THE FORMATION OF LIGNIN IN WHEAT PLANTS

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Abstract
Wheat plants were harvested approximately every two weeks after emergence from the soil. Determination of the lignin content by the 72% sulphuric acid method showed a rapid increase between the period 45–70 days from seeding, and there was a corresponding increase in the yield of vanillin and syringaldehyde, obtained by alkaline nitrobenzene oxidation. The p-hydroxybenzaldehyde yield remained low at all times and seemed to be derived from a different system. It is suggested that the source of this aldehyde is not the lignin, but the tyrosine associated with the protein. The ratios of vanillin to syringaldehyde did not remain constant from plants of different ages, the percentage of syringaldehyde being lower than vanillin in the young plants and higher in those which were more mature. This is qualitatively in agreement with an increase in the methoxyl content of the lignin.When the yield of aldehydes from mature plants is taken as the criterion for the amount of lignin present, the youngest plants have a lignin content of about 0.13%, a value much lower than other bases for calculation would indicate.Oxidation of the whole plant and of the plant which had been extracted with alcohol–benzene, water, and 1% hydrochloric acid, gave similar percentages of aldehydes, owing partly to the removal of aldehyde-producing substances and partly to the degradation of the lignin during the extraction process. Oxidation of the extracts gave no trace of aldehyde in the hydrochloric acid extract, a trace in the water extract, and appreciable percentages in the alcohol–benzene extract. This latter extract consisted of two portions, a red water soluble, alcohol–benzene insoluble fraction and the main alcohol–benzene soluble fraction. The former gave rise to vanillin and p-hydroxybenzaldehyde on oxidation, the amount being a maximum at the first harvest and decreasing to zero after lignification had occurred. The latter, that is, the alcohol–benzene soluble extract, gave rise to syringaldehyde as well as the others, the amounts being low at first and increasing during lignification in the same way as the main body of the plant. It is believed that the alcohol–benzene removes a soluble portion of the lignin since the relative proportions of the aldehydes are very similar to those obtained from lignin itself.