Long-term anoxia tolerance in leaves of Acorus calamus L. and Iris pseudacorus L

Mature green leaves of Acorus calamus and Iris pseudacorus have been shown to survive at least 28 d of total anoxia in the dark during the growing season, increasing up to 75 d and 60 d in overwintering leaves in A. calamus and I. pseudacorus, respectively. During the period of anaerobic incubation the glycolytic rate is reduced, carbohydrate reserves are conserved and ethanol levels in the tissues reached an equilibrium. Prolonged anoxia significantly suppressed leaf capacity for respiration and photosynthesis. After 28 d of anoxia, respiratory capacity was reduced in A. calamus and I. pseudacorus by 80% and 90%, respectively. The photosynthetic capacity of leaves decreased by 83% in A. calamus and by 97% in I. pseudacorus after 28 d of anoxia. This reduction in photosynthetic capacity was accompanied by a modification of the chlorophyll fluorescence pattern indicating damage to the PSII reaction centre and subsequent electron transport. Chlorophyll content was only slightly reduced after 28 d under anoxia and darkness in A. calamus, whereas there was a 50% reduction in I. pseudacorus. On return to air A. calamus leaves that endured 28 d of anoxia recovered full photosynthetic activity within 7 d while those of I. pseudacorus had a lag phase of 3-10 d. This well-developed ability to endure prolonged periods of oxygen deprivation in both these species is associated with a down-regulation in metabolic activity in response to the imposition of anaerobiosis. It is suggested that when leaf damage eventually does take place in these species after protracted oxygen deprivation, it is anoxic rather than post-anoxic stress that is responsible.