A absorção de cálcio pelos tecidos de folha do cafeeiro (Coffea arábica 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 consistiu em submeter cortes de 300µ de espessura a uma série de soluções com concentrações crescentes de cálcio, contendo a ⁴⁵Ca como traçador, luminosidade de 3000 lux, temperatura e arejamento constantes. A série de soluções de cálcio foi constituída das seguintes concentrações de CaCl₂: 5 x 10^-6M, 10^-5M , 2 x 10^-5M , 4 x 10^-5M, 8 x 10^-5M, 1,6 x 10^-4M, 3,2 x 10^-4M, 6,4 x 10^-4M, 1,28 x 10^-3M e 2,56 x 10^-3M. Os ensaios realizados foram: absorção de cálcio na fase vegetativa e de florescimento do cafeeiro, considerados com o padrões; absorção de cálcio na presença de inibidores metabólicos (2,4-DNP 10^-5, borbulhamento com N₂ e temperatura de 10°C); absorção de cálcio na presença de KCl 5 x 10^-3M, KCl 10^-4M, MgC1₂ 5 x 10^-3M e H₃BO₃ 5 x 10^-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 de caráter competitivo.

Studies on calcium absorption by Coffee (Coffea Arabica L. var. Mundo Novo) leaves. Reaearches were conducted in arder 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 tissue used for experiments carne from young coffee trees growing in a Piracicaba grave. The tissue was prepared by cutting it into slices of 300 µ width. Slices after cutted were placed into a large volume of deionized water and rinsed several times to remove ions released from damaged 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 5 x 10^-6M, 10^-5M , 2 x 10^-5M , 4 x 10^-5M, 8 x 10^-5M, 1,6 x 10^-4M, 3,2 x 10^-4M, 6,4 x 10^-4M, 1,28 x 10^-3M e 2,56 x 10^-3M. concentration of ⁴⁵Cacl₂. Experiments were carried out at 30°C. Light was provided by four 20W fluorescent tubes (3000 lux). The possible occurrence of interactions and mutual competitions in the absorption of potassium, magnesium and boron was investigated in a series of experiments. Each solution contained KCl 10^-4M and 10^-3M , MgCl₂ 5 x 10^-3M H₃BO₃ 5 x 10^-3M. The slices were then removed, blotted, dried at 70°C, and weighed. The material was asked at 600°C, dissolved with 2 ml HCl 0,01N. Aliquot were transferred to planchet, dried, and the radioactivity 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 effect on calcium absorption. c. magnesium ions presents a non competitive inhibition interference on calcium.