Diurnal dynamics of photosynthetic parameters of Norway spruce trees cultivated under ambient and elevated CO2: the reasons of midday depression in CO2 assimilation

Diurnal courses of net CO2 assimilation, non-assimilatory processes, electron transport rate, photosystem (PS) II photochemical efficiency, and pigment composition were investigated during summer days for sun exposed needles of Norway spruce trees grown under ambient (AC) and/or elevated (EC; 700 μmol (CO2) mol−1) atmospheric carbon dioxide concentration ([CO2]). Daily courses of net CO2 assimilation (ANact) documented significantly higher values for trees cultivated under EC conditions at irradiances above 250 μmol m−2 s−1. In comparison with the AC variant, EC treatment led to the diminution of midday photosynthesis depression that was predominantly caused by stomatal closure and subsequent decrease of intercellular [CO2] (Ci). A partial recovery of assimilation ability of AC shoots correlated with the partial restoration of stomatal conductivity during the afternoon hours. On the contrary, maximum depression of light-saturated assimilation rate (ANmax), permanent decrease of assimilation capacity (ANsat) over the day and an atypical daily course of gross photosynthesis (Pact) suggested an increased contribution of non-stomatal processes in a down-regulation of carboxylation efficiency in EC shoots during the noon and afternoon hours. The relatively high potential quantum yield of PS II photochemistry (FV/FM ≥ 0.8), estimated on both AC and EC dark adapted needles throughout the day, confirmed that PS II photoinhibition was not the primary reason of the midday depression of photosynthesis. However, the feedback effects of enhanced down-regulation of photosynthesis under EC induced: (a) slight inactivation of PS II and reduction of PS II electron transport rates, (b) enhanced demand on xanthophyll dependent non-radiative dissipation of excess light energy (accumulation of deepoxidized antheraxanthin and zeaxanthin), (c) adjustment of the light harvesting complexes (increase of chlorophyll a/b ratio).