Dark metabolism of carbon monoxide in lettuce leaf discs

In the dark, leaf tissue of crisphead lettuce (Lactuca sativa L.) metabolized ¹⁴CO to ¹⁴CO₂ and acid-stable products. Tissue incubated at 2.5°C for 3.5 hours and 48 hours converted about 1% and 17%, respectively, of the applied ¹⁴CO to ¹⁴CO₂, and incorporated about 0.04% and 0.6% of the ¹⁴C in acid-stable products. Examination of soluble acid-stable products from ¹⁴CO and ¹⁴CO₂-treated leaf tissue revealed that the labeling patterns of both treatments were identical during the 3.5-hour and the 48-hour incubation periods. Malate, citrate, and aspartate together comprised 70% or more of the soluble radioactivity from both treatments. Incorporation of radioactivity from CO into soluble acid-stable products during a 3-hour incubation period at 2.5°C was inhibited 90% by adding 3% nonradioactive CO₂. These results indicate that in head lettuce in the dark ¹⁴CO is metabolized primarily to ¹⁴CO₂ which is the precursor of acid-stable products. In leaf discs at 2.5°C, the apparent K_m for CO oxidation to CO₂ was 5.3 microliters per liter and the V_max was 9.7 nanoliters per gram per hour. The mitochondrial fraction of the leaf homogenate was the most active fraction to oxidize CO to CO₂, and this activity was heat-labile and cyanide-sensitive.     

Experiments on the fluoroacetate metabolism of Dichapetalum cymosum (Gifblaar)

It was shown that application of fluoroacetate to leaf disks of Dichapetalum cymosum (Gifblaar) did not lead to an inhibition of oxygen uptake or accumulation of citrate, in contrast to the 'control plant' Parimarium capense which lacks fluoroacetate. The addition of fluorocitrate did, however, inhibit the oxygen uptake of both plants and caused an accumulation of citrate. From the results it was deduced that either citrate synthetase or acetate thiokinase from D. cymosum had different affinities for the fluorinated derivative and the 'normal' metabolite. The addition of fluoropyruvate to leaf disks caused a decrease in oxygen uptake and no change in the citrate concentration. From this it was deduced that fluoropyruvate inhibited pyruvate oxidase in both plants. It was concluded that the tolerance of D. cymosum to such high concentrations of fluoroacetate may be ascribed to the fact that the 'lethal synthesis' of fluorocitrate does not take place in the plant most probably because citrate synthetase has different affinities for fluoroacetylcoenzyme A and acetylcoenzyme A.     

Defluorination of fluoroacetate in the rat

Rats given 5 ppm F as FAc (equivalent to 26 ppm of NaFac) in the drinking water for approximately four months deposited as much fluoride in the skeletal system as did rats receiving 5 ppm F as NaF in the water. Little evidence could be found for the presence of organically bound fluoride in bone after ingesting FAc, though an appreciable proportion of skeletal fluoride deposited when NaF was ingested was shown not to respond to the fluoride ion electrode. The daily urinary excretion of total fluoride after FAc was somewhat greater than after NaF; about two thirds of this fluoride responded to the electrode, whereas more than 90 percent of the total fluoride after NaF was ionic in nature. The data are interpreted as showing that the rat is capable of splitting the C-F bond in FAc and/or in its fluoride-containing metabolites, with subsequent skeletal storage and renal excretion of the released fluoride ion. The chronic administration of this low level of FAc caused an early but temporary retardation of growth. The Krebs cycle was interfered with, as evidenced by increased concentrations of citrate in the kidney and urine. At termination of the experiment, histological examination of the testes showed that the FAc had induced severe damage characterized by massive disorganization of the tubules, nearly total loss of functional cells, absence of sperm, and damage to the Sertoli cells.     

Activity and metabolism of 9-substituted cytokinins during lettuce seed germination

The activities of N⁶-benzyladenine (BA) and its 9-substituted methyl, methoxymethyl, tetrahydropyranyl, cyclopentyl, and cyclohexyl analogs were determined for the promotion of lettuce seed (Lactuca sativa L. cv. Grand Rapids) germination. Cytokinin concentrations used were 10^?4, 10^?5, 10^?6 and 10^?7M. All seeds were incubated under total dark conditions at 28 ± 1°C. After 48 h the percentage of germination was recorded. A comparison of means based on Duncan's Multiple Range Test allowed for a ranking of cytokinin activities for the promotion of lettuce seed germination. The activities were: BA = 9-tetrahydropyranyl BA > 9-methyl BA > 9-methoxymethyl BA > 9-cyclopentyl BA > 9-cyclohexyl BA. The results were significant at the 95% confidence level as determined by analysis of variance. In order to study the metabolism of a cytokinin, lettuce seeds were incubated with 9-methyl-BA-methylene-¹⁴C. The labeled cytokinin was prepared by refluxing benzylamine hydrochloride (methylene-¹⁴C) with an equal molar ratio of 6-chloro-9-methylpurine. Final cytokinin concentration was 10^?5M. Incubation periods were 2, 4, 8, 12, 16 and 20 h at 28 ± 1°C under total dark conditions. At the end of the various time periods the seeds were extracted with 70 percent methanol. The resulting extracts were purified and radioactive metabolites identified by solvent fractionation, Sephadex LH-20 column chromatography, and paper chromatography. Co-chromatography with authentic standards in the appropriate solvent system revealed that the metabolites were 9-methyl BA, N⁶-benzyladenosine-5′-monophosphate, and N⁶-benzyladenosine. The results lend support to the theory that the cytokinin ribonucleotide serves as a storage form which is converted to the active ribonucleoside as needed during lettuce seed germination.     

Growth and gibberellin a(1) metabolism in excised lettuce hypocotyls

Excised lettuce (Lactuca sativa L. cv. Arctic) hypocotyls retain the ability to elongate in response to exogenously supplied gibberellic acid and gibberellin A(1) (GA(1)). We have studied the relationship between metabolism of GA(1) and elongation in this tissue. In 24 hours at 28 C, hypocotyls treated with 3 mum GA(1) double in length while controls elongate less than 45%. After an exogenous hormone supply is removed, hypocotyls continue to grow faster than untreated controls, although as the hormone application time is decreased, the GA(1) concentration required to effect a given length change increases. [(3)H]GA(1) was used to determine rates of hormone uptake, efflux, and metabolism. In the presence of [(3)H]GA(1), hypocotyls accumulate and metabolize lable for at least 24 hours. When the exogenous label is removed, the amount of acidic GA in the hypocotyl declines rapidly to a constant level while ethyl acetate-insoluble metabolites increase rapidly to a constant level. Lable accumulation and metabolism at any time are proportional to the external GA(1) concentration below 50 mum GA(1). Chromatographic analysis of radioactive compounds present in tissue extracts suggests that unaltered GA(1) is the major component of the acidic ethyl acetate-soluble fraction, and gibberellin A(8) is a minor component. The ethyl acetate-insoluble fraction appears to contain an unidentified GA(1) metabolite with chromatographic properties similar to those of GA(1). The strong retention of accumulated GA(1) confirms the possibility of a continuing requirement for GA(1) during the sustained response to a GA(1) "pulse" but raises the question of accessibility of the stored hormone for growth promotion.     

Involvement of putative chemical wound signals in the induction of phenolic metabolism in wounded lettuce

Cutting leaves of Romaine lettuce ( Lactuca sativa L. cv. Longifolia) produces a wound signal that induces the synthesis of phenylalanine ammonia lyase (PAL, EC 4.3.1.5) and the accumulation of phenolic compounds in cells up to 2 cm from the site of injury, and tissue browning near the site of injury. The response of leaves within a head of Romaine lettuce to putative chemical wound signals [abscisic acid (ABA), jasmonate (JA) and methyl jasmonate (MeJA)] differed significantly with leaf age. Exposure of harvested heads of lettuce to ABA, JA, MeJA, or salicylic acid (SA) did not induce changes in PAL activity, the concentration of phenolic compounds or browning in mature leaf tissue that was similar to the level induced by wounding. Methyl jasmonate applied as vapour (10, 100 or 1000 µl kg⁻¹ FW), or as an aqueous spray or dip (0.01–100 µ M ) at 5 or 10°C did not produce an effect on PAL activity or browning that differed significantly from the untreated controls. In contrast, JA, MeJA and SA did induce elevated levels of PAL activity in younger leaves. However, the levels induced were far lower than those induced by wounding. Wound induced phenolic metabolism in mature leaves appears to be induced by different signals than those functioning in young leaves.     

Exogenous spermidine improves the sucrose metabolism of lettuce to resist high-temperature stress

Plant Growth Regulation - Lettuce is a popular fresh vegetable, and high-temperature stress will reduce the yield of lettuce. Spermidine is an essential phytohormone in plant stress responses....     

Enhancement of pullulan elaboration by fluoroacetate

Summary The addition of fluoroacetate, the precursor of the aconitase inhibitor, fluorocitrate, to resting cell suspensions of Aureobasidium pullulans was found to increase the quantity of extracellular polysaccharide elaborated and to raise the pullulan content to 93%.