MECHANICAL PROPERTIES OF ORANGE PEEL AND FRUIT TREATED PRE–HARVEST WITH GIBBERELLIC ACID

Abstract

Mechanical property data of citrus peel and fruit are limited. Gibberellic acid (GA3), a plant growth regulator,is applied to citrus fruit, in part, because it is believed to improve fruit integrity, yet studies characterizing effects ofGA3treatment on mechanical properties of citrus peel and fruit are lacking. The effect of GA3 treatment on the mechanicalproperties of Valencia orange (Citrus sinensis) peel and fruit was tested. Trees were treated with GA3 before, at, or after peelcolor break in the fall of 1998, or nontreated, and their fruit was harvested on 23 April (midseason) and 13 May(lateseason) 1999. Peel samples were subjected to shear and tensile tests, and whole fruits were subjected to compressionand cutting tests. The magnitude of peel and wholefruit mechanical properties decreased between harvests, but moretreatment effects were observed on the second harvest than on the first.<br><br>Treatments affected more mechanical properties of the peel than the whole fruit. Mechanical properties of the peel werepoorly correlated with thickness, probably because the mechanical properties of the tissues composing the peel are notequivalent. The earliest GA3 application date was the most effective. On the second harvest, the peel of fruit treated beforecolor break had 10% greater rupture force (in shear) and 15% greater tensile strength than the peel of nontreated fruit, whilethe properties of treated and nontreated whole fruits were similar. On the later harvest, the peel of fruit treated with GA3before color break had 10% greater rupture force than the peel of nontreated fruit, while the peel of fruit treated at colorbreak had 15% greater shear strength than nontreated fruit. The peel of treated fruit generally had tensile properties thatwere 10% to 20% greater than the peel of control fruit, especially on the second harvest. Fruit treated before color break alsohad greater apparent modulus of elasticity and higher cutting force than other fruit.