A basic peroxidase isoenzyme from the grape pedicel is induced by gibberellic acid

Soluble peroxidase activity from pedicels of seedless table grape cv. Sulta na was highly stimulated by post-bloom applications of gibberellic acid (GA (3)) to vines. The increase in peroxidase activity was mainly due to the in duction of a basic peroxidase isoenzyme (pI > 9; BPrx-HpI). The activity of two other peroxidase isoenzymes of pI 6.5 and 3.2 was not altered by the h ormone treatment. BPrx-HpI was induced by GA(3) in pedicels and rachis but not in berries, although in berries peroxidase activity was also stimulated by post-bloom GA(3) applications. BPrx-HpI oxidised guaiacol and ortho-phe nylenediamine (o-PDA), while the others peroxidases found in the pedicel an d in the berry oxidised only o-PDA. Hence, BPrx-HpI was characterised as a guaiacol-peroxidase showing no activity towards ascorbic acid (ASC). The po ssible role of BPrx-HpI in pedicel lignification and berry-drop caused by G A(3) applications to cv. Sultana vines is discussed.