Photosynthetic induction

At low oxygen concentrations has been shown to be correlated with steady state electron flow calculated from gas exchange measurements (1). Here we investigate the validity of this relation during photosynthetic induction in dark adapted spinach leaves. Photosynthetic electron flow was estimated from measurements of the rate of oxygen evolution with the photoacoustic technique (2,3,4). Our results indicate that equation [1] holds during induction, but that Op is very small compared to OpQ, i.e. electron flow is almost completely suppressed in "energized open PS2 reaction centers. 

These results confirm the notion that cyclic electron flow Is Involved In the oscillations occurring during the photosynthetic Induction (6,7). Both the date on metabolite additions (PGA) and the effects of Inhibitors Indicate that the oscillations during photosynthetic Induction are the consequence of readjustment of fluxes during a transients Imbalance In the rate of ATP and NADPH turnover. 

Fig 4. Characteristics of photosynthetic induction in low (A control and B mildewed leaves) and high irradiance (C control and D mildewed leaves) following a period in the dark. 

An interesting relationship has been observed between MAA concentration and water flow.143 When exposed to higher water velocities but the same radiation environment, corals accumulate larger amounts of MAAs. This positive correlation has been attributed to the stimulatory effect of increased water velocity on photosynthesis that, in turn, supplies the necessary substrates for MAA synthesis. If this is true, then the observed vertical gradient of MAAs may also be a function of photosynthetic rate relative to the attenuation of PAR and not only a direct signaling of MAA synthesis by radiation intensity. Alternatively, there could be a mechanical stimulatory mechanism related to a damage-induction response for MAA synthesis in the zooxanthellae. 

Interestingly, the levels of all the extractable flavonoids in the leaves of B. napus plants are decreased in a dose-dependent manner in response to UVA exposure. Additionally, the accumulation of the extractable flavonoids was examined by a shift from the photosynthetically active radiation (PAR) + UVA to PAR + UVB to assess whether the preexposure of the UVA affects the UVB-induced flavonoid accumulation. The UVA preexposures inhibited the UVB-induced accumulation of some flavonoids. This down regulation was particularly evident for quercetin-3-O-sophoroside and quercetin-3-O-sophoroside-7-O-glucoside of two anthocyanins. This is very interesting because the induction of quercetin by UVB is correlated with the UVB tolerance in some plant species. The photobiological nature of these UVA-mediated effects on flavonoid accumulation might suggest complex interactions between the UVA and UVB responses [66]. 

D.-P. Hader, H. Herrmann, J. Schafer, R. Santas (1997). Photosynthetic fluorescence induction and oxygen production in two Mediterranean Cladophora species measured on site. Aquat. Bot., 56, 253-264. 

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