Lettuce Cultivars Response To Ratios Of NH₄～+-N/NO₃-N And Their Mechanism

Previous studies showed that partial replacement of nitrate (NO₃) by ammonium(NH₄⁺) in nutrient solution could significantly increase the biomass and decrease the NO₃concentration of aerobically growing crops. NO₃ accumulation, especially in leafyvegetables, was paid much attention because of its hazardous effects on human and animalhealth. The consumption of foods including high NO₃ content has a potential hazardousrisk on health when NO3- is reduced to nitrite (NO₂) form. Thus, enhancing the percentageof NH₄⁺-N in fertilizer application and addition of nitrification inhibitor in vegetableproduction is a potential nitrogen management. However, the mechanism of the effect ofdifferent nitrogen (N) forms on growth of leafy vegetables is still unknown. Thisdissertation is aimed to clarify the mechanism of NH₄⁺ enhancement on lettuce growth fromthe aspects of physiology and biochemistry. Hydroponic experiments were carried out tostudy the effect of different forms of N on photosynthetic characteristics, root growth anddevelopment, NO₃ uptake kinetics, NO₃ release and remobilization in root vacuoles,translation of plant endogenous hormones, as well as the yield and quality of lettuce.The results obtained are listed as follows.1. Five cultivars of lettuce, very popularly cultivated in south-east China, such as Ny,Sx1, Rw, Net and Nrnct, responded in a significantly different way to different ratios ofNH₄⁺-N : NO₃-N. Biomasses of roots and shoots of these cultivars increased with theincreasing of NH4^-N:NO₃-N ratio, and obtained the greatest value at 25:75 ofNH₄⁺-N:NO₃-N. Therefore, different ratios of NH4^oN:NO₃-N in nutrient solution couldinfluence the biomass of lettuce, and the best ratio of NH₄⁺-N:NO₃-N for lettuce growthand development is 25 : 75. Different ratio of NH₄⁺-N:NO₃-N in nutrient solution couldaffect the ratio of root and shoot, which was increased with the increased application ofNH₄⁺-N, and then decreased with the further increasing of applied NH₄⁺-N.2. Different ratios of NH₄⁺-N:NO₃-N in nutrient solution significantly affected thecontents of NO₃ in shoots of five cultivars. NO₃ contents of five cultivars decreased withthe increasing of the percentage of NH₄⁺-N in total N applied. 3. Different cultivar of lettuce responded differently to different NH₄⁺-N:NO₃^--N ratios innutrient solution. Sx1 was a mostly sensitive eultivar and Nrnct was a mostly insensitivecultivar to different ratios of NH₄⁺-N:NO₃^--N in terms of biomass production and NO3₃^-content among the cultivars tested in this experiment.4. The photosynthetic characteristics of cultivars, Sx1 and Nrnct, responded in asignificantly different way to different ratios of NH₄⁺-N:NO₃^--N. The SPAD readings, netphotosynthesis rate (Pn) and stomatol conductance (Cond) of Sx1 and Nrnct cultivarsincreased with the increasing of NH₄⁺-N:NO₃^--N ratio from 0:100 to 25:75, and they werefound to be the highest in 25:75 for NH₄⁺-N:NO₃^--N. However, they decreased with theincreasing of NH₄⁺-N:NO₃^--N ratio further. In contrast to the responses of SPAD readings,Pn and Cond of these cultivars to different ratios of NH₄⁺-N:NO₃^--N, the intercellular CO₂concentration (Ci) of these eultivars decreased with the increasing of, NH₄⁺ percentage andthe minimum Ci was found in the treatment of 25:75 for NH₄⁺-N:NO₃^--N. This indicatedthat NH₄⁺ addition could promote the photosynthesis of lettuce. The enhanced effect ofNH₄⁺ addition on photosynthesis characteristics should be attributed to Ci mainly.5. Results from the photosynthesis experiments showed that the cultivar named as Sx1was a more sensitive genotype to different ratios of NH₄⁺-N:NO₃^--N than the cultivarnamed as Nrnct in this experiment. Therefore, Sx1 was a sensitive genotype and Nrnct wasa insensitive genotype to different ratios of NH₄⁺-N:NO₃^--N.6. Different ratios of NH₄⁺-N:NO₃^--N in nutrient solution could greatly affect the rootgrowth of Sxl and Nrnct. Total root surface area, root volume, total root length and rootnumber increased with increased application of NH₄⁺-N, percentage from 0ï¼…to 25ï¼…, Theywere found to be the highest in 25:75 of NH₄⁺-N:NO₃^--N, and then decreased quickly withthe further increase NH₄⁺ amount. This result indicated that moderate NH₄⁺ addition couldimprove the development of root growth of lettuce. The effects of different N forms anddifferent ratios of NH₄⁺-N:NO₃^--N on total root length of these cultivars were exhibitedmainly on the root length of diameter of 0-0.15mm.7. In terms of the effects of different ratios of NH₄⁺-N:NO3₃^--N on root architectures of Sx1 and Nrnct cultivars, Sx1 had greater response and be more sensitive than that of Nrnct under moderate ratio of NH₄⁺-N:NO₃^--N.8. The Contents of endogenous hormone in Sx1 and Nrnct were significantly affected. The contents of IAA and iPAs of roots and shoots increased with the increasing of NH₄⁺-N:NO₃^--N ratio, and they were found to be the highest in 25:75 of NH₄⁺-N:NO₃^--N. However, they decreased with the further increase of NH-4⁺-N:NO₃^--N ratio from 25:75 to50:50. In contrast to the responses of IAA and iPAs contents to different ratios ofNH₄⁺-N:NO₃^--N, the contents of ABA in roots and shoots decreased as the percent of NH₄⁺increased, and the minimum ABA contents in roots and shoots were found in the treatmentof 25:75 for NH₄⁺-N:NO₃^--N. Then, they increased with the increasing NH₄⁺-N:NO₃^--Nratio from 25:75 to 50:50. With the development of growth from 4-leaves to 6-leaves and8-leaves, the contents of ABA, IAA and iPAs in Sx1 and Nrnct decreased in roots andshoots.9. The contents of IAA and iPAs in roots and shoots of Sx1 were higher than those ofNrnct, and there were significant difference between that in Sx1 and Nrnct. The contents ofABA in roots and shoots of Sx1 were 93.0ï¼…and 96.9ï¼…of that in Nrnct, respectively.10. Results from the NO₃^- uptake kinetics by Sx1 and Nrnct showed that, with theincreasing of NO₃^- content, the NO3₃^- uptake rate by Sx1 and Nrnct increased 100ï¼…NO₃^- -Nnutrient solution, while the increment of NO₃^- uptake decreased. After replaced 10ï¼…NO₃^--N by 10ï¼…NH₄-N and enhanced 10ï¼…NH₄-N in nutrient solution, the rates of NO₃^-uptake by Sx1 and Nmct decreased significantly, compared with that in the solution withoutNH₃⁺-N. Vmax of NO₃^- uptake in 0ï¼…NH₄⁺-N nutrient solution was higher significantlythan that in nutrient solution with 10ï¼…NO₃^--N replacement by 10ï¼…NH₄^--N. The Km ofNO₃^-uptake in 0ï¼…NH₄⁺-N nutrient solution was lower than that in nutrient solution with10ï¼…NO₃^-N replacement by 10ï¼…NH₄⁺-N, but there was no significant difference. Thisindicated that the inhibition of NO₃^- uptake by NH₄⁺-N was mainly due to the higher Vmax.NH₄⁺-N enhancement also inhibited NO₃^- uptake by increased the Cmin.11. Different cultivar has different NO₃^-uptake rate. The NO₃^-uptake rates of Sx1 werehigher than those of Nrnct in solution with NH₄⁺-N and without NH₄⁺-N. With the NO₃^-content increasing, the difference of NO₃^- uptake rates between Sx1 and Nrnct increased.Added NH₄⁺-N, the decreasing percentage of Vmax for Nmct was higher than that for Sx1,and the increasing percentage of Km for Nrnct was lower than that for Sx1.12. The activities of NO₃^- in vacuoles were higher than that in cytoplasms during the firstperiod of being planted in Yamazaki solution. After NO₃^- was removed from culturesolution, the biomasses of shoots and roots increased, and also the ratios of root and shootincreased. After the deprivation of NO₃^-, the activities of NO₃^- in vacuoles decreased greatlyfrom the 1st day to 3rd day, and decreased slowly from 4^th day to 7^th day, while the NO₃^-activities in cytoplasms were 4.0 mmol L^-1 and did not changed. This suggested that vacuolar NO₃^- could be released into cytosol to maintain a steady NO₃^- concentration forensure normal plant growth. From the second day re-supply NO₃^- again, NO₃^-concentrations in vacuoles increased quickly, and the biomasses boomed since the forth day.As the relative growth rate of shoots were higher than that of roots, the ratios of root andshoot decreased. The NO₃^- activities in cytoplasms were hardly changed in either NO₃^-supplied or deprived conditions. NO₃^- in vacuole of Sx1 had more mobilization than that ofNrnct. In the fifth day after deprivation of N, the NO₃^- concentrations in vacuoles of Sx1and Nrnct were 12.97 mg L^-1 and 27.67 mg L^-1, respectively. And the increment of NO₃^-concentration in vacuole of Sx1 was higher than that of Nrnct in re-supply NO₃^- condition.This showed that Sx1 was a high N use efficiency cultivar comparing with Nrnct in term ofthe characteristics of NO₃^- release and remobilization in root vacuoles.In conclusion, different ratios of NH₄⁺-N:NO₃^--N in solution could significantly affectroot architecture, photosynthetic characteristics and endogenous hormones, which, in turn,affected the root and shoot biomasses of lettuce cultivars. Different species of lettuce had adifferent ways to respond to different ratios of NH₄⁺-N:NO₃^--N in solutions. This respondeddifferences were found not only in the changes of root architecture, photosyntheticcharacteristics and endogenous hormone contents, but in NO₃^- uptake characteristics as well.Different cultivars of lettuce had also a different ability to remobilize the NO₃^- in vacuolesafter deprivation and re-supply of N in the solution. The NO₃^- in vacuoles of lettuce plantscould be rcmobilized after deprivation of N in the solution, which could sustain a relativelystable NO₃^- concentration in cytoplasm and thus ensure a normal function of cells.