Fat Metabolism in Higher Plants. III. Enzymatic Oxidation of Glycerol.

Abstract

, 22 and 23). Numerous plants of tropical and subtropical origin are susceptible to such injury and include many horticultural crops now grown in temperate climates. This physiological disorder is often referred to as chilling injury and should not be confused with injury due to freezing. Temperatures in the range from 0 to 100 C will be referred to as chilling temperatures. The svmptoms of this disorder include surface pitting, internal browning, increased susceptibility to decay, and in the case of some fruits, failure to ripen. Although chilling injury is an interesting and important problem in plant physiology, very little is known regarding processes affected by chilling temperatures or possibly explanations of the primary injury. Conflicting reports relative to the respiratory responses of various cold-sensitive plant materials emphasize the need for a thorough investigation of the problem. Jones (5) reported a larger temperature coefficient for CO2 production of papaya fruits between 7.2 and 10° C than for other temperature ranges above or below that of 10 to 7.20 C. This indicated to him a marked break in the nature of the processes having to do with CO2 production of the papaya below 10° C. Gane (3), however, presents data showing only slight deviations in the temperature coefficients for CO2 production of bananas at temperatures of 0, 5, 12.5, and 20° C. Platenius (15) concluded that neither the respiratory rates nor the respiratory quotients of cold-sensitive crops (includ-ing cucumbers) held at 0.50 C showed deviations from the results of those held at 10 or 240 C that would suggest an abnormal rate or course of respiration associated with chilling injury. However, Mack and Janer (8) reported a threefold increase in the rate of CO2 production of cucumbers during a 3-week 1 period in the temperature range of 2.2 to 3.30 C. They also reported a low initial respiratory quotient (0.45) for cucumbers held in this temperature range. Both papers mentioned above are in general agreement regarding the response of cucumber fruits held at temperatures of 100 C or above; i.e., the rate of CO2 production decreased with time, and the respiratory quotients were near unity. The purpose of the present investigation was to study the physiology of chilling injury to cucumbers by a comparative evaluation of the rates of CO2 production and the respiratory quotients of fruits held at various constant chilling and non-chilling temperatures , and the rate of CO2 production at 25° C subsequent to various chilling treatments. MATERIALS AND METHODS Cucumber fruits (Cucumis sativus L.) of the Cubit variety, ranging in length from 12 to 18 cm, were obtained from vigorous plants and handled carefully to minimize mechanical injury. To facilitate comparisons between tests, all experimental fruits, except where otherwise stated, were harvested in the morning. Comparable samples were selected and placed in respiratory chambers at constant temperature between 11:00 A.M. and 2:00 P.M. of the same day. The first determination was usually made the next morning, allowing sufficient time to establish temperature and gaseous equilibrium. Respiratory responses were determined by three methods. Carbon dioxide production was measured by the method of Claypool and Keefer (2) in which an air stream of known flow is equilibrated with a buffered bromthymol blue solution and the percentage CO2 estimated colorimetrically. The method was modified in that the water-saturated air passing over the fruit was not freed of CO2 and the indicator solution was renewed for each determination. It was found that about one liter of air must be bubbled through the 10 ml of the indicator solution to establish equilibrium. To correct for the CO2 content of the air entering the fruit chamber, the air was assumed to contain 0.03 % CO2 at all times. The rate 308