Thidiazuron affects fruit growth, ripening and quality of Actinidia deliciosa

The effects of thidiazuron (TDZ) on growth, quality and ripening of kiwifruits were evaluated by comparison with untreated (control) and CPPU (cytokinin) treated fruits. Two weeks after full bloom (AFB), TDZ at 5, 10 or 20 ppm and CPPU at 20 ppm were applied to fruits by dipping them for about 5 s in aqueous solutions of the two compounds. TDZ remarkably increased fruit growth soon after application. The increase in size levelled off at 10 ppm and no further improvement in fruit size resulted from the 20 ppm treatment. At harvest, TDZ treated fruits, depending on the concentration, were 50% or 60% heavier than untreated ones. All TDZ concentrations used also gave a large (+44%) and similar increase in fruit dry weight. TDZ did not cause significant variations in the number and weight of seeds. TDZ caused some modification in fruit shape. It increased the percentage of fruits having a protruded distal end and tended to decrease the fruit length/average diameter ratio, especially at the higher concentrations. TDZ caused an increase in the size of all the tissues of the fruits without changing their relative proportions at harvest. TDZ treated fruits had an advanced and/or faster ripening (about one week) with respect to the control. This was clearly shown by the lower flesh firmness and total titratable acidity and the higher content of soluble solids and soluble sugars (although the total carbohydrate content was not affected) at harvest. After harvest, once full ripening was reached, all fruits had a soluble solids content of about 14.5 degrees Brix and a flesh firmness of about 2.0 kgf, without differences due to treatments. In general, results of CPPU were similar to those of TDZ treated fruits at the same concentration (20 ppm). TDZ greatly stimulates fruit growth indicating that it can be a powerful tool for improving kiwifruit cropping.