Fig. 2.

Temperature dependencies of light-saturated net assimilation rate (a, b), photosynthetic electron transport rate estimated from chlorophyll fluorescence (Eq. 1, c, d) and dark respiration rate (e, f) per unit leaf area (a, c, e) and per unit dry mass (b, d, f) in hybrid aspen grown at day/night temperatures of 25/20 °C (control) and 35/27 °C (high temperature treatment). Data were fitted by Eq. 3 (a-d) and Eq. 5 (e, f), and the values of the estimated optimum temperature (T_opt, Eq. 4) are indicated for (a) and (c) and the values of the activation energy ΔH_a (Eq. 5) for (e). The optimum temperatures in (a) and (c) are significantly different among the treatments (P < 0.005 for (a) and P < 0.02 for (c)). The measurements in a-d were conducted at an ambient CO₂ concentration of 360 μmol mol^-1 and at an incident quantum flux density of 650 μmol m^-2 s^-1. Data are means ± SE (n = 4).