Table I. Effects of alendronate and fosmidomycin treatments on isoprene emission rate, DMADP pool size, dark pool size, rate constant of isoprene synthase, and in vivo Km, and maximum activity (Vmax) of isoprene synthase in leaves of hybrid aspen

The isoprene synthase rate constant is given as the initial slope of isoprene emission (I) versus DMADP pool size relationship, whereas in vivo K m and V max are calculated from the Hanes-Woolf plots (Fig. 3 for the sample plots). Both DMADP pool size and the dark pool size were determined from the biphasic dark decay of isoprene emission (Li et al., 2011; Rasulov et al., 2011, 2014; Li and Sharkey, 2013a). The initial rapid decay, 200 to 400 s after darkening, characterizes the pool of isoprene substrate (mainly DMADP and to some extent IDP that is rapidly converted to DMADP) that was present prior to darkening. In the darkness, there is a secondary rise of isoprene emission between approximately 400 and 1,200 s after darkening that results from conversion of phosphorylated intermediates to DMADP in the dark (dark pool; Li et al., 2011; Rasulov et al., 2011, 2014; Li and Sharkey, 2013a). This secondary rise occurs earlier in bisphosphonate-inhibited leaves due to a greater contribution of IDP and changed affinity of reducing enzymes for alternative electron donors (Figs. 1 and 2; see also “Discussion”). Two experimental protocols were used. The first protocol involved measurements without inhibitors (control), 40-min treatment with alendronate, and ultimately 30-min treatment with fosmidomycin. The second protocol consisted of measurements without inhibitors (control), 30-min treatment with fosmidomycin, and 40-min treatment with alendronate. Each value represents the mean ± se of five measurements in different plants. Means with different lowercase letters are significantly different at P < 0.05 according to Student's t tests (paired samples t tests for comparisons among treatments involving the same leaves and separate sample t tests for treatments with different leaves). NA, Not available.

TreatmentIsoprene Emission RateDMADP Pool SizeDark Pool SizeInitial SlopeKmVmax
nmol m−2 s−1 nmol m−2 s−1 nmol m−2 nmol m−2 s−1
Control25.6 ± 1.6a1130 ± 120b1340 ± 90d0.0325 ± 0.0021a2400 ± 180a80.6 ± 4.8a
Alendronate (Ald, 40 min)20.2 ± 1.8b1680 ± 100a5860 ± 680a0.0205 ± 0.0013b2800 ± 170a58.2 ± 3.6b
Fosmidomycin (Fmd, 30 min)1.63 ± 0.32d61 ± 7e793 ± 90e0.0331 ± 0.0027aNANA
Ald+Fmd (40 +30 min)4.8 ± 0.5c400 ± 70c4800 ± 460b0.0195 ± 0.0018b2150 ± 190b44 ± 6c
Fmd+Ald (30 +40 min)2.46 ± 0.21d126 ± 10d3240 ± 330c0.0202 ± 0.0014bNANA