The contribution of flowering time and seed content to uneven ripening initiation among fruits within Vitis vinifera L. cv. Pinot noir clusters

MAIN CONCLUSION : Ripening initiation-associated hormonal changes and sugar accumulation for individual fruits differed by seed content and did not depend heavily on flowering time or duration from anthesis to clusters’ onset of ripening. For Vitis vinifera, the ripening initiation of individual fruits in a cluster occurs unevenly. This developmental period is called véraison. Why individual fruits initiate ripening at different times is not well studied, though differences in seed content and unequal developmental durations that arise from asynchronous flowering within a cluster have been proposed. This study examined how much both variables contribute to individual fruits’ ripening progress by mid-véraison, when half of berries in a cluster have initiated ripening, and whether either or both factors affect the timing of characteristic, ripening-initiation associated changes in abscisic acid and auxin before, at, and after véraison. Overall, developmental duration and flowering time did not sufficiently explain how far berries had progressed into the ripening stage because fruits did not require a fixed amount of time to initiate ripening. Fruits from early and late flowers but of similar seed content were able to initiate ripening at the same time despite differences in chronological age. This suggests either an early developmental enhancement occurred for late-initiated fruits or that flowering time is an inappropriate “day zero”. Ultimately, only seed content was linked to the timing and magnitude of ripening-related hormone changes, supporting that seeds have a comparatively larger influence than flowering time on the ripening initiation of individual berries. More specifically, if the fraction of berry weight occupied by seed was high, then the initiation of ripening for that berry and its associated hormone changes were delayed relative to berries with less seed weight versus total berry weight.