Plant adaptations to anaerobic stress

It has been estimated that wetlands occupy approximately 6% of the earth’s land surfaces (Maltby 1991). They comprise fresh, brackish and salt-water marshes, inland and coastal swamps, flood plains, and lowland and upland mires (fen, bogs), as well as agricultural wetlands such as rice paddy. On a smaller scale there are constructed wetlands valued for their role in the purification of domestic, agricultural and industrial effluents (Cooper & Findlater 1990). In addition to these examples, occasional soil flooding and even a degree of submergence can be a common though unwelcome feature of non-wetland agriculture. Wetlands are usually characterized by permanent or long-term soil flooding, but often the vegetation may be wholly or partly submerged; in the lower reaches of coastal marshes this may be a daily or twice daily occurrence. Indeed, since wetlands are found world-wide and at most altitudes, flooding regimes can vary enormously; not only do the seasonal timing, duration and depth of flooding differ, but so too do light and temperature regimes and sediment type—factors which strongly interact with flooding to influence vegetation. The large numbers and wide variety of vascular plants indigenous to wetlands reflect this variety in habitat features; species range from wholly submerged aquatics of pools, rivers, coastal and lake margins through to the large tree species of the flood plain forests of the Americas and Africa. In view of this variety in both plant and habitat, a whole spectrum of flood-related stresses might be anticipated; similarly one might expect to find a number of flood-tolerance strategies.

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Mechanisms of flood tolerance in plants

CHAPTER 3 – Effects of Flooding on Growth and Metabolism of Herbaceous Plants

1. The overall implications of biological stress.- 2. Oxidative stress.- 3. Salt stress.- 4. Chilling stress.- 5. Freezing stress.- 6. Desiccation.- 7. Water and drought stress.- 8. Heat stress.- 9. Anaerobic stress - flooding and ice-encasement.- 10. Environmental pollution stress.- Epilogue.

Stress and stress coping in cultivated plants

Ethylene and responses of plants to soil waterlogging and submergence

The role of ethylene and auxins in flood-induced adventitious root formation and hypocotyl hypertrophy in sunflower (Helianthus annuus L. cv. Russian) plants was studied. Flooding without aeration (F) resulted in a steady increase in ethylene in hypocotyls, and flooding with aeration (FA) caused a transient increase. Low light intensity increased ethylene levels but decreased adventitious root formation. Treatment of shoots with benzyladenine (BA) increased ethylene content in non-flooded (NF) but not in F or FA shoots.



Twenty-four hours of flooding brought about a rise of endogenous indole-acetic acid (IAA) in hypocotyls. 14C-IAA applied to the shoot accumulated more in F and FA hypocotyls than in NF hypocotyls, and BA reduced this accumulation. There was less IAA metabolism in F and FA than in NF hypocotyls.



Tri-iodo benzoic acid (TIBA) applied to the hypocotyls of F plants inhibited root production. Benzyladenine (BA) applied to the leaves had similar effect but was not effective when supplied to the shoot apex. BA did not inhibit flood-induced hypocotyl hypertrophy. Ethrel did not affect adventitious root formation in NF plants but did increase hypocotyl thickening.



It is concluded that flood-induced adventitious root formation is stimulated primarily by an accumulation of auxins in the hypocotyls. Increases in ethylene might cause this auxin build up. Hypocotyl hypertrophy is presently thought to be the result of an interaction of auxin and ethylene with ethylene being the major factor.

The Role of Endogenous Auxins and Ethylene in the Formation of Adventitious Roots and Hypocotyl Hypertrophy in Flooded Sunflower Plants (Helianthus annuus)

The morphological changes of the shoots of Helianthus annuus cv. Russian caused by varying periods of flooding of the roots were studied. Attention was focused on adventitious root formation and hypocotyl hypertrophy which are known to be caused by excess water around the roots in the absence of anoxia. Twenty-four hours of flooding was found to be sufficient to cause both of these responses. Experiments involving flooding, leaf removal, shoot apical bud removal, two different light intensities, and a treatment in which only a small proportion of the roots were flooded suggested that a major factor causing the flood-induced changes in the hypocotyl arises in the root but other factors may come from the leaves. In the longer-term experiments other flood-induced changes in shoot morphology were leaf epinasty, death of shoot apex, extreme chlorosis and hypertrophy of nodes and internodes. There was no apparent contribution of adventitious roots to the survival of flooded plants.

Control of Adventitious Root Production and Hypocotyl Hypertrophy of Sunflower (Helianthus annuus) in Response to Flooding

The effects of flooding, flooding with aeration, and no flooding of the root system on shoot growth was studied in sunflower plants. The responses of shoots appear to be brought about by: (1) The anaerobic condition of the roots which causes stem dwarfing, chlorosis, and petiolar epinasty. (2) The presence of water in excess of field capacity (but not anoxia) around the roots which results in an increase in stem hypertrophy and the formation and growth of adventitious roots.

Effect of aeration on the flood-induced formation of adventitious roots and other changes in sunflower (Helianthus annuus L.)

Vegetative shoots of coastal Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] seedlings rapidly metabolized 1,2[3H]-gibberllin A4 (GA4) to GA2, GA34 and 6 unknown acidie, ethyl acetate-soluble compounds. The kinetics of formation of each compound depended upon the stage of ontogeny, metabolism being most rapid during bud set and bud break and least rapid during elongation. Translocation of radioactivity was greatest at the bud set stage. By hour 72 large amounts (65%) of neutral, ehter-soluble compunds were formed at the stage of bud break and lesser, but significant amounts (40%) at bud set. Less than 15% of the radioactivity went into neutral ether during the elongation stage. Douglas fir appears to have an efficient system for con-version of the relatively non-polar GA34. This, coupled with the formation of neutral, ether-soluble and acidie, n-butanol-soluble substances, may regulate the effective level of biologically active exogenous and/or endogenous GAs in this conifer.

Robert L. Wample

Papers

The Role of Endogenous Auxins and Ethylene in the Formation of Adventitious Roots and Hypocotyl Hypertrophy in Flooded Sunflower Plants (Helianthus annuus)

Control of Adventitious Root Production and Hypocotyl Hypertrophy of Sunflower (Helianthus annuus) in Response to Flooding

Effect of aeration on the flood-induced formation of adventitious roots and other changes in sunflower (Helianthus annuus L.)

Metabolism of Gibberellin A4 by Vegetative Shoots of Douglas Fir at Three Stages of Ontogeny