Fig. 1.

Representative light (Q) dependencies of isoprene emission rate for young (a), mature (c) and old non-senescent (e) leaves and comparison of standardized light dependencies of isoprene emission among young (b), mature (d) and old non-senescent (f) leaves of hybrid aspen (Populus tremula x P. tremuloides). Data in (a), (c) and (e) were fitted by Eq. 2 (r² = 0.989 for the young, r² = 0.991 for the mature and r² = 0.982 for the old leaf) with the parameters defined as: I_max - the isoprene emission capacity, α_I - the initial quantum yield of isoprene emission. In addition, the predicted isoprene emission rate at Q = 1000 μmol m^-2 s^-1, I₁₀₀₀, and the value of Q that is necessary to achieve 90% of the process rate at the quantum flux density of 2000 μmol m^-2 s^-1, Q_I90, (Eq. 3, saturating quantum flux density) are also shown. In (b), (d), and (f) all data (6 light response curves for each leaf age class pooled) were standardized with respect to I₁₀₀₀ and Eq. 2 was fitted to the data again (r² = 0.981 for young, r² = 0.983 for mature and r² = 0.968 for old leaves). Standardized response curve parameters are denoted as i_max (apparent capacity) and α_SI (apparent quantum yield). Young leaves (4th-5th from the apex) were ca. 12 days old, young fully mature leaves (10th-12th from the apex) were ca. 30 days old and old non-senescent fully mature leaves (18th-20th from the apex) were ca. 60 days old.