Arboriculture & Urban Forestry 32(6): November 2006 Arboriculture & Urban Forestry 2006. 32(6):277–285. 277 Sucrose-Induced Tolerance to and Recovery from Deicing Salt Damage in Containerized Ilex aquifolium L. and Quercus robur L. Sulaiman Al-Habsi and Glynn C. Percival Abstract. Two outdoor experiments were conducted using containerized trees of the species holly (Ilex aquifolium L.) and English oak (Quercus robur L.) to determine the effectiveness and feasibility of sucrose to improve tolerance to and recovery from deicing salt (sodium chloride [NaCl]) damage. In experiment one, sucrose plus a water-only control was applied as a root drench to containerized stock. Seventy-two hours later, NaCl was applied to both tree species as a root drench. At day 15 after NaCl application, effects on leaf chlorophyll a and b, carotenoid (lutein, -carotene, neoxanthin, -carotene), xanthophyll (zeaxanthin, antheraxanthin, violaxanthin) concentration, leaf necrosis, and chlorophyll fluores- cence (Fv/Fm) were measured. In experiment 2, NaCl was applied as a root drench. At day 14 after NaCl application, plants were root drenched with sucrose plus a water-only control and recovery rates monitored by recording leaf necrosis, leaf chlorophyll concentration, and chlorophyll fluorescence over the proceeding 8 weeks. At week 8 after sucrose application, effects on tree growth were recorded. Sucrose was found to confer to both tree species a useful degree to tolerance to NaCl and improve recovery from salt-induced damage with promotion of photoxidative antioxidant pigments (carotenoids, xanthophylls) and chlorophylls strongly indicated as significantly contributing to the induction of NaCl tolerance. Key Words. Carbohydrates; carbon compensation; carotenoids; chlorophyll fluorescence; holly; oak; photosynthetic chlorophylls; photosystem II; urban trees. Deicing salts in the form of sodium chloride (NaCl) can be a major chemical pollutant in urban landscapes resulting in substantial tree deaths (Hermans et al. 2003). Symptoms of excess salinity in trees include crown dieback, lesions on the stem or trunk, stomatal closure, and concomitant chlorophyll degradation resulting in leaves yellowing and development of leaf necrosis (Gibbs and Palmer 1994; Percival and Hender- son 2002; Ryan 2005). In addition, symptoms can accumu- late, e.g., tip burn of conifer, leading to necrosis of needles that can lead to dieback of limbs and tree death (Dobson 1991). Subzero temperatures experienced during the winter months are a frequent cause of deicing salt application to prevent the icing up of road networks. With increased traffic volume and road network expansion increasing the quantity of salt used for deicing operations, higher tree death rates are predicted (Percival and Henderson 2002). Attempts to improve salt hardiness of urban trees, through plant breeding, are limited by the nature of salt tolerance, which is a quantitatively inherited trait controlled by many genes. Consequently, traditional breeding cycles requires many years to produce new cultivars with the desired salt- resistant characteristics. Although perceived to be of benefit for major agricultural crops such as wheat or potato, such breeding programs are rare for urban trees. As a consequence, there is a demand for protectant compounds that are both inexpensive and can be applied at relatively short notice to increase the salt tolerance of plants. A number of studies on the involvement of sugars in the response of plants to environmental and chemical stresses have reported protective effects on the leaf photosynthetic apparatus (Sulmon et al. 2004). The positive effect of sugars on reducing environmental stress in rice has been related to modifications and protection of photosystem II (Garg et al. 2002). In addition, enhancement of sugar levels in leaf chlo- roplasts has been shown to result in the protection of photo- system II to salt stress (Fukushima et al. 2001). Sucrose has also been shown to be involved in the thermostability of plant proteins after heat damage and maintaining chlorophyll levels and photosynthetic oxygen evolution after herbicide applica- tion (Robertson et al. 1994). Sugar alcohols have been shown to possess osmoprotectant functions that can help protect plants against salt and photoxidative stress caused by high light intensities (Williamson et al. 2002). Recent work has also shown that application of sugars confer a high degree of ©2006 International Society of Arboriculture
November 2006
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