**5.4 Plant hormones and their application to enhance anthocyanin accumulation**

Ethylene, a hormone involved in maturation processes, plays an important role in regulating anthocyanin synthesis in apples. A close relationship between internal ethylene accumulation and anthocyanin accumulation in "Fuji" apple fruits [27] and in "Jonathan" apple fruits [42] has been reported. Therefore, ethylene can be used commercially to increase anthocyanin accumulation in red apples. Ethephon (2-chloroethyl phosphonic acid), an ethylene releasing compound that acts as a growth regulator, effectively promotes anthocyanin synthesis in "Starking Delicious" [37, 38] and "Fuji" [39] when light and temperature requirements for reddening are met [26]. Awad and Jager [40] compared the effects of the following seven growth regulators on the accumulation of anthocyanin in "Jonagold" apple peel: ethephon, CCC (2-chloroethyltrimethyl ammonium chloride), prohexadione-Ca (3-oxido-4-propionyl-5-oxo-3-cyclohexane-carboxylate), GA4 + 7, plantacur-E (a vitamin E formulation containing 25% alpha-tocopherol), shikimic acid, and galactose. Among these growth regulators, ethephon resulted in the highest anthocyanin concentration in fruit peel at commercial harvest.

The above results differ from reports that did not find a positive relationship between ethylene synthesis and anthocyanin accumulation in apple fruits that were irradiated with UV for "Starking Delicious," "Jonathan," "McIntosh," "Fuji," "Ralls Janet," "Tsugaru," "Jonagold," "Mutsu," and "Golden Delicious" cultivars [29] or subjected to hot climatic conditions [21]. For instance, apple trees of "Misuzu Tsugaru" subjected to hot climatic conditions (29°C 12 h/19°C 12 h) had lower anthocyanin accumulation in fruit at harvest but 9 times the ethylene production of those grown under the control (25°C 12 h/15°C 12 h) conditions [21]. These findings suggest that ethylene does not always play a role in regulating anthocyanin synthesis in apple fruits when the trees were grown under such light and hot temperature conditions.

In addition to ethephon, the plant hormone methyl jasmonate can alter anthocyanin accumulation. Treatment of apple peel disks of "Tsugaru" (yellow-green background covered in red-pink blush with occasional striations) with methyl jasmonate stimulates anthocyanin accumulation regardless of the fruit growth stage [43]. "Fuji" apple fruit after harvest that was treated with increasing concentrations of methyl jasmonate shows decreasing hue angles (Δh°) (redness) of the peel [44], and field application of methyl jasmonate to apple fruit can also decrease the hue angle [45]. Therefore, the application of methyl jasmonate can enhance apple fruit

anthocyanin formation and peel reddening. Daminozide, paclobutrazol, auxins, and auxin analogs, which are growth regulators, can also promote reddening [44].
