MECHANISTIC DIFFERENCES IN UTILIZATION OF ABSORBED EXCITATION-ENERGY WITHIN PHOTOSYNTHETIC APPARATUS OF NORWAY SPRUCE INDUCED BY THE VERTICAL-DISTRIBUTION OF PHOTOSYNTHETICALLY ACTIVE RADIATION THROUGH THE TREE CROWN

An attempt to reveal the causal relation between gradient of photosynt hetically active radiation (PhAR) through Norway spruce crown and diff erentiation of photosynthetic apparatus under field conditions is pres ented. The exposure of exposed (E) needles to higher photosystem (PS) II excitation pressure than for shaded (S) ones led to diminution of l ight harvesting complexes (LHCs), increased photosynthetic capacity (P -Nmax, by 70%) and increased capacity of nonradiative dissipation (NPQ , by 30%). Further analyses of chlorophyll a (Chl a) fluorescence reve aled several differences between S and E needles concerning NPQ: (a) f or E needles the NPQ provides more efficient protection against overre duction of PS II reaction centres; (b) the steeper dependence of F-0 q uenching on NPQ (by 53%) in E needles than in S needles supports the v iew that nonradiative dissipation localized in LHCs was enhanced for E needles; (c) for E needles induction of NPQ was accompanied by a sign ificant decrease in energy transfer from Ch1 b to Ch1 a (by 20% and/or 15%) as estimated from 77 K excitation spectra of Ch1 a fluorescence measured from PS II and PS I, respectively. This implied that down-reg ulation of light harvesting function operates in PS I as well as in PS II. The physiological significance of these results regarding the ada ptation of Norway spruce photosynthetic apparatus to the gradient of P hAR through the tree crown is discussed. (C) 1998 Elsevier Science Ire land Ltd. All rights reserved.