| Cotton(Gossypium hirsutum L.)leaves play vital roles in cotton growth and development,because they supply the major carbohydrates for the cotton plant.A two-year experiment was established to investigate the effects of potassium(K)application on the main metabolism of cotton leaves.The field experiment was conducted using two cotton cultivars with contrasting K sensitivity(Simian 3,low-K-tolerant cultivar and Siza 3,low-K-sensitive cultivar)under three K levels(0,150 and 300 kg K2O ha-1).We explored the effects of K application on carbon metabolism and antioxidant metabolism of the subtending leaf and their relationships with cotton growth and boll development.The pot experiment was carried out using the same cotton cultivars planted in the field experiment with three K levels(0,4.5 and 9.0 g K2O pot-1)which was calculated according to the K levels in the field.In the pot experiment,we examined the effects of K application on nitrogen metabolism of the subtending leaf.In addition,we analyzed the deficiencies in physiological characteristics between the two cotton cultivars in response to K application.The main results are as follows:1.Potassium application affects the accumulation of biomass and potassium in cottonK application had little effect on the number of fruit branch,but increased the height,total fruit nodes and boll setting rate.K application also increased the plant biomass,the maximum and average accumulation rate of biomass.K application did not alter the biomass of root and stem,but increased the biomass of leaf and reproductive organs.The maximum theoretical accumulation of K,the maximum and average accumulation rate of K were increased in the K-supply treatments(150 and 300 kg K2O ha-1).Compared with Simian 3,the sensitivity of Siza 3 to low-K was showed in the following aspects:boll setting rate,the duration of biomass rapid-accumulation period,the duration of K rapid-accumulation period and the percentage of potassium accumulation in reproductive organs were more sensitive to K.2.Potassium application affects carbohydrates metabolism in the leaf subtending the cotton boll and its relationship with boll biomassK application increased leaf K concentration,net photosynthetic rate(Pn),stomatal conductance(Gs),and stimulated boll biomass(capsule wall,seed and lint).K application increased the proportion of lint biomass,decreased the proportion of seed biomass,and did not affect the proportion of capsule wall biomass.Specific leaf weight(SLW),maximum/minimum sucrose contents and nonstructural carbohydrates(hexose,sucrose,starch)decreased,but sucrose transformation rate in the LSCB increased in both cultivars after K application.The critical leaf K levels for hexose content,sucrose content and starch content were 1.1%,1.3%,1.2%-1.4%in Simian 3 and 1.6%,1.7%and 1.7%-1.8%in Siza 3,respectively.The activities of ribulose-1,5-bisphosphate carboxylase-oxygenase(Rubisco),cytosolic fructose-1,6-bisphos-phatase(cy-FBPase),sucrose phosphate synthase(SPS),sucrose synthase(SuSy)and amylase activities were increased by K application,whereas soluble acid invertase(SAI)activity was decreased.SPS and SuSy activities in Siza 3 were more sensitive than those in Simian 3.Correlation analysis revealed that higher Pn,sucrose transformation rate and SPS activity in the LSCB were necessary to improve boll biomass,but the accumulation of sucrose in the LSCB was not beneficial to boll biomass.3.Relationship between K fertilization and nitrogen metabolism in the leaf subtending the cotton bollTotal dry matter increased by 13.1%-27.4%and 11.2%-18.5%under K supply for Simian 3 and Siza 3.Boll biomass and boll weight also increased significantly in the 4.5 and 9.0 g K2O pot-1 treatments.Leaf K content,leaf N content and nitrate(NO3-)content increased with increasing K rate,and leaf N content or NO3-content had a significant positive correlation with leaf K content.Free amino acid content increased in the 0 g K2O pot-1 treatment for both cultivars,due to increased protein degradation caused by higher protease and peptidase activities,resulting in lower protein content in the 0 g K2O pot-1 treatment.The critical leaf K contents for free amino acid and soluble protein content were 14 mg g-1 and 15 mg g-1 in Simian 3,and 17 mg g-1 and 18 mg g-1 in Siza 3,respectively.Nitrate reductase(NR),glutamic-oxaloace transaminase(GOT)and glutamic-pyruvic transaminase(GPT)activities increased in the 4.5 and 9.0 g K2O pot-1 treatments for both cultivars.Glutamine synthetase(GS)and glutamate synthase(GOGAT)activities were increased by K application for Siza 3,but were not affected in Simian 3,indicating that this was the primary difference in nitrogen-metabolizing enzymes activities for the two cultivars with different sensitivities to low-K.4.Potassium application affects antioxidant metabolism in relation to leaf senescence in cottonK application decreased the early season flowering rate,earliness,shedding rate and yellow leaf rate,and increased leaf number,leaf area,boll number,seed cotton weight per boll and lint percentage.The premature senescence of leaf subtending the cotton boll(LSCB)induced by K deficiency was characterized by early chlorophyll degradation and negative chlorophyll fluorescence.Despite higher activity of hydrogen peroxide(H2O2)scavenging enzymes(catalase and peroxidase)and higher content of ascorbic acid(ASC)existed in the K-deficient leaf,higher H2O2 content was observed,which caused higher malondialdehyde content.Although lower dehydroascorbate reductase activity was observed under K deficiency,high ASC content was attributed to lower ascorbate peroxidase activity.The differences between Siza 3 and Simian 3 in response to K were that:(1)the seed cotton weight per boll and lint percentage in Siza 3 increased more obviously under K application,(2)higher leaf K concentration was needed for maintaining chlorophyll content for Siza 3,(3)chlorophyll fluorescence parameters were more sensitive to K in Siza 3,especially non-photochemical quenching,and(4)superoxide dismutase and glutathione reductase activities were markedly changed by K application only in Siza 3.In conclusion,K application increased Pn and the activities of Rubisco,cy-FBPase,SPS,SuSy and amylase,but decreased SAI activity,which was conducive to sucrose accumulation.K application increased the sucrose transformation rate,resulting the low contents of sucrose,hexose and starch in leaves.Decreased protein degradation caused by lower protease and peptidase activities,resulted in higher protein content and lower free amino acid content in leaves.Despite higher activity of H2O2 scavenging enzymes(catalase and peroxidase)and higher content of ASC existed in the K-deficient leaf,higher H2O2 content was observed,which accelerated the process of senescence.Compared with Simian 3,the critical leaf K content of SPAD,Pn,sucrose,hexose,starch,amino acid and soluble protein was higher in Siza 3,indicating Siza 3 was more sensitive to low leaf K content.In addition,the effects caused by different leaf K contents on the activities of SPS、SuSy、GS、GOGAT、SOD and GR were more obvious in Siza 3 than Simian 3. |
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