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.     

Control of barley root respiration

Evidence from barley [ Hordeum distichum (L.) Lam. cv. Maris Mink], and from many other species, suggests that respiration is controlled by either supply of carbohydrate or demand for ATP. The relationship between root respiration rate (measured as O₂ consumption or CO₂ production) and ethanol-soluble carbohydrate content altered with time following selective pruning, and the change could not be accounted for by buffering of the cytoplasmic carbohydrate concentration by sugars in the vacuole. Exogenous sucrose supplied to the roots prevented any decline of the respiration rate in shoot-pruned plants, and if supplied for 24 h stimulated the respiration rate after any treatment. Root extension responded to sucrose in a similar manner. We suggest that respiration is under fine control by adenylates, but the capacity of the respiratory system is fixed by the supply of sucrose, possibly via coarse control of the respiratory machinery, or of the processes requiring metabolic energy.     

Root respiration during grain filling in sunflower: The effects of water stress

Instantaneous rates of (soil + root) respiration were measured periodically during grain filling in sunflower crops that were i) irrigated at weekly intervals and ii) subjected to water stress for the last 25 days of the 40-day grain filling period. Daily (soil + root) respiration was calculated using instantaneous respiration rates, an empirically determined temperature response function, and diurnal records of soil temperature. Daily soil respiration was estimated using empirically determined functions linking soil respiration to soil temperature and water content. Between anthesis and maturity, daily root respiration of the irrigated crop dropped by about one half from ca. 1.8 g C m^-2 d^-1, exhibiting a strong association with daily crop gross photosynthesis. Water stress brought about a rapid decrease in root respiration, which fell to about 0.1 g C m^-2 d^-1 at maturity. Root respiration during grain filling was 46 and 30 g C m^-2 for irrigated and stressed crops, respectively.     

The relationship between phosphate status and cyanide-resistant respiration in bean roots

Bean ( Phaseolus vulgaris L.) seedlings were cultured on complete or phosphate-deficient nutrient medium. After 14 days of culture on phosphate-deficient medium the visible symptoms of P_i deficiency were observed only in the shoot, the fresh and dry weights of the roots were slightly higher than in control plants. The decreased P_i content in the roots had little effect on total respiration rate but had an effect on the level of inhibition of respiration by cyanide. The high resistance of respiration to cyanide observed in P_i-deficient roots was the result of the suppression of cytochrome path activity and an increased participation of the alternative, cyanide-resistant pathway. The cytochrome pathway activity increased when inorganic phosphate was supplied to P_i-deficient roots for 1 or 3.5 h. It is speculated that the suppression of cytochrome pathway in P_i-deficient roots may result from restriction of the phosphorylating capacity or a partial inhibition of cytochrome oxidase activity.     

Heat treatment of ripening apples: Differential effects on physiology and biochemistry

Apples ( Malus domestica Borkh.) were heated for 4 days at 38°C immediately after harvest and then placed at 20°C for 7–10 days. Protein synthesis, ethylene production and fruit softening were reversibly inhibited by the heat treatment. Fruit respiration, membrane permeability and chlorophyll degradation in the fruit peel were enhanced during the treatment. The heat-treated apples ripened normally but more slowly than untreated apple We hypothesize that heat treatment differentially affects processes which normally increase simultaneously during fruit ripening, by inhibiting those processes which require tie novo protein synthesis and enhancing those that do not.     

Effects of three herbicides on soil microbial biomass and activity

Three post-emergence herbicides (2,4-D, picloram and glyphosate) were applied to samples of an Alberta agricultural soil at concentrations of 0, 2, 20, and 200 μg g⁻¹. The effects of these chemicals on certain microbial variables was monitored over 27 days. All herbicides caused enhancement of basal respiration but only for 9 days following application, and only for concentrations of 200 μg g⁻¹. Substrate-induced respiration was temporarily depressed by 200 μg g⁻¹ picloram and 2,4-D, and briefly enhanced by 200 μg g⁻¹ glyphosate. It is concluded that because changes in microbial variables only occurred at herbicide concentrations of much higher than that which occurs following field application, the side-effects of these chemicals is probably of little ecological significance.     

NONEQUILIBRIUM RATES OF PHOTOSYNTHESIS AND RESPIRATION UNDER DYNAMIC LIGHT SUPPLY1

To identify processes that might account for differences in growth rates of rhodophytes under constant and dynamic light supply, we examined nonequilibrium gas exchange by measuring time courses of photoinduction, loss of photoinduction, and respiration rates immediately after the light–dark transition. Using the rhodophyte species Palmaria palmata (Huds.) Lamour and Lomentaria articulata (Huds.) Lyngb., we compared the effects of growth-saturating constant photon flux density (PFD) (95 μmol photons · m^?2· s^?1) to those of a dynamic light supply modeled on canopy movements in the intertidal zone (25 μmol photons · m^?2· s^?1 background PFD plus light flecks of 350 μmol photons · m^?2· s^?1, 0.1 Hz). The time required for P. palmata and L. articulata to be fully photoinduced was not affected by the dynamics of light supply. L. articulata required only 6 min of illumination with either fluctuating or constant light to be completely induced compared to 20 min for P. palmata. The latter species also lost photoinduction more rapidly than did L. articulata in the dark. There was no significant decline in photoinduction state for either species at the background PFD. The time courses of respiration after illumination with constant and fluctuating light were significantly different for P. palmata but not for L. articulata when the total photon dose was equal. In general, gas exchange of P. palmata appeared to be particularly sensitive to the temporal distribution of light supply whereas that of L. articulata was sensitive to the amplitude of variations, being photoinhibited at high PFD. These results are discussed in terms of the different mechanisms of inorganic carbon acquisition in the two species.     

Cloning by functional complementation, and inactivation, of theSchizosaccharomyces pombe homologue of theSaccharomyces cerevisiae geneABC1

TheSaccharomyces cerevisiae geneABC1 is required for the correct functioning of thebc ₁ complex of the mitochondrial respiratory chain. By functional complementation of aS. cerevisiae abc1 ^– mutant, we have cloned aSchizosaccharomyces pombe cDNA, whose predicted product is 50% identical to the Abc1 protein. Significant homology is also observed with bacterial, nematode, and even human amino acid sequences of unknown function, suggesting that the Abc1 protein is conserved through evolution. The cloned cDNA corresponds to a singleS. pombe geneabc1Sp, located on chromosome II, expression of which is not regulated by the carbon source. Inactivation of theabc1Sp gene by homologous gene replacement causes a respiratory deficiency which is efficiently rescued by the expression of theS. cerevisiae ABC1 gene. The inactivated strain shows a drastic decrease in thebc ₁ complex activity, a decrease in cytochromeaa3 and a slow growth phenotype. To our knowledge, this is the first example of the inactivation of a respiratory gene inS. pombe. Our results highlight the fact thatS. pombe growth is highly dependent upon respiration, and thatS. pombe could represent a valuable model for studying nucleo-mitochondrial interactions in higher eukaryotes.     

Herbivore-induced changes in plant carbon allocation: assessment of below-ground C fluxes using carbon-14

Effects of above-ground herbivory on short-term plant carbon allocation were studied using maize (Zea mays) and a generalist lubber grasshopper (Romalea guttata). We hypothesized that above-ground herbivory stimulates current net carbon assimilate allocation to below-ground components, such as roots, root exudation and root and soil respiration. Maize plants 24 days old were grazed (c. 25–50% leaf area removed) by caging grasshoppers around individual plants and 18 h later pulse-labelled with¹⁴CO₂. During the next 8 h,¹⁴C assimilates were traced to shoots, roots, root plus soil respiration, root exudates, rhizosphere soil, and bulk soil using carbon-14 techniques. Significant positive relationships were observed between herbivory and carbon allocated to roots, root exudates, and root and soil respiration, and a significant negative relationship between herbivory and carbon allocated to shoots. No relationship was observed between herbivory and¹⁴C recovered from soil. While herbivory increased root and soil respiration, the peak time for¹⁴CO₂ evolved as respiration was not altered, thereby suggesting that herbivory only increases the magnitude of respiration, not patterns of translocation through time. Although there was a trend for lower photosynthetic rates of grazed plants than photosynthetic rates of ungrazed plants, no significant differences were observed among grazed and ungrazed plants. We conclude that above-ground herbivory can increase plant carbon fluxes below ground (roots, root exudates, and rhizosphere respiration), thus increasing resources (e.g., root exudates) available to soil organisms, especially microbial populations.     

The relationship between the relative growth rate and nitrogen economy of alpine and lowland Poa species

This study investigates the nitrogen economy of six altitudinally contrasting Poa species which differ in their relative growth rate (R). Two alpine (Poa fawcettiae and P. costiniana), one sub-alpine (P. alpina)and three temperate lowland species (P. pratensis, P. campressa and P. trivialis) were grown hydroponically under identical conditions in a growth room. The low R exhibited by the alpine species was associated with lower plant organic nitrogen concentration (n_p) and lower nitrogen productivity (Π_p, amount of biomass accumulation per mol organic nitrogen and time). The differences in Π_p between the alpine and lowland species did not appear to be due to differences in the carbon concentration or the proportion of total plant organic nitrogen allocated to the leaves, stems or roots. Variations in Π_P were also not due to variations in photosynthetic nitrogen use efficiency (Ψ_N, the rate of photosynthesis per unit organic leaf nitrogen) or shoot or root respiration rates per unit organic nitrogen (Λ_SH and Λ_R, respectively) per se. Rather, the lower Λ_p in the alpine species was probably due to a combination of small variations in several of the parameters (e.g. slightly lower Ψ_N, slightly higher Λ_SH and Λ_R, and slightly higher proportions of total plant organic nitrogen allocated to the roots). The alpine species exhibited lower organic acid and mineral concentrations. However, no differences in whole-plant construction costs (grams of glucose needed to synthesize one gram of biomass) were observed between She alpine and lowland Poa species. The lack of sub-stantial differences in Ψ_N between the alpine and lowland species contrasts with the large differences in Ψ_N between slow- and fast-growing lowland species that have been reported in the literature. The reasons for the unusually high Ψ_N values exhibited by the alpine Poa species are discussed.