Symbiotic nitrogen fixation and vegetative growth of cowpea (Vigna unguiculata (L.) walp.) in waterlogged conditions

Summary Various periods of waterlogging (up to 32 days duration) were imposed upon cowpea plants grown in pots under controlled glasshouse conditions. Particular attention was paid to treatment effects on nodule cortication, nitrogenase activity and fixation efficiency, and the consequent differences in plant dry weight and nitrogen content.All waterlogging treatments increased nodule cortication as compared with the unstressed controls; a 16-day stress period being of critical duration with respect to the bi-phasic nature of this anatomical response. Conspicuous lenticel-type protuberances were present on nodules formed under waterlogged conditions but were markedly reduced, or indeed absent, in the controls. Total dry weight of nodules per plant was reduced by 60 per cent after only 8 days waterlogging, but nitrogen fixation efficiency of nodules which persisted was only 18 per cent less than those on control plants; mean nodule cortex having increased from 39.8 (control) to 51.5 per cent. After 16 days waterlogging, total plant dry weight was decreased by ca 60 per cent as compared with control plants; reflecting similar adverse changes in leaf, stem and root dry weight. The most severe treatment (32 days waterlogging) did not further reduce plant dry weight but mean nodule cortex area increased from 55.9 (16 days) to a maximum of 59.3 per cent. With the exception of nodules, percentage nitrogen content of various plant components was unaffected by the treatments imposed.Both the formation of enlarged lenticles and increased nodule cortication are regarded as adaptive anatomical responses which facilitate continued symbiotic nitrogen fixation and vegetative growth of this legume under waterlogged conditions.One of a series of papers describing work undertaken in a collaborative project with the International Institute of Tropical Agriculture, Nigeria, sponsored by the U.K. Ministry for Overseas Development.