Translucent Scale and Leathery Scale Disorders of Onion Bulbs: Describing Symptoms and Causes

摘要

In Northern Europe, onion (Allium cepa L.) is grown from sets planted out in early May and lifted in early September. After a field-curing period the bulbs are put into storage and dried at 25-30°C until the neck is completely dry before the temperature is gradually reduced to 0°C for long-term storage. Watery scale (translucent and leathery scale) is a postharvest problem in our area causing loss of yield and quality. Watery scale includes the following categories of disorders: leathery scale, which appears as thick, dark scales between outer dry and inner fleshy scales, and translucent scale, which appears as glassy, firm fleshy scales further into the bulb. Symptoms of watery scale were clarified based on visual appearance, tissue pH, electrical conductivity, and contents of ethanol, lactic acid, and acetic acid. The disorders were linked to disturbances in gas exchange of bulbs with high internal CO_2 and/or low internal O2 levels. Bulbs were exposed to handling treatments leading to increased respiration and/or reduced gas exchange from the interior of bulbs to the outside atmosphere. Internal bulb atmosphere, gas flux, and gas permeability (of O_2) of onion scale and tissues were measured to explain why damage occurred. Translucent scale generally occurred in the middle and upper parts of the outer two fleshy scales but could include the entire bulb. A clear reduction in electrical conductivity of translucent tissue was measured, indicating leaching. A rise in pH occurred in translucent scales with weak symptoms followed by a reduction in pH. Leathery scale tissues had a pH around 3.5 and a rise in ethanol content, indicating anaerobic respiration. Dry scale and epidermis of fleshy scale had very low oxygen permeability. Transport of gasthrough the neck region was 10-20 times higher than transport through the scale. Gas flux was related to need of O2 in respiring tissue and to problems of adverse internal atmosphere and development of disordered scales. Normal cultivation of onion andhandling of bulbs resulted in low occurrence of disordered bulbs. Weak symptoms of translucent scale could disappear in storage, but usually symptoms became more severe after long-term storage. Dropping and pressure treatments produced more translucent scale but had no effect on leathery scale. Internal CO_2 concentration, as high as 22.3%, occurred after combineddropping and pressure treatments. The results explained the induction of translucent scale. Leathery scale was not affected by the treatments examined in this study. Further work is needed to understand why leathery scale develops and how it can be minimized.