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 Pho tosystem II (PSII) quantum yield to light availability in the short term (q uantum flux density integrated over the measurement day, Q(d)) and in the l ong term (Q(d) averaged over the season, Q(s)) 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 t remula L. in the upper canopy. In both species, intrinsic efficiency of PSI I in the dark-adapted state (F-v/F-m) was lower during the day than during the night, and the difference in F-v/F-m between day and night increased wi th increasing Q(s). Although the capacity for photosynthetic electron trans port increased with increasing Q(s) in both species, maximum quantum effici ency of PSII in the tight-adapted state (alpha) decreased with increasing Q (s). At a common Q(s), alpha was lower in T. cordata than in P. tremula pri marily because of a higher fraction of closed PSII centers, and to a smalle r extent because of limited, non-radiative, excitation energy dissipation i n the pigment bed in T. cordata. Across both species, photochemical quenchi ng (q(p)), which measures the openness of PSII centers, varied more than fi vefold, but the efficiency of excitation energy capture by open PSII center s (F-v' /F-m'), which is an estimate of non-radiative excitation energy dis sipation in PSII antennae, varied by only 50%. Chlorophyll turnover rates i ncreased with increasing irradiance, especially in T. cordata, possibly bec ause of increased photodestruction. Diurnal measurements of PSII quantum yi elds (Phi (PSII)) indicated that, under similar environmental conditions, P hi (PSII) was always lower in the afternoon than in the morning, and the fr action of daily integrated photosynthetic electron transport lost because o f diurnal declines in Phi (PSII) (Delta) increased with increasing Q(d). At a common Q(d), mean daily PSII center reduction state, the fraction of lig ht 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 a ttributed to greater stomatal closure during the day, which led to a greate r reduction in the requirement for assimilative electron flow in T. cordata . Across both species, Delta scaled negatively with the fraction of light u tilized photochemically, demonstrating the leading role of PSII center open ness in maintaining high PSII efficiency. Because photosynthesis (A) at cur rent ambient carbon dioxide concentration is limited by CO2 availability in high light and mainly by photosynthetic electron transport rates in low li ght, overall daily down-regulation of Phi (PSII) primarily influences A in low light. Given that foliar water stress scales positively with Q(s) in bo th 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 i s predicted by decreases in stomatal conductance alone.