Figure 1.

Age-dependent changes in (a) the size of main stem leaves, leaf dry mass per unit area (LMA), and (b) contents of total chlorophyll (Chl a+b) and total carotenoids (Car) in aspen (Populus tremula L.). In (b), the data were fitted by exponential relationships by minimizing the sum of the squares between predicted and measured values (Eq. 8; r² = 0.994 for chlorophyll and r² = 0.986 for carotenoids, P < 0.001 for both). The fitting coefficients (Eq. 8) obtained were for chlorophyll (a+b): a₁ = 601 μmol m⁻², a₂ = 0.443 d and a₃ = 0.125 d⁻¹ ; for carotenoids: a₁ = 185 μmol m⁻², a₂ = 0.775 d and a₃ = 0.130 d⁻¹. Data are means ± SE of at least four replicate experiments. Day 0 corresponds to the day of bud-burst (May 18). Data in the inset demonstrate variation in daytime minimum, maximum and mean temperature during the study period (derived from the data of the weather station of the Laboratory of Environmental Physics, University of Tartu, http://meteo.physic.ut.ee).