Russeting in Apple Is Initiated After Exposure to Moisture Ends—I. Histological Evidence

Abstract

Russeting (periderm formation) is a critical fruit-surface disorder in apple (Malus × domestica Borkh.). The first symptom of insipient russeting is cuticular microcracking. Humid and rainy weather increases russeting. The aim was to determine the ontogeny of moisture-induced russeting in ‘Pinova’ apple. We recorded the effects of duration of exposure to water and the stage of fruit development at exposure on microcracking, periderm formation and cuticle deposition. Early on (21 or 31 days after full bloom; DAFB) short periods (2 to 12 d) of moisture exposure induced cuticular microcracking—but not later on (66 or 93 DAFB). A periderm was not formed during moisture exposure but 4 d after exposure ended. A periderm was formed in the hypodermis beneath a microcrack. Russeting frequency and severity were low for up to 4 d of moisture exposure but increased after 6 d. Cuticle thickness was not affected by moisture for up to 8 d but decreased for longer exposures. Cuticular ridge thickness decreased around a microcrack. In general, moisture did not affect cuticular strain release. We conclude that a hypodermal periderm forms after termination of moisture exposure and after microcrack formation. Reduced cuticle deposition may cause moisture-induced microcracking and, thus, russeting.

Russeting in Apple is Initiated after Exposure to Moisture Ends: Molecular and Biochemical Evidence

“…Second, despite a marked and consistent up-regulation of genes involved in synthesis of monomers for suberin, there was no clear corresponding increase in suberin monomers 8 d after discontinuation of moisture exposure. At this stage, a periderm had begun to develop in the hypodermal cell layers, in this and also our earlier study, as inferred from cross-sections of skin patches [ 16 ]. However, when skin patches were incubated in pectinase and cellulase, the cell layers separating the periderm from the epidermis were digested and, hence, the developing islands of periderm were lost to the isolation medium.…”

“…These included genes involved in the synthesis of monomers and constituents ( GPAT6 , KCS10 , SHN3 , WSD1 , CER6 ) and their transport across the plasma membrane ( ABCG11 ). The down-regulation occurred at the same time as microcracks formed [ 15 , 16 ], as cuticle thickness around microcracks decreased [ 16 ] and as the amounts of the amounts of the major constituents of cutin and wax decreased. These observations suggest a causal relation between moisture exposure, a reduction in the expressions of genes involved in cuticle synthesis, a decrease in cuticle mass and the subsequent formation of a periderm and the onset of suberin synthesis and deposition.…”

“…The histological, biochemical and molecular results demonstrate that moisture-induced russet is limited to the early stages of fruit development [ 16 ]. This is consistent with field observations where the first four weeks after full bloom are considered critical [ 1 , 10 , 11 , 12 , 13 , 14 ].…”

“…Microscopic cracks (‘microcracks’) in the cuticle are the first visible symptom of russeting [ 10 , 15 , 16 ]. Microcracks form when the fruit skin is strained during periods of rapid surface expansion [ 17 ].…”

“…It thus allows critical comparisons to be made by eliminating a range of potential sources of variability in russet formation, such as by the stage of fruit development, the microenvironment of the fruit in the canopy, etc. Using this system, the effect of moisture exposure on the histology of russet formation was investigated in greater detail [ 16 ]. Several findings were reported: (1) Microcracking of the cuticle occurred during moisture exposure, but there was no periderm formation during moisture exposure.…”