Phosphoenolpyruvate Carboxylase from the Crassulacean PlantBryophyllum fedtschenkoiHametet Perrier: ACTIVITY CHANGES AND KINETIC BEHAVIOUR IN CRUDE EXTRACTS

The total activity (capacity) of phosphoenolpyruvate carboxylasein crude extracts of leaves of Bryophyllum fedtschenkoi, measuredon a fresh weight basis, varies considerably with leaf positionand with photoperiodic treatment. No diurnal variation in capacitywas detected under long – day or short – day conditions.The enzyme in freshly prepared crude extracts is about 50 timesmore sensitive to inhibition by malate at pH 7 than that inaged extracts, or in the fully purified state. Desensitizationin extracts proceeds rapidly unless protective measures aretaken, and appears to be irreversible. A pH-dependence studyshows that the effect of desensitization on the kinetic parametersof malate inhibition is identical to that of increasing thepH by 2.0 units over a wide range, but the maximum velocityat pH 7.8 is virtually unaffected. The significance of the resultsfor both the experimental determination of cyclic changes inenzyme capacity, and the theories concerning the mechanism ofa circadian rhythm of phosphoenolpyruvate carboxylase activityin vivo are discussed.     

Influence of rodent density on nesting associations involving the Bar-tailed Godwit Limosa lapponica

The Bar-tailed Godwit Limosa lapponica rarely defends its nest aggressively against predators, but rather associates with other more aggressive species for protection. I studied the distribution of Bar-tailed Godwit nests relative to nests of the aggressive Longtailed Skua Stercorarius longicaudus, as well as to nests of two slightly less aggressive alternative 'protective umbrella' species, the Whimbrel Numenius phaeopus and the Grey Plover Pluvialis squatarola. Rodents are the main prey of the Long-tailed Skua but, in years when numbers of rodents are low, Long-tailed Skuas also eat eggs and chicks of other birds. Data on rodent densities and nest distribution were collected on Finnmarksvidda, Northern Norway, in 1987, 1989, 1990, 1992 and 1997, and on Taimyr, Siberia, in 1994 and 1995. It is concluded that the Bar-tailed Godwit is able to evaluate the probability of nest predation from Long-tailed Skuas, based on direct or indirect information about the status of local rodent populations. This information is used to choose between Long-tailed Skuas and other potential species for protection against nest predators.