Physiological and biochemical responses of overwintering alfalfa to anoxia
A. Bertrand, Y. Castonguay, P. Nadeau, P. Rochette, G. Belanger, R. Michaud, S. Laberge, and L. Couture. Agriculture and Agri-Food
Canada Research Centre, Sainte-Foy, Quebec, Canada G1V 2J3.
The formation of an impermeable ice sheet over perennial plants during winter induces modifications of the atmosphere around the plants. The resulting anaerobic conditions can affect the winter survival of economically important forage crops. The sensitivity of alfalfa exposed to a progressively developing anoxic stress was assessed in an experiment performed under simulated winter conditions at a site near Quebec City.
Alfalfa was exposed to a progressively developing anoxic stress by enclosing potted plants in gas-tight plastic bags in late fall and exposing them to simulated winter conditions in an unheated greenhouse. Furthermore, four pots filled only with soil were used to assess the contribution of the microflora to atmospheric changes. Gas composition in bags, regrowth potential and biochemical components related to cold acclimation and anoxic metabolism were measured at different intervals after the initiation of the anoxia treatment. Anoxic conditions (0% O2) were reached after 60 d of enclosure for alfalfa and 106 d in bags with bare soil. This indicates that a large proportion of the O2 consumption was attributable to the soil microflora activity. Long-term exposure to anoxia caused a striking decrease in regrowth potential. Carbohydrate concentrations were modified by anoxia: starch and sucrose declined whereas stachyose and raffinose increased as compared to controls. Ethanol, an end product of anaerobic metabolism, increased under anoxia and reached the highest level at the end of the anoxia treatment. The activity of alcohol dehydrogenase, a key enzyme in the biosynthesis of ethanol, increased under anoxia. Furthermore, the expression of the alcohol dehydrogenase gene transiently increased during fall acclimation and was markedly induced by the anoxia treatment as early as December when plants were under hypoxic conditions (6% O2).
We conclude that the presence of a lasting impermeable cover such as ice may affect winter survival and yield of alfalfa if extreme concentrations of 0% for O2 and 15% for CO2 are reached. The development of anoxic conditions under these conditions depends on heterotrophic activity as well as plant metabolism.
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