Regulatory Effect Of CO₂ Enrichment Coupled With Water And Nitrogen On Melon Growth

The increase of CO₂ concentration in the atmospheric environment and the unreasonable utilization of water and nitrogen have brought both opportunities and severe challenges to the sustainable production of greenhouse vegetables.In order to cope with the impact of the increase of CO₂ concentration coupled with water and nitrogen and realize the precise management of water,CO₂ and fertilizer under CO₂ fertilization,this study used oriental melon（Cucumis melo L.cv.Qianyu No.6）under facility agriculture planting as the test material to explore the coupling effect of three factors and their interaction of CO₂concentration,irrigation and nitrogen fertilizer on the comprehensive growth of melon.The experiment was carried out in three crops:spring 2020（April-July,2020S）,fall 2020（August-October,2020F）,and spring 2021（April-July,2021S）.A split-plot test design was adopted,with CO₂ concentration as the main plot,irrigation amount as the split plot,and nitrogen fertilizer amount as the secondary split plot,and two CO₂ concentration levels（C₁:400 ppm and C₂:800 ppm）,two nitrogen fertilizer levels(N₁:184.8（spring）,147.2（fall）kg ha^-1;N₂:369.5（spring）,294.4（fall）kg ha^-1)and three irrigation levels（I₁:80%ET,I₂:100%ET and I₃:120%ET）,a total of 12 treatments.The main stem length and stem diameter were measured in three consecutive periods:stretching period,blooming and fruiting period,with maturity period;leaf number,single leaf area and whole plant leaf area were measured in stretching period,blooming and fruiting period,expanding period,with maturity period;and the root,stem,leaf,fruit and whole plant dry matter were obtained,a total of 10 apparent growth indicators.The net photosynthetic rate,transpiration rate,stomatal conductance,instantaneous water use efficiency,carboxylation efficiency,stomatal limitation of melon at stretching period,blooming and fruiting period,expanding period,with maturity period;chlorophyll a,chlorophyll b and total chlorophyll at the stretching period,blooming and fruiting period,expanding period;as a total of 9 gas exchange indicators.There were 2 yield indicators of yield per hectare and the ratio of marketable fruit;8 quality indicators of total soluble solids,total soluble sugar,soluble protein,free amino acid,vitamin C and nitrate content;water use efficiency and nitrogen use efficiency were 2 water and nitrogen use indicators.The regulation effect of CO2 concentration coupled with water and nitrogen on each indicator was analyzed,and important contribution indicators to the yield of melon was obtained,and the Fuzzy^-Borda model integrating Fuzzy^-AHP,GRA and TOPSIS method was introduced to comprehensively evaluate 15 melon growth indicators,and the optimal water and nitrogen management strategy under CO₂ enrichment was determined.The specific results were as follows:（1）CO₂ concentration and irrigation had significant effects on main stem length,main stem diameter and leaf number.In 2020F,CO₂ enrichment coupling with N₁ interactive significantly increased main stem diameter and leaf number at 10.3%,18.5%,respectively.800 ppm CO₂ coupling with I₁ significantly improved main stem lengthen at two spring growing seasons of expanding period by 20.8%,13.5%in 2020S,2021S,respectively.With the increase of irrigation amount,the leaf area was increased.In 2020S,increased CO₂concentration coupling with I₁ promoted whole plant leaf area at the blooming and fruiting period,expanding period and maturity period by 33.6%,34.4%and 33.5%,respectively.Increasing CO₂ concentration significantly promoted the accumulation of dry matter,which increased the whole plant dry matter by 11.7%,7.36%and 19.6%in three growing seasons,respectively.Irrigation amount also had a significant effect on the dry matter accumulation in different organs and whole plant,and the best performance of dry matter accumulation at the I₃ level.（2）CO₂ concentration significantly affected net photosynthetic rate and instantaneous water use efficiency,especially the instantaneous water use efficiency of three growing seasons was increased by 52.0%,41.1%and 28.0%,respectively,while transpiration rate,stomatal conductance and stomatal limitation were significantly reduced under CO₂enrichment.Decreasing irrigation amount could increase stomatal limitation while inhibited carboxylation efficiency.Increasing CO₂ application concentration could improve carboxylation efficiency at I₁ level.Irrigation amount significantly affected chlorophyll component content,which increasing with the increase of irrigation amount.The increase of nitrogen fertilizer inhibited the stomatal conductance,but CO₂ enrichment had a mitigating effect on the stomatal conductance of N₂ at the spring growing seasons.（3）Yield per hectare was significantly improved by increasing CO₂ concentration and irrigation amount,and CO₂ enrichment improved yield per hectare of I₁.Nitrogen fertilizer amount significantly affected the ratio of marketable fruit,which showed N₂ improve the ratio of marketable fruit under spring growing seasons,while was better in N₁ under autumn growing season.Increasing irrigation amount significantly affected total soluble solids and improved Vitamin C.The increased application concentration of CO₂ significantly improved soluble protein,free amino acid and vitamin C,especially increased soluble protein by 1.8%,14.7%and 6.8%under three growing seasons,respectively.Total soluble sugar at N₁ lever was better.Increasing application concentration of CO₂ significantly increased free amino acids of N₁ by 5.5%,8.3%and 17.0%under three growing seasons,respectively.In 2020F and 2021S,CO₂ enrichment significantly decreased nitrate content of I₁,which was achieved in 30.9%,14.9%,respectively.CO₂ concentration significantly affected water use efficiency and nitrogen use efficiency.When N₂ was applied,water use efficiency increased.CO₂×I had a significant interactive effect on water use efficiency,especially in 2020S and 2021S,CO₂enrichment improved nitrogen use efficiency of I₂ by 40.5%and 43.9%,respectively.（4）Total 15 indicators were significantly correlated with yield per hectare,and the biggest correlation coefficients with yield per hectare was achieved in whole plant dry matter,net photosynthetic rate and leaf dry matter in three growing seasons,which were 0.778,0.905and 0.902,respectively.Based on path analysis,the direct determinants of yield at three growing seasons were obtained,including whole plant dry matter in 2020S,fruit dry matter and net photosynthetic rate in 2020F,and total chlorophyll and leaf dry matter in 2021S.（5）Fuzzy^-Borda model evaluate system integrating Fuzzy^-AHP,GRA and TOPSIS method was established by selecting 15 indicators,including yield per hectare,the ratio of marketable fruit,total soluble solids,total soluble sugars,soluble protein,free amino acids,vitamin C,nitrate content,leaf dry matter,fruit dry matter,whole plant dry matter,net photosynthetic rate and total chlorophyll,and finally obtained that the best treatment under the Fuzzy^-Borda method was T10（C₂ I₂ N₂）,at 400 ppm CO₂ concentration,the optimal treatment was T5（C₁ I₃ N₁）in spring growing seasons and T6（C₁ I₃ N₂）in autumn growing season.Therefore,under increasing CO₂ concentration in the future,irrigation was based on 100%ET and appropriately incresea nitrogen fertilizer in melon cultivation can increase resource utilization efficiency and improve growth performance,stabilize yield and improve quality.