| Sumario: | The sesonal trends of nitrate reductase activily, of NO-3 induced activity of the same enzyme, and of the quotient between them, aRNi/aRNb, known as Nitrate Assimiletion Potential, were investigated. Assays were carried out in actively producing coffee trees, routinely fertilized and located in two different climatic zones: Central Valley and Atlantic Region of Costa Rica. Leaves from fruit-bearing bianches and from fruitless branches were analysed separately. Nitrate Reductase activily was found to vary in both locations according to rainfall and water content of soils and leaves. Induced activily of the enzyme was highest diving the dry season and in coincidence wilb firit maturation and flowering. Nitrale Assimilation Potential registered normal values for well nourished plants throughout the year, except when trees flowered or presented mature fruits.
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