Effect of O2 on the Kinetics of the Flash-Induced 518 nm Absorbance Change in Vivo

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Abstract
The kinetics of the flash induced 518 nm absorbance change (δA518) in lettuce leaves were found to be dependent on O2 concentration. (1) Either a lower O2 partial pressure or the addition of weak red background illumination accelerated the decay of (δA518) while far-red background light induced a transient acceleration. (2) In the presence of background red light the accelerated decay could be restored to the original dark level by the addition of O2. A linear relationship was found between the intensity of red background light and the O2 pressure required for this restoration. (3) The O2 dependence of (δA518) decay halftime was biphasic, the sensitive phase saturating at 0.3 atmospheres O2 independent of input light energy while the O2 concentration needed to saturate the second phase increased with increasing input light energy (increasing flash frequency). (4) Treatment with N, N′-dicyclohexylcarbodiimide (DCCD) or KCN eliminated all O2 and background light effects and DCMU treatment inhibited all but the sensitive phase of the O2 dependence on (δA518) decay halftime. (5) The extent of the lag phase in the dark recovery of (δA518) normally present after preillumination induced acceleration of decay was decreased with added O2 or KCN. (6) It was concluded that O2 competes directly with background red light induced electron transport to PS I acceptors to influence the (δA518) decay. A possible mechanism involving the O2 sensitive ferredoxin-thioredoxin-reductase activation of chloroplast coupling factor 1 ATP-hydrolase activity was discussed.