Absorção de cálcio pelos tecidos da folha do cafeeiro (Coffea arabica L., var. Mundo novo)

Researches were conducted in order to: a. detect mechanisms involved on the calcium absorption by leaves. b. calcium uptake as function of potassium, magnesium an boron in the external solution. The leaf tissued used for experiments came from young coffee trees growing in a Piracicaba grove. The tissue was prepared by cutting it into slices of 300 m width. Slices after cuted were placed into a large volume of deionized water and rinsed several times to remove ions released from demaged cells. Slices were randomly allocated to batches, and placed to into 500 ml of 45CaCl2 labelled areted solution for 60 minutes. The experimental solutions contained 5x10-6M, 10-5M, 2x10-5M, 4 x l0-5M, 8 x 10-5M, 1,6 x 10-4M, 3,2x10-4M, 6,4x10-4M, 1,28x10-3M e 2,56x10-3M. concentration of 45CaCl2 Experiments were carried out at 30°C. Light was provided by four 20W fluorescent tubes (3000 lux). The possible occurence of interactions and mutual competitions in the absorption of potassium, magnesium and boron was investigated in a series of experiments. Each solution contained KC1 10-4 and 10-3M, MgCl2 5 x 10-3M and H3BO3 5 x 10-3M. The slices were then removed, blotted, dried at 70°C, and weighed. The material was ashed at 600°C, dissolved with 2 ml HC1 0,01N. Aliquot were transferred to planchet, dried, and the radioactivety was assayed. The data from all experiments were run through statistical estimations (regression analysis and Student's «t-test»). Calcium influx was reduced and inhibited by DNP, N2 and also by temperature (10°C). The theory and terminology of active transport used was the same as applied by Epstein and Hagen (1952) for root tissues. The concluding comments are: a. the absorption of calcium is an actif process, involving a single mechanism. b. potassium and boron ions induces a stimulatory efect on calcium absorption. c. magnesium ions presents a non competitive inhibition interference on calcium.A absorção de cálcio pelos tecidos de folha do cafeeiro (Coffea arabica L., var. Mundo Novo) foi estudada em cortes transversais feitos na porção média do limbo foliar de amostras de folhas colhidas em plantas cultivadas em condições de campo. O estudo teve como objetivos, caracterizar o mecanismo de absorção de cálcio e avaliar os efeitos de K, Mg e B na absorção daquele nutriente. A técnica experimental padrão empregada em todos os ensaios constituiu em submeter cortes de 300 m de espessura a uma série de soluções com concentrações crescentes de cálcio, contendo 45Ca como traçador, luminosidade de 3000 lux, temperatura e arejamento constantes. A série de soluções de cálcio foi constituída das seguintes concentrações de CaCl2: 5x10-6M, 10-5M, 2x10-5M;, 4 x l0-5M, 8 x 10-5M, 1,6 x 10-4M, 3,2x10-4M, 6,4x10-4M, 1,28x10-3M e 2,56x10-3M. Os ensaios realizados foram: absorção de cálcio na fase vegetativa e de florescimento do cafeeiro, considerados com os padrões, absorção de cálcio na presença de inibidores metabólicos (2,4-DNP 10-5M, borbulhamento com N2 e temperatura de 10°C); absorção de cálcio na presença de KC1 5x10-3M, KC1 10-4M, MgCl2 5x10-3M e H3BO3 5x10-3M. Os resultados obtidos foram submetidos a análise de regressão e comparados estatisticamente pelo teste «t». Após a análise estatística, foram calculados os valores dos parâmetros Km e Vm, os quais demonstraram que a absorção de cálcio, pelos tecidos foliares, é metabólica e que os íons K e B provocam um efeito estimulatório, enquanto que o Mg tem um efeito inibidor de caráter não não competitivo