78 Banks and Percival: Failure of Foliar-Applied Biostimulants in Urban Trees Arboriculture & Urban Forestry 2014. 40(2): 78–83 Failure of Foliar-Applied Biostimulants to Enhance Drought and Salt Tolerance in Urban Trees Jonathan M. Banks and Glynn C. Percival Abstract. Urban environments present an array of environmental conditions detrimental to the biology of trees. Two major problems include deicing salts, a common soil pollutant, and drought. One potential option for managing these environ- mental disorders may be through the application of commercially available biostimulants, as enhance a plant's resistance to environmental stresses. Trials used containerized stock of evergreen oak (Quercus ilex), holly (Ilex aquifolium), rowan (Sorbus aucuparia), and beech (Fagus sylvatica) to evaluate the effectiveness of these products are reported to seven commercially available biostimulants as drought and salt protectant compounds. Results conclude that none of the biostimulants evalu- ated in this study provided any significant degree of salt or drought damage protection compared to water-treated controls. Key Words. Drought Tolerance; Environmental Stress; Fagus sylvatica; Ilex aquifolium; Landscape Disorders; Plant Health Care; Quercus ilex; Salt Tolerance; Sorbus aucuparia; Tree Physiology. With increased global average temperature and extremes in global weather patterns recorded (Pachauri and Reisinger 2007), those involved in managing urban trees are dealing with an ever in- creasing number of weather-related tree disorders, such as drought and subzero temperatures resulting in the application of deicing salts (Gibbs and Palmer 1994; Black 2012). Drought- and salt-related injuries appear as leaf yellowing, tip and/or peripheral leaf burn, stunted growth, defoliation, and potentially tree death (Cresswell and Weir 1997; Roberts et al. 2006). Salt damage to plants is oſten associated with deicing salt applications to roads and pavements (sidewalks) that manifests itself in two forms. One mechanism reduces water availability to plant roots through reduced soil osmotic potential (i.e., water flows from the root system back into the soil), while the other directly damages aboveground plant parts, causing direct toxicity burn symptoms. Drought stress is also of interest because it is regarded as one of the major abiotic stress factors (Akıncı and Lösel 2012). Drought resistance also displays cross- resistance (i.e., “resistance to one stress, induced by acclimation to another”) (Taiz and Zeiger 1991, ©2014 International Society of Arboriculture p. 347); therefore, it could be considered that a sub- stance found to counter drought stress could prove to influence other plant stresses, thus proving use- ful in the search for holistic solutions to tree care. Control measures for environmental disor- ders generally involve modifying the external growing environment, including, for example, mulching to reduce evaporative soil water losses caused by high temperatures and to prevent percolation of deicing salts into the soil (Chalker- Scott 2007; Percival et al. 2009). Alternatively, modifying a plant’s internal chemical environ- ment has shown promise. One example of this is the concept of sucrose-induced tolerance to salt damage (Al-Habsi and Percival 2006). Bios- timulants differ from traditional nitrogen, phos- phorous, and potassium fertilizers in that their active ingredient consists of a range of organic compounds, such as plant hormones, humic acids, marine algae extracts, sea kelp, vitamins, and other chemicals that vary according to the manufacturer (Ferrini and Nicese 2002). Biostim- ulants, it is claimed, influence plant metabolism and subsequently enhanced plant stress toler-
March 2014
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