Investigation of the processes involved during the photoinhibition of Zea mays L. seedlings.
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Date
1990
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Abstract
It has been proposed that the protective systems (photorespiration, the anti-oxidant
system and non-radiative energy dissipation) alleviate or reduce photoinhibitory
damage under high light conditions. To investigate the role of these mechanisms in
C4 photosynthetic species, nine day old Zea mays seedlings were photoinhibited (30
minutes of 2500 J,Lmol m-2 s-1 PPFD) in the presence of various concentrations of 02
or CO2; or by photoinhibiting leaves in N2 after they had been fed glycolate or
phosphoglycerate via the transpiration stream. The extent of the photoinhibition
and the subsequent recovery from the photoinhibitory treatments was monitored
with both CO2 gas exchange and chlorophyll fluorometry.
Photoinhibitory treatments resulted in both a decrease in the rate of CO2 fixation
and an interruption of PSII electron transport. CO2 response curves were used to
monitor the efficiency of the carboxylation processes and the level of carbon metabolism
substrate cycling during recovery following photoinhibitory treatments. Both
were decreased by the treatment and recovered once leaves were returned to
normal conditions. Low concentrations of 02 (2%) markedly reduced the extent of
the photoinhibition. This protection could not be accounted for by photorespiration,
which would be inoperative at such a low 02 concentration. Leaves fed glycolate
exhibited enhanced photoinhibtion. It is also unlikely that the anti-oxidant system
(Mehler reaction and associated glutathione and ascorbate reactions) could
utilize sufficient reductant at such low 02 concentrations to produce the observed
protection.
Leaves inhibited in the presence of 02 had decreased maximum fluorescence yields
(Fm) and little altered initial fluorescence yields (F0)' resulting in decreased PSlI efficiency
(Fv/Fm)' Photoinhibition resulted in a small increase in the slow relaxing
component (60 minute) of non-radiative energy dissipation. This component became
more predominant as the 02 concentration was increased. The rate constant
for photochemistry was also decreased by the inhibitory treatment.
Leaves supplied with CO2 at a concentration above 50 J,Lmol mol-1 exhibited little
photoinihibition suggesting that the protection was not due to a quantitative utilization
of energy. PGA, fed via the transpiration strea~ enhanced the photoinhibition,
suggesting that more than just the Benson-Calvin cycle is required to protect
C4 plants from photoinhibition. At CO2 concentrations below this, the Fv/FID ratio
was decreased due to large increases in the F0 values. Fm was little altered. These
changes are characteristic of a decrease in the rate constant for photochemistry.
The rate constant for non-radiative energy dissipation was little altered by the
photoinhibition.
The protection observed in the presence of either CO2 or 02 was not due to a
quantitative utilization of energy and the different responses of F0' Fm and the rate
constants KD and Kp, suggest that different mechanisms were operative in the
presence or absence of oxygen.
Description
Thesis (M.Sc.)-University of Natal, Durban, 1990.
Keywords
Photosensitization, Biological., Photosynthesis., Theses--Botany.