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S. Mark Goodwin, Christopher J. Edwards, Matthew A. Jenks, and Karl V. Wood

The fungal pathogen Diplocarpon rosae causes rose blackspot disease, a serious problem for roses (Rosa) in the managed landscape. To prevent this disease, homeowners and professional growers often apply chemical fungicide. However, increased use of fungicides poses an environmental hazard and an economic burden to the user. New landscape rose cultivars like ‘Knockout’ possess increased disease resistance, but the biological basis for this resistance is still unknown. To investigate the potential role of leaf cuticle in blackspot resistance in rose, five rose cultivars known to vary greatly in blackspot resistance were examined for variation in the major lipids of the leaf cuticle, specifically the monomers of the cutin polyester and the free cuticular waxes. This is the first report of cutin monomers in the Rosa genera. The rose cultivars selected for this study were ‘Knockout’, ‘Mister Lincoln’, ‘Garden Party’, ‘Purple Passion’, and ‘Bicolor’. ‘Knockout’ and ‘Garden Party’ had significantly lower total cutin monomer amount per leaf area than the other cultivars, whereas the most cutin monomers were observed on ‘Purple Passion’, ‘Bicolor’, and ‘Mister Lincoln’. Five major cutin monomers (mostly hydroxylated 16 carbon fatty acids) dominated the cutin profiles of both adaxial and adaxial surfaces of all cultivars, with the 10,16-dihydroxy hexadecanoic acids being most abundant. The proportion of 10,16-dihydroxy hexadecanoic acids was slightly higher in the adaxial than abaxial leaf cuticles of all cultivars. Correspondingly, other cutin monomers were relatively lower in the adaxial cuticle, except 16-hydroxy hexadecanoic acid that differed little. Uniquely, this is the first report of cutin monomer composition of isolated abaxial and adaxial leaf cuticles of any plant. Total leaf cuticular wax amounts were lowest on ‘Purple Passion’ and ‘Knockout’, intermediate in ‘Mister Lincoln’ and ‘Garden Party’, and highest on ‘Bicolor’, with alkanes as the most abundant wax class. Consistent with previously published disease susceptibility ratings, our visual scores showed that ‘Knockout’ was most resistant to blackspot pathogen infection with a visual disease rating score of 1.0, followed by ‘Mister Lincoln’ at 1.8, ‘Garden Party’ at 5.4, ‘Bicolor’ at 7.5, and ‘Purple Passion’ with the most visible disease damage at 8.8. Regression analysis revealed that the alkane and ester proportions were most closely associated with blackspot disease susceptibility ratings, being inversely (R2 = 0.63, P = 0.05) and directly (R2 = 0.81, P = 0.05) correlated, respectively. More studies on the role of cuticle in rose susceptibility to blackspot are now clearly warranted.

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Matthew A. Jenks, Carole H. Gaston, Mark S. Goodwin, Jessie A. Keith, Rebecca S. Teusink, and Karl V. Wood

Cuticular waxes were analyzed on abaxial and adaxial leaf surfaces of three Hosta genotypes differing in leaf surface glaucousness; the glossy-leaved Hosta plantaginea, the glossy-leaved Hosta lancifolia, and the glaucous-leaved Hosta `Krossa Regal'. All three hosta had their highest total leaf wax quantity in the spring soon after full leaf expansion. The major wax constituent class on these hosta was primary alcohols, comprising up to 84.6% of the total wax. Many hosta leaves had unusually high C24 length primary alcohols, especially in the spring. However, the dominant chain length in this alcohol class varied with development and genotype. A unique class of ß-diketones were present on the glaucous `Krossa Regal', comprising as much as 28.7% of the total waxes on abaxial leaf surfaces in the summer. Interestingly, these ß-diketones were only 0.9% of total waxes on adaxial leaf surfaces of `Krossa Regal' in the summer. Studies are under way to determine whether the dramatic seasonal changes in the waxy leaf coatings described in this report are associated with biotic and abiotic stress resistance in hosta.

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Maalekuu Kissinger, Sharon Tuvia-Alkalai, Yavin Shalom, Elazar Fallik, Yonatan Elkind, Matthew A. Jenks, and Mark S. Goodwin

Fruit of pepper (Capsicum annuum L.) is hollow by nature, which limits its water reservoir capacity, and as such, small amounts of water loss result in loss of freshness and firmness, which reduce fruit quality, shelf life, and market value. In order to understand the basis for water loss from fruit, 10 pepper accessions with wide variation in water loss rate were used to study physiological and biochemical factors associated with postharvest water loss in ripe pepper fruit during storage. Postharvest water loss rate in ripe pepper fruit stored at 20 °C, and 85% relative humidity, was found to be associated with cell membrane ion leakage, lipoxygenase activity, and total cuticular wax amount. Total cuticular wax amounts were highest in the high-water-loss pepper fruit, and lowest in the low-water-loss fruit. However, total cuticle amount (isolated enzymatically and quantified gravimetrically), total cutin monomer amount, and the amount of individual cutin monomer and wax constituents (determined using gas chromatography mass spectrometry) indicated no direct association with postharvest water loss rates. Fruit fresh weight, pericarp weight, pericarp surface area, pericarp thickness, initial water content, and dry matter were highly associated with each other, but less so with water loss rate. Fruit of accessions displaying high fruit water loss rate matured and ripened earlier than fruit of accessions displaying low-water-loss rate. Cell membrane ion leakage and lipoxygenase activity were higher after storage than immediately after harvest. Pepper fruit total cuticle wax amount, lipoxygenase activity, and cell membrane ion leakage were directly related to postharvest water loss rate in pepper fruit during storage.