Net photosynthetic rate of cocksfoot leaves under continuous and fluctuating shade conditions in the field P. L. Peri*† , D. L. McNeil*, D. J. Moot*, A. C. Varella* and R. J. Lucas* *Soil, Plant and Ecological Sciences Division, Lincoln University, Canterbury, New Zealand, and †Universidad Nacional de la Patagonia Austral-INTA, Argentina Abstract Maximum light-saturated photosynthetic rate (Pmax) and stomatal conductance (gs) of field-grown cocksfoot (Dactylis glomerata L.) leaves in a silvopastoral system were measured at different times under moderate (850– 950 lmol m )2 s )1 photosynthetic photon flux density, PPFD) and severe shade (85–95 lmol m )2 s )1 PPFD). Also Pmax and gs were measured after 30, 60 and 180 min of severe shade to determine the lag in the rise of photosynthesis rate from low to high irradiance levels (induction state). The highest Pmax and gs values obtained were 26Æ5 lmol CO 2 m )2 s )1 and 0Æ41 mol H 2 Om )2 s )1 in non-limiting conditions with full sun- light (1900 lmol m )2 s )1 PPFD). These values were defined as standardized dimensionless Pmax s ¼ 1 and gs s ¼ 1 for comparison of treatment effects. The Pmax s under severe shade decreased by 0Æ004 units per minute from 1 to 180 min and reached a steady-state of 0Æ37 units after 140 min. Under moderate shade, Pmax s decreased by 0Æ002 units per minute from 1 to 120 min and reached a steady-state of 0Æ76 units. The time required to reach full induction on return to full sun (Pmax s ¼ 1) was 15 min after 30 min of severe shade and 37 min after 180 min of shade. Mathematical equations were derived to describe the changes in Pmax s and gs s under severe and moderate shade and during induction. The rate of change of gs s was slower than for Pmax s on entering shade and also slower during the subsequent induction process. This indicated other factors in addition to gs were operating in the reduction and increment of Pmax and a two-step model to explain this is proposed. The defined photosynthetic responses of cocksfoot leaves to fluctuating light regimes could be used to develop quantitative predic- tions of Pmax for inclusion in a canopy photosynthesis model of silvopastoral systems. Keywords: induction phase, leaf photosynthetic rate, light regime, shade, stomatal conductance Introduction In field environments plants can experience frequent fluctuations in irradiance from full sun to shade caused by cloud cover, overstorey shading (e.g. silvopastoral systems) and within-canopy shading (Chazdon and Pearcy, 1986; Knapp and Smith, 1987). The extent of shading can alter the efficiency of conversion of energy to dry matter (DM) by affecting light interception and the photosynthetic activity of individual leaves (Sheehy and Cooper, 1973). Therefore, to quantify changes in carbon gain (or DM production) of a canopy experi- encing fluctuating light regimes, responses of photo- synthetic activity of individual leaves under this regime must be understood. In this study, the focus is on silvopastoral systems where understorey plants experi- ence frequent and rapid fluctuations in irradiance from full sun to shade caused by tree canopy shading. The understorey plant used is cocksfoot (Dactylis glomerata L.), for which the effects of different uniform light energy levels on growth (Eagles and Treharne, 1969; Eagles, 1973; Sheehy and Cooper, 1973) and photosynthesis (Singh et al., 1974; Frank and Barker, 1976) have been reported. In addition, maximum light- saturated leaf net photosynthesis rate (Pmax) has been used as one physiological variable to predict the effect of environmental factors on cocksfoot pasture production (Peri et al., 2002). Of interest in the present study are the environmen- tal and physiological controls on photosynthesis rate Correspondence to: Dr P.L. Peri, Soil, Plant and Ecological Sciences Division, Lincoln University, PO Box 84, Canter- bury, New Zealand. E-mail: moot@lincoln.ac.nz Received 19 March 2001; revised 10 October 2001 Ó 2002 Blackwell Science Ltd. Grass and Forage Science, 57, 157–170 157