Effect of pre-bloom GA application on pollen tube growth in cv. Delaware grape pistils

Pre-bloom application of 100 ppm gibberellin A3 (GA) to Delaware grape clusters (Vitis labrusca Bailey) induces the set of seedless berries when treated about 14 d before full bloom (FB). By contrast, GA application less than 10 d before FB results in the set of both seeded and seedless berries. In order to clarify the mechanism underlying the production of seedless berries by GA treatments, pollen tube transmitting tissue (TT) development and pollen tube growth in pistils were investigated in 2003 and 2004. Clusters were treated with GA 14 d before FB (normal GA treatment) or 8 and 7 d before FB (late GA treatment), and were hand-pollinated with Muscat of Alexandria pollen at FB. Untreated clusters (control) were also pollinated. No significant difference in TT development in various parts of style and ovary was found among the two treatments and control. On the other hand, at the upper part of the ovary, pollen tube growth in normal GA treatment was significantly inhibited 8 and 24 h after pollination compared with those in late GA treatment and control. Pollen tubes reached the micropyle 24 h after pollination in control and 72 h after pollination in late GA treatment, whereas no pollen tube reached the micropyle after a normal GA treatment. Most pollen tube tips in GA-treated ovary tissues were found to be coiled up. These results indicate that inhibition of pollen tube growth in pistils after normal GA treatment may be due to biosynthesis of pollen tube inhibitor(s), leading to unfertilized ovules. By contrast, late GA treatment allows pollen tube penetration into the lower ovary and, in rare cases, into the micropyle which leads to seed formation.