Responses to soil flooding and oxygen shortage were studied in field, glasshouse and controlled environment conditions. Established stools of Salix viminalis L, were compared at five field sites in close proximity but with contrasting water table levels and flooding intensities during the preceding winter. There was no marked effect of site on shoot extension rate, time to half maximum length or final length attained. When rooted cuttings were waterlogged for 4 weeks in a glasshouse, soil redox potentials quickly decreased to below zero. Shoot extension was slowed after a delay of 20 d, while, in the upper 100 mm of soil, formation and outgrowth of unbranched adventitious roots with enhanced aerenchyma development was promoted after 7 d. At depths of 100-200 mm and 200-300 mm, extension by existing root axes was halted by soil flooding, while adventitious roots from above failed to penetrate these deeper zones. After 4 weeks waterlogging, all arrested root tips recommenced elongation when the soil was drained; their extension rates exceeding those of roots that were well-drained throughout. Growth in fresh mass was also stimulated. The additional aerenchyma found in adventitious roots in the upper 100 mm of soil may have been ethylene regulated since gas space development was inhibited by silver nitrate, an ethylene action inhibitor. The effectiveness of aerenchyma was tested by blocking the entry of atmospheric oxygen into plants with lanolin applied to lenticels of woody shoots of plants grown in solution culture. Root extension was halved, while shoot growth remained unaffected.

No abstract

No abstract

No abstract

Willow is common in riparian zones that are frequently waterlogged. There is interest in fast-growing willow for biomass, and land that is subject to flooding may be suitable for this purpose. Accordingly, we have studied the ability of willow to withstand poorly aerated root environments under controlled conditions. Rooted cuttings of Salix viminalis L. cv. Bowles Hybrid were grown in aerated or deoxygenated nutrient solution. They showed considerably reduced root growth when dissolved oxygen partial pressures were reduced from 21 kPa to 0-2 kPa. At below 1 kPa, adventitious roots extended at about one third the rate of aerated roots. Growth in dry mass was also curtailed. Root growth under these conditions could have been sustained either by oxygen transported to the roots internally from the shoot or by the root's ability to respire and grow without oxygen. These possibilities are being examined further. Aerenchyma (internally interconnected gas-filled space) is the most likely pathway for oxygen transport to the roots, from aerial parts. Therefore roots were investigated for the presence or absence of aerenchyma. Roots of all treatments had similar amounts of aerenchyma (20-30% of the cortex). Aerenchyma formation in Salix viminalis therefore appears to be constitutive and is not induced by poorly aerated conditions, as happens in cereal roots such as wheat and maize. In the latter species, ethylene (ethene) is thought to be responsible for inducing aerenchyma by promoting lytic breakdown of cortical cells. To test whether ethylene is active in willow, silver nitrate, a well-known inhibitor of ethylene action was applied. Silver nitrate was toxic at concentrations of 0.05 mg I" 1 and above but did not inhibit aerenchyma formation at lower non-toxic concentrations. Ethylene therefore is unlikely to be involved in aerenchyma development.