Net photosynthetic rate of cocksfoot leaves under continuous and fluctuating shade conditions in the field

Abstract

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 µmol m −2 s −1 photosynthetic photon flux density, PPFD) and severe shade (85–95 µmol 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 26·5 µmol CO 2 m −2 s −1 and 0·41 mol H 2 O m −2 s −1 in non‐limiting conditions with full sunlight (1900 µmol m −2 s −1 PPFD). These values were defined as standardized dimensionless Pmax s =1 and gs s =1 for comparison of treatment effects. The Pmax s under severe shade decreased by 0·004 units per minute from 1 to 180 min and reached a steady‐state of 0·37 units after 140 min. Under moderate shade, Pmax s decreased by 0·002 units per minute from 1 to 120 min and reached a steady‐state of 0·76 units. The time required to reach full induction on return to full sun (Pmax s =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 predictions of Pmax for inclusion in a canopy photosynthesis model of silvopastoral systems.