Abstract:
The number of P700 (the reaction centre of
Photosystem I) converted to P700+, in winter rye, was
determined by measuring the absorbance change at 820nm . It
was found, with a single turnover flash, that thylakoids
isolated from cold grown plants have a 50% greater number of
P700 oxidized than thylakoids isolated from warm grown plants.
Incubation of thylakoids in the dark at 35 C did not change
the number of P700 oxidized. The conversion of P700 to P700+
with a single flash can be compared to a steady state rate of
electron transport using a Clark electrode. The results for
P700 oxidation using the absorbance change at 820 nm measure
effects within the PSI complex whereas the results obtained
from a Clark electrode measures steady state electron
transport between the cytochrome blf complex and the PSI
complex. In contrast to the results for P700 oxidation it was
shown, using a Clark electrode, that both thylakoids from cold
grown plants and thylakoids incubated at in the dark 35 C
exhibited 50% higher rates of electron transport than
thylakoids from warm grown plants. The correlation between the
higher rate of steady state PSI electron transport observed in
thylakoids isolated from cold grown winter rye and number of
active PSI reaction centres localizes the site of the increase
to the PSI reaction centre. In contrast the lack of
correlation after incubation at 35 C indicates the increase in
the rate of light saturated electron transport in thylakoids
isolated from cold grown plants and thylakoids incubated in
the dark at 35 C occur by different mechanisms.
