46 Zimmerman and Jull: Sodium Chloride Injury on Buds and honeylocust (Gleditsia triacanthos L.) tend to preclude salt (Sinclair et al. 1987), whereas naked buds (lacking scales) of glossy buckthorn (Rhamnus frangula L.) are sus- ceptible to salt spray (Lumis et al. 1973, 1975). Norway maple (Acer platanoides L.), littleleaf linden (Tilia cordata Mill.), and wayfaringtree viburnum (Viburnum lan- tana L.) are common taxa in the U.S. urban landscape; there- fore, so information about their salt tolerance would be use- ful. Norway maple [mature height of 12.2 to 18.3 m (40 to 60 ft)] and littleleaf linden [mature height of 15.2 to 21.3 m (50 to 70 ft)] are shade trees commonly used in street locations, parks, and residential and commercial landscapes. Wayfar- ingtree viburnum [mature height of 2.4 to 4.6 m (8 to 15 ft)] is a flowering ornamental shrub used for hedges, screens, and shrub borders. These species were chosen for their differ- ences in bud morphology. Norway maple vegetative buds are small [3 to 6 mm (0.12 to 0.24 in) long] and have numerous, tightly arranged bud scales. Dissection of Norway maple buds reveals four outer, waxy bud scales and four or five fleshy, sticky, inner bud scales. Littleleaf linden buds are 6 to 9 mm (0.24 to 0.35 in) long and have four outer bud scales and three or four inner bud scales. Wayfaringtree viburnum vegetative buds are large [13 to 25 mm (0.52 to 1 in) long] and have a naked bud morphology (entirely lack bud scales). The objective of this study was to quantify the concentra- tions of NaCl that produce injury on dormant buds of Norway maple, littleleaf linden, and wayfaringtree viburnum. Bud morphologies of each species were also examined for use as a potential selection criterion for salt spray tolerance. MATERIALS AND METHODS Plant Materials Stem nodal samples containing lateral, vegetative buds were collected from Norway maple, littleleaf linden, and wayfar- ingtree viburnum. Samples were collected at McKay Nursery in Waterloo, Wisconsin, U.S. (43°11 N latitude, 88°59 W longitude). The collection sites were several hundred meters from a secondary road and thus were likely minimally af- fected by de-icing salts. One lateral branch node was taken per plant at each collection. A total of 162 plants were sampled at each collection period for electrolyte leakage and visual observation tests. In addition, eight branch samples from each species were placed in vases with tapwater, at ambient temperature, 21°C (70°F), for 16 hr per day for 3 weeks at all three collection periods, as a means of determin- ing the stage of dormancy. The fluorescent light used pro- vided a photosynthetic photon flux [PPF (400 to 700 nm)] of 24.1 mol · m−2 · s−1 (1.78 klx), as measured at the tops of the branch samples with a cosine-corrected LI-COR LI-189 quantum/radiometer/photometer (LI-COR, Lincoln, NE). Dormancy was estimated from the number of days to bud- ©2006 International Society of Arboriculture break, which was recorded when immature leaves became visible. A factorial combination of three species (Norway maple, littleleaf linden, and wayfaringtree viburnum) and nine salt concentrations [0, 500, 1,000, 2,000, 4,000, 8,000, 16,000, 32,000, and 64,000 mg/L (0, 500, 1,000, 2,000, 4,000, 8,000, 16,000, 32,000, and 64,000 ppm)] of A.C.S.-certified crystalline NaCl was used. The experiment was repeated three times during 2001–2002 (4 December, 30 January, and 12 March). Temperature was recorded at a weather station in Madison, WI (43°8 N latitude, 89°20 W longitude) (Figure 1). One centimeter (0.4 in) nodal samples were cut for Norway maple and littleleaf linden buds, and 2 cm (0.8 in) samples were cut for larger, wayfaringtree viburnum buds. Nodal samples were rinsed in deionized water for 30 sec, placed into 60 mL (1.8 oz) vials containing one of the nine NaCl solu- tions, and shaken on an Innova 2100 platform shaker (New Brunswick Scientific Co., Inc., Edison, NJ) at 140 rpm for 24 hr at 4°C (39°F). Buds were left on stem nodes to reduce NaCl uptake via xylem and phloem tissues. Samples were removed from solutions and rinsed in deionized water for 1 min to remove exterior salinity. Bud injury was determined using electrolyte leakage and visual observation methods, similar to methods reported by Shirazi and Fuchigami (1993). Electrolyte Leakage One bud was cut from each node and placed into individual 25 mL (0.75 oz) vials containing 8 mL (0.24 oz) deionized water (six buds per species/NaCl treatment were used). Vials were shaken for 20 hr at ambient temperature to facilitate electrolyte leakage from injured tissues. Initial electrical con- ductivity measurements were recorded for each vial using an Acromet AR20 electrical conductivity meter (Fisher Scien- tific, Chicago, IL). Vials were then immersed in a hot water Figure 1. Daily maximum and minimum air temperatures from November 2001 to March 2002 in Madison, Wiscon- sin (43°8 N latitude, 89°20 W longitude). Arrows indicate collection dates.
March 2006
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