| Abstract: | Nodulated, pot-grown plants of white clover (Trifolium repenscv. Katrina) were subjected to different soil moisture regimesand the effect of these treatments on dry matter production,nitrogenase activity, aerenchyma formation, and bacteroid distributionwas determined. In the first experiment, after 9 weeks growthshoots were significantly (P<0.01) heavier for clover plantswhich were flooded from germination compared with those subjectedto normal watering, indicating that clover can adapt to long-termwaterlogging. In the second experiment, time-courses of acetylenereduction showed a lag phase of only 10 min for waterloggedplants, suggesting that gas exchange was occurring through aerationpathways rather than the water. Flooded plants that were drained24 h prior to acetylene reduction assays had substantially greaternitrogenase activity than normally watered or continually floodedplants. However, there was a marked decrease in nitrogenaseactivity when plants which had been watered normally were flooded,suggesting a sensitivity of white clover to sudden changes inmoisture conditions. Morphological studies of nodules from plantsgrown in normally watered and continually flooded soil showedincreased aerenchyma production around roots and nodules ofwaterlogged plants. In addition, the infected cells of submergednodules were larger and had larger vacuoles than those fromnodules of normally watered plants. This increase in vacuolevolume to protoplast volume in infected cells may play a rolein the tolerance of white clover nodules to waterlogging. Key words: White clover, waterlogging, hypoxia, nitrogen fixation, root nodule morphology. |
