Anoxia tolerance of Acorus calamus, Iris pseudacorus and Vaccinium macrocarpon has been investigated by incubating whole plants under anaerobic conditions in the dark. Long-term survival of rhizomes under anoxia has been described in previous studies, but this study has shown that green leaves can also endure anoxia for prolonged periods. Leaves of A. calamus, I.pseudacorus and V.macrocarpon remained green and turgid under anoxia for up to 75d, 60d and 45d respectively. All growth processes ceased in leaves under anoxia. Anaerobic energy production via ethanol fermentation was active in all investigated plant organs as shown by the accumulation of ethanol. Low rates of anaerobic CO2 production indicated however, that the overall metabolic activity in the leaves was low under prolonged anoxia. The leaves seemed to adapt to the anaerobic conditions by an overall reduction of energy consumption rather than acceleration of the glycolytic rate. The demands for fermentable substrate were met by the mobilisation of internal carbohydrate reserves in leaves of V.macrocarpon. A.calamus and I.pseudacorus leaves contained only small amounts of carbohydrates, and these leaves possibly received carbohydrates from the stores in the rhizome. Prolonged anoxia considerably affected the leaf capacity for respiration and photosynthesis. After 28d of anoxia, respiratory capacity was reduced in A.calamus and V.macrocarpon by 80%, and in I.pseudacorus by 90-95%; this corresponded with a decline in the activity of the cytochrome c oxidase. The photosynthetic capacity of leaves was decreased after 28d of anoxia by 83% in A.calamus, by 97% in I.pseudacorus and by 80% in V.macrocarpon. The reduction in the photosynthetic capacity was accompanied by alterations in the chlorophyll fluorescence pattern indicating damage to the PSII reaction centre and the subsequent electron transport; only minor changes occurred in the chlorophyll content of anaerobic leaves. On return to air and light, recovery of respiration and photosynthesis occurred in the leaves, but species-specific differences were observed in the speed of recovery. Among the three investigated species, A.calamus leaves endured the anoxic conditions longer than leaves of the other two species; and on return to air, A.calamus leaves showed the most rapid recovery. A.calamus was characterised by efficient carbohydrate utilisation under anoxia. Cellular membranes and organelle ultrastructure appeared to be stable in A.calamus leaves for at least 28d of anoxia.

The relative iron (II) tolerance of a range of wetland plants was determined and compared with some species characteristic of well drained soils. A wide range of tolerance occurred amongst the wetland species but they were generally more tolerant than those from well drained soils. No correlation was found between iron (II) tolerance and the amount of air space {% v/v) (aerenchyma) in the roots of these species. There was a significant negative correlation between air space and iron uptake by roots. This may have been caused by iron (II) oxidation in the rhizosphere resulting in decreased availability. There was evidence that differential iron (II) tolerance of excised root tips was maintained under aerobic and anaerobic conditions. It was thus suggested that iron (II) tolerance may not be dependent on iron exclusion or oxidation of iron (II) by oxygen diffusing through the aerenchyma. Levels of malic and citric acids in roots were altered by iron (II) sulphate, but the absolute levels and changes in levels had no correlation with the iron (II) tolerance, of the species. Peroxidase and catalase activities in root tips of plants gown in drained and flooded sand culture were measured and considered in relation to the oxidising power of roots. Activity was detected in all species examined but was generally I unaffected by flooding. Evidence from the literature suggested that these enzymes of peroxide metabolism are unlikely to be active in flooded roots and so could not mediate their oxidising power. The structure of root aerenchyma had great variability between species. The Cyperaceae had the most complex and well organised structure. Growth under flooded conditions increased air space in most species, but there were exceptions. In Eriophrum angustifolium and E. vaginatum air space was high under drained conditions and was not increased by flooding. In Filipendula ulmaria the small amount of air space was not increased by flooding. Low nutrient levels increased air space production in Nardus stricta. The function of aerenchyma and the influence of environmental factors on its production are discussed.