title: Fluorescence F 0 of photosystems II and I in developing C3 and C 4 leaves, and implications on regulation of excitation balance.
abstract: This work addresses the question of occurrence and function of photosystem II (PSII) in bundle sheath (BS) cells of leaves possessing NADP-malic enzyme-type C4 photosynthesis (Zea mays). Although no requirement for PSII activity in the BS has been established, several component proteins of PSII have been detected in BS cells of developing maize leaves exhibiting O2-insensitive photosynthesis. We used the basal fluorescence emissions of PSI (F 0I) and PSII (F 0II) as quantitative indicators of the respective relative photosystem densities. Chl fluorescence induction was measured simultaneously at 680 and 750 nm. In mature leaves, the F m(680)/F 0(680) ratio was 10.5 but less in immature leaves. We propose that the lower ratio was caused by the presence of a distinct non-variable component, F c, emitting at 680 and 750 nm. After F c was subtracted, the fluorescence of PSI (F 0I) was detected as a non-variable component at 750 nm and was undetectably low at 680 nm. Contents of Chls a and b were measured in addition to Chl fluorescence. The Chl b/(a + b) was relatively stable in developing sunflower leaves (0.25-0.26), but in maize it increased from 0.09 to 0.21 with leaf tissue age. In sunflower, the F 0I/(F 0I + F 0II) was 0.39 ± 0.01 independent of leaf age, but in maize, this parameter was 0.65 in young tissue of very low Chl content (20-50 mg m(-2)) falling to a stable level of 0.53 ± 0.01 at Chl contents >100 mg m(-2). The values of F 0I/(F 0I + F 0II) showed that in sunflower, excitation was partitioned between PSII and PSI in a ratio of 2:1, but the same ratio was 1:1 in the C4 plant. The latter is consistent with a PSII:PSI ratio of 2:1 in maize mesophyll cells and PSI only in BS cells (2:1:1 distribution). We suggest, moreover, that redox mediation of Chl synthesis, rather than protein accumulation, regulates photosystem assembly to ensure optimum excitation balance between functional PSII and PSI. Indeed, the apparent necessity for two Chls (a and b) may reside in their targeted functions in influencing accumulation of PSI and PSII, respectively, as opposed to their spectral differences.
authors.affiliation: Department of Biochemistry and Genetics, The Connecticut Agricultural Experiment Station, 123 Huntington St., New Haven, CT, 06511, USA, Richard.Peterson@ct.gov.
authors.forename: Richard B
authors.initials: RB
authors.lastname: Peterson
authors.forename: Vello
authors.initials: V
authors.lastname: Oja
authors.forename: Hillar
authors.initials: H
authors.lastname: Eichelmann
authors.forename: Irina
authors.initials: I
authors.lastname: Bichele
authors.forename: Luca
authors.initials: L
authors.lastname: Dall'Osto
authors.forename: Agu
authors.initials: A
authors.lastname: Laisk
mesh_terms: D002734:Chlorophyll / Q000378:metabolism; D004579:Electron Transport; D005453:Fluorescence; D006368:Helianthus / Q000502:physiology* / Q000528:radiation effects; D008027:Light; D008291:Malate Dehydrogenase / Q000378:metabolism; D058503:Mesophyll Cells; D010788:Photosynthesis / Q000502:physiology*; D045331:Photosystem I Protein Complex / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000378:metabolism*; D018515:Plant Leaves / Q000502:physiology / Q000528:radiation effects; D010940:Plant Proteins / Q000378:metabolism; D058526:Plant Vascular Bundle / Q000502:physiology / Q000528:radiation effects; D013050:Spectrometry, Fluorescence; D003313:Zea mays / Q000502:physiology* / Q000528:radiation effects
chemical_list: D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D002734:Chlorophyll; D008291:Malate Dehydrogenase; C031917:malate dehydrogenase (oxaloacetate-decarboxylating) (NADP+)