Sulfur nutrition of trees. A comparison of spruce (Picea abies L.) and beech (Fagus sylvatica L.)

Sulfur nutrition of herbaceous plants is characterized by sulfate uptake and xylem loading of sulfate in the roots, assimilatory sulfate reduction in the leaves, and a cycling pool of both, sulfate and glutathione, the main long distance transport form of reduced sulfur. Also in spruce and beech sulfate aquisition by the roots, sulfate transport to, and assmiliatory sulfate reduction in the leaves are important processes required to fulfil the tree's needs for sulfur in growth and development. However, sulfate reduction for protein synthesis may take place in roots of beech as well as in roots and stems of spruce trees. In addition, storage of reduced sulfur and its mobilization is required in spruce and beech to support the early growth of the new flush. In spruce reduced sulfur in the form of glutathione is stored in the older needles during winter and is transported in phloem and xylem in acropetal direction to the new generation in spring. In beech glutathione exported from the leaves is removed from the phloem during basipetal transport; its sulfur seems to be stored as protein in the stem and apparently undergoes mobilization in spring during bud break. As a consequence, high amounts of cysteine are transported to the developing leaves in the xylem.