INTRODUCTION Express methods of monitoring the physiologi- cal state of plants grown under various environmental conditions are presently in enormous demand. Lumi- nescence analysis is most commonly used for this purpose. One of the methods of luminescence analysis of plant leaves is based on comparison of two main spectral peaks of fluorescence in the red spectral re- gion. One spectral peak is within the wavelength range 682–686 nm (F 682 ) and another spectral peak is at 730–742 nm (F 734 ) [1–3]. The fluorescence inten- sity is measured separately at each peak and then the fluorescence intensity ratio (F 734 /F 682 ) is calculated. The F 734 /F 682 ratio was shown to depend on the fluo- rescence excitation wavelength and sample tempera- ture [2, 5]. Some researchers believe that this ratio correlates with chlorophyll concentration in leaves [2–4], whereas other authors suggest it to be a spe- cies-specific character [1]. It was reported in [5] that the F 734 /F 682 ratio depends on the physiological state of plant. However, thorough analysis of the results re- ported in [1–5] showed that there is no unambiguous interpretation of correlations between the F 734 /F 682 ra- tio and other characteristics of plant growth. There are several viewpoints on the mechanisms of generation of luminescence at 680 and 730 nm in plant leaves. Some researchers believe that the fluo- rescence spectrum is determined by the degree of chlorophyll aggregation and reabsorption of light quanta [6,7]. According to an alternative viewpoint, the fluorescence peaks at 685 and 730 nm are due to chlorophyll of photosystem (PS) II and PS I, respec- tively [2]. Yet another author suggests that fluores- cence represents natural energy losses and it is emit- ted by any pigment molecules incorporated in light- harvesting and focusing complexes, whereas both 685 Biophysics, Vol. 45, No. 4, 2000, pp. 685–692. Translated from Biofizika, Vol. 45, No. 4, 2000, pp. 704–711. Original Russian Text Copyright © 2000 by Zavoruev, Zavorueva, Shelegov. English Translation Copyright © 2000 by MAIK “Nauka / Interperiodica” (Russia). COMPLEX SYSTEMS BIOPHYSICS Fluorescence of Cucumber Leaves Induced within the Photoexcitation Wavelength Range 380–540 nm and Its Dependence on Vegetation Time and Illumination Regime V. V. Zavoruev 1 , E. N. Zavorueva 2 , and A. V. Shelegov 1 1 Institute of Computer Modeling, Siberian Division, Russian Academy of Sciences, Akademgorodok, Krasnoyarsk, 660036 Russia 2 Institute of Biophysics, Siberian Division, Russian Academy of Sciences, Akademgorodok, Krasnoyarsk, 660036 Russia Received September 12, 1997; in final form, April 6, 2000 Abstract—A model of slow fluorescence induction excited in cucumber leaves by broad-band light (380–540 nm; 180 W/m 2 ) was considered. The ratio of fluorescence peaks in the red spectral region (F 734 /F 682 ) was found to be close to two in both young and mature leaves. This value is supposed to be deter- mined by the spillover-mediated increase in the contribution of long-wavelength fluorescence. The F 734 /F 682 value in plants grown under natural conditions correlated with chlorophyll and carotenoid concentrations. In plants grown under exposure to red and blue light, such correlation was not observed and the F 734 /F 682 ratio was maintained at a constant level. It is concluded that the F 734 /F 682 ratio depends on both the intensity and the spectral composition of actinic light used for growing plants. Key words: slow fluorescence induction, cucumber leaves, vegetation time, illumination regime Abbreviations: SFI, slow fluorescence induction; PS I and PS II, photosystem I and photosystem II; LHC, light-harvesting complex; PAR, photosynthetically active radiation