them were studies on NaCl stress. The main anion of the soil salinization of the greenhouse was NO₃^-, little work have been done on the photosynthesis and light energy allocation in cucumber caused by NO₃^- accumulation in greenhouse soil. Based on the previous achievements, cucumber plants were grown in nutrient solution with different NO₃^- concentration for 7 d at hydroponics system, so that to study the changes of photosynthetic rate and the related parameters, such as the photochemical efficiency of PSâ…¡and light energy allocation as well as the thermal dissipation and NPQ triggering depending on xanthophyll cycle. The results of present will provide theoretical basis for the improvement of rhizosphere condition lightening the alkali-saline injury. The main results obtained were as follows:1. When NO₃^- in the nutrient solution was supplied by Ca(NO3)2 and KNO3(1∶1), and the available NO₃^- was at lower level (14 to 98 mmol·L-1 NO₃^-) in the medium, photosynthetic rate (Pn), corboxylation efficiency (CE), light saturation point (LSP) and absolute increment of leaves area were increased by proper supplement of NO₃^-. Although these parameters were all higher than that of the control, but there were no significant difference when compared to the control. The apparent quantum yield (AQY) and CO2 compensation point (CCP) were lower than those of control plants, but the differences were not significant. So supplement of NO₃^- at appropriate range improved the photosynthesis, the accumulation of photosynthates and the growth of plants.2. When the available NO₃^- was at higher level (140 to 182 mmol·L-1 NO₃^-) in the medium, significant differences in photosynthetic rate (Pn) and related parameters were found compared to the control:(1) The photosynthetic rate (Pn) significantly decreased. Factors caused reduction of Pn transferred from stomatal limitation to non stomatal limitation with the further increase of NO₃^- concentration.(2) With further increase of NO₃^- concentration, AQY and CE and LSP all decreased; meanwhile, light compensation point (LCP) and CCP all increased. The capabilities of cucumber seedling to use weak light and high intensive light were lower than the control. The activity of Rubisco and the capability of using lower concentration of CO2 were lower as well.(3) When NO₃^- concentration was increased to as high as 182 mmol·L-1, the ultrastructure of chloroplasts were damaged; number of grana in cells and number of amella in granas decreased; the number of starch grains in cells and starch grain size decreased as well.(4) Leaf area and plant height were significantly lower than the control plants as the NO₃^- concentration was further increased. When NO₃^- concentration was increased to 182 mmol·L-1, leaf area was reduced, dry weight was significantly lower than that of control plants.3. When cucumber seedlings were grown in the nutrient solution with NO₃^- concentrations of 140 and 182 mmol·L-1 for 7 days, the spreading obstruction of mesophyll cell were increased, the maximal PSâ…¡efficiency (Fv/Fm) at open centers in the absence of NPQ, the antenna efficiency at open centers in the presence of NPQ (Fv'/Fm'), the actual PSâ…¡efficiency (ΦPSâ…¡) and the photochemical quenching (qP) were all decreased. Electron transferring was restrained and the devivation from full balance between PSâ… and PSâ…¡(β/α-1) were improved significantly. The photochemical efficiency of PSâ…¡and photosynthesis decreased, the growth of cucumber seedling was restrained by higher concentration of NO₃^-. At the same time, The absorbed light energy allocated to the photochemical reaction of PSâ…¡(P) was reduced by high light intensity and high NO₃^- concentration, the proportion allocated to the antenna heat dissipation (D) increased significantly, and the photochemical efficiency of leaves decreased.4. When the cucumber seedling was stressed by both higher NO₃^- concentration (140 to 182 mmol·L-1 NO₃^-) and high light intensity spontaneously, the nonphotochemical quenching (NPQ) was increased. However, when 1, 4-dithiothreitol (DTT) was applied, NPQ decreased. The changing trend of NPQ consisted with that of (A+Z)/(V+A+Z)% in cucumber seedling leaves. It could be inferred that thermal dissipation was mainly depending on xanthophyll cycle and could be important photoprotection mechanisms to alleviate strong light damage.5. When cucumber seedling was stressed by both higher NO₃^- concentration and high light intensity spontaneously, excessive energy was more easily produced; the degree of NPQ triggering depending on xanthophyll cycle in cucumber seedling leaves were bigger than that in control, so that the excessive energy was dissipated effectively. The triggering process of thermal dissipation depending on xanthophyll cycle was slowed in cucumber seedling leaves under 182 mmol·L-1. NO₃^- stress.6. There were three kinds of nitrate to supply 140 mmol·L-1 NO₃^- separately, they were KNO3, Ca(NO3)2, mixture of KNO3 and Ca(NO3)2 (1∶1). The cucumber seedling appeared NO₃^- stress symptom(1) Under NO₃^- stress came from three different sources, Pn, AQY and CE were all lower than that of the control; growth of cucumber seedling was decreased.(2) The photochemical efficiency of PSâ…¡and photosynthesis decreased in cucumber seedling under three different NO₃^- stresses. The absorbed light energy allocated to thermal dissipation increased. The degree of NPQ triggering depending on xanthophyll cycle in cucumber seedling leaves was bigger than that of the control.(3) The photosynthesis and chlorophyll fluorescence parameters had no significant difference under Ca(NO3)2 and mixture stress. But when the cucumber seedling was under KNO3 stress, the fluctuant range of these photosynthesis parameters, such as Pn and AQY and CE, was increased , amella in grana was damaged, Fv'/Fm'andΦPSâ…¡decreased. The triggering process of NPQ depending on xanthophyll cycle was slowed.