Delayed Light Emission in Green Plant Meterials:Temperature-Dependence and Quantum Yield

The discovery of the delayed light emission of plant materials by Strahler and Arnold in 1951 has stimulated a good deal of interest in this rather remarkable property. The emitted light has been shown to be due to an electronic transition between the first excited singlet state of chlorophyll and the ground state. At room temperature, a luminescence is observable from about 0.01 seconds to several minutes after excitation. Thus, the electronic transition cannot be rate-determining and the process represents neither normal fluorescence nor normal phosphorescence. Indeed, there is some evidence that the decay curve of the luminescence is the resultant of more than one rate-limiting process. Strahler and co-workers have been able to demonstrate the existence of many relationships between delayed light emission and photosynthesis and thus have been led to interpret the luminescence phenomena as a consequence of the reversibility of some of the enzymatic photosynthetic reactions. Moreover, Tollin and Calvin have shown that the faster decaying components of the delayed light are present to as low a temperature as -100 C, suggesting that the early processes following light-absorption are non-enzymatic in nature. These latter observations, in conjunction with several other types of experimental and theoretical information, have suggested an interpretation of the physical processes leading to delayed light emission, and, by analogy, to photosynthesis, in terms of semiconductor theory. The earlier investigations in this laboratory have been limited to the study of the light emitted approximately 0.1 seconds after excitation by a flash discharge. The recent reports of luminescences at still shorter times after excitation have prompted the construction of a device capable of continuously observing the light emission of a sample of plant material from 0.0015 seconds to about 30 seconds after the onset of flash excitation. The present work describes a series of experiments carried out with this apparatus