Abstract:
In determinations made with a porometer it was verified that the stomatal opening of coffee leaves increases in size immediately after the leaves are detached from the plant (fig. 1). This fact indicates that the method of rapid weighings of detached leaves is not suitable for studies on the transpiration of the coffee plant. The transpiration of coffee plants has been determined by the weighing potted plant method. Pots with a capacity for 51 liters were used. They were made of galvanized iron plate and had no drainage hole. When the moisture content of the soil approached the wilting point water was added to bring all the soil mass to fiel capacity. In order to avoid oxygen depletion the pots were not sealed. To decrease the direct evaporation at the soil surface a two-centimeter layer of rice hulls was used as mulch, and a celotex cover was placed on top of the pot. Three similar pots with the same type of protection, but without plants were used to measure the direct evaporation from the soil surface. The weight losses of these pots were subtracted from the weight losses of the pots with plants. Two to throe-year old plants of Coffea arabica var. bourbon were used. Three plants were employed each month and after this time their leaves were picked off, and the total leaf area was determined. The tests were carried out for a whole year. The transpiration rates were calculated in g/dm2/day. The total leaf area of an adult coffee plant was found to be 3,146 dm2 (average of seven trees). Based on this average and on the transpiration rate of the experimental plants it was possible to calculate approximately the amount of water that an adult coffee plant would transpire under the same conditions. Since the plants submitted to the experiment did not suffer water shortage at any time, the present results show the approximate amount of water lost by the coffee plant under optimal soil-water conditions. The data are presented in table 1. The figure 1 shows the monthly amount of water lost by transpiration in a coffee* plantation compared with the monthly rainfall during a year. The environmental factors that influence plant transpiration vary much less between the same months of different years than between the different months of the same year. Hence the results obtained should not be far from the average; that would be obtained if the transpiration had been measured during a number of years. Comparing the water lost by transpiration of coffee trees with the average rainfall in'Campinas, State of São Paulo, Brazil, it is seen that from October to March rainfall exceeds water consumption. In April, June, and September the excess is small, and in May, July, and August the amount of water transpired exceeds the average rainfall (fig. 2). Since the rainfall distribution is variable, this figure may be different in a given year. The highest daily rate of transpiration encountered was 17.6 mg/dm2/m (over twelve; hours). Previous results obtained by the writers showed 18.2 mg/dm2/m to be the rate of transpiration in a very sunny and hot day. Nutman working in Africa (2) found a maximum daily rate of 20.67/dm2/m (over nine hours). These data are comparable and must represent approximately the maximum rate of transpiration of the coffee plant. The average daily transpiration encountered was 6.20 g/dm2/day and the total amount of water transpired by a coffee tree in one year was found to be 7,273 liters.
