Physiological investigations of leaf mucilages. 2, The mucilage of Taxus baccata L. and of Thuja occidentalis L.

Abstract

Seasonal variations of the content of leaf mucilages of Taxus baccata and Thuja occidentalis were determined. In Taxus the content is highest during the late winter and early spring and lowest in the summer. In Thuja it is highest in the winter, decreasing during spring and low in the summer months. Under the light microscope, Taxus mucilage was located in the vacuoles of mesophyll cells and that of Thuja in mucilage idioblasts. Temperature experiments show that storage of mucilage is not temperature dependent and is not reduced by prolonged darkness. Taxus mucilage consists of the sugars galactose, rhamnose, glucose, arabinose and xylose and contains a low percentage (<5%) of uronic acid. Furthemore, a peptidic component is found and shows annual variations from 6% to 15% of total mucilage. Seasonal variations of the sugar components are pronounced only during the spring in the period of intensive mucilage synthesis. Purified Taxus mucilage could not be separated into different components by gel chromatography. All fractions showed a similar composition of sugars and peptide. These findings, in addition to the IR spectra, lead to the conclusion that the mucilage is a proteoglycan and perhaps a mixture of polymers of similar molecular weights. Isolated and dried mucilage has a high water-binding capacity; at 96% relative humidity it equilibrates to 180%, and at 100% rh to 280-300% of its dry weight. From mucilage content and cell volume the mucilage concentration of the vacuoles can be estimated as being higher than 5-6%, which must give rise to a remarkable matric potential. It is suggested that the water-binding capacity of the mucilage plays an important role in stabilizing the water relations of the needles, thereby increasing frost resistance.