Flower abscission and fruit set on table grapes (Vitis vinifera L.): unraveling physiological and molecular mechanisms

Doutoramento em Engenharia Agronómica - Instituto Superior de Agronomia - UL

Despite the importance of grapevine (Vitis vinifera L.) as one of the most cultivated species, the molecular events occurring during the critical period of fruit set, are far from elucidated. Aiming at providing a new insight on flower-to-fruit transition and flower abscission regulation, transcriptomic (RNA-Seq) and metabolomic analyzes were performed in the inflorescences and vine physiological alterations were investigated. Regarding flower-to-fruit transition regulation the results showed involvement of nutrient transport regulation and alterations on carbohydrates, secondary and hormone metabolism. In particular, induction of indole-3-acetic acid accumulation and activation of ethylene and sugar signaling were hypothesized to induce bioactive gibberellins biosynthesis, stimulating cell division within inflorescences. Assays with gibberellic acid (GAc) spraying and reduction of light interception during bloom allowed to promote flower abscission and suggested that growth regulator application and C-starvation resulted in distinct effects on inflorescence metabolism. GAc response involved stimulation of photosynthetic and respiratory machinery, nucleotide biosynthesis and carbon metabolism. Conversely, shading repressed photosynthesis, induced carbon/nitrogen imbalance and comprehensive alterations on hormone-related pathways, resulting in repression of cell division and induction of senescence. Candidates as common pathways leading to abscission were putrescine catabolism regulation, auxin biosynthesis induction, gibberellin biosynthesis repression and ROS signaling/detoxification although often through changes on specific transcripts and metabolites levels. Aiming at optimizing thinning methods, mandatory on table grapes production for guarantee bunch quality, GAc spray and shading during bloom were tested in seedless and seeded cultivars growing under field and greenhouse conditions. 'Thompson Seedless' showed to be sensitive to both thinning methods resulting in increased flower drop and reduced bunch compactness, but only GAc spray enhanced berry quality. Both treatments induced flower abscission in 'Black Magic' growing in late cycle on greenhouse production system, whereas during early cycle, only shade enhanced flower drop, bunch aspect and berry quality, resulting in an effective thinning method