Study On Inhibiting Effect Of Chlorine To Watermelon Growth And Its Regulation By Nitrogen

The objective of this study is to clarify the inhibition mechanism of chlorine on watermelon growth and the regulating mechanism of nitrogen under chlorine stress,and then to provide theoretical basis for the safe utilization of brackish water.In this study,Jincheng No.5 variety of watermelon was selected as the test crop,and the effects of different kinds of chlorine salts(NaCl,NH4Cl,CaCl2,KCl)and their concentrations(0,80,160 mmol·L-1)on the growth and physiology of watermelon were investigated by hydroponic experiments in order to determine the chlorine toxic concentration of watermelon.Then,different nitrogen concentrations(0,10,15,20,25 mmol·L-1,recorded as N0,N10,N15,N20,N25)were set on the basis of this toxic concentration to explore the regulation of nitrogen on physiological resistance and tolerance of watermelon under chlorine stress.Moreover,soil culture experiments were conducted to further explore the effects of different nitrogen application rates(0,0.1,0.15,0.2,0.25 g·kg-1)on soil enzyme activities and physiological resistance of watermelon under chlorine stress,and to further verify the regulation mechanism of N in hydroculture and recommend the appropriate nitrogen application rate for regulating chlorine stress.The main research results are as follows:(1)Compared with the control,the biomass and dry matter of watermelon treated with medium and high concentration of chlorine(80 mmol·L-1 and 160 mmol·L-1)decreased by 31.50%,46.58%,and 22.09%,53.42%,and chlorophyll content had no significant change.Different concentrations of exogenous chlorine could promote the absorption of N and Cl-,inhibit the absorption of P and K,and promote the transport of Cl-,P and K to the shoot.The nutrient absorption and transport showed incoherence.The content of chloride ions increased significantly by 3.63 and 4.70 times,and the total nitrogen content increased first and then decreased with the increase of chlorine concentration.Medium chlorine treatment reduced total potassium content in roots by 41.09%and increased total potassium content in stems by 27.26%.The proportion of nutrient elements was disordered,nutrient absorption and utilization were blocked,and the growth of watermelon plants was inhibited.Medium concentration of CaCl2 and medium concentration of KCl treatment had no significant effect on watermelon dry matter.Compared with other chlorine sources,CaCl2 treatment could reduce Cluptake by roots by 4.54%～28.81%,promote the transport amount of K+to shoot by 2.07～3.28 times,and the total phosphorus of high concentration plants was 13.75%～38.69%higher than other chlorine sources.KCl can promote root potassium ion absorption by 27.98%～189.51%,and high concentration KCl treatment has the lowest Cl-transport ratio,the lowest chloride ion content and the highest total potassium content.Chlorophyll content of NH4Cl treatment is significantly different from CaCl2 and KCl treatment,but 94.84%and 87.84%higher than NaCl treatment.Chloride ion absorption transport and nitrogen and phosphorus transport increased compared with NaCl,CaCl2 and KCl,and total nitrogen content was the highest under NH4Cl treatment.It concludes that exogenous chlorine increases the chlorine content of plants by promoting the absorption and transport of chlorine by roots,causing ion toxicity,and inhibits the absorption of P and K elements,thus promoting the transport to the above-ground part.Nutrient absorption in plants is blocked and nutrient element imbalance inhibits the growth of watermelon.(2)The membership function analysis showed that the tolerance of watermelon under chlorine salt stress was N10>N15>N20>NO>N25 under different nitrogen concentrations.Compared with N0,N10 treatment significantly increased the activities of superoxide dismutase(SOD)and catalase(CAT)by 19.65%and 40.00%,soluble sugar and proline contents by 55.13%and 49.90%,and decreased malondialdehyde(MDA)contents in leaves and roots by 34.55%and 52.51%.Nitrogen application increased root length,number of root tips and specific root length by 117.15%～226.15%,91.05%～118.78%and 86.68%～181.34%.Compared with N0,the root surface area and root activity of N10 and N15 were significantly increased by 48.83%and 57.32%,40%and 22.22%,and the content and accumulation of P and K were increased by 58.76%and 75.66%,54.32%and 97.01%,respectively,under N10 treatment.The content and accumulation of Na+decreased by 33.90%and 22.62%,and the K/Na ratio increased by 132.75%.At the same time,Na+content and cumulant decreased by 33.90%and 22.62%,and K/Na increased by 132.75%.Nitrogen application significantly increased NO3-content and accumulation 2.50～3.10 times and 3.43～3.57 times,NO3-/Clratio increased 3～4 times,N10,N15,N20 significantly reduced chlorine content 25.34%,22.06%,21.27%,and N20 significantly reduced Cl-accumulation 29.17%.It shows that suitable nitrogen concentration can enhance the selective absorption of N,P,and K,increase the contents of N,P,and K,increase the ratio of K/Na,N/Cl,SOD,CAT,proline,and soluble sugar,and thus weaken the primary stress ion toxicity and osmotic stress,the secondary stress nutrient deficit and oxidative stress caused by chloride stress,and alleviate chloride stress.Increase the biomass of watermelon.Curve fitting showed that when N concentration was 12.35 mmol·L-1 to 15.05 mmol·L-1,the chlorine toxicity of watermelon growth was alleviated better.(3)The contents of nitrate nitrogen and ammonium nitrogen in soil increased by 35.90%～66.23%and 14.14%～40.15%under nitrogen application.Compared with no nitrogen application,urease activity increased by 46.51%in 0.15 g·kg-1 treatment and 89.58%in 0.2 g·kg-1 treatment.0.15 g·kg-1 and 0.2 g·kg-1 could reduce Na+/K+ratio by 44.44%and 55.56%,increase NO3-/Cl-ratio by 3 and 3.5 times,increase proline content by 44.18%and 75.54%,and increase SOD activity by 55.82%and 54.94%.While the MDA content decreased by 43.30%and 27.69%,the biomass increased by 96.93%and 29.03%,and the dry matter accumulation increased by 73.91%and 17.51%.Moreover,0.15 g·kg-1 nitrogen could significantly increase soluble sugar 75.54%,CAT activity 54.94%,photosynthetic rate and stomatal conductivity 70.67%and 152.17%of leaves,and root activity 29.44%.Total root length,total root surface area,number of root tips,and number of branches were 103.42%,46.41%,64.44%,87.80%.Intercellular CO2 concentration was reduced by 28.43%,root MDA content by 40.30%,total root volume by 23.05%,and total root number by 40.15%.At the same time,nitrogen application increased nitrogen accumulation by 64.07%～95.28%,nitrate reductase(NR)activity by 56.69%and 78.71%at 0.15 g·kg-1 and 0.20 g·kg-1,respectively,and nitrogen absorption efficiency by 2.39 and 2.43 times,respectively.The highest nitrogen use efficiency was 97.10%in 0.15 g·kg-1 treatment.The curve fitting results showed that when the nitrogen application rate was 0.15～0.19 g·kg-1 under chlorine stress,watermelon had ideal root configuration and higher root vitality.The physiological resistance of watermelon plants could be enhanced by improving ion homeostatin,osmotic regulatory substances,antioxidant enzyme activity,photosynthesis and reducing oxidative damage.In this way,the inhibition effect of chloride stress on watermelon growth was alleviated,and nitrogen uptake efficiency and biomass accumulation were increased.In conclusion,exogenous chlorine can inhibit the growth of watermelon by inhibiting the absorption of nitrogen,phosphorus and potassium,resulting in nutrient deficit,and increasing the accumulation of sodium,chlorine and other salt ions,resulting in chloride ion toxicity.However,appropriate nitrogen could alleviate the inhibition effect of chloride on watermelon growth by adjusting root morphology,increasing the content of osmotic regulatory substances,improving the enzymatic antioxidant system and maintaining ion homeostasis.