Sulfur nutrition of deciduous trees.

Sulfur in its reduced form (-II) is an essential nutrient for growth and development, but is mainly available to plants in its oxidised form as sulfate. Deciduous trees take up sulfate by the roots from the soil solution and reduce sulfate to sulfide via assimilatory sulfate reduction in both roots and leaves. For reduction in the leaves, sulfate is loaded into the xylem and transported to the shoot. The surplus of sulfate not reduced in the chloroplast or stored in the vacuole and the surplus of reduced S not used for protein synthesis in the leaves is loaded into the phloem and transported back to the roots. Along the transport path, sulfate and glutathione (GSH) is unloaded from the phloem for storage in xylem and phloem parenchyma as well as in pit and ray cells. Remobilised S from storage tissues is loaded into the xylem during spring, but a phloem to xylem exchange does not appear to exist later in the season. As a consequence, a cycling pool of S was only found during the change of the seasons. The sulfate:glutathione ratio in the phloem seems to be involved in the regulation of S nutrition. This picture of S nutrition is discussed in relation to the different growth patterns of deciduous trees from the temperate climate zone, i.e. (1) terminated, (2) periodic and (3) indeterminate growth patterns, and in relation to environmental changes.