1021-4437/01/4804- $25.00 © 2001 MAIK “Nauka /Interperiodica” 0473 Russian Journal of Plant Physiology, Vol. 48, No. 4, 2001, pp. 473–477. Translated from Fiziologiya Rastenii, Vol. 48, No. 4, 2001, pp. 551–555. Original Russian Text Copyright © 2001 by Christov, Stefanov, Goltsev, Abrasheva. INTRODUCTION Grapevine fanleaf and stem pitting diseases are widespread in wine countries and cause substantial economic damage. These virus diseases have a detri- mental effect on the growth, development, yield, and longevity of grapevine [1]. The pathogen of grapevine fanleaf disease (GFLV) is a polyhedral virus measuring about 30 nm in size. A yellow mosaic and the yellowing of leaf veins are the symptoms that appear in leaves at a certain stage of growth and development. The virus is sufficiently well characterized by molecular biology studies [2]. The causal agent of the Rupestris stem pitting dis- ease (RSP) has not yet been determined. However, it is supposed that the pathogen is a rod-shaped virus struc- turally similar to the apple-tree stem pitting virus [3]. Apart from pits and grooves occurring on the wood cyl- inder, this disease causes the curling, yellowing, or red- dening of leaves at a certain period of plant develop- ment. Little is known about the physiological conse- quences of these diseases. In most cases, plants with evident symptoms were examined. Photosynthesis is the process most sensitive to biotic and abiotic stresses [4]. Therefore, changes in photosynthetic reactions rep- resent a convenient model for studying the responses of grapevine plants to biotic stresses. Under natural envi- ronments, grapevines are subject to attack from a wide range of pathogens, which complicates the distinction between their effects on photosynthetic processes. For this reason, in vitro cultured grapevine plants represent an appropriate model system for studying the physio- logical responses caused by virus diseases. In addition to commonly used variable fluorescence of chlorophyll as a means for analyzing the photosyn- thetic apparatus in native systems [5], measuring delayed fluorescence is a very informative approach [6, 7]. The delayed fluorescence parameters are used as indi- cators of (a) the photosynthetic capacity for utilizing light energy [8] and (b) the energized state of thylakoid membranes [9]. There is little data in the literature concerning the effects of GFLV and RSP on photosynthetic reactions [10]. In this study, we employed the aforementioned luminescence-based methods for investigating the Effects of Grapevine Fanleaf and Stem Pitting Viruses on the Photosynthetic Activity of Grapevine Plants Grown in vitro I. K. Christov*, D. Stefanov**, V. N. Goltsev***, and P. Abrasheva* *Institute of Viticulture and Enology, Pleven, Bulgaria **Popov Institute of Plant Physiology, Sofia, Bulgaria ***Department of Biophysics and Radiobiology, Faculty of Biology, Sofia University “St. Kliment Ohridski,” bul’v. Dragana Tsankova 8, Sofia, 1421 Bulgaria; e-mail: goltsev@biofac.uni-sofia.bg Received March 9, 2000 Abstract—The effects of viral diseases on the photosynthetic activity of grapevine (Vitis rupestris var. Rupes- tris du Lot) leaves were investigated. The third and sixth leaves used for measurements were obtained from in vitro grown healthy plants and plants affected by grapevine fanleaf and rupestris stem pitting viruses. The induction curves of prompt and delayed chlorophyll fluorescence, as well as the temperature characteristics of steady-state, prompt, and delayed emissions, were investigated. Age-dependent changes were found, which were related, on the one hand, to the acceleration of electron transport and the enhancement of thylakoid ener- gization and, on the other hand, to a smaller extent of transmembrane ∆μH + in the younger sixth leaf compared to that in the third leaf. The infected plants characteristically showed faster electron transport, an elevated ener- getic efficiency of photosynthesis, and the suppression of CO 2 fixation owing to a presumable activation of the adenylate metabolism. An analysis of the thermograms of prompt and delayed fluorescence revealed the shifts in the position of the M 1 peak and a half-inhibition temperature T 50 towards a higher temperature in infected plants, which indicates a certain increase in the thermal tolerance of thylakoid membranes. The data suggest that the viral metabolism affects the functional activity and stability of thylakoid membranes. Key words: Vitis rupestris - grapevine fanleaf virus - stem pitting virus - photosynthesis - delayed fluorescence - variable fluorescence - thermograms of delayed and prompt fluorescence Abbreviations: DF—delayed fluorescence; PAR—photosyntheti- cally active radiation; PF—prompt fluorescence; PS—photosys- tem; GFLV—grapevine fanleaf virus; RSP—Rupestris stem pit- ting virus.