Patterns of peroxidative ethane emission from submerged rice seedlings indicate that damage from reactive oxygen species takes place during submergence and is not necessarily a post-anoxic phenomenon.

Using ethane as a marker for peroxidative damage to membranes by reactive oxygen species (ROS) we examined the injury of rice seedlings during submergence in the dark. It is often expressed that membrane injury from ROS is a post-submergence phenomenon occurring when oxygen is re-introduced after submergence-induced anoxia. We found that ethane production, from rice seedlings submerged for 24-72 h, was stimulated to 4-37 nl gFW(-1), indicating underwater membrane peroxidation. When examined a week later the seedlings were damaged or had died. On de-submergence in air, ethane production rates rose sharply, but fell back to less than 0.1 nl gFW(-1) h(-1) after 2 h. We compared submergence-susceptible and submergence-tolerant cultivars, submergence starting in the morning (more damage) and in the afternoon (less damage) and investigated different submergence durations. The seedlings showed extensive fatality whenever total ethane emission exceeded about 15 nl gFW(-1). Smaller amounts of ethane emission were linked to less extensive injury to leaves. Partial oxygen shortage (O(2) levels <1%) imposed for 2 h in gas phase mixtures also stimulated ethane production. In contrast, seedlings under anaerobic gas phase conditions produced no ethane until re-aerated: then a small peak was observed followed by a low, steady ethane production. We conclude that damage during submergence is not associated with extensive anoxia. Instead, injury is linked to membrane peroxidation in seedlings that are partially oxygen deficient while submerged. On return to air, further peroxidation is suppressed within about 2 h indicating effective control of ROS production not evident during submergence itself.

Kinetics of ethanol and acetaldehyde release suggest a role for acetaldehyde production in tolerance of rice seedlings to micro-aerobic conditions.

BACKGROUND AND AIMS: This paper examines the basis of the greater tolerance of an indica rice cultivar FR13A to complete submergence compared with relatively intolerant japonica rice CT6241. We study whether this superior tolerance is related to its greater tolerance to O2 shortage and to an ability to run a more favourable rate of alcoholic fermentation during and after O2 deprivation. METHODS Fermentation products were analysed using sensitive laser-based photoacoustics at high time resolution to establish patterns and rates of ethanol and acetaldehyde emission by intact rice seedlings exposed to micro-aerobic (0.05-0.5 % O2) or zero O2 supply, and also during their return to air. Oxygen and CO2 emission or uptake was also quantified. KEY RESULTS: In the dark, no acetaldehyde and ethanol emission was observed until external O2 concentration in a gas phase decreased to

Dynamic aspects of alcoholic fermentation of rice seedlings in response to anaerobiosis and to complete submergence: relationship to submergence tolerance.

Rice plants are severely damaged by complete submergence. This is a problem in rice farming and could be the result, in part, of tissue anoxia imposed by a reduced availability of oxygen. To investigate this possibility we monitored alcoholic fermentation products as markers for tissue anaerobiosis using sensitive laser-based spectroscopy able to sense ethanol and acetaldehyde down to 3 nl l(-1) and 0.1 nl l(-1), respectively. Acetaldehyde emission began within 0.5 h of imposing an oxygen-free gas phase environment followed closely by ethanol. As treatment progressed, ethanol output increased and came to exceed acetaldehyde emission as this stabilized considerably after approx. 3 h. On re-entry of air, a sharp post-anaerobic peak of acetaldehyde production was observed. This was found to be diagnostic of a preceding anoxic episode of 0.5 h or more. When anaerobiosis was lengthened by up to 14 h, the size of the post-anaerobic acetaldehyde outburst increased. After de-submergence from oxygen-free water, a similarly strong but slower post-anaerobic acetaldehyde upsurge was seen, which was accompanied by an increase in ethanol emission. Light almost, but not completely, eliminated fermentation in anaerobic surroundings and also the post-anaerobic or post-submergence peaks in acetaldehyde production. All photosynthetically generated oxygen was consumed within the plant. There was no substantial difference in acetaldehyde and ethanol output between FR13A and the less submergence-tolerant line CT6241 under any submergence treatment. In some circumstances, submergence damaged CT6241 more than FR13A even in the absence of vigorous fermentation. We conclude that oxygen deprivation may not always determine the extent of damage caused to rice plants by submergence under natural conditions.