Potassium effects on irrigated table grapevine resiliency under drought stress

Uri Hochberg, Or Sperling, Uri Yermiyahu, and Alon Ben-Gal

Background

Potassium (K⁺) is the most abundant cation in plant tissues and an essential macro-element in plant nutrition. It is the major inorganic cation in the cytoplasm, involved in electrical neutralization of anions and macromolecules, pH homeostasis, control of membrane electrical potential, and the regulation of cell osmotic pressure. Thus, plant growth requires large quantities of K⁺ ions that are taken up by roots from the soil solution, and then distributed throughout the plant (Marschner, 2011, Nieves-Cordones et al., 2016). Yet, under many scenarios, precise understanding of K⁺ requirements in plants is elusive. In fact, not constituting organic molecules or participating in phloem carbon transport, K⁺ deficiencies are often obscured by vital growth or biomass accumulation (Erel et al., 2008). Even at critical deficit levels, K⁺ could be partially substituted by sodium to mitigate physiological hazards (Erel et al., 2014). Nevertheless, since K⁺ contributes to stomatal regulation, hydraulic flow and osmotic adjustment (some of the most important physiological responses to drought stress), over time its deficiency could result in susceptibility to abiotic stresses, low yields, and even plant mortality (Marschner, 2011). When K⁺ is abundant, its accumulation in the cytoplasm increases the cell osmotic potential and enables the plant to maintain turgor (and many subsequent processes) even under low water potential. Hence, K⁺ supplement to several crops (e.g. maize (Premachandra et al., 1991) and wheat (Gupta and Berkowitz, 1987)) improved their osmotic adjustment and drought tolerance. As K⁺ acted similarly on eucalyptus trees (Battie‐Laclau et al., 2014), we postulate that it would improve drought tolerance in perennial crops as well (i.e. horticulture and viticulture), despite the lack of studies on the subject.

Grapevine, one of the most cultivated perennial fruit crops (after only coffee and olives) with over 7 million hectares of production (FAO stats for 2016), is largely cultivated in semi-arid environments and is often exposed to drought stress. Therefore, vineyard productivity, ranging between 5 tons per hectare in rain-fed vineyards and up to 40 tons per hectare in well-watered vineyards, largely depends on irrigation to mitigate the potential stress (Chaves et al., 2010). However, even in well-watered vineyards, vines are occasionally exposed to drought stress, e.g. in between irrigations (if soil water reservoir is depleted) or during peak transpiration. In such cases, a vine’s ability to maintain stomata open and photosynthesize largely depends on its osmotic potential (Hochberg et al., 2016). Thus, we suggest that K⁺ fertilization can be used to improve plant drought tolerance and productivity, and should be particularly important in arid and semi-arid regions.

Xylem embolism and the loss of hydraulic function presents the major cause for tree mortality under drought stress (Anderegg et al., 2016). Plants normally prevent xylem embolism by tight stomatal control of transpiration, which leaves only a narrow safety margin between the minimal water potential and the cavitation threshold (Martin‐StPaul et al., 2017). Hence, shifting the stomatal response curve towards lower water potential values could present a hydraulic threat unless xylem vulnerability to cavitation is also shifted. Accordingly, hydraulic vulnerability should be coordinated with osmotic adjustment and stomatal regulation, but this has yet to be tested. This knowledge gap is largely due to methodological limitations in quantifying xylem cavitation (Rockwell et al., 2014).
A new method developed by Brodribb et al., 2016 presents a simple and affordable technique to measure xylem cavitation and should allow us to explore its coordination with K⁺ availability and osmotic adjustment.

Hypothesis:

Abundant K⁺ levels enable vines to be hydraulically adjusted to low water, thus improving plant assimilation and yield in episodic drought conditions.

Specific hypotheses: Under drought conditions high K⁺ availability will lead to:

1. Improved osmotic adjustment (lower turgor loss point) and lower vulnerability to embolism.
2. Higher stomatal conductance, photosynthetic rates, and soluble sugars at the leaf level.
3. Higher yields with increased sugar level.

Goals:

To evaluate the effect of K⁺ deficiency on the physiological development and yield level of table grapes (Early sweet) under drought stress.

Specific goals:

• To evaluate the plasticity of turgor loss point and hydraulic vulnerability in respect to drought stress and K⁺
• To assess the differences in stomatal regulation in vines under drought stress in respect to K⁺
• To characterize advantages in yield quality and quantity for K⁺ fertigation in drought stressed grapevines.