VAPOR PRESSURE GRADIENTS, WATER DISTRIBUTION IN FRUITS, AND SO‐CALLED INFRA‐RED INJURY

Abstract

IT HAS long been recognized that plant tissues exposed to strong sunlight or to light from artificial sources may become so heated by the absorbed radiation as to become severely injured or killed. It has also been recognized that ultra-violet light, at intensities greater or at wave lengths shorter than those received at the earth's surface, is likely to be distinctly injurious. Arthur (1932) suggested that some of his experiments with apples indicate specific infrared injury which seems independent of any heating effect. Later (1936), referring to these same experiments, he gave much the same interpretation but stated that possibly an equal amount of energy in the visible region may have produced similar injury. His conclusions were based chiefly on the following observations. Apples in a cold storage room if exposed to radiation from a General Electric S-1 lamp for 5 days or more developed a wrinkled necrotic area on that side of each apple exposed to the lamp. This was first thought to be due to ultra-violet injury but, since apples placed in a cold box with air temperature of 2?C. and exposed to light from an ordinary 500 watt incandescent lamp filtered through window glass showed similar injury, it was obviously not due to ultra-violet radiation. Repeating the experiment, using heat-transmitting black glass, the same type of injury resulted. Since the injured side of the fruit reached a temperature of only about 220C., Arthur concluded that this was not sufficiently high to have caused the injury, but that, injury was probably due to the direct action of infra-red on a tissue which absorbs it freely. In a recent paper (Curtis, 1936) the great importance of temperature gradients in their effects on vapor pressure gradients and the movement of water in vapor form has been emphasized. The fruits Arthur worked with were in a cold chamber with an air temperature of 2?C. He reported an internal fruit temperature of 220C. but did not specify where this temperature was taken. Presumably it was taken near the upper irradiated surface. The irradiated side of the fruit, therefore, was probably at a temperature nearly 200C. above that of the shaded side. With such a temperature gradient and resulting steep vapor-pressure gradient, one wouldexpect a distillation of water from the warm side to the cool side. In order to test this hypothesis-that is, to determine if the withering and necrosis could be due to internal distillation of water from one side to the other and not to any specific infra-red injury-we carried out two types of experiments, one somewhat similar to that of Arthur in which the fruits were in