Vitis 49 (3), 145–146 (2010) and the eight nodes were randomly chosen for measure- ments. The IL received ≤ 300 µmol m -2 s -1 of PPFD while SL received ≥ 1500 µmol m -2 s -1 . Curves were performed between 09:00 and 12:00 h (solar time) by exposing leaves to decreasing levels of PPFD for 2-3 min at each level before measurements were taken. Leaf temperature was between 27 and 30 °C and CO 2 concentration was set at 370 µmol mol -1 . A non-rectangular hyperbola (MARSHALL and BISCOE 1980) was used to model the leaf photosynthetic-light re- sponse. This model has four variables (Pn sat , α, θ and R d ) which provide a large degree of flexibility in relation to the shape of the curve (ZUFFEREY et al. 2000). According to these authors, curves fit better with θ values between 0.7 and 1. The photochemical efficiency was calculated as the slope of the lineal portion of the response curve when PPFD ≤ 100 µmol m -2 s -1 (PASIAN and LIETH 1989). Light saturation (I s ) and compensation indexes (I c ) were calcu- lated following PASIAN and LIETH (1989). Curve fitting was performed with Table & Curve 2D v.2.03 software (Jandel Scientific, San Rafael, USA). Data analysis was carried out through confidence interval calculation for a p ≤ 0.05. Photosynthetic measurements were performed with a portable open circuit infrared gas analyser (CIRAS-2, PP Systems, Hertfordshire, U.K.), with an automatic leaf-chamber with 2.5 cm 2 measurement area (PLC6 (U) CRS121, PP Systems, Hertfordshire, U.K.) and a led unit (PLC6 (U) Broad CRS131, PP Systems, Hertfordshire, U.K.). Leaf temperature was measured with a built-in in- frared sensor. Results: For all type of leaves, net photosynthesis increased with PPFD until light saturation (I s ) (Figure). The SL showed higher Pn, I s , and Pn sat values than IL. At bloom, I s was 884 µmol m -2 s -1 for SL and 397 µmol m -2 s -1 for IL (Figure, Table). The I s remained relatively con- stant along the season on IL, while on SL it decreased to 617 µmol m -2 s -1 at veraison and increased to 806 at harvest. Nevertheless, SL showed higher I s values than IL. On IL, the light compensation index decreased about 40 % from bloom to harvest (Table 1). The SL showed a higher I c than IL. However, I c values were relatively low reaching about 5 µmol m -2 s -1 . The Pn sat decreased along the season for all types of leaves. The SL had higher Pn sat than IL all along the season. Photochemical efficiency (α) was higher for IL than SL, except at veraison when SL showed higher val- ues. Calculated values of α were around 0.042 µmol CO 2 µmol photon -1 until veraison, and thereafter they decreased to about 0.020 at harvest. Discussion: Photosynthetic-light response curves were similar to those observed on previous research conducted on grapevines (CARTECHINI and PALLIOTTI 1995, SCHULTZ 2003, GARCIA DE CORTÁZAR et al. 2005). Values of I s may change with cultivar (ZUFFEREY et al. 2000), leaf type, age, and leaf temperature (ZUFFEREY and MURISIER 2000). Ac- cording to these authors, healthy adult leaves can reach I s values of 1500 µmol m -2 s -1 with temperatures between 30 and 35 ºC. In our conditions, leaf temperature dur- ing photosynthetic-light responses curves were between Research Note Modelling photosynthetic-light response on Syrah leaves with different exposure J. A. PRIETO, E. GALAT GIORGI and J. PEREZ PEÑA Estación Experimental Agropecuaria Mendoza, Instituto Nacio- nal de Tecnología Agropecuaria (INTA), Mendoza, Argentina K e y w o r d s : light saturation and compensation index, photosynthesis. A b b r e v i a t i o n s : α: Photochemical efficiency [μmol (CO 2 ) µmol (photon) -1 ]; θ: Light-curve convexity, dimension- less; I s : Light saturation index [μmol (photon) m -2 s -1 ]; I c : Light compensation index [μmol (photon) m -2 s -1 ]; Pn sat : Net saturated photosynthetic rate [μmol (CO 2 ) m -2 s -1 ]; PPFD: Photosynthetic photon flux density [μmol (photon) m -2 s -1 ]; R d : Dark respiration [μmol (CO 2 ) m -2 s -1 ]. Introduction: Radiation intercepted by leaves var- ies significantly within a grapevine canopy affecting leaf photosynthesis. Light intensity and quality received during growth affects the biochemical composition of the pho- tosynthetic machinery (CARTECHINI and PALLIOTTI 1995), the light saturation and compensation indexes, and the saturated photosynthetic rate (GARCIA DE CORTÁZAR et al. 2005). Spatial and temporal radiation variability within the canopy is difficult to characterize due to the number of measurements required with the methods commonly used. Simulation models have become the main research tool to overcome this problem, to scale up from single leaf func- tioning to whole canopy performance (SCHULTZ 2003), and to study the influence of different cultural practices on leaf and canopy physiology. It is therefore important to under- stand the response to different light intensities of different type of leaves (ZUFFEREY et al. 2000). The objective of this work was to analyse the photosynthetic response of leaves with different location within the canopy to different light intensities. Materials and Methods: The experiment was con- ducted during the 2006 /07 season in an own rooted 'Chen- in blanc' vineyard planted in 1981 and re-grafted in 2001 with 'Syrah' in Mendoza (33º 00’S, 68º 51’W). The vine- yard was planted at 2.5 m between rows and 1.25 m be- tween plants in NS oriented rows. Vines were conducted in a divided canopy trellis system and pruned to 4 fruiting canes and 4 spurs with 22-28 buds per vine. Photosynthetic-light response curves were performed at bloom, veraison and harvest on 5 interior leaves of the canopy (IL) and 5 sun-exposed leaves (SL) that had been initially selected at bloom. Leaves located between the fifth Correspondence to: Dr. J. PEREZ PEÑA, EEA Mendoza INTA. San Martín 3853, Luján de Cuyo, Mendoza, Argentina. Fax: +54-261- 496-3320. E-mail: patagua@mendoza.inta.gov.ar