PHOTOSYSTEM-II QUANTUM YIELD AS A MEASURE OF RADICAL SCAVENGERS IN CHILLING INJURY IN CUCUMBER FRUITS AND BELL PEPPERS - A STATIC, DYNAMIC AND STATISTICAL MODEL

Cucumber fruits (Cucumis sativus L. cv. Jessica) and green bell pepper s (Capsicum annuum L. cvs. Lokas and Medeo) were stored at different t emperatures ranging from 2 to 12 degrees C. After three different stor age periods, fruits from each temperature were transferred to 20 degre es C for 7 d to allow for the development of visual symptoms of chilli ng injury (CI). During storage, the photochemical quantum yield of pho tosystem II (PSII) in peel tissue adapted to darkness, was calculated from measurements of pulse-amplitude-modulated chlorophyll fluorescenc e. The decrease in PSII quantum yield during storage at low temperatur es in darkness can be described as a temperature-dependent inhibition of an enzyme according to Arrhenius, assuming a negative activation en ergy. By comparison with the radical-scavenger measurements of Hariyad i and Parkin (1991, Postharvest Biol. Techn. 1, 33-45) it is postulate d that the time and temperature dependence of the quantum yield parall els the diminution of radical-scavenging activity at lower temperature s in cucumber and capsicum fruits. This is combined with an equation f or the process of radical scavenging itself and an equation for the au to-catalytic radical-producing lipid peroxidation reaction. These thre e basic processes lead to both a static and a dynamic model for the oc currence of chilling injury in low-temperature-sensitive plant tissue. A statistical fit of the measured data using the static model leads t o the estimates of the different activation energies and reaction rate s with a high degree of accuracy. The estimated values are in accord w ith what one would expect on the basis of knowledge of the processes l eading to chilling injury, and directly point to meaningful physico-ch emical parameters.