Submitted to: Acta Horticulturae
Publication Type: Proceedings
Publication Acceptance Date: 11/15/2010
Publication Date: N/A
Interpretive Summary: The California raisin grape industry is using mechanical harvest methods to reduce production costs. Fruiting canes must be cut to cause the grapes to dry into raisins on the trellis before they are harvested. This is expensive and requires specially trained people to do the pruning. Grape germplasm (natural DOV) has been identified in which the fruit dries into raisins without cutting canes. Understanding the cause of fruit drying into raisins would allow breeders to select this type of raisin grape with less field testing. Fruit was dried under controlled conditions to determine the drying rate of the different germplasm available. There was no difference between the drying rates of the natural DOV and regular raisin selections. The germplasm did not differ in cuticle or wax thickness, except one DOV selection had more wax which should have slowed drying. The causes that induce natural drying of grape fruit into raisins remain to be found. These natural DOV raisin types have the potential to change the way growers manage and dry their raisin crop in to future.
Technical Abstract: The California raisin industry has turned to mechanical harvest to increase production efficiency. Fruiting canes of raisin cultivars must be cut to induce the fruit drying process. Grape germplasm that has its fruit dry-on-the-vine (DOV) without having their canes cut has been identified. Identification of fruit or plant characteristics that cause the drying process to start would be useful in aiding the selection of seedlings with the natural DOV raisin trait. The drying rates at 38C of two natural DOV selections were compared to Summer Muscat (fast berry drying rate) and Thompson Seedless (slow drying rate). All berries compared were 14.3 mm diameter and at 22o Brix as determined by density flotation. The drying rate of one genotype was similar to Summer Muscat and the other dried slower than Thompson Seedless, even though both dry into raisins on the vine without cutting canes. Drying was slower for all genotypes when the pedicle was covered with wax to prevent water loss through the pedicle compared to two years when the pedicle was not waxed. Thompson Seedless drying rate was dramatically decreased by the wax compared to the other genotypes. The drying order was the same for all years except Thompson Seedless was the slowest when waxed. The drying rate does not explain why the two natural DOV genotypes dry without cutting canes. The cuticle, wax and skin thickness was similar for genotypes, except one natural DOV selection had more wax. This should slow drying rates but this was not the case. The DOV genotypes do not develop higher sugar levels early as might be expected. There may be other factors such as water transport out of the berry and through the leaves that might be causing the initiation of the natural DOV phenomena.