Photosynthesis In Intact Chloroplasts Research Articles

Atrazine and 1-phenylethyl (or 1-methyl-1-Phenylethyl) amino-s-triazine effects on photosynthesis in intact chloroplasts and mesophyll cells of spinach.

Atrazine and 1-phenylethyl (or 1-methyl-1-Phenylethyl) amino-s-triazine effects on photosynthesis in intact chloroplasts and mesophyll cells of spinach.

The influence of inorganic phosphate on photosynthesis in intact chloroplasts from Mesembryanthemum crystallinum L. plants exhibiting C3 photosynthesis or crassulacean acid metabolism

Intact chloroplasts were obtained from mesophyll protoplasts isolated from Mesembryanthemum crystallinum in the C3 or Crassulacean acid metabolism (CAM) photosynthetic mode, and examined for the influence of inorganic phosphate (Pi) on aspects of bicarbonate-dependent O2 evolution and CO2 fixation. While the chloroplasts from both modes responded similarly to varying Pi, some features appear typical of chloroplasts from species capable of CAM, including a relatively high capacity for photosynthesis in the absence of Pi, a short induction period, and resistance to inhibition of photosynthesis by high levels of Pi. In the absence of Pi the chloroplasts retained 75-85% of the (14)CO2 fixed and the total export of dihydroxyacetone phosphate was low compared with the rate of photosynthesis. In CAM plants the ability to conduct photosynthesis and retain most of the fixed carbon in the chloroplasts at low external Pi concentrations may enable storage of carbohydrates which are essential for providing a carbon source for the nocturnal synthesis of malic acid. At high external Pi concentrations (e.g. 10 25 mM), the amount of total dihydroxyacetone phosphate exported to the assay medium relative to the rate of photosynthesis was high while the products of (14)CO2 fixation were largely retained in the chloroplasts which indicates starch degradation is occurring at high Pi levels. Starch degradation normally occurs in CAM plants in the dark; high levels of Pi may induce starch degradation in the light which has the effect of limiting export of the immediate products of photosynthesis and thus the degree of Pi inhibition of photosynthesis with the isolated chloroplast.

Effects of long‐chained aliphatic amines on photosynthetic reactions in isolated spinach chloroplasts

The effect of six long‐chained aliphatic amines on 14CO2‐reduction, electron transport and the 515 nm absorbance change and shrinkage in isolated intact and broken chloroplasts from spinach (Spinacia oleracea L. cv. Weibulls Medania) was investigated. Five of the six investigated amines affected photosynthesis in intact chloroplasts by inhibiting 14CO2‐reduction. In broken chloroplasts the same amines uncoupled electron transport. When added to intact chloroplasts the five amines induced a light‐dependent oxygen uptake leading to (he formation of hydrogen peroxide. The oxygen uptake was not due to the amines acting as Mehler reactants. The mode of action, different from that of simple aliphatic amines, was an effect on membrane integrity, first affecting the membrane potential. At higher amine concentrations a more general effect on the ion conditions in the thylakoids was evident.

Evidence for Cyclic Photophosphorylation during 14CO2 Fixation in Intact Chloroplasts

This study examines the effect of antimycin A and nitrite on (14)CO(2) fixation in intact chloroplasts isolated from spinach (Spinacia oleracea L.) leaves. Antimycin A (2 micromolar) strongly inhibited CO(2) fixation but did not appear to inhibit or uncouple linear electron transport in intact chloroplasts. The addition of small quantities (40-100 micromolar) of nitrite or oxaloacetate, but not NH(4)Cl, in the presence of antimycin A restored photosynthesis. Antimycin A inhibition, and the subsequent restoration of photosynthetic activities by nitrite or oxaloacetate, was observed over a wide range of CO(2) concentration, light intensity, and temperature. High O(2) concentration (up to 240 micromolar) did not appear to influence the extent of the inhibition by antimycin A, nor the subsequent restoration of photosynthetic activity by nitrite or oxaloacetate. Studies of O(2) exchanges during photosynthesis in cells and chloroplasts indicated that 2 micromolar antimycin A stimulated O(2) uptake by about 25% while net O(2) evolution was inhibited by 76%. O(2) uptake in chloroplasts in the presence of 2 micromolar antimycin A was 67% of total O(2) evolution. These results suggest that only a small proportion of the O(2) uptake measured was directly linked to ATP generation. The above evidence indicates that cyclic photophosphorylation is the predominant energy-balancing reaction during photosynthesis in intact chloroplasts. On the other hand, pseudocyclic O(2) uptake appears to play only a minimal role.

Photosynthesis under osmotic stress

1. Increasing the sorbitol concentration in a suspension of intact chloroplasts induced a fast, transient and not very specific efflux of metabolites from chloroplasts to the medium. Stroma proteins were retained by the chloroplasts. 2. Within the first 30 s following hypertonic stress, the chloroplast volume decreased according to the Boyle-Mariotte relation. A subsequent and transient increase suggested some influx of external solute. 3. Dark reactions of intact chloroplasts such as starch degradation and formation of labelled 3-phosphoglycerate from dihydroxyacetone phosphate or ribose-5-phosphate and (14)CO2 were inhibited at low water potentials. After chloroplast rupture, the activity of stromal enzymes was decreased by high solute concentrations. Ribulose bisphosphate carboxylase exhibited a decrease of Vmax, while KmCO 2 remained unaltered. With sorbitol, sucrose, glycerol or glycinebetaine, 50% inhibition of enzymes was observed at osmotic potentials between 40 and 50 bar, with ethyleneglycol at about 70 bar. With salts such as KCl, 50% inhibition was found at 15 to 20 bar. 4. A comparison between inhibition of photosynthesis in intact chloroplasts and inhibition of enzymes in stroma extracts by solutes supports the notion that inhibition of photosynthesis at high osmotic potentials is mainly a solute effect. Another factor contributing to inhibition of photosynthesis in isolated chloroplasts is the loss of intermediates and cofactors which occurs during rapid osmotic dehydration.

Uncouplers Stimulate Photosynthesis in Intact Chloroplasts by Enhancing Light-Activation of Enzymes Regulated by the Ferredoxin-Thioredoxin System

Some uncouplers stimulate CO(2)-dependent O(2) evolution by intact spinach chloroplasts at pH 8.6. This effect is not due to alkalinization of the stroma. The stimulation is observed only when photosynthesis has been partly inhibited by the presence of H(2)O(2), generated in a Mehler-type reaction by the broken chloroplasts which always contaminate the intact chloroplast preparations. The addition of methyl viologen increases the Mehler-type reaction and results in greater inhibition of photosynthesis. The addition of excess catalase stimulates photosynthesis by preventing accumulation of H(2)O(2). The uncouplers stimulate photosynthesis primarily by enhancing the light-activation of enzymes that are regulated by the ferredoxin-thioredoxin system, and this effect results from the influence of the uncouplers on the redox poising of the ferredoxin in the intact chloroplasts.