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Antioxidant activity and phenolic content of sweetpotato root and leaf tissues were quantified at different developmental stages. 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical method was used to measure antioxidant activity and total phenolic content was quantified by spectrophotometry using Folin-Denis reagent. Individual phenolic acids were quantified using reversed phase high performance liquid chromatography. Antioxidant activity and phenolic content decreased with root development and leaf maturity. Roots at the initial stages of development (about 4.0 g root weight) had a higher antioxidant activity and phenolic content compared to fully developed roots. Phenolic content in fully developed roots was significantly higher in the cortex tissue than internal pith tissue. The highest total phenolic content and antioxidant activity was found in cortex tissue at the initial stage of development (10.3 mg chlorogenic acid eq/g dry tissue weight and 9.7 mg Trolox eq/gdry tissue weight, respectively). Sweetpotato leaves had a significantly higher phenolic content and antioxidant activity than roots. Immature unfolded leaves had the highest total phenolic content (88.5 mg chlorogenic acid eq/g dry tissue weight) and antioxidant activity (99.6 mg Trolox eq/g dry tissue weight). Chlorogenic acid was the major phenolic acid in root and leaf tissues with the exception of young immature leaves in which the predominant phenolic acid was 3,5-dicaffeoylquinic acid.

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Authors: and

Antioxidant activity and phenolic content in sweetpotato root and leaf tissues were quantified at different developmental stages. The 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical method was used to measure antioxidant activity and total phenolic content was quantified by spectrophotometry using Folin-Denis reagent. Individual phenolic acids were quantified using reversed phase high performance liquid chromatography. Antioxidant activity and phenolic content decreased with root development and leaf maturity. Roots at the initial stages of development (about 4 g root weight) had a higher antioxidant activity and phenolic content compared to fully developed roots. Phenolic content in fully developed roots was significantly higher in the cortex tissue than internal pith tissue. The highest total phenolic content and antioxidant activity was found in cortex tissue at the initial stage of development (10.3 mg chlorogenic acid eq/g dry tissue weight and 9.7 mg Trolox eq/gdry tissue weight, respectively). Sweetpotato leaves had a significantly higher phenolic content and antioxidant activity than roots. Immature unfolded leaves had the highest total phenolic content (88.5 mg chlorogenic acid eq/g dry tissue weight) and antioxidant activity (99.6 mg Trolox eq/g dry tissue weight). Chlorogenic acid was the major phenolic acid in root and leaf tissues with the exception of young immature leaves in which the predominant phenolic acid was 3,5-dicaffeoylquinic acid.

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Phenolic acids are one of several classes of naturally occurring antioxidant compounds found in sweetpotato. Simplified but reliable methodologies were developed to quantitate total and individual phenolic acids in sweetpotato roots. Total phenolic acid content was measured using both Folin-Denis and Folin-Ciocalteu reagents. The Folin-Ciocalteu reagent gave an overestimation of total phenolic acids due to the absorbance of interfering compounds (i.e., reducing sugars and ascorbic acid). The average total phenolic acid content in `Beauregard' sweetpotatoes was 60.9 mg/100 g fresh weight. Individual phenolic acids were separated with two reversed-phase C18 columns of different dimensions and particle size. The columns tested were a 7 × 53 mm, 3 μm, Alltima Rocket (Alltech Assoc.) and a 3.9 × 150mm, 4 μm, Nova-Pak (Waters Corp.). Different mobile phases were also evaluated. The Alltima C18 column using a mobile phase of 1% (v/v) formic acid aqueous solution: acetonitrile: 2-propanol, pH 2.5 (70:22:8) provided the best separation of individual phenolic acids. Total analysis time was less than 5 minutes. Chlorogenic acid was the major phenolic acid found in sweetpotato root tissue (15.8 mg/100 g fresh weight). In a comparison of different tissue preparation states (fresh, frozen, freeze-dried), fresh tissue gave the highest concentration of total and individual phenolic acids. Among the 3 extraction solvents tested (80% methanol, 80% ethanol, and 80% acetone), 80% methanol and 80% ethanol gave higher, but similar, phenolic acid extraction efficiency.

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Sweetpotatoes may be potentially high in concentration of certain phytochemical compounds, including phenolics. Low temperature stress-induced phenolic compounds may enhance the nutraceutical value of sweetpotatoes. However, extended exposure to low temperature results in chilling injury. Cured and non-cured roots of `Beauregard' sweetpotatoes were exposed to low temperature storage (5 °C) for up to 4 weeks. The total phenolics and individual phenolic acid contents were determined at weekly intervals using Folin-Denis reagent and reversed-phase HPLC, respectively. Total phenolics and individual phenolic acids increased with length of low temperature exposure. Non-cured roots had a higher phenolic content than cured roots after 4 weeks. A 3-day exposure period to room temperature (22 °C) following removal from low temperature storage typically resulted in increased phenolics. In a comparison of different tissue locations, the highest phenolic content was found in peel tissue and the lowest in the pith tissue. The major individual phenolic acid in all root tissues was chlorogenic acid.

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Abstract

Temperature and photoperiod affected time of flowering in ‘Red Kidney’ and ‘Great Northern UI 1’ varieties of dry beans. ‘Great Northern-1’ was delayed in flowering under 18-hr photoperiod and the low temperature of 60°F night and 70° day. However, at higher temperatures it flowered normally under all photoperiods. ‘Red Kidney’ on the other hand was delayed under 18-hr photoperiod and high temperatures, above 85° day and night temperatures 70° or higher. It was relatively insensitive to photoperiod under the low temperature, and under the medium temperature of 70° night and 80° day.

Studies of the flowering response of parental and hybrid populations under 18-hr photoperiod and the high, medium and low temperature conditions indicated that ‘Red Kidney’ and ‘Great Northern-1’ differ by 2 major temperature-sensitive genes. ‘Red Kidney’ has a dominant gene Ht which causes delayed flowering at temperatures above 85° and long photoperiod while ‘Great Northern-1’ has a dominant gene Lt which causes delayed flowering at temperatures below 75° and long photoperiod. The F1 Htht Ltlt is delayed at all temperatures under long photoperiod.

Anatomical studies indicated that flower initiation had occurred under all photoperiod and temperature conditions but further development of floral primordia into flowers was delayed or completely inhibited under the conditions responsible for delay.

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