Fruit-set of cultivar Regal responds to source-sink manipulation

Background and Aims In grapevine, reproductive development and vegetative shoot growth occurs simultaneously during the early part of the season and may compete for stored and current assimilates. Until fruit set, clusters are weak sinks for assimilates (Motomura 1990) compared with the growing vegetative tips and the young leaves. Any disturbance, caused by various environmental or physiological conditions, may even lead to an excessive flower abscission. The competition between reproductive and vegetative organs may be manipulated by removing the growing shoot tips or by cane girdling. Moreover, different authors reported that Titanium dioxide (TiO2) positively affects the yield and the quality of various crops (Mansour and Mubarak, 2014). The aims of this trial was to improve fruit set, yield and berry size of cultivar the ‘Regal’ by changing the shoot organs source-sink relationships. Experimental Procedure and Results The trial was carried-out during 2016 and in a 6-year old drip-irrigated vineyard located in southern Italy. Vines from cultivar Regal/140 Ru were cane pruned and trained in a Y trellis system with a vine density of 1,600 vines per ha. The following treatments were imposed: NT – tips were not removed; G_NT – girdling without tipping; Ti_NT – shoots organs sprayed with a solution of TiO2 and tips were not removed; T0 – shoot tips were removed at stage EL18 (Coombe 1995); T1 – Shoot tips were removed at stage EL23. Cane girdling and TiO2 were applied at stage EL23. Vegetative and reproductive growth was evaluated by destructive measurements. Fresh and dry weight of leaves, berries, clusters and shoots were performed at the following stages: flowering (EL23), fruit set (EL27), berry pea size (EL31), veraison (EL35) and harvest (EL38). Leaf gas exchange measurements were also performed at the same stages, while LAI was measured at veraison using the Viticanopy smartphone application. Fruit set was significantly improved by removing the tips at EL18 and EL23 stages, while G_NT and Ti_NT treatments were not significantly different from untipped shoots (NT treatment). T1 and G_NT treatments showed a significantly higher clusters weight. T1, G_NT and Ti_NT treatments produce a significantly higher berry size if compared to NC and T0 treatments. Some other statistical differences among treatments were also measured in leaf gas exchange, LAI and the ratio of laterals/main leaves. Discussion and Significance of the Study In our experimental conditions, when the tips were removed, fruit set was improved. However, clusters and berry size improved only when this manipulation was made at full bloom. In constrast, girdling and TiO2 made no contribution to the improvement of fruit set. This could be due to a different physiological regulation of the source-sink relationships. Acknowledgements We would like to thank Marino De Caro, member of the farm ‘Ponterosa’, for his hospitality and contribution to the trial. We are grateful to Bartolomeo De Tommaso and Antonio Romito, agronomists of the Agriproject srl Society.