Needles of un-hardened and frost-hardended seedlings of P&us sylvestris and Pinus contorta were exposed to photoinhibitory photon flux densities at temperatures between 0 and 35°C under laboratory conditions. Photoinhibition of photosynthesis was assayed by measuring oxygen evolution under saturating CO2 in a leaf disc oxygen electrode or by recording of photosystem II fluorescence induction kinetics at 77 K. It was demonstrated that frost hardening of pine did not affect the susceptibility of photosynthesis to short time (2 h) photoinhibition at 15°C. The two pine species irrespective of acclimative state were equally sensitive to photoinhibition as assayed by apparent photon yield analyses of photosynthetic oxygen evolution. Plots of the apparent photon yield of oxygen evolution vs. Fv/Fm revealed a non-linear relationship.
In the temperature range of 15-20°C short term photoinhibition caused a loss of Fv without effect on F 0. However, photoinhibition at temperatures lower or higher caused F 0 to increase and decrease, respectively. In fact the decrease of Fv/Fm upon lowering the temperature was mainly caused by the temperature effect on F 0. Besides photoinhibition causing the well established quenching of Fv by increased radiationless decay somewhere in the reaction center-antenna complex, it is suggested that F0 generally increases as a result of loss of functional reaction centers causing decreased trapping of excitation energy. However, the high temperature induced quenching of F 0 suggests that the quenching process (or processes) induced under photoinhibitory conditions is temperature dependent; i.e. it increases with the increase of temperature.
In pine the photon yield of photosynthesis was much more sensitive to short term photoinhibition than was the rate of light saturated photosynthesis. This difference is explained by photosystem II and electron transport having surplus capacity relative to that of reductive carbon metabolism.