A double-sigmoid model for grapevine bunch compactness development

Aim Bunch compactness is the result of a complex dynamic process in which many bunch, berry and rachis features interact during the whole grapevine reproductive cycle. The aim of this work is to identify the different developmental stages involved in bunch compactness determination during the second growing season, from full-flowering to harvest time. Methods and Results In this work, bunch features from ten wine grapevine cultivars with diverse bunch compactness (including very loose and very compact bunches) have been periodically described. Experimental data allowed us to quantify changes in bunch compactness from berry set until harvest time in an objective manner. Our results indicate that bunch compactness development follows a double-sigmoid model, characterized by two consecutive cycles, each one with a growing stage followed by a latent one. Differences in the slope factor of the growing stages and in the duration of each developmental phase can explain part of the bunch compactness variation observed in our set of cultivars. Conclusions Two consecutive sigmoid cycles with four different stages can be observed in the determination of grapevine bunch compactness. Bunch architecture before véraison plays an important role in the final compactness of the bunch at harvest time. Significance and impact of the study The double-sigmoid model for bunch compactness development, and the four different stages identified will aid in future analyses aimed to unravel the underlying mechanisms affecting this complex trait