Importance of oxidative electron transport over oxidative phosphorylation in optimizing photosynthesis in mesophyll protoplasts of pea (Pisum sativum L.)

The role of mitochondrial respiration in optimizing photosynthesis was assessed in mesophyll protoplasts of pea ( Pisum sativum L., cv. Arkel) by using low concentrations of oligomycin (an inhibitor of oxidative phosphorylation), antimycin A (inhibits cytochrome pathway of electron transport) and salicylhydroxamic acid (SHAM, an inhibitor of alternative oxidase). All three compounds decreased the rate of photosynthetic O₂ evolution in mesophyll protoplasts, but did not affect chloroplast photosynthesis. The inhibition of photosynthesis by these mitochondrial inhibitors was stronger at optimal CO₂ (1.0 m M NaHCO₃) than that at limiting CO₂ (0.1 m M NaHCO₃). We conclude that mitochondrial metabolism through both cytochrome and alternative pathways is essential for optimizing photosynthesis at limiting as well as at optimal CO₂. The ratios of ATP to ADP in whole protoplast extracts were hardly affected, despite the marked decrease in their photosynthetic rates by SHAM. Similarly, the decrease in the ATP/ADP ratio by oligomycin or antimycin A was more pronounced at limiting CO₂ than at optimal CO₂. The mitochondrial oxidative electron transport, through both cytochrome and alternative pathways, therefore akppears to be more important than oxidative phosphorylation in optimizing photosynthesis, particularly at limiting CO₂ (when ATP demand is expected to be low). Our results also confirm that the alternative pathway has a significant role in contributing to the cellular ATP, when the cytochrome pathway is limited.     

Regulation of the cytosolic adenylate ratio as determined by rapid fractionation of mesophyll protoplasts of oat

Adenylate levels in chloroplasts, mitochondria and the cytosol of oat mesophyll protoplasts were determined under light and dark conditions, in the absence and presence of plasmalemma-permeable inhibitors of electron transfer and uncouplers of phosphorylation. This was achieved using a microgradient technique which allowed an integrated homogenization and fractionation of protoplasts within 60 s (Hampp et al. 1982, Plant Physiol. 69, 448–455), under conditions which quench bulk activities of metabolic interconversion in less than 2 s. In illuminated controls, ATP/ADP ratios were found to be 2.1 in chloroplasts, about unity in mitochondria, and 11 in the cytosol; whereas, in the dark, this ratio only showed a large drop in chloroplasts (0.4). None of the compounds used [carbonylcyanide m-chlorophenylhydrazone (CCCP), carbonylcyanide p-trifluoromethoxy-phenylhydrazone (FCCP), antimycin A, dibromothymoquinone (DBMIB), dichlorophenyldi-methylurea (DCMU), or salicylhydroxamic acid (SHAM)] affected the stroma adenylate ratio in the dark. Under illumination, however, the ATP/ADP ratios were partly reduced in the presence of antimycin (inhibitor of cyclic photophosphorylation) and of DCMU (inhibitor of linear electron flow), while in the presence of DBMIB, DCMU+ antimycin (inhibition of both cyclic and linear electron flow), and CCCP (uncoupling) the ratio obtained was the same as that occurring in the dark. In contrast, mitochondrial adenylate levels did not exhibit large variations under the various treatments. The cytosolic ATP/ADP ratio, however, showed dramatic changes: in darkened protoplasts, cytosolic values dropped to 0.2 and 0.1 in the presence of uncouplers and antimycin, respectively, while SHAM did not induce any significant alteration. In the light, a similar pronounced decrease in ATP levels was observed only after the application of uncouplers or inhibitors of both mitochondrial and photosynthetic electron transport, whereas selective inhibition of the latter was largely ineffective in reducing the cytosolic ATP/ADP ratio. Thus, the results show that the antimycin-sensitive electron transport is, potentially, equally active in light and darkness. In addition, they indicate that antimycin-insensitive electron transport in mitochondria (alternative pathway) does not significantly contribute to the cytosolic energy state.Abbreviations CCCP carbonylcyanide m-chlorophenylhydrazone - DBMIB dibromothymoquinone (2,5-dibromo-3-methyl-6-isopropy-p-benzoquinone) - DCMU dichlorophenyldimethylurea - FCCP carbonylcyanide-p-trifluoromethoxy-phenylhydrazone - SHAM sancylhydroxamic acid     

Glycolate metabolism in algal chloroplasts: inhibition by salicylhydroxamic acid (SHAM)

Unicellular green algae such as Chlamydomonas and Dunaliella excrete small amounts of glycolate during active photosynthesis. This phenomenon has been explained by the fact that these algae do not have leaf-type peroxisomes and glycolate oxidase; instead, they have a limited capacity to metabolise glycolate in their mitochondria by a membrane-associated glycolate dehydrogenase. Salicylhydroxamic acid (SHAM), an inhibitor of alternative oxidase in plant and algal mitochondria, stimulates glycolate excretion by the algae or their isolated chloroplasts 5-fold. In the presence of SHAM, cells of Chlamydomonas or Dunaliella grown with high-CO₂ (5% CO₂ in air, v/v) or adapted with air levels of CO₂ excreted glycolate at a rate of about 14 µmol glycolate mg⁻¹ Chl h⁻¹. Aminooxyacetate (AOA), an inhibitor of aminotransferases, also increases glycolate excretion by the algal cells or chloroplasts but at a lower rate (about 50%) than SHAM. The algal, light dependent, SHAM-sensitive glycolate oxidizing system in the chloroplasts appears to be the primary site for glycolate oxidation, and it is different and more active then the minor mitochondrial glycolate dehydrogenase.     

Further support for the involvement of phytoehrome in stomatal movement

The involvement of phytochrome in stomatal movement in Commelina communis L. is indicated by the following observations: 1) Short irradiation with red or blue light causes opening, of isolated stomata and swelling of guard cell protoplasts. This is reversed by subsequent far red irradiation. 2) In a similar way, stomatal response to prolonged irradiation with red or blue light is decreased by concomitant far red irradiation. 3) Pretreatment with filipin, which interferes with phytochrome binding to membranes, decreases stomatal opening in red and blue light. The stomatal responses to blue and red light are modified by DCMU, N₂, CO_2-enriched atmosphere, and CO_2-free air, which are known to affect, among other processes, chlorophyll fluorescence. Increased chlorophyll fluorescence by DCMU, N₂ and CO₂-enriched atmosphere enhanced stomatal opening in blue light and inhibited it in red light. CO₂-free air, which decreases chlorophyll fluorescence, had the opposite effect.     

Direct and indirect effects of Cd2+ on photosynthesis in sugar beet (Beta vulgaris)

Sugar-beet plants ( Beta vulgaris L. cv. Monohill) were cultivated for 4 weeks in a complete nutrient solution. Indirect effects of cadmium were studied by adding 5, 10 or 20 μ M CdCl₂ to the culture medium while direct effects were determined by adding 1, 5, 20, 50 or 2 000 μ M CdCl₂ to the assay media. The photosynthetic properties were characterized by measurement of CO₂ fixation in intact plants, fluorescence emission by intact leaves and isolated chloroplasts, photosystem (PS) I and PSII mediated electron transport of isolated chloroplasts, and CO₂-dependent O₂ evolution by protoplasts. When directly applied to isolated leaves, protoplasts and chloroplasts. Cd²⁺ impeded CO₂ fixation without affecting the rates of electron transport of PSI or PSII or the rate of dark respiration. When Cd²⁺ was applied through the culture medium the capacity for, and the maximal quantum yield of CO₂ assimilation by intact plants both decreased. This was associated with: (1) decreased total as well as effective chlorophyll content (PSII antennae size), (2) decreased coupling of electron transport in isolated chloroplasts, (3) perturbed carbon reduction cycle as indicated by fluorescence measurements. Also, protoplasts isolated from leaves of Cd²⁺-cultivated plants showed an increased rate of dark respiration.     

The role of the mesophyll in stomatal responses to light and CO2

Stomatal responses to light and CO₂ were investigated using isolated epidermes of Tradescantia pallida , Vicia faba and Pisum sativum . Stomata in leaves of T. pallida and P. sativum responded to light and CO₂, but those from V. faba did not. Stomata in isolated epidermes of all three species could be opened on KCl solutions, but they showed no response to light or CO₂. However, when isolated epidermes of T. pallida and P. sativum were placed on an exposed mesophyll from a leaf of the same species or a different species, they regained responsiveness to light and CO₂. Stomatal responses in these epidermes were similar to those in leaves in that they responded rapidly and reversibly to changes in light and CO₂. Epidermes from V. faba did not respond to light or CO₂ when placed on mesophyll from any of the three species. Experiments with single optic fibres suggest that stomata were being regulated via signals from the mesophyll produced in response to light and CO₂ rather than being sensitized to light and CO₂ by the mesophyll. The data suggest that most of the stomatal response to CO₂ and light occurs in response to a signal generated by the mesophyll.     

Photoinhibition of respiratory CO2 release in the green alga Scenedesmus

¹⁴CO₂ evolution of prelabeled Scenedesmus obliquus Kütz, has been followed in the dark and in the light. In the light, no carbon dioxide is evolved. Addition of unlabeled NaHCO, leads to ¹⁴CO₂ release attaining 20 to 30% of the dark rate. Double-reciprocal plots of NaHCO₃ concentrations vs ¹⁴CO₂ release results in a straight line, indicative of competition between exogenously supplied bicarbonate and endogenously evolved carbon dioxide. With this method, it is possible to measure CO₂ evolved by respiration in the light and to show that true photoinhibition of respiration occurs in Scenedesmus . In the light. DCMU substantially increases ¹⁴CO₂ evolution; in the presence of the uncoupler carbonyl cyanide- m -chlorophenylhydrazone. ¹⁴CO₂ evolution is comparable to that in the dark. ¹⁴CO₂ release and oxygen uptake in the dark are only slightly affected by cyanide, indicative of a cyanide-resistant respiration and/or fermentation as the essential CO₂-yielding processes in the presence of cyanide. These results, compared with concurrent ATP levels, lead us to assume that energy charge is not the only factor responsible for photoinhibition of respiration.     

The role of photorespiration in redox and energy balance of photosynthetic plant cells: A study with a barley mutant deficient in glycine decarboxylase

Protoplasts and mitochondria were isolated from leaves of homozygous barley ( Hordeum vulgare L.) mutant deficient in glycine decarboxylase complex (GDC, EC 2.1.2.10) and wild-type plants. The photosynthetic rates of isolated protoplasts from the mutant and wild-type plants under saturating CO₂ were similar, but the respiratory rate of the mutant was two-fold higher. Respiration in the mutant plants was much more strongly inhibited by antimycin A than in wild-type plants and a low level of the alternative oxidase protein was found in mitochondria. The activities of NADP- and NAD-dependent malate dehydrogenases were also increased in mutant plants, suggesting an activation of the malate-oxaloacetate exchange for redox transfer between organelles. Mutant plants had elevated activities of NADH- and NADPH-dependent glyoxylate/hydroxypyruvate reductases, which may be involved in oxidizing excess NAD(P)H and the scavenging of glyoxylate. We estimated distribution of pools of adenylates, NAD(H) and NADP(H) between chloroplasts, cytosol and mitochondria. Under photorespiratory conditions, ATP/ADP and NADPH/NADP ratios in the mutant were higher in chloroplasts as compared to wild-type plants. The cytosolic NADH/NAD ratio was increased, whereas the ratio in mitochondria decreased. It is concluded that photorespiration serves as an effective redox transfer mechanism from the chloroplast. Plants with a lowered GDC content are deficient in this mechanism, which leads to over-reduction and over-energization of the chloroplasts.     

Inhibitory effect of sodium bicarbonate on the motility of sperm of Japanese eel

Sperm motility of Japanese eel Anguilla japonica was depressed from 59.1 to 1.1% when NaHCO₃ concentration increased from 0 to 150 mM. In 25 mM NaHCO₃, when pH of the medium was 6.2, 7.2, 8.2, 9.2 and 10.2, relative sperm motility was 0, 0, 55.8, 93.7 and 136.6%, respectively to that of the control (0 mM NaHCO₃). The remarkable effect in acid or neutral condition indicates that free-CO₂ (liberated CO₂ and H₂CO₃) is a key factor for the motility inhibiting effect of NaHCO₃.     

Beneficial interaction between photosynthesis and respiration in mesophyll protoplasts of pea during short light-dark cycles

The respiratory uptake or photosynthetic evolution of oxygen by mesophyll protoplasts of pea ( Pisum sativum L. cv. Arkel) were monitored during successive short. (3–5 min) cycles of darkness and illumination. The rate of respiration was nearly doubled after 3–4 short periods of illumination while there was a 15–20% enhancement in photosynthesis with cycles of illumination and darkness preceding illumination. Such interaction between photosynthesis and respiration was statistically significant when bicarbonate was present in the reaction medium. The inhibitors of photosynthesis [3(3,4–dichlorophenyl)-l,l-dimethylurea (DCMU), glyceraldehyde] decreased respiration after periods of illumination, whereas inhibitors of respiratory electron transport (Rotenone, antimycin A, NaN₃) suppressed photosynthesis, as well. We suggest that a rapid beneficial interaction exists between photosynthesis and respiration in protoplasts, even during short cycles of light and darkness.     

Effect of sink size on photosynthesis and carbohydrate content of leaves of three spring wheat varieties

Effects of CO₂ on stomatal movements of Commelina communis L. were studied with plants, epidermal strips and guard cell protoplasts. With plants, the stomatal response induced by a blue light pulse was studied for different ambient CO₂ concentration ranging from CO₂-deprived air to 100 Pa in darkness or under red light. It was observed that the blue light response could be obtained not only under a red light background but also in darkness and CO₂-free air, the two responses being quite similar.
With epidermal strips, the effect of CO₂ on ferricyanide reductase activity at the guard cell plasmalemma was studied by transmission electron microscopy. In the presence of ferric ions, reduced ferricyanide gives an electron dense precipitate of Prussian Blue. In darkness and air, no precipitate was observed. In darkness and CO₂-free air as well as under light and normal air, a precipitate was found along the plasmalemma of the guard cells, indicating a ferricyanide reductase activity. With guard cell protoplasts suspended in a medium either in equilibrium with air or in a CO₂-free medium the H⁺ extrusion induced by a blue light pulse added to a red light background was measured. A low CO₂ content was obtained by adding photosynthetic algae to the suspension of guard cell protoplasts. In a CO₂-free medium the rate of H⁺ extrusion was enhanced.
The results are discussed on the basis of a possible competition for reducing power between CO₂ fixation and a putative blue light dependent redox chain located on the plasma membrane.     

Studies on the pathway of cyclic electron flow in mesophyll chloroplasts of a C4 plant

1. Cyclic photophosphorylation driven by white light, as followed by ¹⁴CO₂ fixation by mesophyll chloroplast preparations of the C₄ plant Digitaria sanguinalis, was specifically inhibited by disalicylidenepropanediamine (DSPD), antimycin A, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIb), 1-ethyl-3(3-dimethylaminopropyl)-carbodiimide (EDAC), and KCN suggesting that ferredoxin, cytochrome b₅₆₃, plastoquinone, cytochrome f, and plastocyanin are obligatory intermediates of cyclic electron flow. It was found that 0.2 μM DCMU and 40 μM o-phenanthroline blocked noncyclic electron flow, stimulated cyclic photophosphorylation, and caused a partial reversal (40–100%) of the inhibition by DBMIB and antimycin A, but not DSPD.

2. Cyclic photophosphorylation could also be activated using only far-red illumination. Under this condition, however, cyclic photophosphorylation was much less sensitive to the inhibitors DBMIB, EDAC and antimycin A, but remained completely sensitive to DSPD and KCN. Inhibition in far-red light was not increased by preincubating the chloroplasts with the various inhibitors for several minutes in white light.

3. The striking correspondence between the effects of photosystem II inhibitors, DCMU and o-phenanthroline, on cyclic photophosphorylation under white light and cyclic photophosphorylation under far-red light (in the absence of photosystem II inhibitors) suggests that electrons flowing from photosystem II may regulate the pathway of cyclic electron flow.     

High yield isolation of mesophyll protoplasts from wheat, barley and rye

Efficient procedures are described for high-yield isolation of mesophyll protoplasts from spring wheat ( Triticum aestivum L. cv. Glenlea), winter wheat ( Triticum aestivum L. cv. Frederick), barley ( Hordeum vulgare L. cv. Bruce) and rye ( Secale cereale L. cv. Puma). Factors such as plant age, composition of the incubation medium during isolation, purification procedures and culture medium affect protoplast yield, viability and metabolic competence, as measured by light-dependent CO₂ fixation. Optimal osmolarity of the isolation medium was equivalent to 1.8 times that measured in the leaves of all plant material used. The presence of 2 m M ascorbic acid in the preincubation and isolation medium increased the yield by 50% and conserved viability and metabolic competence. The protoplasts were stable for up to 48 h without loss of either viability or of original activity of CO₂ fixation, which was in the order of 100 μmol CO₂ (mg chl)⁻¹h⁻¹.
In our MC-56 liquid medium these protoplasts regenerated cell walls within 72 h and a few divided.     

Kinetics of the reactivation of the Hill reaction in CO2-depleted chloroplasts by addition of bicarbonate in the absence and in the presence of herbicides

In isolated broken chloroplasts photosynthetic electron transport requires the presence of CO₂ and/or bicarbonate. This bicarbonate effect on electron flow was measured in a medium containing 100 m M sodium formate. In this medium a dark incubation time with bicarbonate is required for the reactivation of the Hill reaction. We have measured the kinetics of the reactivation of electron flow by varying the dark incubation of CO₂-depIeted pea ( Pisum sativum L., cv. Rondo) chloroplasts with bicarbonate. The half-time of this reactivation appears to be 25 s when 2 m M bicarbonate is added.
The dinitrophenol herbicide, i -dinoseb, is shown to be a competitive inhibitor of the bicarbonate dependent Hill reaction with an inhibitor constant (K_i) of 31 n M . In the presence of 100 n M i-dinoseb or 100 n M DCMU the half-time of the reactivation by 2 m M bicarbonate appears to increase to about 58 s. We provide an explanation for these phenomena by analyzing the bicarbonate-thylakoid interaction on the basis of a simple reaction scheme. The binding of bicarbonate to the thylakoids appears to be a second order reaction with pseudo-first order kinetics. According to our analysis, any inhibitor, which is competitive with respect to the bicarbonate stimulation of the Hill reaction, should increase the half-time of the reactivation of the Hill reaction.     

Relationships between CO2 concentration within the leaf and photosynthesis during deacidification in a CAM plant, Kalanchoë pinnatum

The CO₂ concentration within the leaf of Kalanchoë pinnatum (Persoon) was measured during deacidification in the light. When the acidified leaf was treated with DCMU, the CO₂ concentration within the leaf was increased about 3-fold as compared to that of the non-treated leaf, concomitant with a decline in deacidification. Low light intensity during deacidification also increased CO₂ concentration within the leaf. From these experiments, it was concluded that in order for deacidification to proceed, CO₂ released from malate must be fixed continuously by photosynthesis. Otherwise, an equilibrium for malate decomposition is soon established and results in suppression of deacidification. Thus, the CO₂ concentration within the leaf seems to be one of the regulatory factors of deacidification.     

Volume changes of Commelina communis guard cell protoplasts in response to K+, light and CO2

The effects of K⁺ concentration, light intensity and CO₂ levels on the volume of Commelina communis L. guard cell protoplasts were studied. Two degrees of swelling response were observed, both dependent on an external supply of K⁺, but not necessarily on the supply of a permeant anion. The presence of K⁺ itself, independent of light or CO₂ level, stimulated swelling at a relatively slow rate. When K⁺, light and low CO₂ conditions were supplied together, the swelling was relatively rapid and of high magnitude. The rapid swelling was specific for K⁺ and Rb⁺ giving a half maximal effect after 2 h at a KCl concentration of about 18 mmol m⁻³. The addition of CaCl₂ at 1 mol m⁻³ inhibited K⁺-dependent swelling under all conditions tested. The response to light and low CO₂ levels by Commelina guard cell protoplasts is thought to reflect a high degree of physiological integrity.     

Leaf structure and specific leaf mass: the alpine desert plants of the Eastern Pamirs, Tadjikistan

This study examines interrelationships between eight leaf attributes (specific leaf mass, area, dry mass, lamina thickness, mesophyll cell number per cm², mesophyll cell volume, chloroplast volume, and number of chloroplasts per mesophyll cell) in field-grown plants of 94 species from the Eastern Pamir Mountains, at elevations between 3800 and 4750 m. Unlike most other mountain areas, the Eastern Pamirs, Karakorum system, Tadjikistan provide localities where low temperatures and radiation combine with moisture stress at high altitudes. For all the attributes measured, significant differences were found between plants with different mesophyll types. Leaves with dorsiventral palisade structure (dorsal palisade, ventral spongy mesophyll cells) had thicker leaves with larger but fewer mesophyll cells, containing more and larger chloroplasts. These differences in mesophyll type are reflected in differences in the total surface of mesophyll cells per unit leaf area ( A _mes/ A ) or volume ( A _mes/ V ). Plants with isopalisade leaf structure (palisade cells under both dorsal and ventral surfaces) are more commonly xerophytes and their increased values of A _mes/ A and A _mes/ V decrease CO₂ mesophyll resistance, which is an important adaptation to drought. Path analysis shows the critical importance of mesophyll cell volume in leading to the covariance between the different leaf attributes and hence to specific leaf mass (SLM), even though mesophyll cell volume is not itself strongly correlated with SLM. This is because mesophyll cell volume increases SLM through its effects on leaf thickness and chloroplast number per cell, but decreases SLM through its negative effect on mesophyll cell density.     

Distribution of two isoforms of glutamine synthetase in bundle sheath and mesophyll cells of corn leaves

Localization of two isoforms of glutamine synthetase (GS; EC 6.3.1.2) was investigated in different cell types, mesophyll cells and bundle sheath cells, of corn ( Zea mays L. var. W64A × W182E) leaves by using ion exchange chrotnatography. In whole leaf extracts, relative activities of GS1 (cytosolic GS) and GS2 (chloroplastic GS) were almost equal. Purified mesophyll protoplasts and bundle sheath strands also showed similar proportions of GS1 and GS2. Methionine sulfoximine (1 mM ) enhanced the accumulation of ammonia when mesophyll protoplasts were incubated with nitrite or when bundle sheath strands were incubated with glycine. This clearly indicates a spatial separation of metabolism of NH⁺₄ derived from photorespiration and from reduction of NOJ.     

The regulation of photosynthetic rate and activation of extracellular carbonic anhydrase under CO2-limiting conditions in the marine diatom Skeletonema costatum

In the marine diatom Skeletonema costatum , carbonic anhydrase activity exterior to the plasma membrane (CA_ext) was detected only when the available CO₂ concentration was less than 5·0 mmol m^–3, this activity being unaffected by the total dissolved inorganic carbon concentration. The inhibition of CA_ext by dextran bound sulphonamide (DBS) demonstrated the key role of this enzyme in maintaining photosynthetic rate under CO₂-limited conditions. Treatment with trypsin followed by affinity chromatography on p-aminomethylbenzene-sulphamide agarose and subsequent SDS-PAGE analysis revealed a polypeptide from carbon-replete cells of identical molecular mass to the CA_ext released by trypsin from CO₂-limited cells. Redox activity in the plasma membrane of intact cells was measured by following the light-dependent reduction of ferricyanide or NADP, the greatest activity being shown by CO₂-limited cells. Overall the results suggest that high rates of redox activity under conditions of CO₂-limitation were required for the activation of CA_ext.     

Response of photosynthetic carbon assimilation in mesophyll protoplasts to restriction on mitochondrial oxidative metabolism: Metabolites related to the redox status and sucrose biosynthesis

The patterns of cellular metabolites related to redox status and sucrose biosynthesis in mesophyll protoplasts of pea (Pisum sativum L.) were examined in the absence or presence of oligomycin (inhibitor of oxidative phosphorylation) or antimycin A (inhibitor of cytochrome pathway) or salicylhydroxamic acid (SHAM) (inhibitor of alternative pathway). The increase on illumination in the rate of photosynthesis or cellular metabolites was more at optimal CO₂ (1.0 mM NaHCO₃) compared to that at limiting CO₂ (0.1 mM NaHCO₃). Furthermore, the inhibition of photosynthesis in presence of mitochondrial inhibitors was more pronounced at optimal CO₂ than that at limiting CO₂. There was a marked increase in steady-state levels of triose-P/PGA (phosphoglyceric acid) and glucose-6-phosphate (Glc-6-P) in the presence of oligomycin and antimycin A. In contrast, SHAM caused a marked increase in malate/OAA (oxaloacetate). We suggest that dissipation of excess redox equivalents generated in photosynthesis occurs through both cytochrome and alternative pathways, while sucrose biosynthesis is backed up by cytochrome pathway alone. Thus, mitochondrial respiration (through both cytochrome and alternative pathways of mitochondrial electron transport) optimizes chloroplast photosynthesis by modulating cellular metabolites related to both intracellular redox state and sucrose biosynthesis.