Abstract
Recently, colder weather has been occurring frequently in winter due to climate change, and this has a negative effect on the production of late maturing citrus cultivar such as Shiranuhi mandarin. Therefore, we investigated how freezing at a low temperature affects cold tolerance, fruit characteristics, amino acid content, volatile compounds, and antioxidant activity of Shiranuhi mandarin. A tree in a non-heated plastic greenhouse was surrounded on five sides by a self-assembly expanded polystyrene panel; the temperature was controlled using an air conditioner at –5°C, and –7°C for 6 h. An untreated tree was used as the control. In the –5°C treatment, there was no difference among organs according to the number of days after treatment (DAT), but –7°C-treated leaves showed high damage compared to the branches. The total soluble solid (TSS) content was no difference among treatments at 7 DAT, but low-temperature treatments reduced the TSS content at 14 DAT compared with the control, showing a significant difference between the treatments. The color space values and firmness were significantly lower in the –7°C treatment than in the other treatment. Furthermore, the freezing low-temperature treatment affected the sucrose, citric acid, and malic acid contents. The leucine content significantly increased by approximately six-fold compared with the control levels, depending on temperature and DAT. Low -temperature treatments elevated amino acid combinations associated with bitterness. Among volatile compounds, the monoterpenes contents increased under the treatments, whereas the sesquiterpenes and other compounds content decreased. The total phenol and flavonoid content increased with DAT, but differed with tissue and temperature. Therefore, it seems necessary to improve heating facilities or develop new varieties with strong low-temperature tolerance to prepare for cold winter conditions in plastic greenhouses.
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