Light management and gibberellic acid spraying as thinning methods in seedless table grapes (Vitis vinifera L.): Cultivar responses and effects on the fruit quality

- Gibberellic acid (GAc) and shade were tested as thinning methods on seedless cultivars.
- Experiments were conducted on 'Sugraone', 'Crimson Seedless' and 'Thompson Seedless'.
- Shade reduced leaf gas exchange rates, leaf area and sugar content in the phloem sap.
- Shade imposed at 50% bloom resulted in reduced bunch compactness in all cultivars.
- GAc caused reduced bunch compactness and improved quality only in 'Thompson Seedless'.

In seedless table grapes, the excessive natural fruit set leads to compact bunches, small berries with poor colour, low uniformity of maturation and higher incidence of diseases, requiring flower and fruit thinning to attain a profitable production. Gibberellic acid spraying (GAc) and imposition of shading nets, cutting incident irradiance by 90, 98 or 100% at flowering, were tested in consecutive years aiming to examine their influence on flower drop and consequent quality of 'Sugraone', 'Thompson Seedless' and 'Crimson Seedless' grapes. Shade treatments reduced leaf gas exchange and leaf growth rates while no differences were found in GAc-treated vines. Effects on leaf gas exchange showed to be reversible after shading net removal. Flower drop percentages increased in 'Sugraone' in response to 90% light reduction imposed at 50% bloom, and in 'Thompson Seedless' in response to GAc spraying and shade imposition, when compared to untreated vines. Bunches were significantly less compact by reducing 90% and 100% of the incident light at 50% bloom, during 15 and 24 days, on 'Sugraone' and 'Crimson Seedless', respectively, when compared to untreated vines. In 'Thompson Seedless' vines, bunches were less compact after shade imposed both at 50 and 100% bloom, during 14 and 10 days, respectively. In general, shade-promoted thinning induced a decrease on berry weight and diameter compared to untreated vines. GAc treatment resulted in lower bunch compactness and improved berry quality in 'Thompson Seedless', while the effects on berry quality observed in 'Sugraone' and 'Crimson Seedless' were not consistent across the trials conducted in different years. 'Thompson Seedless' was the more sensitive cultivar to GAc and shade thinning methods.