Physiological studies of light-induced greening in fresh market potatoes

Laura Grunenfelder

Exposure of tubers to light induces chlorophyll synthesis in cortical parenchyma tissue directly beneath the periderm, resulting in undesirable greening. There are two main issues associated with potato greening: human health and marketability. Human health is considered a major factor because of the parallel and independent development of toxic glycoalkaloids. Although no metabolic connection between greening and glycoalkaloid development has been established, green potatoes are considered less fit for consumption and are discriminated against by consumers. While produce managers routinely cull greened potatoes, there are currently no specific grading criteria, resulting in highly variable quality within and among stores. Accordingly, greening indices were developed for the cultivars White Rose, Dark Red Norland, Yukon Gold, Norkotah Russet and Reba. Tuber color changed rapidly, and the full range of greening (8-10 levels) for each cultivar was obtained within 6 to 10 days of exposure to light intensities found in retail stores. Fluorescence microscopy of sections of tuber tissue from ‘White Rose’ revealed chlorophyll development within the outer 15 layers of parenchyma cells directly beneath the periderm. The fluorescence coincided with development of chlorophyll and increased as periderm color changed from green stage 3 to 7. Regardless of cultivar, concentrations of total glycoalkaloids in the flesh samples never exceeded the upper limit for consumption of 20 mg/100 g fresh wt established by the FDA. Glycoalkaloids increased in parallel with greening, ranging from 6.9 mg/100 g dry wt prior to greening to 46.8 mg/100 g dry wt (post-greening), well within the limits established by the FDA (approximately 1mg/g dry wt, assuming dry weight is 20% of fresh weight). Reduction of light intensity from 6.5 to 3.8 [mu]mol quanta m-2 s-1 through packaging in perforated polyethylene bags had no effect on greening. However, the rates of chlorophyll accumulation and greening were less at 3.5 [mu]mol quanta m-2 s1 than at 10.2 and 17.9 [mu]mol quanta m-2 s-1. Chlorophyll development in tubers is thus highly sensitive to low light levels, likely as a consequence of phytochrome involvement. Reducing the photoperiod from 24 to 18 hours had no effect on the rate of greening. Low temperatures (7 and 10 °C) slowed the greening process over a 5-day interval. Surveys implementing the greening scales showed a high degree of variation among and within retail outlets for the extent of greening, underscoring a need for more consistent grading procedures. The grading scales covered the full range of greening found in stores, demonstrating their potential utility as an aid to quality control. Collectively, these results emphasize the sensitivity of tubers to very low light intensities and illustrate the difficulties faced by retailers in developing displays and packaging that will extend shelf-life through attenuation of the greening process.

Physiological studies of light-induced greening in fresh market potatoes

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