Cyanide-Resistant Respiration in Higher Plants

Experiments were undertaken in order to test the mechanism of energy coupling for amino acid uptake proposed in the cotransport hypothesis. According to the hypothesis an electrochemical potential difference in H+ is established by active H+ extrusion. That potential difference then drives the cotransport of H+ and amino acids into the cells. Application of amino acids to oat (Avena sativa var. Victory) coleoptiles induced transient depolarizations of the cell membrane electrical potentials considered to reflect the joint uptake of H+ and amino acids followed by an enhanced H+ extrusion. In the presence of KCN, cysteine induced strong depolarizations, but the rate of repolarization depended linearly upon the cyanideadjusted ATP level of the tissue. At an ATP level 44% of normal, the membrane potential was 74% of normal, but the repolarization after cysteine-induced depolarization was practically nil. Sudden transitions from room temperature to temperatures below 15°C induced sharp depolarizations of the membrane which then repolarized within 3 min; the ATP content of the tissues was unaffected. Cysteine and alanine induced strong depolarizations at temperatures between 5 and 25°C, and the Qio for the rate of depolarization was 1.5 for cysteine and 1.6 for alanine. The QLo for

Section: Discussionmentioning

confidence: 89%

Energy Coupling in H⁺-Amino Acid Cotransport

Kinraide¹,

Etherton²

1982

“…Vol. 81,1986 light can be lower at low 02 concentrations could involve the operation of the mitochondrial alternative electron transport pathway (see [29] for a review) in the light. This nonphosphorylating, cyanide-resistant pathway is present (in addition to the phosphorylating, cyanide-sensitive Cyt pathway) in wheat and bean leaves (3,4), and it has been shown that the alternative pathway can operate in the dark under certain conditions in these leaves, and that the alternative pathway is apparently less restricted in vivo by oxidative phosphorylation than the Cyt pathway (4,5,22).…”

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confidence: 99%

“…given that the affinity of the alternative pathway for 02 is lower than that of the Cyt pathway (29). Although this is an attractive hypothesis, it remains to be demonstrated that the alternative pathway operates in leaves in the light.…”

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confidence: 99%

Effect of Oxygen on the Contribution of Respiration to the CO₂ Compensation Point in Wheat and Bean Leaves

Azcón-Bieto

1986

The CO2 compensation point at 21% 02 (r2l) and at 2% 02 (r2), and the rate of dark CO2 efflux at 21% 02 (R.) were measured in adult wheat (Triticum aestivum L, cv Gabo) leaves at the end of the night and after a period of photosynthesis of 5 h at 800 ;tbar CO2. Ref. 7). The contribution of Rd to r at low 02 concentration (e.g. 2%) seems to be large in some cases (2) and very small in other cases (15,21,27).The rate of respiratory CO2 efflux in the dark (R.) measured at 21% 02 is not significantly different (although it is normally slightly higher) than that measured at low 02 concentration (1-3%) (1,7,10,15), although there are some exceptions to this (see Sharp et al. [28], using sunflower leaves). On this basis, several leaf CO2 exchange models (13,26) assume that the contribution of Rd to r should not depend on 02 concentration above about 1 %, and several workers have tried to estimate the rate of respiration in the light in leaves using these models (24,26,27). However, Day and Parkinson (9) In the present investigation, some responses of the CO2 compensation point at 21 and 2% 02 were examined in wheat and bean leaves, and the results suggest that the rate of respiration in the light contributing to r can be smaller at low 02 levels than at ambient levels.MATERIALS AND METHODS Plant Material. Triticum aestivum (cv Gabo) and Phaseolus vulgaris (cv Hawkesbury Wonder) plants were grown in a controlled environment in pots of soil. They were watered twice a day, and were fertilized every other day with nitrate-type Hewitt's solution (4). Total nitrate concentration was 12 mm. Quantum flux density (400-700 nm) was 600 to 700 uE.m 2 * s-'. The day/night temperature regime was 25/20°C with a daylength of 13 h. Only mature leaves of T. aestivum were used. Wheat plants were selected from the cabinet at the end of the night period. Leaflets of the fourth trifolium (in chronological order of appearance) of P. vulgaris were used after several hours in the light. Leaf age was calculated in days, and "day zero" was arbitrarily established as the 1st d in which the leaflets of the selected trifolium were fully unfolded; this occurred approximately 3 or 4 d after the appearance of the corresponding foliar bud. Leaf area at day zero was less than 5% ofmaximal area which occurred at about d 6 to 7.Gas Exchange Apparatus. The open gas exchange system used for measuring CO2 and H20 exchanges of intact attached leaves has been described (1). The gas exchange apparatus was modified by the inclusion ofa closed system. A metal bellows pump (Metal Bellows Corp., Sharon, MA) circulated the air through the system. Plug valves (Nupro Co., model B-4P4T, Cleveland, OH) were used to manually switch' from open to closed system or vice versa. The 02 concentration in the circuit was changed by mixing air with N2 from cylinders, and was measured with an 02 electrode (Yellow Spring Instruments, model 5331).Comparison of Two Methods for Measuring the CO2 Compensation Point. r was either measured by using a closed system and allowi...

“…However, SHAM alone at 10 mMfor 30 min has no detectable effect on pelletability (data not shown). This effect suggests that possible involvement of a functional cyanide-insensitive electron transport pathway but that it is only activated when the normal pathway is blocked (13). The uncoupler FCCP is also effective in inhibiting the pelletability response (Fig.…”

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confidence: 93%

Irradiation-enhanced Phytochrome Pelletability

Quail¹,

Briggs²

1978

Short, high intensity pulses of red and far red light are used to study, at room temperature, the kinetics of the In vivo dark reaction responsible for irradiation-enhanced phytochrome pelletability. The t1/2 for this reaction is 2 seconds at 25 C in both Avena shoots and Zea mays coleoptiles. This is the most rapid phytochrome-far red-absorbing form (Pfr)-mediated cellular response thus far reported. Anoxia, KCN, NaN3 and carbonyl cyanide p-trifluoromethoxyphenylhydrazone reduce the rate (but not the final extent) of the reaction by more than an order of magnitude. The rate of the reaction under these conditions is strongly correlated with the inhibitor-induced reductions in cellular ATP levels. Likewise, recovery in ATP levels upon withdrawal of the inhibitors is accompanied by a parallel recovery in the rate of the reaction. Cytochalasin B blocks cytoplasmic streaming without diminishing the pelletability response. Colchicine is likewise without effect. These data suggest a requirement for phosphorylative energy in one or more of the Pfr-dependent intracellular events leading to enhanced phytochrome pelletability. The possibility that this event might represent an ATP-dependent modification of the pigment protein itself in the Pfr form is discussed.