Sensitivity of photosynthetic electron transport to photoinhibition in a temperate deciduous forest canopy: Photosystem II center openness, non-radiative energy dissipation and excess irradiance under field conditions

We used chlorophyll fluorescence techniques to investigate responses of Photosystem II (PSII) quantum yield to light availability in the short term (quantum flux density integrated over the measurement day, Qd) and in the long term (Qd averaged over the season, Qs) in a mixed deciduous forest comprising shade-tolerant and water-stress-sensitive Tilia cordata Mill. in the lower canopy and shade-intolerant and water-stress-resistant Populus tremula L. in the upper canopy. In both species, intrinsic efficiency of PSII in the dark-adapted state (Fv/Fm) was lower during the day than during the night, and the difference in Fv/Fm between day and night increased with increasing Qs. Although the capacity for photosynthetic electron transport increased with increasing Qs in both species, maximum quantum efficiency of PSII in the light-adapted state (alpha) decreased with increasing Qs. At a common Qs, alpha was lower in T. cordata than in P. tremula primarily because of a higher fraction of closed PSII centers, and to a smaller extent because of limited, non-radiative, excitation energy dissipation in the pigment bed in T. cordata. Across both species, photochemical quenching (qP), which measures the openness of PSII centers, varied more than fivefold, but the efficiency of excitation energy capture by open PSII centers (Fv'/Fm'), which is an estimate of non-radiative excitation energy dissipation in PSII antennae, varied by only 50%. Chlorophyll turnover rates increased with increasing irradiance, especially in T. cordata, possibly because of increased photodestruction. Diurnal measurements of PSII quantum yields (PhiPSII) indicated that, under similar environmental conditions, PhiPSII was always lower in the afternoon than in the morning, and the fraction of daily integrated photosynthetic electron transport lost because of diurnal declines in PhiPSII (Delta) increased with increasing Qd. At a common Qd, mean daily PSII center reduction state, the fraction of light in excess (1 - fractions of light used in photochemistry and dissipated as heat) and Delta were higher in T. cordata than in P. tremula. This was attributed to greater stomatal closure during the day, which led to a greater reduction in the requirement for assimilative electron flow in T. cordata. Across both species, Delta scaled negatively with the fraction of light utilized photochemically, demonstrating the leading role of PSII center openness in maintaining high PSII efficiency. Because photosynthesis (A) at current ambient carbon dioxide concentration is limited by CO2 availability in high light and mainly by photosynthetic electron transport rates in low light, overall daily down-regulation of PhiPSII primarily influences A in low light. Given that foliar water stress scales positively with Qs in both species, we conclude that the inverse patterns of variation in water and light availabilities in the canopy result in a greater decline in A than is predicted by decreases in stomatal conductance alone.