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Abstract

Quinones are naturally occurring lipid-soluble compounds some of which have the structures shown in Fig. 1. The isoprenoid side chain bestows considerable lipophilic properties on these compound and it is not surprising that they are nearly always found in the membrane fraction of cells. This review will deal with various aspects of just 4 quinones; ubiquinone, (UQ); plastoquinone, (PQ); Vit. K or phylloquinone, (K); and Vit E, or tocopherol, (T). The aspects of horticultural interest to be dealt with here are: the biochemical function of quinones in plants and animals; the biosynthesis of quinones; and the nutritional aspects of quinones.

Abstract

Investigation of optimum conditions for extraction, titrimetric assay and activity of avocado (Persea americana Mill cvs. Fuerte, Hass) pectinmethylesterase (PME) showed maximum extraction was obtained from lyophilized mesocarp using 0.4 m NaCl. Best assay conditions required a substrate of 0.5% pectin in 0.1 m NaCl. The enzyme was released from the tissue by salt but not by nonionic detergents. The Arrhenius plot between 3 and 32°C was a straight line which indicates no involvement with membrane lipid. Incubation with air or ethylene did not affect the enzyme. PME activity declined rapidly as ripening was initiated and reached a minimum shortly before the peak in respiration and ethylene production.

During the harvest season apples ripen and develop scald resistance. In the Northeast they usually are also exposed to cool temperatures as they mature and ripen. Experiments were conducted to study the effects of cool temperature, light and maturity on the endogenous antioxidants and subsequent scald development in Cortland and Delicious apples. Total lipid-soluble antioxidant activity in apple peel at harvest generally increased as scald incidence after storage decreased. Yet, α tocopherol, ascorbic acid and total water-soluble reducing capacity were not closely related to scald development. The absence of light (bagged fruit) decreased all measured antioxidants and increased scald development. However, ethephon applied in mid-August to induce ripening increased the levels of these antioxidants but had little effect on scald incidence in the absence of cool temperatures (hours <10°C). Cool temperatures, which decreased scald susceptibility, increased lipid-soluble antioxidant activity but had little influence on the other measured antioxidants. These data suggest that the endogenous antioxidants may be only partly responsible for natural scald resistance.

Strawberry fruit (Fragaria ×ananassa `Chandler') were evaluated at five different stages of growth and development for changes in the senescence process in fruit tissues. Levels of total antioxidant activity, hydrogen peroxide (H₂O₂), lipid peroxidation product, malondialdehyde, and ethylene production were determined. Total antioxidant activity (TAA) was measured in terms of in situ antioxidants to scavenge the ABTS.superscript +superscript radical cation. With the progression of ripening and senescence, there was a significant decline in TAA that coincided with increased concentration of H₂O₂, lipid peroxidation and increased production of ethylene. Our results illustrate that the senescence process in strawberry fruit is associated with the decline of TAA and the potential initiation and accumulation of reactive oxygen species. These results are additionally discussed in terms of potential processes associated with abiotic and biotic environmental stresses. Moreover, although strawberry fruit are typically classified as nonclimacteric, this study illustrates that the free radical mediated senescence process is similar to that of climacteric fruits.

Mexican redbud (Cercis canadensis var. mexicana) exhibits resistance to leaf cutter bees (Megachile spp., LCB). Resistant trees (CMG) have glossy leaves and sustain little LCB damage as compared to dull-leaf Mexican redbud (CMD) and the closely related eastern redbud (Cercis canadensis, CC). On average, LCB made 35 times as many cuts per week on CC as on CMG and CMD, even though there were half as many leaves available. Mexican redbud leaves are twice as thick as CC leaves, which may account for LCB preference for the latter. However, leaves from CMG and CMD are similar in leaf thickness, cuticle wax content, and resistance to penetration, yet LCB had an even stronger preference for the former. More than 83 times as many cuts per week were made on CMD over CMG, even though the number of leaves was comparable. CMG leaves have a thicker cuticle on the upper surface that lacks wax crystals present in the CMD and CC. The upper cuticle from CMG leaves also contains fewer lipids and an altered lipid composition (notably fewer long-chain alcohols) compared to CMD.

Bermudagrass (Cynodon dactylon) cultivars may exhibit increased tolerance to cold following periods of exposure to moderately cold temperature (i.e., acclimation). We are evaluating biochemical changes and the regulation of gene expression in two cultivars—'Midiron' and U-3—during this acclimation period. Total membrane lipid fatty acids per unit of total lipids (MLFA/TL; μg·mg^–1) increased in crowns over the 4-week exposure to chilling temperatures (8C day/2C night). Of the fatty acids comprising 95% of total MFLA, concentrations of short-chain and saturated FAs declined significantly while unsaturated longer-chain FA concentration increased. As a result, the double bond index (percent of each FA x number of double bonds in the FA) increased during the period of low temperature exposure, indicative of increasing membrane fluidity. Changes in MFLA were evident as early as 4 days following exposure to chilling temperatures. Identification of mRNA species expressed in response to low temperature utilized differential display-PCR. Initial screening with paired T11N1N 2 3'-anchor and 5'-random decamer primers has identified transcripts differentially expressed as early as 23 h post-exposure and was maintained for at least an additional 36 h. Isolation, reamplification, and cloning of these identified PCR products is in progress.

Catkin external morphological characteristics of a protogynous (`Stuart') and a protandrous (`Desirable') cultivar of pecan [Carya illinoensis (Wangenh.) C. Koch] were related temporally to the differentiation of microspore and pollen grains. Reproductive cell development was divided into seven periods based on evaluations of number, location, and intensity of staining of the nucleus and/or nucleolus; and vacuolization and staining intensity of the cytoplasm. Catkins with anthers and bracteoles enclosed by bracts did not have reproductive cells that were matured to free microspore. Free microspore developed only after bracteoles became externally visible. The Period 1 nucleus was at the periphery of the cell and a large central vacuole was present; at Period 2, the nucleus was at the center and vacuolation had been reduced. As the angle between the bract and catkin rachis increased to 45°, vacnolation was reduced as the nucleus enlarged and moved to a central location in the microspore (Periods 3 and 4). The majority of the pollen grains were binucleate, and the generative nucleus became elliptical (Periods 5 and 6) by the time anthers became externally visible. Acetocarmine staining intensity of cellular components masked the presence of the generative nucleus (Period 7) just before anther dehiscence. Staining reaction for protein was positive from Period 1; starch from Period 3; lipids and polyphenols from Period 5. The mature pollen grain was abundant in stored reserves of starch and lipids and had a wall with a thicker exine than intine as demonstrated by acetolysis.

A major postharvest problem of Protea neriifolia is premature leaf blackening. Carbohydrate stress, due to floral sink demand, may lead to cellular disorganization and leaf blackening. Leaf blackening, nonstructural carbohydrates, ethylene, carbon exchange rates, stomatal conductance and lipid peroxidation were measured on leaves of vegetative and floral stems preharvest, and during a 7 day dark postharvest period. Postharvest treatments were: 0 or 0.5% sucrose in the vase solution, 20% sucrose pulse, or floral decapitation. Leaf blackening was significantly reduced in vegetative stems and floral stems in the 20% pulse treatment, in comparison to all other treatments. Ethylene production and lipid peroxidation were not associated with leaf blackening in any treatment and leaf respiration rates declined for all treatments over time. The magnitude and rate of leaf blackening was inversely related to leaf starch concentrations, with greatest carbohydrate depletion occurring within 24 h of harvest (by 75-85%). Leaf starch from the 20% pulse treatment increased by 300%, in contrast to declining starch concentrations in all other treatments. The data suggest that the flowerhead functions as the major sink for carbohydrate depletion leading to subsequent leaf blackening.

Leaf surface compounds of pecan [Carya illinoensis (Wangenh.) C. Koch] were analyzed with regard to developmental stage and to susceptibility to infection by Cladosporium caryigenum (Ell. et Lang. Gottwald). Immature and mature leaves of two resistant (`Elliott' and `Sumner') and two susceptible (`Wichita' and `Schley') cultivars were extracted with methylene chloride. Extracts were separated by silicic acid chromatography into polar and nonpolar fractions. Constituents of each fraction were subsequently separated by gas chromatography and were identified by gas chromatography-mass spectroscopy. Leaf surface constituents characterized included long-chain aliphatic hydrocarbons, aliphatic wax esters, triterpenoid constituents, aliphatic alcohols, fatty acids, and diacyl glycerides. The predominant surface compounds on immature leaves were lipids such as fatty acids, fatty alcohols, and glycerides. On mature leaves, lipids declined and aliphatic hydrocarbons and triterpenoids became predominant leaf surface constituents. The changes were observed for all cultivars, regardless of genotypic response to C. caryigenum. Thus, we conclude that cuticular chemicals change dramatically during leaf maturation but do not correlate with resistance to scab disease common to certain pecan cultivars.

The relationship between fruit maturation and accumulation of hydrogen peroxide (H₂ O₂), lipid peroxidation, ethylene (C₂ H₄) production, antioxidant activity (hydrophilic, lipophilic and total) and the antioxidant enzyme ascorbate peroxidase (APX, EC 1.11.1.11) in fruit pericarp tissue of `Chandler' (Fragaria × ananassa Duch.) strawberry were measured. `Chandler' fruit pericarp maturation and ripening were accompanied by a decline in H₂ O₂ content early in fruit development followed by a rapid accumulation. An increase in membrane lipid peroxidation (thiobarbituric acid reactive substances, TBARS) coincided with accumulation of H₂ O₂, which preceded a rise in C₂ H₄ production. In general, antioxidant activity declined as fruit matured and ripened. APX enzyme activity increased by 2-fold and peaked at the pink stage of development and then gradually declined with ripening. H₂ O₂ may serve as a signal molecule to initiate the cascade of oxidative processes during maturation and ripening. APX enzyme activity during maturation and ripening was not substantial and thus, may not have a role in alleviating accumulation of H₂ O₂ and subsequent events related to oxidative senescence in fruit pericarp. To our knowledge, this is the first study to present fractionated antioxidant activities (HAA, LAA and TAA) from strawberry pericarp as assessed by the ABTS∼+ radical cation assay. A fundamental understanding of the mechanisms involved in the senescent related-oxidative changes during strawberry fruit ontogeny in relation to quality and nutrition is discussed.