Leaf age-dependent differences in sulphur assimilation and allocation in poplar (Populus tremula x P-alba) leaves

S-35-sulphate was flap-fed to poplar leaves of different leaf development stages - young developing, expanding, mature, and old mature poplar leaves. S-35-sulphate was taken up independent of the leaf development stage. Whereas young development leaves did not export the S-35 taken up, export increased with increasing leaf development stage. Expanding leaves allocated the exported 35S mainly into apical tree parts (73-87%) and only to a minor extent (13-27%) in basipetal direction. Neither lower trunk sections nor the roots were sinks for the exported S-35. Expanding and developing leaves, but not the shoot apex, were the main sinks for the S-35 allocated in apical direction. In contrast, mature and old mature leaves exported the S-35 taken up mainly in basipetal direction (65-82%) with the roots constituting the main sinks. The S-35 allocated into apical tree parts was found in expanding and developing leaves, hut only to a minor extent in the shoot apex. Apical allocated S-35 was identified as sulphate. Apparently the demand of young developing leaves for reduced sulphur was not fulfilled by mature leaves. Therefore, reduced sulphur for growth and development of young developing leaves must be supplied from other sources. In vitro activity of enzymes involved in assimilatory sulphate reduction was measured to investigate whether demand for reduced sulphur by young leaves is met by their own sulphate reduction. ATP sulphurylase and APS reductase activities were not significantly lower in developing than in mature leaves. Sulphite reductase and serine acetyltransferase activities were highest in developing leaves; O-acetylserine (thiol) lyase activity was similar in all leaf developing stages. Apparently, young developing poplar leaves are able to produce their own reduced sulphur for growth and development. Whether other sources such as storage tissues and/or roots are involved in reduced sulphur supply to developing leaves remains to be elucidated.