Seasonal Fluctuations in Chlorophyll Content in Birch Stems with Special Reference to Bark Thickness and Light Transmission, a Comparison between Sprouts and Seedlings

Abstract

A comparison is made of the structural properties of the stems of sprouts and seedlings of Betula pubescens and B. pendula with respect to cortical photosynthesis by studying the thickness and chlorophyll content of the chlorenchyma in parts of the stem of differing age at various times in the year, paying particular attention to the thickness and light transmission capacity of the phellem. The birch stems had a mean chlorophyll content of approx. 0.5–0.8 mg (gFW)-1 in the tissue lying immediately beneath the phellem all the year round. This total chlorophyll concentration remained virtually constant in the middle of winter, increased in early spring, and fell sharply upon emergence of the leaves. It then rose considerably again at the beginning of the growing season, especially in the younger annual shoots, falling once more in late summer and the early part of the winter. A distinct annual rhythm was also detected in the chlorophyll a/b ratio, which was similarly connected with the emergence and shedding of the leaves, the ratio being substantially higher during the leafless period of the year than in the summer (maximum in December and minimum in May). Both the chlorophyll a/b ratio and total chlorophyll fell markedly with age of the shoots, along with a thickening of the cork cell layers and a decline in its light transmission properties. The amount of chlorophyllous tissue increased with the age of the tree (1–20 years), however. Differences in the light transmission capacity of the phellem were observed between the plants of sprout and seed origin and to some extent between the species of birch, and these were reflected in the stem chlorophyll concentrations. The cork of the sprouts usually transmitted light better than that of the seedlings, this being more marked in Betula pubescens than in B. pendula. In all cases the best transmission capacity was found with respect to infra-red radiation of wavelength 725 mm, but large amounts of light at the photosynthetically active wavelength of 665 nm was also transmitted. The transmission properties altered with the thickening of the phellem and changes in its colour. The present pilot study reveals that young birch shoots would seem to possess a capacity for photosynthesis in their stem chlorenchyma which could be of ecological significance particularly in the leafless periods in early spring and late autumn. The differences between Betula pubescens and B. pendula and between the sprouts and seed origin shoots are nevertheless not of a magnitude that they could explain the more rapid early growth of the sprouts or the differences in sprouting capacity between the two species.