| With the global warming,climate change will become an important limiting factor for agricultural development in the future.In recent years,extreme weather has occurred frequently,especially in the south of China.The continuous high temperature in summer is very easy to cause the yellowing and wilting of Cucumber in fruit stage,resulting in the reduction of yield.This study was conducted in the Venlo-type greenhouse at the Agrometeorological Experimental station of Nanjing University of information Science and Technology from May 2020 to August 2021.The“Jinyou 101”cucumber was used as the tested material and grown in the intelligent artificial climate box.The nitrogen（N）application levels were set as:0 kg·hm-2（N0）,160 kg·hm-2（N1）,240 kg·hm-2（N2）and 320 kg·hm-2（N3）;the day/night air temperature levels were set as:28℃/18℃（control;CK）,35℃/25℃,38℃/28℃,41℃/31℃in the fruit growth stage.There were 16 treatments in total and each treatment was replicated thrice.During the experiment,under different treatments of combined high temperature and nitrogen application levels,the dynamic changes in plant height,stem diameter,biomass,plant nitrogen concentration,and yield of cucumber were observed;the changes in chlorophyll content,antioxidant characteristics,photosynthetic characteristics,fluorescence characteristics,and nitrogen metabolic enzyme activity of cucumber leaves were measured.Moreover,the effects of nitrogen application combined with high temperature stress on chlorophyll fluorescence characteristics of leaves and nitrogen absorption and metabolism of cucumber plants in the fruit stage were discussed.In summary,the physiological mechanism underpinning the mitigative effects of nitrogen application under high temperature stress was clarified,and the nitrogen critical concentration dilution model of cucumber to copy with high temperature stress was constructed.The main results are as follows:（1）High temperature stress in the fruit growth stage cause significant damage to the photosystem reaction center in cucumber leaves.N2 and N3 treatment could significantly increase the chlorophyll content of cucumber leaves,and enhance the activity of PSII donor side oxygen releasing complex and the energy connectivity between thylakoids.Furthermore,the inhibition of PSI receptor side terminal electron receptor library was alleviated.Therefore,the appropriate nitrogen application can effectively alleviate the damage to the photosynthetic organs of cucumber under high temperature stress.（2）High temperature stress of 35℃,38℃,and 41℃increased the content of malondialdehyde（MDA）in cucumber leaves.N1 treatment could significantly increase the activities of catalase peroxidase and soluble protein content in cucumber leaves under air temperature of 35℃,while N2 treatment could improve the activities of catalase,peroxidase,and soluble protein content in cucumber leaves under air temperature of 38℃and 41℃.Moreover,optimal nitrogen application could timely remove MDA and other harmful substances,thus enhancing the stress resistance of cucumber plants,and alleviating the damage caused by membrane lipid peroxidation.（3）Nitrogen application could promote the growth of cucumber and improve the shoot dry matter accumulation under high temperature stress.However,only N2 and N3 treatments were beneficial to the recovery of cucumber growth after high temperature stress.With the increase of temperature,the nitrogen application rate of cucumber increased at first and then decreased.The highest yield under stress of 35℃,38℃and 41℃could be obtained when the nitrogen application rates were 236 kg·hm-2,283 kg·hm-2,and 177 kg·hm-2,respectively.The aboveground dry matter mass,stem nitrogen transport,leaf transpiration rate,and nitrate reductase activity of cucumber had a great positive effect on cucumber yield,whereas leaf nitrogen distribution and stem dry matter distribution negativelu affected cucumber yield.（4）High temperature stress significantly reduced the shoot nitrogen accumulation,fruit nitrogen distribution rate,and nitrogen transport from stems and leaves to fruits,resulting in the"source-sink imbalance"of cucumber.N2 and N3 treatments increased the nitrogen transport from leaves to fruits by increasing the activity of nitrogen metabolism enzymes in cucumber leaves,thereby improving the nitrogen distribution rate in cucumber fruits.The amount of dry matter transported from leaves and stems to fruits of cucumber increased at first and then decreased with the increasing of nitrogen application.The dry matter transport capacity of cucumber vegetative organs treated reached the highest value under N2.The stem and leaf dry matter transport of cucumber was most easily to be affected by plant height,leaf number,dry matter accumulation,dry matter distribution,leaf transpiration rate,net photosynthetic rate,and stomatal conductance.Among them,the dry matter distribution rate of stem and leaf had a negative impact on the dry matter transport,and other indicators showed opposite effects.（5）With the increase intensity of high temperature stress,the nitrogen concentration per unit dry matter mass of cucumber plant（value of“a”）increased,and the decreasing speed of critical nitrogen concentration（value of“b”）decreased with the increase of aboveground dry matter mass.The variation range of model parameter“a”is 3.27～3.71,and the variation range of parameter“b”is 0.09～0.16.Under high temperature stress,the nitrogen nutrition index（NNI）of cucumber without nitrogen application was about 0.5,while it was increased to ca.0.8 and 1.2 after nitrogen application.Based on the demand for nitrogenfertilizer in cucumber plants and the optimal economic yield,the nitrogen application thresholds of cucumber were 240～248 kg·hm-2,240～297 kg·hm-2and 160～186 kg·hm-2under air temperature of 35℃,38℃,and 41℃,respectively. |
