Crop losses to freeze injury represent a major economic threat to horticultural production. In the United States, economic losses related to cold-temperature injury are greater than to any other weather-related phenomenon (Snyder and de Melo-Abreu, 2005). Freeze injuries to perennial fruit crops can occur before dormancy in the fall, during the dormant season, and in the spring during and after budburst (Rodrigo, 2000). To date, studies on plant acclimation to low temperatures have mostly focused on tolerance of dormant tissues to low winter temperatures, a major environmental constraint on the distribution of fruit crops in cold-climate regions (Wolf, 2008). Freeze injury to vulnerable, green, and actively growing plant tissues can also negatively impact production of many fruit crops all over the world (Barranco et al., 2005; Kappel, 2010; Molitor et al., 2013; Rowland et al., 2013). For example, in 2007, widespread spring frosts in the eastern and midwestern United States resulted to estimated $2 billion in crop losses (Warmund et al., 2008).
Factors affecting the extent of freeze injury in young and developing plant tissues include cultivar, dew point and surface moisture, probability of an ice nucleation event, prefrost environmental conditions, and stage of development, with budburst considered the onset of the most susceptible period (Trought et al., 1999). To avoid or reduce the risk of a post-budburst frost-damaging event, growers preferentially plant cultivars with delayed phenological development in more frost-prone areas. However, cultivar differences in frost resistance appeared to be related to factors other than bud phenology. Small but consistent differences in frost resistance of buds at the same growth stage were reported between ‘Concord’, ‘Vidal Blanc’, and ‘Baco Noir’ vines (Johnson and Howell, 1981a). Severity of freeze injury was affected by cultivar in olive (Barranco et al., 2005), almond (Imani et al., 2011), blueberry (Rowland et al., 2013), and sweet cherry (Kappel, 2010) trees.
In addition to cultivar selection, growers can adopt other frost protection methods to reduce the risk of spring freeze injury. Frost protection methods have been developed to either modify microclimate conditions in the orchard or vineyard (i.e., wind machines, heaters, etc.), inhibit formation of ice in plant tissues (i.e., over-vine/tree sprinkling irrigation), or decrease the probability or severity of frost damaging events (i.e., site selection, application of chemical compounds to delay budburst, and cultural practices) (Evans, 2000). Installation and operation of frost protection methods may require large investments and growers managing small size orchards or vineyards cannot justify the costs involved. Therefore, there is an extensive need for low-cost methods for protecting crops against spring freeze injury.
A potentially affordable approach to decrease freeze injury to young plant tissues is through exogenous application of surface or systemic cryoprotectant products. Surface cryoprotectants are thought to cover green tissues with a physical barrier, which may prevent the formation of ice crystals inside the plant (Fuller et al., 2003). Systemic cryoprotectants are used to mimic natural frost tolerance or avoidance mechanisms (Wilson, 2001) by stabilizing proteins and membrane functions (Nuccio et al., 1998) and/or by cellular changes in Ψs. Indeed, a decrease in Ψs resulting from increased intracellular solute concentration may lower the freezing point of the cytoplasm and reduce cell dehydration caused by extracellular freezing (Sakai and Larcher, 1987).
Agrochemical companies have promoted the application of chemical compounds for frost protection, and many products have been screened over the last 30 years for cryoprotectant activity with inconsistent results. Foliar applications of antitranspiring compounds (surface cryoprotectants) were ineffective to decrease post-budburst freeze injury in peach (Matta et al., 1987), young citrus (Burns, 1973), and tomato and pepper (Perry et al., 1992). On the contrary, the application of a hydrophobic particle film (CM-96-018) led to a decrease in freeze injury in potatoes, grapevine, and citrus plants (Fuller et al., 2003).
Foliar applications of systemic cryoprotectants (e.g., glycine betaine, ethylene glycol, and BRIJ 35) increased freeze resistance in grapevines (Himelrick et al., 1991) and in Arabidopsis thaliana (Xing and Rajashekar, 2001). However, another commercial product advertised as a systemic cryoprotectant (e.g., frostgard) was ineffective in decreasing freezing temperature in ‘Pinot Noir’ vines (Gardea et al., 1993) or ‘Arking’ strawberry plants (Anderson and Whitworth, 1993). Similarly, several cryoprotectants (e.g., frostgard, frostfree, and KDL) were ineffective in reducing flower freeze damage of ‘Honeoye’ strawberry plants (Warmund and English, 1994).
A foliar macronutrient, Agro-K’s Potassium Dextrose-Lac® (KDL; 0N–0P–24K) (Agro-K Corporation, Minneapolis, MN) has received increased attention for its potential cryoprotectant activity. Foliar application of KDL within 36 h of a predicted frost event is advertised to protect green tissues of plants against freeze injury. Although the mechanism of action is not clear, the product is attractive to growers due to its low cost and several positive testimonials (Agro-K Corporation, 2016). The product label indicates that KDL derives from potassium carbonate (K2CO3) and Ascophyllum nodosum. Therefore, a hypothetical mechanism of action may include increased frost resistance due to A. nodosum extract and/or to an increase of potassium (K) concentration. Potassium is one of the major osmotic solutes of plant cells, therefore its accumulation in the cell could increase tolerance to cellular freezing (Keller, 2010). An extract of the brown seaweed A. nodosum increased tolerance in A. thaliana to cold temperatures (−5.5 °C) when the plants were irrigated 48 h before the freezing treatment (Rayirath et al., 2009).
In this study, we used four grapevine cultivars (V. vinifera L.) to test the hypothesis that post-budburst freeze injury varies between genotypes and in response to application of KDL. To assess if the mechanism of action of the KDL was related to an increase of intracellular solute concentration, Ψs was measured. A second experiment was conducted on ‘Noiret’ (Vitis hybrid) vines to assess the impact of KDL on post-budburst freeze injury on a hybrid grapevine cultivar and to explore the relationship between shoot mortality induced by freezing temperature and stage of phenological development.
Agro-K Corporation 2014 Get a jump on frost with KDL. Agro-K Corporation, Minneapolis. 12 Apr. 2016. <http://www.agro-k.com/wp-content/uploads/2014/08/kdl-frost-flyer.pdf>.
Agro-K Corporation 2016 Agro-K Corporation, Minneapolis. 12 Apr. 2016. <http://agro-k.com/>.
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