Growth strains cause vascular browning and cavities in ´Nicoter´ apples

Abstract

'Nicoter' apples (Malus × domestica Borkh.) occasionally develop a disorder referred to as vascular browning. Symptomatic fruit are perceived as being of low quality. The objective was to identify the mechanistic basis of this disorder. The frequency of symptomatic 'Nicoter' apples differed between growing sites and increased with delayed harvest. Typical symptoms are tissue browning and cavities in the ray parenchyma of the calyx region, and occasionally also of the stem end. Cavity size is positively correlated with the extent of tissue browning. Cavities were oriented radially in the direction of the bisecting line between the radii connecting the calyx/pedicel axis to the sepal and petal bundles. Microscopy revealed cell wall fragments in the cavities indicating physical rupture of cell walls. Immunolabelling of cell wall epitopes offered no evidence for separation of cells along cell walls. The growth pattern in 'Nicoter' is similar to that in its parents 'Gala' and 'Braeburn'. Allometric analyses revealed no differences in growth in fruit length among the three cultivars. However, the allometric analyses of growth in diameter revealed a marked increase in the distance between the surface of the calyx cavity and the vascular bundle in 'Nicoter', that was absent in 'Braeburn' and 'Gala'. This increase displaced the petal bundles in the ray parenchyma outwards and subjected the tissue between the petal and sepal bundles to tangential strain. Rupture of cells results in tissue browning and cavity formation. A timely harvest is a practicable countermeasure for decreasing the incidence of vascular browning.