Factors Affecting the Appearance of Russet Spotting in Iceberg Lettuce and Anatomical Study of its Development
Date Issued
2014
Date
2014
Author(s)
Ou, Yu-Shan
Abstract
Iceberg lettuce (Lactuca sativa L. var. capitata) is the most cultivated type within all the vegetable lettuces. Due to its inherited nature for long distance shipment, it occupies a significant position in the world market of fresh produce. Exposed of iceberg lettuce to ethylene during shipping or cold storage will induce the physiological disorder russet spotting (RS). The production of iceberg lettuce in Taiwan expanded rapidly during the past 10 years, and has become an important crop for export market. The object of this research was to study the development of russet spotting in a domestic-grown iceberg lettuce cultivar ‘Number 6’. Tissue segments excised from lettuce midrib tissue were exposed to ethylene to induce russet spotting and the development of the disorder is studied by section staining technology. Exposing to 0.1 μL‧L-1 ethylene for 4 days at 5°C will induce RS with slight lesion. Exposing to 1.0 μL‧L-1 ethylene for 3 days at 5°C will induce RS, which developed rapidly and became severe on the 7th day. RS development was most severe at 5 ℃, un-noticeable at 0°C, and less severe at 10 and 15℃. At 5 ℃, treating mid-rib sections with 1 μL‧L-1 ethylene for 1 or 2 days did not induce the development of russet spotting. It requires at least 3 days to induce the RS, and the symptom will continue to development even after the removal of ethylene. Lettuce heads weighing more than 600 g developed RS more rapidly and severely than less mature heads. Within a single head, the out wrapper leaves were more sensitivity to ethylene than the inner heart leaves. Ethylene induced a 2.67 fold increase in the phenylalanine ammonia lyase (PAL) activity in the mid-rib tissue. RS appeared mostly on the surface of tissue. In the early stages these are light yellow; later they become deep red-brown and the predominant color is russet. In advanced stages, the epidermis and mesophyll cells near the surface collapsed and pitting appeared. Most of the russet spots occurred as depression of surface cells, but some occurred below the surface. The latter appear diffuse and dark below the surface layers of normal white cells in the midribs. In order to verify whether cell wall lignifications took place in the disordered cell, the section of lettuce midribs were histologically stained with three lignin-specific stains: the Wiesner reagent test, the Maule reation test and the Safranin-Aniline blue double stain test. The results showed that the lignified xylem was stained by all three tests, but the color at the RS lesion did not change significantly. This result indicated that little or no lignifications in the RS affected cells.
Subjects
結球萵苣
鏽斑病
乙烯
木質化
褐變
Type
thesis
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