Physiology Mechanism Of Temperature Affecting Chemical Defoliation And Its Application In Cotton

Cotton(Gossypium hirsutum L.)is an important cash crop in China.Due to the sharp rise in labor costs,machine picking cotton has become an inevitable choice for the development of the cotton industry.Cotton leaves must be removed with a chemical defoliant before machine harvesting.At present,Thidiazuron(TDZ)is a commonly used defoliant in cotton.Some studies also suggested a compound of two or more defoliants to achieve better defoliation effects.However,TDZ is more sensitive to the ambient temperature,and its defoliation effect would disappear when the ambient temperature is lower than 15?.To date,there are few reports of a compound defoliant that can make cotton leaves fall off at low temperatures.During cotton harvest,low-temperature often occurs,which would have defoliant not working and affect the machine harvest in cotton and subsequently decrease seriously the cotton yield.Therefore,it is necessary to elucidate the mechanism of temperature in the chemical defoliation of cotton,and develop a cotton defoliant that can defoliate at low temperature,so as to overcome the deficiencies of TDZ,and provides a pathway to improve the defoliation rate and to promote the machine picking in cotton harvest.To this aim,the optimal concentration for the experiment by statistically applying different concentrations of TDZ(0%,0.005%,0.01%,0.05%,0.1%,0.3%)to investigate the leaf defoliation rate was investigated.The LC-MS was used to measure the auxin(IAA),ethylene precursor(1-Aminocyclopropane-1-Carboxylic Acid,ACC),Abscisic acid(ABA),Salicylic acid(SA),Jasmonic acid(JA),and Gibberellins(GA3)in cotton leaves abscission layer.The colorimetric method was used to detect the defoliation associated enzyme activities including cellulase(CEL),polygalacturonase(PG),and pectin methylesterase(PME).Real-time fluorescent quantitative PCR(RT-qPCR)was used to detect the expression of key enzyme genes(GhCEL,GhPG,GhPME)in the formation of cotton leaves defoliation to unravel the mechanism of temperature on cotton chemical defoliation.The optimal concentration of ZMC was determined by the statistical analysis of the defoliation rate of cotton leaves after applying ZMC with different concentrations(0 ppm,50 ppm,100 ppm,300 ppm,500 ppm,1000 ppm)at 15?.And then the optimal concentration of ZMC and TDZ was combined to apply on the cotton leaves at a low temperature(15?)and the defoliations of cotton leaves was observed and analyzed by applying TDZ and TDZ+ZMC.The main results are as follows:1.Variation of defoliation rate of cotton leaves under different temperaturesBased on the statistical analysis of the difference in leaf removal rate after applying TDZ at different concentrations,the dada indicated that the optimal TDZ treatment concentration was 0.1%.By applying 0.1%TDZ at different temperatures to observe the defoliation,the results showed that applying TDZ at 25? on the third day,the defoliation rate(22%)was about 8 times that at 15?(by the defoliation rate of 2.7%).These results indicated that TDZ was significantly affected by ambient temperature.2.The effect of temperature on endogenous hormones in cotton abscission zoneBy measuring the content of endogenous hormones in cotton abscission zone,the results indicated that the content of endogenous hormones ACC(after 72 hours)and JA in cotton abscission zone prayed with TDZ at 15? was significantly lower than that treated at 25?,while the contents of IAA(after 48 hours),ABA,SA,and GA3 were significantly higher than that at 25?.The balance analysis of endogenous hormones in cotton petiole defoliation showed that the ratios of IAA/ACC,ABA/ACC,SA/ACC and GA3/ACC treated with TDZ at 15? were significantly higher than those treated at 25?.However,the ratios of JA/ACC,IAA/ABA,SA/ABA,GA3/ACC and JA/ABA treated at 15? were significantly lower than those treated at 25?,which might be the main reason that applying TDZ at a low temperature of 15? could not quickly remove leaves.3.The effect of temperature on key enzyme activities in cotton abscission zoneBy measuring the enzyme activities related to the formation of cotton abscission zone,it was found that when applying TDZ at 15?,the CEL activity and PG activity level of cotton abscission zone were significantly lower than those treated at 25?,which was the main reason that TDZ at low temperature could not defoliate leaves quickly.However,the PME activity level is higher,which suggested that there was no significant correlation between PME and shedding of cotton leaf.4.The effect of temperature on enzymic genes associated with the abscission zone formation in cotton leavesBy measuring the expression of enzymes related to the formation of cotton abscission zone,it was found that the gene expressions of GhCEL1,GhPG,and GhPME037 in cotton abscission zone treated with TDZ at 15? were significantly lower than those at 25?,which would be the main reason for no quickly defoliate at low temperature of 15?.5.Development and application of new cotton defoliantBased on the analysis of the difference in leaf removal rate between cotton treated with different concentrations of ZMC and 0.1%TDZ at 15?,the optimal ZMC treatment concentration was selected as 300 ppm.By applying TDZ(0.1%)+ZMC(300 ppm)and TDZ(0.1%)at 15?on the 3rd day,the results showed that the defoliation rate(11.7%)of TDZ+ZMC treatment was increased by 9%compared with TDZ treatment(2.7%).On the 7th day after treatment,the defoliation rate of TDZ+ZMC treatment(64.7%)was higher by 18.5%than that of TDZ treatment(45.2%).On the 10th day,the cotton leaves of TDZ+ZMC treatment were completely removed,which was significantly improved the defoliation rate of cotton leaves.The determination of hormone content showed that the content of IAA in the cotton abscission zone with TDZ+ZMC treatment was significantly lower than that with TDZ treatment,which was the main reason why TDZ+ZMC could promote the cotton leaf shedding at 15? better than TDZ treatment.In summary,the statistical analysis of cotton leaf defoliation rate,determination of endogenous hormone content,the key enzyme activity in cotton abscission zone and qRT-PCR detection presented the knowledge for why TDZ treatment could not induce cotton leaf shedding at 15?.What was more,a new defoliant which could efficiently remove the cotton leaves at lower temperature was created.