Effect Of Cotton (Gossypium Spp.) Seedling Gamma Aminobutyric Acid Metabolic Response To Drought Stress

Drought influence crop production greatly more than the sum of other stress factors. Therefore,improving crop drought resistance has become a critical issue of agricultural sustainable development inarid areas. Xinjiang is a typical arid area in China and cotton is the most important economic crops in thisarea. Research on the ability to adapt to drought stress in cotton, discuss physiological and ecologicalmechanism in drought resistance and cultivate varieties which are drought resistant, these have becomethe urgent question to resolve in Xinjiang.The metabolism of sugar mainly through glycolysis and the TCA cycle in plants. Gamma aminobutyricacid （GABA） branch is closely linked to TCA cycle. The GABA branch has a central control role in C/Ndistribution and amino acid metabolism GABA is considered to be a signaling molecule in plant. Underbiotic and abiotic stress conditions, plants will accumulate a large number of GABA rapidly. ExogenousGABA can weak the plant abiotic stress. Hence, it is necessary to study GABA metabolism in cottonseedlings response to drought stress, it will help us understand the nitrogen metabolism and improve cottonproduction under drought stress. In this experiment, we studied the response of two cotton varietiesseedlings with different drought stress under soil culture and hydroponics in artificial climate chamber.Some results as follows:1. Under sustaining drought stress,"Xinluzao No.7（drought-resistant variety）" and "Xinluzao No.24"（sensitive to variety） leaf water potential, the relative water content of leaves, roots water content andactivity decreased gradually. After5d treatment, it decreased to lowest level, compared with the controlgroup, the difference reached a significant level. The activity of glutamic decarboxylase （GAD） reachedthe maximum value after4d treatment, but it decreased after5d treatment and re-watering. With theextension of drought stress time, the content of GABA increased, and reached to maximum after5d stress.The GABA content increased by290.56%and200.31%in "Xinluzao No.7" and "Xinluzao No.24" leaves,respectively; and in roots, GABA content increased by320.63%and243.81%. Leaf GABA-T has higheractivity in the early stress period, but it decreased with the stress time prolonged in "Xinluzao No.7" and"Xinluzao No.24". Compared with the control, the3d decreased significantly and5d reached the lowestlevel. GABA-T activity of roots and leaves had the similar trend in two varieties. After re-watering, theGAD activity, GABA-T activity and GABA content recover in leaves and roots of two varieties, but"Xinluzao No.7" was recover faster than "Xinluzao No.24". It was suggested that drought-tolerant speciescotton has stronger recover ability.2. There are at least3kinds of endopeptidase in cotton leaves and roots, namely the thiol protease,serine protease and metalloprotease. The thiol protease is most active among them. Exogenous GABA candecrease significant endopeptidase activity in cotton leaves and roots.3. The results of GAD activity are as follows after treated by Ca（NO₃）₂、W7、TFP and different organicacid and amino acid:Under-1.0MPa PEG treatment, GAD activity increased significantly in cotton leaves. But under10M Ca（NO₃）₂treatment, GAD activity was lower than the control in cotton leaf. Under-1.0MPaPEG+100mMW7+10M Ca（NO₃）₂treatment, GAD activity was significantly higher than that of control incotton leaf, which suggested that GAD activity was restrained obvious under W7treatment. For5mMalanine,5mM glycine,5mM glutamic acid,5mM aspartic acid and5mM ammonium ion treatment, GADactivity of cotton leaves reached to the highest under alanine treatment. GAD activity was slightly lowerthan that of control under other amino acids treatment. Under5mM ammonium ion treatment, GAD activity of cotton leaves was the lowest. For10mM butyric acid、citric acid、malic acid and oxalic acidtreatment, GAD activity of cotton leaf was slightly higher than that of control, oxailc acid> citric acid>malic acid> butyric acid. The activity of GAD and GABA content in cotton roots has the similar trendswith leaves under Ca（NO₃）₂、 W7、TFP、different amino acid treatment,but which reduced slowly.4. Under-0.5MPa,-1.0MPa and-1.5MPa water stress conditions, putrescine （Put）, spermidine （Spd）and spermine （Spm） contents in cotton leaf increased with decreasing stress potential, among them Put>Spd> Spm. The increase range of "Xinluzao No.7" is higher than "Xinluzao No.24". Under-1.5MPaPEG water stress condition, Put content increased significantly, Spd and Spm changed slightly. Afteradding aminoguanidine （AG）, which was DAO activity inhibitor, Put, Spd and spermine Spm contentincotton leaf increased slightly with the decrease of water potential, the most change extent is Put, then Spd,and Spm changed slightly. DAO activity was highest under-1.0MPa, but lowest under-0.5MPa stress.After joining the AG,DAO activity in cotton leaf was almost completely inhibited. Under-1.5MPa PEGstress, GABA content reached the highest and it was the lowest under-0.5MPa PEG stress.Under three kinds of PEG potential stress, Put, Spd, Spm, DAO activity and GABA content in cottonroots are similar trend with leaves. But the Put, Spd, Spm and GABA content was much lower than theleaves, and DAO activity was slightly lower than the leaves. The polymine degradation pathway canprovide30%to40%GABA product under-1.0MPaPEG stress both in cotton leaves and roots.5. Under the continuous drought stress and re-watering, nitrogen metabolism key enzyme activity,soluble protein, total nitrogen, pure nitrogen and free amino acid content major for "Xinluzao No.7" and"Xinluzao No.24" leaves and roots. Under PEG stress, two varieties leaves water content decreasedgradually,"Xinluzao No.7" decreased more obviously;Compared with the control water condition,glutamine synthetase （GS） activity and nitrate reduction enzyme （NR） activity in leaves and rootsdecreased significantly in "Xinluzao No.7" and "Xinluzao No.24",The endopeptidase activity, glutamatedehydrogenase （GDH） activity of leaves and roots increased markedly. The content of soluble protein, totalnitrogen and ammonium ion content also increased markedly than that of control. In addition, free aminoacid content increased under continuous water stress in leaves and roots for two varieties of cotton. Freeamino acid content of "Xinluzao No.7" increased slowly in the early stage under stress and then increasedquickly. Free amino acids change trend of " Xinluzao No.24" was opposite to " Xinluzao No.7". Afterre-watering, water content, activity of nitrate reductase and glutamine synthetase activity recover faster for"Xinluzao No.7" roots and leaves. Glutamate dehydrogenase activity, endopeptidase activity, free aminoacid, ammonium ion, soluble protein and total nitrogen content decreased faster than the "Xinluzao No.24". The correlation analysis showed that the GABA content of cotton seedling was significantly orextremely significantly related to glutamic acid, proline and total amino acid content.