Action spectra of photo phosphorylation 2. atp formation catalyzed by phenazine methosulfate suggesting the involvement of long wavelength pigment forms in the light harvesting process for photosystem ii and photosystem i
Van-Ginkel, G.; Hammans, J.W.K.
Photochemistry and Photobiology 31(4): 385-396
Action spectra of phenazine methosulfate (PMS)-catalyzed photophosphorylation, [Spinacia oleracea cv. Noorman], measured under aerobic conditions without added reductants, show the involvement of both photosystem 2 (PS II) and photosystem 1 (PS I). Addition of low concentrations 3-(3,4)dichlorophenyl-1, 1-dimethylurea (DCMU) changes the shape of the curves and causes formation of typical PS II action spectra. This can be understood by the partial inhibition of PS II electron flow by DCMU, which therefore becomes rate-limiting. The presence of DCMU causes a strong decrease of photochemical activity in these spectra at 638, 660 and 690 nm, which are attributable to decreasing activity of PS I pigments. Curve analysis of the action spectra suggests the presence of different Chl [chlorophyll] forms at room temperature peaking at: 641.6; 650.3; 661.2; 669.8; 677.1; 684.0 and 690.6 nm. Comparison of our data with published data from low temperature absorption spectra indicates that lowering the temperature causes a decrease in halfwidth of the absorption bands but no significant change in peak position. The data support the hypothesis of French and coworkers that there are 4 universal forms of Chl a also at 25.degree. C. Careful investigation of data in literature does not give evidence that the pigment peaking at 641 nm is a 2nd form of Chl b, as is sometimes assumed. Because our action spectra show 2 small but distinct maxima at 703-705 and 718-720 nm when PS II is rate-limiting, we also measured action spectra of PS I (reduced PMS + DCMU) and PS II (ferricyanide-ions + 2,5-dibromo-3-methyl-6-isopropylbenzoquinone) separately. They show, in agreement with published data, that PS I is operating more efficiently in the spectral region 690-725 nm than PS II. The Chl form peaking at 690 nm is absent in the PS II action spectrum, whereas this curve has 2 small but distinct maxima around 703-705 and 718-720 nm, respectively, which are also present in the PS I action spectrum. It is suggested that light absorbed by these pigments can be transferred to either of the 2 photosystems, and not to PS I exclusively as is sometimes assumed.