Composition, ultrastructure and function of the cutin- and suberin-containing layers in the leaf, fruit peel, juice-sac and inner seed coat of grapefruit (Citrus paradisi Macfed.)

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Abstract
Cutin and suberin polymers from various anatomical regions of grapefruit were analyzed chemically and ultrastructurally. The leaf, fruit peel and juice-sac showed an amorphous cuticular layer. The cutin in the leaf was composed of 10,16-dihydroxy C16 acid and its positional isomers as the major monomers whereas 16-hydroxy-10-oxo C16 acid was a major component in the fruit peel. Juice-sac cutin, on the other hand, contained the dihydroxy C16 acids, hydroxyoxo C16 acids, hydroxyepoxy C18 acids and trihydroxy C18 acids. Ultrastructural examination of the inner seed coat showed that an amorphous cuticular layer encircled the entire seed except in the chalazal region which showed several layers of cells with lamellar suberin structure throughout the cell walls. Consistent with the ultrastructural assignment, the compositions of the aliphatic components of the polymers from the chalazal region and the non-chalazal region indicated the presence of suberin and cutin, respectively. The aliphatic portion of the polymer from the chalazal region of the inner seed coat contained C16, C18:1, C22 and C24 ω-hydroxy acids (46% combined total) and the corresponding dicarboxylic acids (43%) as the major components. ω-Hydroxy-9,10-epoxy C18 acids and 9,10,18-trihydroxy C18 acids were the major components (77%) of the polymer from the non-chalazal portion of the inner seed coat. The main portion and the chalazal region of the inner seed coat yielded 17 and 342 μg/cm2 of aliphatic monomers, respectively, and the diffusion resistance of these two portions of the inner seed coat were 62 and 192 sec/cm, respectively. The inner seed coat was shown to be the major moisture diffusion barrier influencing imbibition and germination.