Fluctuations of endogenous gibberellin A4 and A7 content in apple fruits with different sensitivity to russet

Skin russet of 'Golden Delicious' apples (Malus domestica Borkh.) is the result of an interaction between an epidermal structure, more or less sensitive to the disorder, which is the predisposing cause, and external environmental factors, such as high relative humidity, rain, low temperature, or some pesticide treatments. However, treatments with GA(4) or GA(4+7) in the first period of fruit development reduce russeting. Trees of four 'Golden Delicious' clones ('Clone B', 'Smoothee', 'Crielaard', and 'Reinders') with different sensitivity to russeting were covered, in order to protect the fruitlets from russeting factors, for I week in weeks 2 to 8 after full bloom, or for the whole period, or kept uncovered as a control. Russeting was checked at harvest. The trial confirmed that the period of maximum sensitivity to russeting lies between the second and the fourth week after full bloom, when the fruit diameter is in the range between 15 to 30 mm. Gibberellin concentration was analyzed in uncovered fruitlets picked in the period of maximum sensitivity to russeting. In that period, fruits of the more sensitive clones showed a higher GA(7) amount and a lower GA(4) amount than fruits of less sensitive clones, where GA(4) was predominant. Similar results were obtained analyzing the gibberellin concentration, during the same stage of development, in a completely russeted cultivar ('Reinette Grise') and in a cultivar that never shows russeting ('Rosmarina Bianca'). Therefore, the resistance to russeting seems to be related to a high ratio of endogenous GA(4)/GA(7) during the period of maximum sensitivity.