BIOLOGIA PLANTARUM 48 (1): 149-152, 2004 BRIEF COMMUNICATION Photosynthetic responses for Vitis vinifera plants grown at different photon flux densities under field conditions M. BERTAMINI and N. NEDUNCHEZHIAN* Istituto Agrario di San Michele all' Adige, I-38010 San Michele all' Adige, Italy Abstract In grapevine (Vitis vinifera L.) leaf chlorophyll (Chl) a and Chl b and carotenoid contents were higher in plants grown at low photon flux densities (PFD) than in those grown at medium and high PFD. The highest Chl a variable to maximum fluorescence ratio F v /F m was observed in plants grown at medium PFD while the minimum fluorescence F 0 was highest in those at high PFD. In isolated thylakoids, both high and low PFD caused marked inhibition of whole chain and photosystem 2 (PS2) activities. The artificial exogenous electron donor diphenyl carbazide significantly restored the loss of PS2 activity in low PFD leaves. Additional key words: carotenoids, chlorophyll, donor side, electron transport, grapevine, photosystems 1 and 2. ⎯⎯⎯⎯ Acclimation of the photosynthesis to photon flux density (PFD) has been of long-standing interest. The differences in ultrastructure as well as biochemical and physiological properties between leaves of plants grown in full sun and of those grown in deep shade have been well documented (e.g., Lichtenthaler et al. 1984, Anderson et al. 1988, Evans et al. 1993, Yin and Johnson 2000). Variation in radiation quality and duration or irradiance cause significant change in pattern of leaf growth and senescence. Plants grown at high irradiance have small leaf size and dry mass, and low water content, but high chlorophyll (Chl) a/b ratio and sun type chloroplasts (Meier and Lichtenthaler 1981, Lichtenthaler et al. 1984). Grapevine is normally planted at densities that result in canopy closure. During the development of a closed grapevine canopy, many leaves expand under full sun, but later function in extreme shade following the development of leaves at higher nodes. The extreme shade conditions can induce rapid senescence of lower canopy leaves several weeks in advance of senescence of the whole plant (Secor et al. 1984, Wells 1991). The second type of response is acclimation of photosynthesis in shaded leaves that remain on the plant until senescence. The grapevine canopy consists of leaves of different ages, which are subjected to variable irradiances during the entire growth season (Hunter and Visser 1988). According to Boardman (1977) a leaf photosynthetic productivity is primarily governed by its position in the plant canopy. In the present paper, we report the effect of photon flux densities (PFD) on the changes in the leaf pigments and photosynthetic activities in field grown grapevine leaves. Leaves were collected from a selected 10-years-old grapevine (Vitis vinifera L.) grown under field on training system with upright growing shoots (Cordon Royet) condition in the Istituto Agrario di San Michele all' Adige, Italy. In order to simplify the experimental procedure, we classified the leaves into three groups according to the mean irradiation they received on the leaf surface. This was 200, 850, and 1 750 µmol m -2 s -1 for low, moderate (control) and high PFD, respectively. The maximum temperature was 32 - 33 °C during the ⎯⎯⎯⎯ Received 11 September 2003, accepted 7 January 2004. Abbreviations: Car - carotenoids; Chl - chlorophyll; DCBQ - 2,6-dichloro-p-benzoquinone; DCPIP - 2,6-dichlorophenol indophenol; DPC - diphenyl carbazide; E - transpiration rate; F 0 - minimum fluorescence; F v - variable fluorescence; g s - stomatal conductance; MV - methyl viologen; PFD - photon flux density; P N - leaf net photosynthetic rate; PS - photosystem; SiMo - silicomolybdate. Acknowledgements: This work was in part supported by a grant from Provincia Autonoma of Trento and National Council of Research (CNR): project “Analisi e Ricerche per il Sistema Agri-Industriale” sub-project “Prometavit”. * Author for correspondence, present address: Government Higher Secondary School, Vellimedupettai-604207, Tindivanam, India, e-mail: nedu2000@yahoo.com 149