Determinação da temperatura letal de folhagem de cafeeiro em noite de geada

Abstract

In order to determine lethal temperatures on coffee trees, leaves taken off from these plants were exposed in frost nights at different micro and topoclimatic conditions. If the humidity gradient of the exposed surfaces during the night is to the effect of water condensation, the leaves nor- mally keep turgid. As the micro and topoclimatic conditions of exposure can be very much variated during the night, the most varied conditions of thermic minima can be obtained, for the study of frosts in an elevated and cold region. Results obtained were the following: a) the first damages on the coffee three leaves apprared whan mi- nimum temperature reached —2°C;b) serious and generalized damages on the leaves only occurred when temperature on minimum thermometers fell below —3°C; c) such leaves as covered by ice crystals, typical of frosts, or by frozen dew, were not normally damaged once minimum temperature did not fall below —2°C; d) direct insolation on frozen coffee trees and their leaves did not show any effect on the frost damages, neither aggravating nor attenuating them; e) no consistent results were observed in regard to frost susceptibility of four different varieties of coffee trees, although two of them were ori­ ginated from seeds of plants which had previously shown resistance to the phenomenon; f) when leaves, taken off plants, were exposed on poles at different heights above the soil, the method of exposure presented satisfactory results. T h e effects of low temperature and low intensity of damages, on leaves taken off plants as well as those naturally attached to plants, were practi­ cally identical. The method based on leaves taken off plants render's possible a great flexibility and facilities in the work as it allows to submit them to different minimum temperatures, at field conditions, all in an only night •which is judged favourable to the work.