Photosynthesis In Spinach Chloroplasts Research Articles

Action of Some Organomercury Compounds on Photosynthesis in Spinach Chloroplasts

Abstract The effects of five organomercury compounds (methylmercuric chloride, phenylmercuric acetate, phenylmercuric borate, phenylmercuric citrate and diphenylmercury) on photosynthetic electron transport (PET) in spinach chloroplasts were investigated. The IC50 values of organomercury compounds related to PET inhibition in spinach chloroplasts varied in the range from 468 mmol dm-3 to 942 mmol dm-3 and were approximately by one order higher than the corresponding value determined for HgCl2 applied also in DMSO solution (IC50 = 58 mmol dm-3). Due to extremely low aqueous solubility of diphenylmercury, the corresponding IC50 value could not be determined. Using EPR spectroscopy as probable sites of action of organomercury compounds in photosynthetic apparatus ferredoxin on the acceptor side of PS 1 and the quinone electron acceptors QA or QB on the reducing side of PS 2 were suggested.

Cobalt(II) Coordination Compounds of Ethyl 4‐Methyl‐5‐Imidazolecarboxylate: Chemical and Biochemical Characterization on Photosynthesis and Seed Germination

In this work we present the synthesis, structural and spectroscopic characterization of Co2+ coordination compounds with ethyl 4-methyl-5-imidazolecarboxylate (emizco). The effects of emizco, the metal salts CoCl2.6H2O, CoBr2, Co(NO3)2.6H2O and their metal coordination compounds [Co(emizco)2Cl2], [Co(emizco)2 Br2].H2O, [Co(emizco)2 (H2O)2(NO2)2.2H2O were evaluated on photosynthesis in spinach chloroplasts. Seed germination and seedling growth of the monocotyledonous species Lolium multiflorum and Triticum aestivum and the dicotyledonous species Trifolium alexandrinum and Physalis ixocarpa were also assayed under the effect of the compounds and salts. The results showed that cobalt(II) salts and their emizco coordination compounds inhibit photosynthetic electron flow and ATP-synthesis, behaving as Hill reaction inhibitors. Coordination compounds are more potent inhibitors than the salts. It was found that the salts target is at the b6f level while the complexes targets are at QB(D1)-protein and b6f level. The QB inhibition site was confirmed by variable chlorophyll a fluorescence yield. On the other hand, emizco inhibits seed germination, root and shoot development, in both weed and crop species. Cobalt(II) coordination compounds are the most effective photosynthesis inhibitors, but they are less potent than emizco in germination and seedling growth, while the metal salts are the least active of all.

Effects of pH and other factors on the phosphate dependence of photosynthesis in spinach chloroplasts

Chloroplast stromal volume and pH influenced the phosphate (Pi)-dependence of photosynthesis of spinach (Spinacia oleracea L.) chloroplasts. Decreasing the sorbitol concentration in the reaction mixture from 0.35 to 0.25 M, or decreasing the external pH from 8.3 to 7.3, extended the induction period of photosynthesis and decreased both the optimal [Pi] and the minimal [Pi] required to inhibit O2 evolution completely. At least part of the effect of external pH was attributable to changes in stromal pH on the basis of effects of NH4Cl and sodium acetate at a constant external pH. When the external pH was increased from 7.3 to 8.3, the stromal pH changed only about 0.6 pH units. Hence, the pH gradient across the envelope was diminished and the efflux of phosphoglycerate relative to dihydroxyacetone phosphate was enhanced.Calvin-cycle activity, varied with light intesity or electron transport inhibitors, affected the rate of photosynthesis but not the induction period or the Pi optimum for photosynthesis. Relatively low Calvin-cycle activity was apparently sufficient to fill metabolite pools and thus terminate the induction period. The results indicate that pH does not affect the Pi dependence of photosynthesis by reducing Calvin-cycle activity. Rather, it is postulated that at low stromal pH, larger metabolic pools are required to maintain maximum rates of photosynthesis because of changes in substrate affinity of some Calvin-cycle enzymes. Consequently, chloroplast photosynthesis would be more sensitive to exogenous Pi.

Polychlorinated biphenyls and their effects on photosynthesis and respiration

The effects of short-term exposures to polychlorinated biphenyls were investigated using Chlorella vulgaris and isolated spinach chloroplasts as experimental material. When Chlorella were immersed in a Warburg-Buck medium to which 10 mM glucose had been added. Aroclor 1221 was found to cause a significant inhibition of the respiration rate at concentrations above 10−7 M. When Chlorella were immersed in the same Warburg-Buck medium to which no glucose had been added, Aroclor 1221 caused a stimulation of the respiration rate at concentrations above 10−5 M. There appeared to be little effect on photosynthesis in Chlorella when Aroclor 1221 was present even at concentrations as high as 10−3 M. In contrast, oxygen evolution from isolated spinach chloroplasts using potassium ferricyanide as the electron acceptor was inhibited by Aroclor 1221. This inhibition was also apparent when the uncoupler ammonium chloride was present. Since there was no inhibition when photosystem I was examined on its own, it is suggested that Aroclor 1221 can inhibit photosynthesis in spinach chloroplasts at a site on the electron transport chain close to photosystem II.