Challenging the potassium deficiency hypothesis for induction of the ripening disorder berry shrivel in grapevine

Berry shrivel (BS) is a ripening dysfunction resulting in grapes with low sugar content, high acidity, reduced anthocyanins and flaccid berries. In this study we challenge the K+-deficiency hypothesis as underlying cause for BS in grapevine. The hypothesis is based on empirical vineyard studies and proposes that K+-deficiency or a disbalance of K+/Mg2+ in plant content cause of BS. Recent studies on more grapevine varieties and further geographical locations lack supporting evidence for the K+ hypothesis. Here we review existing evidence from the field and apply physiological analyses to study processes involved in K+ and nutrients transport in BS and non-symptomatic clusters. For the first time the molecular background of K+ transportation is being studied over the course of BS disorder. Our objectives were (1) to determine the distribution of nutrients in BS vines and clusters and (2) to evaluate the role of selected potassium transport proteins and channels during grape berry development and BS induction in pedicels and berries. Our results with ICP-MS show a strong and significant reduction of K+ concentration in rachis and pedicels of BS grapes, whereas boron, zinc, copper and aluminum were increased. Concentration of nutrients in BS berries were either not changed or increased compared to non-symptomatic clusters. Expression analyses with qPCR in pedicels revealed no pre-symptomatic differences of genes involved in potassium transport (VviKUP1, VviKUP2 and VviK1.2), but later during ripening reduced expression was observed. In BS berries the expression of VviK1.2 was reduced before veraison. We show significant K+ deficiency in BS rachis and pedicels along with partial reduced expression of K+ transporter genes. Consequently K+ phloem transport is involved in BS induction, however our study did not provide conclusive evidence to support the K+ deficiency hypothesis as a single factor for BS induction. Instead the idea that a combination of further stress factors influences K+ and assimilates translocation towards sink organs before veraison is proposed. (C) 2016 Elsevier B.V. All rights reserved.