| The composition of biochemical structure is the foundation of cotton fiber quality formation. It has been found that phenylpropanoid compounds exist in cotton fiber and may be related to the formation of cotton fiber quality. However, the chemical properties of the compounds especially the structure monomers are still lack of research. The cotton fiber is thin, soft and cellulose content is high, which is different from other plant cell wall. The methods of analyzing other plant materials cannot be directly used for the analysis of cotton fiber. In the study, the conventional chemical treatments methods by other plant cell wall were optimized. From analyzing dynamic change of the content of phenylpropanoid compounds in cotton fiber, it will disclose the characteris of phenylpropanoid compounds and lay a basis for further revealing the biochemical mechanism, which control the formation of cotton fiber quality. The main results were as follows:1. The strong autofluorescence of the cut cotton fiber was observed by using fluorescence microscopy. Due to cotton fiber be coated by primary wall, it was found that there were strong fluorescence at the two ends of cotton fiber cutting. The fluorescence phenomenon was universal in the observed cotton fiber. After the phenylpropanoid compounds were extracted by using thioglycolic acid, the autofluorescence in cotton fiber weakened along with the increase of extraction times, which revealed the phenylpropanoid compounds mainly exists in the secondary wall of cotton fiber.2. The classical Klason method, determining lignin in plant cell wall, was optimized to make it suitable for analyzing lignin in cotton fiber. The dynamic accumulation of lignin in different development stages of cotton fiber was analyzed by using the optimized Klason method. The results, from the Klason and the histochemical staining, showed the percent of lignin was the largest at 20 DPA （Day Post-anthesis）. The percent of lignin showed a decreasing trend with the development of cotton fiber and mature fiber with the least. The results of histochemical staining showed the deepest color dyeing at 20 DPA and the color became shallow with the development of cotton fiber, which was in consistent with the results of the percent of Klason lignin. The results showed the percent of lignin was the highest in the development of 20 DPA, the percent of lignin cotton fiber became less with the cotton fiber development. Presumably, there may be G （Guaiacyl unit） and S （Syringyl unit） lignin monomer existed in cotton fiber according to the results of the histochemical staining.3. The suitable analysis for determing phenylpropanoid monomers in cotton fiber was established. Phenylpropanoid compounds, extracted from washed cotton fiber by using thioglycolic acid, were cleaned twice with 5 mL distilled water and ground into powder. Then, the phenylpropanoid compounds was degraded by DFRC （Derivatization Followed by Reductive Cleavage） and GC （Gas Chromatograph） analysis, the phenylpropanoid monomers of cotton fiber were determined. The results showed that guaiacyl monomer （G-lignin） and syringyl monomer （S-lignin） existed in cotton fibers, and G response was significantly higher than S. The improved method for phenylpropanoid monomers was suitable for the analysis of other special materials with the low content of phenylpropanoid compounds.4. After the plant cell walls were treated by thioglycolic acid and HC1, free reagents were cleaned, their components were analyzed by FTIR （Fourier Transform Infrared Spectroscopy）. The results showed the plant cell walls can be broken without causing componential damage by using thioglycolic acid treatment. The wavenumber of untreated fiber and the fiber extracted by thioglycolic acid approximately overlapped in 1500～1100 cm-1, while the wavenumber of treated cotton fiber was significantly higher than the untreated in 1730 cm-1,1350～1150 cm-1. The plant cell wall, treated by thioglycolic acid, showed more absorption peak than non-chemical treatment that the results of FTIR were more abundant. FTIR analysis showed that G and S structural units existed in the cotton fiber. According to the ratio of absorption peak strength at 1270 cm-1 and 1230 cm-1 （A1270/A1230=1.69）, we deduced the types of lignin were guaiacyl-syringyl lignin （G-S） and the relative content of guaiacyl lignin more than syringyl lignin. For the same amount of cotton fiber, the most of the absorption peaks at 20 DPA were higher than subsequent developmental stages, especially the absorption peak at the 3340 cm-1,2920 cm-1,1730 cm-1,1620 cm-1,1510 cm-1,1430 cm-1,1270 cm-1,1230 cm-1, but the absorption peak was slightly lower at 1330 cm-1. With the development of cotton fiber, the absorption peaks were gradually reduced. However, the absorption peak increased at 1330 cm-1 and decreased at 1230 cm-1.5. It can be inferred that cotton fiber lignin belong to guaiacyl lignin and syringyl lignin （GS-type lignin） by using 1H-NMR （Nuclear Magnetic Resonance） analysis. The connection ways between G and S were the following:C-C bond of β-5 and β-β, and ether linkage of β-O-4. |
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