Inadequate supplies of K can result in reduced shoot, root, and fruit growth as a result of reduced xylem sap flow, and can also increase the risk of drought stress [1]. Potassium deficiency leads to inhibition of photosynthesis and to sugar (sucrose) being "trapped" in the leaves which adversely affects yield, fruit ripening, and berry soluble solid concentration [1]. While grape growers should monitor vine health to avoid K deficiency, at the same time they should also be on the lookout for an excessive concentration of K in vine tissues (e.g., leaf, berry) because of its potential negative impact on vine health and wine quality.
When looking back at past inquiries received from Pennsylvania wine grape growers related to vine nutrient or nutrient-related problems, we found that the number of those concerning excessive concentration of K and related issues (e.g., high/unstable wine pH) were more common than inquiries related to K deficiency, which mostly occurred in young vineyards. In-depth information on the mode of uptake and transport of K in the plant and functions of K can be found in "A review of potassium nutrition in grapevines with special emphasis on berry accumulation" by Mplelasoka et al. 2003 [2].
Why might a high/excessive concentration of K in the grape berries negatively impact wine quality? Grape berries are a strong sink for K during ripening. Potassium accumulates mainly in the berry skin tissues (Figure 1) and is the most abundant cation (K+, hereafter referred to as K) in grape juice [2]. Mature grapes may have, indeed, almost twice as much K as nitrogen [1]; for example, one ton of mature grapes contains about 11 lbs (5 kg) of K.
In the pre-planting stage, if the soil selected for planting the vineyard has high exchangeable K levels, an option is to select rootstocks that accumulate low concentration of K. Rootstocks, and grapevine varieties in general, differ in their capacity of K uptake and translocation [2]. For example, rootstocks with V. berlandieri genetic background tend to have reduced K uptake as compared to others, as those with V.champini parentage [12]. In northern California, Chardonnay, Cabernet Sauvignon and Zinfandel vines grafted on 101-14 Mgt and 3309C (V. riparia x V. rupestris), two commonly-used rootstocks in the eastern US, consistently had leaf petiole K concentrations within the intermediate range compared to those of the same varieties grafted on V. berlandieri (lowest K concentrations) and V. champinii (highest K concentrations) crosses [12]. A study conducted at Winchester, VA, by Tony Wolf research group found that the use of 420-A (V. berlandieri x V. riparia) rootstock reduced juice pH in Cab Sauvignon vines as compared to those grafted on 101-14 Mgt and Riparia [6].
However, it is important to consider that the performance of rootstocks in terms of K uptake varies depending on the rootstock-scion combination (i.e., the same rootstock may have variable effects on different scion varieties), soil type, climate, and management practices.
Another aspect to consider when selecting rootstocks is their vigor or growth-potential. Vigorous rootstocks or rootstocks that convey high vegetative growth and yield potential to the scion may cause increased K uptake as a result of increasing vine demand.
Growth drives K uptake: Factors such as high vine vigor, leaf area, and extensive root system can enhance K uptake, translocation, and accumulation in the grape berries. Soil moisture increases the dissolution of K from clay particles, thus facilitates K supply and uptake by the roots. High soil water availability also leads to increase vegetative growth which may indirectly affect K uptake and its accumulation in the berries.
Shaded leaves are a source of K translocation to the grape berries: Canopy microclimate and mainly foliage shading can affect the accumulation of K in the berries. For example, artificial (shading cloths) [13] or natural (canopy) shading [14] was found to increase K concentration in berries and juice. We don't know exactly why this happens yet, but it is possible that in conditions of low sugar accumulation, as under foliage shading, the increasing accumulation of K in the berries helps to regulate osmotic potential, maintaining cell turgor and thus minimizing reduction in berry growth which may occur with low sugar content.
