Influence of anoxia on whole plant sulphur nutrition of flooding-tolerant poplar (Populus tremula x P-alba)

The investigations reported herein were performed to test whether anoxic soil conditions changed the sink strength of the roots for reduced sulphur and therefore transport of sulphur in the phloem, and whether increasing sulphide contents in anoxic soils due to bacteria activity causes changes in sulphate assimilation of the roots. Anoxic conditions caused by flooding of the flooding-tolerant tree species Populus tremula x P. alba led not only to changes in carbon metabolism, but also influenced sulphate assimilation. In the roots, activity and transcript of adenosine 5'-phosphosulphate (APS) reductase, the key enzyme of the sulphate assimilation pathway, completely disappeared, but the Cys content increased 6.7 times after 15 d of flooding. A higher glutathione content of phloem exudates after 3 and 7 d of flooding indicates an increased transport of reduced sulphur to the roots. However, the export rate of S-35-sulphur out of mature leaves after flap feeding S-35-sulphate was not different between flooded and control poplars. Since the allocation to the roots was diminished, enhanced phloem contents of GSH indicate a diminished sink strength of the roots rather than an enhanced sink strength. Therefore, a higher transport of GSH in the phloem could not be responsible for the higher Cys content in the roots. As protein contents were unaffected the higher Cys content in roots could also not originate from protein breakdown. Enhanced O-acetylserine (thiol) lyase (OASTL) activity were measured in the roots demonstrating an uncoupling of sulphate reduction and Cys synthesis. A possible contribution in detoxification of sulphide produced in the anoxic soil is discussed.