THE EFFECT OF WATERLOGGING ON NITROGEN-FIXATION AND NODULE MORPHOLOGY IN SOIL-GROWN WHITE CLOVER (TRIFOLIUM-REPENS L)

Nodulated, pot-grown plants of white clover (Trifolium repens cv. Katrina) were subjected to different soil moisture regimes and the effect of these treatments on dry matter production, nitrogenase activity, aerenchyma formation, and bacteroid distribution was determined. In the first experiment, after 9 weeks growth shoots were significantly (P<0.01) heavier for clover plants which were flooded from germination compared with those subjected to normal watering, indicating that clover can adapt to long-term waterlogging, In the second experiment, time-courses of acetylene reduction showed a lag phase of only 10 min for waterlogged plants, suggesting that gas exchange was occurring through aeration pathways rather than the water, Flooded plants that were drained 24 h prior to acetylene reduction assays had substantially greater nitrogenase activity than normally watered or continually flooded plants. However, there was a marked decrease in nitrogenase activity when plants which had been watered normally were flooded, suggesting a sensitivity of white clover to sudden changes in moisture conditions. Morphological studies of nodules from plants grown in normally watered and continually flooded soil showed increased aerenchyma production around roots and nodules of waterlogged plants. In addition, the infected cells of submerged nodules were larger and had larger vacuoles than those from nodules of normally watered plants. This increase in vacuole volume to protoplast volume in infected cells may play a role in the tolerance of white clover nodules to waterlogging.