Spring phenology and bud fruitfulness of a grapevine segregating population (Rhine Riesling × Cabernet-Sauvignon)

Abstract

The rise of average temperatures worldwide influences grapevine (Vitis vinifera L.) phenology by accelerating its development, starting from anticipated budbreak dates. This shift exposes buds to an increased risk of late spring frosts exposure, which are expected to persist in the future despite global warming. Beyond phenology, bud fruitfulness is also significantly influenced by climate-driven environmental factors, such as radiation, water availability, and temperature. The genetic control of these complex traits remains largely unknown; a better understanding of these mechanisms would benefit selection efforts for slow- or late-developing varieties.

The primary objective of this study, carried out in the 2025 and 2026 seasons, was to characterize the phenotypic variation of traits associated with bud phenology and fruitfulness in a segregating population of 166 individuals derived from a controlled cross between internationally renowned Rhine Riesling (RR) and Cabernet Sauvignon (CS), performed in 2005. All plants are currently located at Fondazione Edmund Mach in San Michele all’Adige (46°18′N, 11°13′E, Italy). Specifically, budbreak date, intended as reaching of stage BBCH 07 (“green tip”), was recorded for each genotype. Bud fruitfulness was assessed in the second week of May at stage BBCH 57 (“inflorescences fully developed”). Overwintering buds on the fruiting cane alone were considered, and the number of inflorescences was counted for each node. Temperature, radiation and precipitation data were collected by a weather station in the vicinity of the vineyard and used for thermal requirements (GDD10) calculations.

Budbreak across the RR×CS population spanned an interval of approximately 20 days. Since RR and CS exhibited intermediate phenotypes, the substantial variation observed in budbreak DOY and thermal requirements indicates transgressive segregation in the progeny, with several individuals exceeding the parental range. Fruitfulness of the entire cane was compared to that of basal buds; values appeared normally distributed in both cases, highlighting the complexity of the genetic control of these traits. Bud fruitfulness of the entire fruiting cane was found to span from a minimum of 0 to a maximum of 3 inflorescences per shoot. The number of lowly fruitful buds was substantial when fruitfulness of basal buds alone was considered, despite the presence of extreme individuals displaying high positive values. Residual bud fruitfulness, namely of apical and median buds combined, and that of basal buds appeared positively correlated.

This study provides a comprehensive screening of the spring phenology of the RR×CS segregating population, revealing significant diversity in phenological and fruitfulness characteristics. It establishes the foundation for subsequent genetic analyses, as well as for the employment of notable individuals for focused physiological studies.