Excessively high temperatures frequently occur between spring and late fall because of global warming. These problems have a negative effect on fruit production capacity and actual production. However, there is a lack of information on the effects of excessively high temperatures (exceeding the optimum range) on the growth of citrus fruits. Thus, the objective of this present study was to determine the effects of excessively high temperatures on Shiranuhi mandarin fruit growth, physiological characteristics, amino acid contents, and antioxidant activity. In this study, five different high-temperature treatments were applied: normal atmospheric temperature (Con), Con + 4 °C during the day (D4), Con + 8 °C during the day (D8), Con + 4 °C during the night (N4), and Con + 4 °C during the day and night (DN4). The total soluble sugar contents were lowest in D8, and the acid content under DN4 was significantly lower than that under Con. Rising temperature during the day or night affected the colouration of the rind, and the free sugar composition ratio under D8 was lower than those under the other treatments. The total amino acid content tended to increase with increasing temperature during the day. The total flavonoid and total phenolic contents in the peel were significantly higher under Con and N4 than the other treatments and in the pulp under Con.

Sunburn causes fruit browning and other physiological symptoms, reducing fruit production and quality. Therefore, we aimed to investigate the anatomical differences and abiotic stress responses in ‘Nichinan 1 gou’ satsuma mandarin (Citrus unshiu Marc.) according to the severity of sunburn damage (five grades: control, no sunburn; I to IV, increasing severity of sunburn). Additionally, the quality of sunburned and non-sunburned fruits was compared, and the sunburn-inducing temperature was estimated. Anatomical observations confirmed that with increased severity of symptoms, the damage to fruit rind surface and oil glands was increased. In the analysis of peel pigments, chlorophyll content in the rind gradually decreased compared with IV, whereas the carotenoid content gradually increased up to III. The flavonoid content in the peel and pulp was the highest in III. In the 1,1-diphenyl-2-picrylhydrazyl and 2,2′-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) radical analyses, the IC50 (the concentration of compound at which the percentage of inhibition is 50%) value was the lowest in grade III in peel or IV in pulp, indicating a high free radical scavenging ability. The fruit quality analysis between sunburned and non-sunburned fruits showed differences in total soluble solid content, total acidity, firmness, coloration, and free sugar and organic acid contents, indicating a significant effect on fruit quality. In the heat tolerance tests on fruit rind in the laboratory and field, the damage was confirmed at temperatures above 47 °C.

We identified the minimum temperature limits to ensure Shiranuhi mandarin growth and fruit quality and provided overwintering temperature management guidelines. Expanded polystyrene panels with air conditioners were exposed to −1, −3, and −5 °C in the greenhouse for 15 h to determine the overwintering temperature. Leaves and fruits were analyzed at intervals for physiological response, fruit quality, and aromatic components. The low temperature treatment groups showed 1.3 to 1.4 times increased malondialdehyde content in leaves and 1.1 to 1.3-fold higher electrolyte linkage rates in the −5 °C treatment group alone. The sugar/acidity ratio was 1.1 to 1.3 times higher in the −5 °C treatment than in the control due to rapid acid reduction. The fruit firmness and citrus color index decreased notably after 21–28 days of treatment. Ascorbic acid content significantly decreased 17.3%–41.1% at −5 °C. Among the aromatic compounds, decanal levels notably increased with −5 °C treatment; −5 °C cold treatment notably affected oxidative stress in leaves and the sugar/acid ratio, ascorbic acid content, and aromatic compounds in fruits. If prolonged exposure to temperatures <−5 °C is expected, preharvest heating facilities are critical. We recommend maintaining greenhouse temperatures >0 °C during fruit growth and >−3 °C after harvest as the minimum temperature to preserve fruit set and quality.