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

Nodulated, pot-grown plants of white clover (Trifolium repens cv. Katr ina) were subjected to different soil moisture regimes and the effect of these treatments on dry matter production, nitrogenase activity, ae renchyma 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 comp ared with those subjected to normal watering, indicating that clover c an adapt to long-term waterlogging, In the second experiment, time-cou rses of acetylene reduction showed a lag phase of only 10 min for wate rlogged plants, suggesting that gas exchange was occurring through aer ation pathways rather than the water, Flooded plants that were drained 24 h prior to acetylene reduction assays had substantially greater ni trogenase activity than normally watered or continually flooded plants . However, there was a marked decrease in nitrogenase activity when pl ants which had been watered normally were flooded, suggesting a sensit ivity of white clover to sudden changes in moisture conditions. Morpho logical studies of nodules from plants grown in normally watered and c ontinually flooded soil showed increased aerenchyma production around roots and nodules of waterlogged plants. In addition, the infected cel ls of submerged nodules were larger and had larger vacuoles than those from nodules of normally watered plants. This increase in vacuole vol ume to protoplast volume in infected cells may play a role in the tole rance of white clover nodules to waterlogging.