192 Percival and Barnes: Calcium-Induced Freezing and Salinity Tolerance chloride. These fertilizers are widely used by the orchard indus- try to reduce fruit disorders such as bitter pit of apples and cork spot of pears (Raese 1997). The efficacy of these calcium fer- tilizers has not been evaluated for their potential in improving the freezing and salt tolerance of urban trees. Objectives of this investigation were to determine the effec- tiveness and feasibility of seven commercially available calcium fertilizers to improve the freezing and salt tolerance of two tree species, apple (Malus cv. ‘Golden Crown’; deciduous) and ev- ergreen oak (Quercus ilex L.; evergreen). MATERIALS AND METHODS Plant Material The apple trial site consisted of a 1.5 ha (3.75 ac) block of apple (Malus cv. ‘Golden Crown’) interspersed with individual trees of Malus ’Red Delicious’ and ‘Gala’ as pollinators. Planting dis- tances were based on 3 m (9.9 ft) by 3 m (9.9 ft) spacing. The trees were planted in 1979 and trained as an open leader, i.e., bush-shaped tree, by periodically pruning the central leader. All experimental trees had an average height of 2 ± 0.2 m (6.6 ± 0.7 ft) with mean butt diameters of 33±5cm (13.2 ± 2 in). The evergreen oak trial site consisted ofa1ha (2.5 ac) block of evergreen oak located opposite to the apple trial site. Planting distances were based on 1 m (3.3 ft) by 1 m (3.3 ft) spacing. The trees were planted in 2001 and trained to produce a central leader system to an average height of 1.2 ± 0.15 m (4 ± 0.5 ft) with mean butt diameters of9±1cm (3.6 ± 0.4 in). The soil of both sites was a sandy loam containing 4% to 6% organic matter, pH of 6.1, available phosphorus, potassium, magnesium, sodium, and calcium were 55.3, 702.4, 188.2, 52.9, and 1888 mg/L (0.0001, 0.004, 0.002, 0.0001, and 0.02 oz/gal). Site manage- ment consisted ofa2m (6.6 ft) wide weed-free strip beneath the trees maintained with glyphosate (Roundup; Green-Tech, Sweet- hills Park, Nun Monkton, York, U.K.). Trees were irrigated with undertree impact sprinklers at approximately 10 day intervals throughout the growing season. No supplementary fertilization was applied during the trial. In the case of apple, a minimal fungicide (myclobutanil, product name Systhane [Landseer Ltd., Chelmsford, Essex, U.K.]) and insecticide (deltamethrin, product name Bandu; Headland Agrochemicals Ltd., Saffron Walden, Essex, U.K.) program was applied every 2 months during the growing season beginning in May 2004 (Nicholas et al. 2003), a standard practice followed at the University of Reading experi- mental site for orchard pest and disease control. All sprays were applied using a Tom Wanner Spray Rig sprayer (Wanner Engi- neering, Inc., Minneapolis, MN) at 30 mL (0.9 fl oz) myclobu- tanil + 40 mL (1.2 fl oz) deltamethrin per 100 L (26 gal) of water. Trees were sprayed until runoff, generally 3.5 L (0.91 gal) per tree. All studies were conducted at Reading University, Shin- field experimental research site, Reading, Berkshire (51°43N, –1°08W). Calcium Treatments A randomized complete block was used in the experimental design. Nine treatments (seven calcium products: calcium chlo- ride, calcium sulphate, calcium nitrate, calcium nitrate borate, calcium hydroxide, calcium metalosate, calcium magnesium complex [United Agri-Products, Alconbury Weston, Hunting- don, U.K.]; 1 Silwett L77 [an 80% polyalkylene oxide 20% allyloxypolyethylene glycol methyl ether plant penetrant; De ©2008 International Society of Arboriculture Sangosse, Swaffham Bulbeck, Cambridge U.K.]; and a water control) were assigned to eight single tree replications giving a total of 72 observations per response variable. Calcium sprays were applied usinga5L (1.3 gal) pump action sprayer and trees were sprayed until runoff. Single-row borders prevented over- spray of calcium treatments and double-row borders prevented calcium drift. Calcium treatments were applied on 15 July, 1 August, 18 August, and 15 September 2004. Each calcium spray was calibrated so that each tree received 2 g (0.07 oz) calcium per spray (8 g [0.28 oz] in total). Poor absorption and uptake of foliar applied calcium is a widely recognized problem within the orchard industry. Consequently, calcium sprays are routinely ap- plied in combination with a plant penetrant such a Silwett L77. Silwett L77 was applied singly and in combination with all cal- cium fertilizers at the manufacturer’s recommended rate, 0.5 mL (0.015 fl oz) per liter (0.26 gal). Freezing Treatments Treatments were carried out during the fall and winter (18 De- cember 2004, 18 January 2005, 9 February 2005). In the case of evergreen oak, six fully expanded nonsenescing leaves per tree were selected at random from throughout the canopy and excised at the base of the petiole using a razor blade and placed abaxial surface down in a Petri dish on moist Watmans filter paper (Cole-Parmer Instrument Company, Ltd., Hanwell, London) sealed with a thin polythene film permeable to air but not water. In the case of apple, because no leaves were present on the tree at each sampling date, 3 cm (1.2 in) stem sections, three sections per tree selected at random from throughout the canopy, were placed into 50 mL (1.5 fl oz) Universal bottles (Fisher Scientific, Loughborough, Leicestershire, U.K.) containing 30 mL (0.9 fl oz) distilled water of a known conductivity and gently shaken by hand. All leaf and stem samples were placed in darkness in a programmable Merck Environmental Freezing Chamber (Merck Pharmaceuticals, West Drayton, Middlesex, U.K.). The tempera- ture was reduced by 2°C (3.6°F) per hr from 10°C (50°F) and maintained for 4 hr at a set temperature, after which the tem- perature was raised by 2°C (3.6°F) per hr to 10°C (50°F; Sakai and Larcher 1987). A series of minimum set temperatures was selected to bracket the temperature causing 50% electrolyte leak- age from stem and leaf tissue. In the case of leaf tissue derived from evergreen oak, set temperature treatments ranged from 2°C (3.6°F) to –12°C (10°F). In the case of twig tissue derived from apple, set temperature treatments ranged from –24°C(–11°F) to –40°C(–39°F). To clarify, in the case of evergreen oak, each individual freezing treatment required six leaves per tree, eight trees per calcium treatment. i.e., 48 leaves in total per freezing (2°C [3.6°F], 0°C [32°F], –2°C [28°F], –4°C [25°F], –6°C [21°F], –8°C [18°F], –10°C [14°F], –12°C [10°F]) treatment. In the case of apple, each individual freezing treatment required three twig sections per tree, eight trees per calcium treatment, i.e., 24 twigs in total per freezing (–24°C[–11°F], –26°C [–15°F], –28°C[–18°F], –30°C[–22°F], –32°C[–26°F], –34°C [–30°F], –36°C[–34°F], –38°C[–38°F], –40°C[–42°F]) treat- ment. All leaf and stem material was prepared within 2 hr of collection from the field. Root samples of both tree species were collected and prepared as described by McKay (1992) before placing into 50 mL (1.5 fl oz) Universal bottles containing 30 mL (0.9 fl oz) distilled water of a known conductivity. As a result of the problems of obtaining consistency in root sampling size, it was not possible to prepare
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