The generation of H2O2 in thylakoids isolated from secondary
barley leaves was measured indirectly by employing a POBN spin
trap. POBN reacts with the secondary radicals which are generated
by the interaction of OH• with ethanol, and hydroxyl radicals
are formed via the Fenton reaction. The secondary radicals formed
are relatively stable -hydroxyethyl-POBN adducts, which can be determined by EPR spectroscopy. Fig. 5a shows the EPR spectrum
of the H2O2-derived carbon-centred radicals trapped by POBN
obtained from isolated thylakoids exposed to 500 mol quantam−2
s−1 of white light for 3 min. No signal was obtained in the dark
(Fig. 5b) and in the illuminated sample in the absence of FeEDTA
(Fig. 5c), which indicates that OH• radicals originate from the H2O2
generated during the illumination. Fig. 6 shows the production of
H2O2-derived radicals during the development of barley secondary
leaves between 18 and 35 DAS with (+) and without (−) NaN3.
It is apparent that the production of H2O2 increases during the
development of barley reaching the highest value 25 DAS. Thereafter,
production of H2O2 was temporally decreased and increased again in the late stages of senescence when chl levels had already
diminished substantially. Comparison of H2O2 production in samples
illuminated with and without NaN3 showed a strong increase
in hydrogen peroxide production when NaN3 was present in the
reaction medium. The effect of uncoupling was more pronounced
in younger thylakoids. Expressed in numbers, production of H2O2
was 2.48 times higher for day 18; 2.46 for day 21; 2.14 for day 25;
1.81 for day 29; 1.78 for day 32 and 1.78 for day 35. In parallel, the
production of H2O2 was determined by using Amplex Red which
reacts with H2O2 creating a product that can be detected with a
fluorimeter. The determination of H2O2 with Amplex Red showed
similar results (Fig. 7), H2O2 increases during the development of
barley reaching the highest value 29 DAS. Thereafter, the production
of H2O2 decreases and finally increases at the late stages of
senescence. Moreover, these results further confirm that OH• radicals
measured with EPR spectroscopy indeed originate from the
H2O2 generated during the illumination.