The sugar-potassium nexus within the grape berry

Zelmari Coetzee

    Research output: ThesisDoctoral Thesis

    213 Downloads (Pure)


    Grape berries import a suite of organic and inorganic substances from the vine and a functional link in the accumulation of potassium and sugars, in the form of hexoses, has been proposed for the ripening grape berry. This link has not yet been adequately defined or characterised, but may be related to the osmoregulatory function of these solutes or through the role that potassium plays in the loading and unloading of sugar from the phloem both at the source and sink. While sugar transporters in relation to phloem unloading have received some attention in grapevines, the study of potassium transporters is still in its infancy.
    Three strategic experiments were conducted to better characterise the sugar-potassium nexus in grape berries. The first experiment was conducted on field-grown Sauvignon Blanc grape berries sampled weekly from pre-véraison to maturity. The aim of this experiment was to determine if the relationship that was previously observed for pooled berry samples, was apparent in single grape berries and within the individual grape berry tissues. A further objective was to assess the relative accumulation patterns of potassium and the sugars during ripening.
    A second experiment was conducted on own-rooted Shiraz grapevines exposed to a controlled environment during ripening. The aim of this study was to determine if the proposed sugar-potassium relationship is upheld under altered carbon and potassium availability to the vine. The net photoassimilation rate of the leaves was reduced by removing CO2 from the atmosphere, thereby limiting the photoassimilates for translocation to the berries. Conversely the available potassium was raised by increasing the potassium in soil-applied fertiliser. Once berries were mature, the carbohydrate and potassium content of the individual vine organs were assessed. In a third experiment, the expression patterns of previously characterised sugar and potassium transport proteins were analysed in the grape berry pericarp to investigate the possibility of co-regulation at a transcriptional level.

    Hexose sugars and potassium accumulation into the grape berry pericarp were closely correlated during ripening. Moreover, a third component, water, was also strongly associated with the sugars and potassium to form a ternary relationship. Plasticity was however evident in this relationship as there was extensive variability between individual grape berries in the content of these solutes. Sugar accumulation was up to ten-fold greater than potassium, highlighting that these solutes were not accumulated in proportional quantities.
    The sugar-potassium relationship persisted under low CO2 and high potassium treatments. Attempts to manipulate sugar accumulation in grape berries resulted in similar quantities of sugar in all treatments even though photoassimilation was drastically reduced. This physiological response, concurrent with a decrease in the starch content of the remainder of the grapevine, emphasises that berry ripening was prioritised over the storage of carbohydrates, likely to ensure seed development and dispersal.
    Of the nine sugar and potassium transporter genes assessed, a hexose sugar and potassium transporter responsible for solute transport over the plasma membrane were closely correlated to each other. The changes in sugar and water content over ripening were positively related to the transcription of not only the hexose transporter, but also to the relative expression of an aquaporin, reconfirming the ternary relationship between sugar, potassium and water.
    This study was novel in that the sugar-potassium relationship was, for the first time, illustrated in individual grape berries and in the grape berry tissues. A strong ternary relationship of these solutes with water was also alluded to. The relationship persisted during the entire ripening phase, and was upheld when sugar and potassium availability were altered. This study has thus advanced our understanding of the underlying mechanisms for sugar and potassium accumulation into grape berries.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Awarding Institution
    • Charles Sturt University
    • Rogiers, Suzy, Principal Supervisor
    • Deloire, Alain, Principal Supervisor
    • Walker, Rob, Principal Supervisor, External person
    • Clarke, Simon, Principal Supervisor
    Award date04 Aug 2017
    Publication statusPublished - 2017


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