Fig. 4.

Correlations of saturating quantum flux density for isoprene emission (Q_I90, Eq. 3) with DMADP pool size (a), isoprene synthase rate constant (b) and normalized light-dependent increase of DMADP pool size [Δ_S, Eq. 5; (c)] in hybrid aspen (P. tremula x P. tremuloides) young, mature and old leaves (symbols as in Fig. 3, and definition of leaf ages as in Fig. 1). The saturating quantum flux density is defined as the value of Q required to reach 90% of the emission rate at Q = 2000 μmol m^-2 s^-1. All data pooled were fitted by non-linear (a) and linear (b, c) regressions (P < 0.01). In (a), DMADP pool size is calculated as the average of measurements at high light intensities of 500 and 1850 μmol m^-2 s^-1, while in (b), the rate constant is the average value for all the three light intensities (250, 500 and 1850 μmol m^-2 s^-1) used for the measurements (Fig. 2 for the definitions and average values at different light intensities). Δ_S is calculated for light intensities of 500 (Q₁) and 1850 (Q₂) μmol m^-2 s^-1 (Eq. 5).