EVAPORATION OF WATER PROM APPLES IN RELATION TO TEMPERATURE AND ATMOSPHERIC HUMIDITY

Abstract

SUMMARY.A technique has been described for determining by a weighing method the rate of evaporation of water from certain isolated plant organs (apples and pears) under conditions of constant temperature and constant atmospheric humidity.The rate of evaporation from Bramley's Seedling apples at constant temperature and humidity changes steadily throughout the storage phase; decreasing at first rapidly and then more slowly towards a more or less steady state. It is evident that the rate of evaporation is conditioned by certain internal factors, the nature of which is at present unknown; the magnitude of the control exerted by these factors is observed to change with time. No sharp changes in rate occur during this period, and so far it has not been possible to correlate the smooth transition with phenomena of metabolic activity, such as change in osmotic pressure of the cell solutions.The relation between evaporation rate and R.H. at constant temperature shows a marked deviation from the linear, as exhibited by a free‐water surface. There is an increase in the rate of evaporation with decreasing R.H. down to about 65 per cent. R.H.; a gradual decline in the rate of increase being shown for each decrement of R.H. The resistance to outward movement of water vapour would therefore appear to change with change of external vapour pressure. Below about 65 per cent. R.H. it would seem that a steady rate might be expected, when internal resistances would become Hmiting.At constant R.H. and temperatures between the limits of 3 and 15° C. the rate of evaporation is more nearly approximate to the vapour‐pressure deficit of the external atmosphere.The variation in evaporation rate between individual apples of the same variety under the same conditions of temperature and humidity is considerable. These variations cannot be attributed to differences in the thickness of the cuticles, since a marked negative correlation between evaporation rate and thickness of cuticle was found in Cox's Orange Pippin apples. Other factors such as frequency of distribution and size of lenticels and the structure of the stem and calyx ends of the fruit probably exert a controlling influence over the magnitude of evaporation rate.The increase in rate of evaporation which occurs with fungal invasion of the tissues would seem to indicate that, although in the healthy fruit some control over the supply of water to the evaporating surface is exerted by the cortical tissues, the extent of this control is small in comparison with that exerted by the epidermis.Grateful acknowledgment is made to Drs Franklin Kidd and Cyril West for their very helpful criticism and advice, and to Miss Ursula Tetley for her co‐operation.