Ultrastructure of the onset of chilling injury in cucumber fruit

Abstract

The onset of the symptoms of disorders provoked by low temperature storage or chilling injury (CI) in pickling cucumber fruit (Cucumis sativus L. cv. 'Trópico' and in the cv. 'Perichán 121') and associated rot were monitored by cryoscanning electron microscopy. Fruit were stored at 4°C for different times (4 to 12 d) and samples were transferred to 20°C every 2-3 d (cv. Trópico). In a second experiment, fruit cv. 'Perichán 121' were stored at 6°C. Macroscopic CI symptoms included small-flattened areas with sunken but externally sound tissue. Later, the damage was manifested as pitting and decay due to the presence of necrotrophic fungi (Pleospora herbarum, Alternaria sp.) on the surface of the broken areas, and Botrytis cinerea in cv. 'Perichán 121'. The period of induction of CI becoming apparent lasted about 4 d at 4oC followed by a phase of slow increase of around 4-5 d prior to an exponential increase in CI. Micro fractures of 45-250 μm length developed in CI tissue around the stomata of 20 μm Ø. The first response to chilling was the sinking of stomata accompanied by 10 μm Ø fractures with collapse of hypodermal cells. These small fractures expanded into a sink of around 40 μm Ø and more than 50 μm depth. Refrigerated tissue had visibly collapsed in 4-6 d, starting with a small brown area indicating collapse of parenchymatous cells, followed by sinking and collapse of epidermal cells, particularly around the micro fractures. The pitted tissue showed flattening of the cell walls, plasmalemma and middle lamella region, with severity increasing with increasing depth in the parenchymatous tissue. Translucent water soaked areas were also located depth in the mesocarp tissue with similar cell damages.