Studies on lichen-dominated systems. XXII. The environmental control of nitrogenase activity inStereocaulon paschalein sprucendash;lichen woodland

摘要

Stereocaulon paschale(L.) Hoffm. collected from sprucendash;lichen woodland (Northwest Territories) at different times of the year was assayed for nitrogenase activity (acetylene reduction) under factorial combinations of photosynthetically active radiation (PAR), thallus temperature, and moisture. Marked seasonal changes in C2H2-reducing capacity were recorded, potential activity being minimal in samples from beneath snow beds in March and maximal in summer collections. Physiologicalndash;environmental relationships established in the laboratory were largely consistent with diurnal and seasonal variations in nitrogenase activity inS. paschalemats recordedin situ. When thallus water content was not limiting (i.e., values ensp;120 of thallus dry weight), diurnal variation in nitrogenase activity was correlated with changes in PAR and thallus temperature or, under isothermal conditions, with changes in PAR alone when C2H2reduction continued throughout dark periods at ca. 40 of daylight rates. However, while nitrogenase activity inS. paschaleat any point in time was dependent upon concurrent environmental conditions, it also was strongly influenced by pretreatment: both winter frost and thermal stress during summer droughts impeded recovery of nitrogenase activity following the onset of ameliorative conditions. It is suggested that a large fraction of the total annual N assimilation byS. paschalein drier, continental subarctic habitats may be achieved in summer during a few comparatively short episodes of suitable moisture conditions.