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

PERI P.L.; MCNEIL D.L.; MOOT D.J.; VARELLA A.C.; LUCAS R.J.

GRASS AND FORAGE SCIENCE (PRINT)

WILEY-BLACKWELL PUBLISHING, INC

Año: 2002 vol. 57 p. 157 - 170

0142-5242

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 minutes 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 CO2 m-2 s-1 and 0.41 mol H2O m-2 s-1 in non-limiting conditions with full sunlight (1900 µmol m-2 s-1 PPFD). These values were defined as standardised dimensionless Pmaxs= 1 and gss= 1 for comparison of treatment effects. The Pmaxs under severe shade decreased by 0.004 units per minute from 1 to 180 minutes and reached a steady-state of 0.37 units after 140 minutes. Under moderate shade, Pmaxs decreased by 0.002 units per minute from 1 to 120 minutes and reached a steady-state of 0.76 units. The time required to reach full induction on return to full sun (Pmaxs= 1) was 15 minutes after 30 minutes of severe shade and 37 minutes after 180 minutes of shade. Mathematical equations were derived to describe the changes in Pmaxs and gss under severe and moderate shade and during induction. The rate of change of gss was slower than for Pmaxs 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.