Chlorophyll a fluorescence response of Norway spruce needles to the long-term effect of elevated CO2 in relation to their position within the canopy

The long-term impact of elevated CO, concentration on photosynthetic activity of sun-exposed (E) versus shaded (S) foliage was investigated in a Picea abies stand (age 12 years) after three years of cultivation in adjustable-lamella-domes (ALD). One ALD is supplied with either ambient air [ca. 350 mumol(CO2) mol(-1); AC-variant) and the second with elevated CO2 concentration [ambient plus 350 mumol(CO2) mol(-1); EC-variant). The pronounced vertical profile of the photo-synthetically active radiation (PAR) led to the typical differentiation of the photosynthetic apparatus between the S- and E-needles in the AC-variant estimated from the irradiance-responses of various parameters of the room temperature chlorophyll (Chl) a fluorescence parameters. Namely, electron transport rate (ETR), photochemical efficiency of photosystem 2, PS2 (Phi(PS2)), irradiance-saturated values of non-photochemical quenching of minimum (SV0) and maximum (NPQ) fluorescence levels, and photochemical fluorescence quenching (q(P)) at higher irradiances were all significantly higher for E-needles as compared with the S-ones. The prolonged exposure to EC did not cause any stimulation of ETR for the E-needles but a strongly positive effect of EC on ETR was observed for the S-needles resulting in more than doubled ETR capacity in comparison with S-needles from the AC-variant. For the E-needles in EC-variant a slightly steeper reduction of the Phi(PS2) and q(p) occurred with the increasing irradiance as compared to the E-needles of AC-variant. On the contrary, the S-needles in EC variant revealed a significantly greater capacity to maintain a high Phi(PS2) at irradiances lower than 200 mumol m(-2) s(-1) and to prevent the over-reduction of the PS2 reaction centres. Moreover, compared to the AC-variant the relation between SV0 and NPQ exhibited a strong decrease (up to 72 %) of the SV0-NPQ slope for the E-needles and an increase (up to 76 %) of this value for the S-needles. Hence the E- and S-foliage responded differently to the long-term impact of EC. Moreover, this exposure was responsible for the smoothing of the PAR utilisation vertical gradient in PS2 photochemical and non-photochemical reactions within the canopy.