| China is the country with the largest production and consumption of potato(Solanum tuberosum L.)in the world.The government of China is engaged in implementing the strategy of“potato as a staple food”in 2015,and its important research content is to consider both yield and quality improvement.However,the potato yield per unit area in China is relatively low,only 18.76 t/hm2,which is lower than the world average yield value of 20.06 t/hm2.In addition,sugar accumulation caused by cold storage,green skin caused by light and sprouting caused by broken dormancy are still the main reasons for potato quality decline and loss in postharvest storage and processing in China,which are closely related to the physiological characteristics of tubers at harvest time.All these have greatly restricted the process of potato staple food.Nitrogen is a mineral nutrient with the great demand for potatoes.Excessive application of nitrogen fertilizer is common problems in potato production at present,which affect the yield and physiological characteristics of tubers to varying degrees,and may directly affect the postharvest storage characteristics.Therefore,it is urgent to establish the relationship between the nitrogen application rate on the production of fresh potato and the characteristics of postharvest storage while studying the regulation of potato yield formation by nitrogen fertilizer application,to provide a scientific theoretical basis for optimizing potato nitrogen nutrient management through feedback and guidance of postharvest storage results.Three commercial potato cultivars(E shu3,Z shu5 and H shu3)were used as the research material in this study,using field experiment,potted plant experiment and postharvest storage(such as cold storage test,tuber greening experiment,dormancy and sprouting,and multiple planting test)were carried out.We used the Logistic equation y=K/(1+ae-bt)to analyze the effect of nitrogen application on potato dry matter accumulation,and clarified the yield formation mechanism of nitrogen application rate regulating source-sink relationship.We also used untargeted and targeted metabonomic techniques to analyze the differences in carbohydrate and free amino acid metabolism of potato tubers during harvest and after cold storage under optimal and excessive nitrogen levels.Besides,fluorescence microscope(FM),confocal laser scanning microscope(CLSM),transmission electron microscope(TEM),scanning electron microscope(SEM)and laser particle size analyzer were used to analyze the ultrastructural changes of tuber greening and the physiological mechanism of nitrogen increasing greening.Moreover,the regulation mechanism of nitrogen application rate on tuber development,dormancy and sprouting was analyzed by paraffin section,carbohydrate,and hormone.The main results of this study are as follows:1)The effect of nitrogen application rate on tuber yield formation was achieved by affecting source,sink development and regulating source-sink relationship.Results showed that the average yield per unit area of tuber yields of the three commercial potato cultivars(E Shu3,Z Shu5 and H Shu3)under optimal nitrogen level was 31.69t/hm2,increased by 9.98 t/hm2,which was 47.2%higher than that under low nitrogen and no nitrogen application.However,excess application of nitrogen result in the tuber yields of the three commercial potato cultivars were decreased by 9.6%,only 28.64 t/hm2.Pearson correlation showed that the maximum tuber weight per plant,tuber number of per plant,large-size tuber rate was significantly positively correlated with yield,while small tuber rate was significantly negatively correlated with yield.Compared with no nitrogen and low nitrogen application,the maximum tuber weight per plant increased by 75.0 g,the number of tubers per plant increased by 1.2,and the percentage of large tubers increased by 4.5%under optimal nitrogen application,which resulted in an increase in tuber yield.Compared with optimal nitrogen application,the maximum tuber weight per plant decreased by 40.0 g,the percentage of large tubers decreased by 1.3,and the percentage of small tubers increased by 2.8%under excessive nitrogen application.,which resulted in a decrease in tuber yield.The number of petioles,lesf number per petiole,total number of leaves,leaf area and photosynthetic rate was significantly positively correlated with yield.Compared with low nitrogen and no nitrogen application,the number of petioles increased by 4.6,lesf number per petiole increased by 2.0,leaf area increased by 34.94 cm2,and the photosynthetic rate of functional leaves by 7.5?mol m-2s-1(tuber bulking stage)and 7.4?mol m-2s-1(tuber maturation stage)in key growth stages under optimal nitrogen application,which resulted in an increase in tuber yield.The logistic equation ofdry matter accumulation showed that nitrogen application increased the duration of rapid dry matter accumulation before tuber bulking,and promoted the establishment of shoot and leaf(source).In addition,in the late growth period(starch accumulation stage),the duration of rapid dry matter accumulation and the maximum growth rate of dry matter in tubers were significantly higher than those in the non-nitrogen treatment,which resulted in the increase of tuber yield.However,the duration and maximum growth rate of dry matter accumulation in shoot and leaf under excessive nitrogen treatment continued to increase,resulting in prosperous leaf growth,imbalance of source and sink,and decreased yield.2)The regulation of the nitrogen application rate on the development of tubers(sink)is mediated by hormones and carbohydrates.The results of potted plant experiment showed that the initial formation time of the stolon did not change with the application of nitrogen,but the swelling of subapical region of the stolon to tuberization was delayed by about 6.0-15.0 days compared with no nitrogen application.Under nitrogen application treatment,the elongation rate of subapical non-expanded solons was much higher than that without nitrogen treatment,increased by 1.3 times,and the average length of stolon increased by 2.72 cm.The results of paraffin section showed that the number of cells per unit area decreased by 36 per 10000?m2,by 57.1%,and the cell length increased by 90.9%under nitrogen application treatment.The content of GAs was significantly increased under nitrogen application compared with no nitrogen application,and the content of GA1 was the most abundant and increased by 2.2 times,which may be closely related to the elongation of stolon and the delay of tuber formation.3)The changes of the metabolic pathways of free amino acids and sugars induced by excessive nitrogen,and exacerbated cold–induced sweetening in potato tubers.We identified 56 metabolites in tubers by GC-MS analysis,including sugars,free amino acids,and other organic acids,and found that there were significant differences in the relative contents of metabolites between different nitrogen levels(optimal,ON;and excessive,EN)at harvest(AH)and cold storage(CS).The results of multivariate statistical analysis including PCA,PLS-DA and T test showed that there were 15 different metabolites between ON and EN at harvest(VIP>1 and P<0.05),mainly including sucrose,glucose,galactose,fructose,asparagine,glutamic acid,aspartic acid,phenylalanine,glutamine,tryptophan,valine,citric acid,fumaric acid,organic acid.These metabolites were involved in alanine,aspartate,and glutamate metabolism,glycine,serine,and threonine metabolism,starch and sucrose metabolism,citrate cycle(TCA),and phenylalanine metabolism.After cold storage,there were 15 metabolites between the ON and EN treatments,mainly including free amino acids and sugars(sucrose-6-phosphate,glucose-1-phosphate,sucrose,glucose,fructose,phenylalanine,aspartic acid,and asparagine).These metabolites are involved in starch and sucrose metabolism,alanine,aspartate and glutamate metabolism,and phenylalanine metabolism.In summary,the metabolic pathways of differential metabolites indicated that EN application induced metabolic changes in alanine,aspartate and glutamate metabolism,and starch and sucrose metabolism as compared to the control tubers(ON treatments),especially in interactions involving cold stress.The absolute content of amino acids related to the metabolic pathways of alanine,aspartic acid and glutamate was verified by target metabolism.The results showed that the absolute content of free asparagine,and other amino acids were higher(by 1.3–1.5 fold)in the EN treatment than that in ON treatment,and this difference was maintained after cold storage.The results of absolute sugar content and enzyme activity determination showed that EN reduced tuber maturity and accumulated higher sucrose at harvest,while the activity of vacuolar invertase was increased after cold storage,reducing sugar also accumulated.This also leads to the upregulation of starch sugar metabolism,which accelerated cold induced sweetening.4)Nitrogen application increased the percentage of small granule starch(<30.0??m)in tuber skin and thereby promoted tuber greening under light.Three commercial potato tubers(E shu3,Z shu5 and H shu3)were exposed to light,causing greening of the tuber skin.Chlorophyll accumulation gradually increased with increasing number of days of light exposure and chlorophyll content was 5–8 times higher than that under darkness.The chlorophyll content of tuber skin under light for 7.0 d was increased by 20.0%–32.0%in the nitrogen fertilization application treatments compared with in the no nitrogen application treatment for the three cultivars.The green epidermal tissue showed obvious red fluorescence under ultraviolet light through fluorescent microscope.These results indicate that chloroplasts were developed in the tuber skin under light.Interestingly,the flaming red fluorescence was concentrated around the starch with small particle size through confocal laser scanning microscope.Through TEM,we observed the complete process by which amyloplasts were developed into chloroplasts.After light-induced greening of the tuber skin,the amyloplast membrane disappeared,while grana occurred around the amyloplast and starch granule shrank.Then many grana lamellae and osmiophilic bodies were formed and starch was completely dissolved,with the amyloplast developing into a mature chloroplast.We observed the formation of grana lamellae around amyloplasts within the diameter range of 9.0–30.0?m.The results indicate that these amyloplasts were involved in the transformation of amyloplasts into chloroplasts.The mean starch gradual size decreased by 4.0–5.0?m with increasing level of nitrogen application for the three cultivars,and this trend was mainly reflected by an increase in the percentage of small starch granules,the percentage of small starch granules(<30.0?m)increased by 10%–13%.These results were consistent with starch granule morphology and size observed by SEM.Therefore,the application of nitrogen increased the greening of tubers under light,which was associated with a significant increasing the percentage of small granule starch(<30.0?m)in the tuber skin.5)Nitrogen management can inhibit or induce the sprouting of potato tubers:consequences of regulation of tuberization.The dormancy period and sprouting rate of three commercial cultivars were significantly affected by nitrogen application rate.The dormancy period was significantly extended by 3.0-6.0 weeks when the nitrogen application rate was reduced,thus prolonging the sales period,which was conducive to storage and sales.Under the condition of sufficient nitrogen supply,the number of bud eyes,the diameter and length of bud increased,the number of main stem and the number of fruiting tuber was increased,and the yield was higher.Moreover,it is beneficial to shorten the breeding cycle of seed tuber.These results have important guiding significance for seed tuber production.Regarding the regulation mechanism of dormancy and sprouting,our study found that nitrogen application promoted bud initiation and growth in apical meristem,increased starch,sucrose,and?-amylase activities in apical meristem,and provided more energy for bud sprouting compared with no nitrogen application.Nitrogen application significantly increased the content of GAs in the bud apical meristem,especially GA1,by 6.0 times.These results are consistent with the hormone regulation of tuber development by nitrogen application observed in Chapter 4of this study,suggesting that increased gibberellin levels contribute to dormancy breaking.The effects of nitrogen application on tuber development,sprouting was similar,both of which are closely related to changes insucrose and GAs level,and the regulation of dormancy is the result of regulation of tuber formation. |
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