**5.2 Effect of temperature on anthocyanin synthesis**

Temperature has a major effect on anthocyanin synthesis in apple plants. As a result of recent global warming, there are increasing concerns that global warming may be detrimental to fruit reddening.

In general, apples redden best in climates with clear bright days and cool nights during the preharvest period [26]. Night temperatures below 18°C enhance reddening in "Fuji" apples [26]. Reay [23] reported that a cool temperature of 4°C in the dark period followed by 20°C with UVB-visible irradiation was the most effective condition for anthocyanin accumulation in apple peel of "Granny Smith." This is the equivalent of cool nights followed by warm clear days. With continuous 20°C temperature in both dark and UVB-visible irradiation periods, the anthocyanin accumulated was less than half that with the former temperature condition. This result shows the importance of a low-temperature period in the development of the red blush on apple peel.

Generally, high temperatures increase the respiration rate and carbohydrate consumption of apple trees, thus reducing anthocyanin synthesis in the fruit. Wang et al. [30] reported that anthocyanin accumulation during the ripening period in the peel of "Royal Gala" red-skinned cultivar grown in hot climates (17–35°C, using artificial heating of fruit on the tree over two 7-day periods) caused a dramatic reduction in peel anthocyanin concentration and decrease in redness compared with those in unheated control fruit (7–22°C for unheated apples). Proctor [24] had earlier found that redness cultivars of "McIntosh," "Northern Spy," and "Cortland" did not color with supplementary light when the 48-h average temperatures were greater than 20°C, suggesting that anthocyanin accumulation requires temperatures lower than 20°C. Consistent with this notion, Honda et al. [21] showed that 15°C was generally the optimal temperature for anthocyanin accumulation during fruit ripening of redness cultivars, "Tsugaru," "Tsugaru Hime," and "Akibae," whereas anthocyanin accumulation was repressed under 30°C.

Iglesias et al. [18] have noted that poor fruit coloration has already become a problem owing to frequent periods of high temperatures (>30°C) with very low rainfall in summer in many apple-producing areas in Spain. Seasonal differences in temperature and rainfall in the period before harvest often cause harvesting to be delayed in order

### *Anthocyanins in Apple Fruit and Their Regulation for Health Benefits DOI: http://dx.doi.org/10.5772/intechopen.85257*

to attain a certain degree of color, a practice that has negative effects on quality—especially firmness and flavor. This lack of color is an important cause of reduction in grade and is generally associated with poor consumer acceptance. To overcome this problem, Iglesias et al. [18] applied overtree microsprinkler irrigation for 25–30 days preceding the harvest to improve fruit redness and increase anthocyanin concentration in "Topred Delicious" cultivars. Similarly, the warm climate of California can delay harvest owing to inadequate reddening of "Fuji" apples; this has been associated with physiological problems such as skin cracking and internal browning [26].

On the basis of 30–40 years of climate records, Sugiura et al. [31] pointed out that the annual mean air temperature in Japan has risen by 0.31–0.34°C per decade. They proposed that since such climate warming brings earlier blooming and higher temperatures during the maturation period, it is likely to change the taste (acidity and soluble solids) and texture (fruit firmness and watercore development) of apple fruits. They suggest that the slower advance toward the benchmark blush rate caused by warming during the maturation period will offset the advance in fruit maturity induced by the earlier blooming. Honda and Moriya [11] demonstrated that the temperature during fruit ripening of apple trees is an important factor in anthocyanin synthesis in both apple fruit peel and fruit flesh. They observed that the anthocyanin concentrations in the flesh of mature "Pink Pearl" (normal fruit flesh color, pale pink or red; fruit color, red and orange flush) harvested in 2016 were considerably lower than the typical level, probably because of the high temperatures recorded during the ripening period in that year. This suggests that the higher temperatures caused during fruit ripening by recent climate warming may be detrimental to fruit reddening.

Many reports about the effects of treatment-induced changes (e.g., light, temperature) on the characteristics of fruit peel color in apples indicate that production areas for apple fruits will likely increasingly suffer from inadequate reddening owing to ongoing climate warming. Therefore, further research is needed to clarify the effects of ongoing climate warming on the rate of anthocyanin synthesis in apple peel at each growth stage.
