A double-sigmoid model for grapevine bunch compactness development

Abstract

Aim: Bunch compactness results from 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 berry set 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 extension of each developmental phase can explain part of the bunch compactness variation observed at harvest time in our set of cultivars. Conclusions: Bunch compactness development after berry set is mainly driven by berry growth, and differences in compactness between loose and compact bunches can be majorly attributed to variation in berry number and rachis length. Accordingly, bunch architecture before véraison plays an important role in bunch compactness 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.