242 advisable for urban plantings than A. pseudoplatanus (Fini at al. 2008). Several Norway maple cultivars are available for sale, but some such as ‘Deborah’, ‘Emerald Queen’, and ‘Summershade’ are particularly appreciated due to reddish colour of new sprouts, intense green colour of foliage, and heat tolerance, respectively. The aim of this work was to: 1) investigate the effects of irrigation on growth and physiology of Tilia and Acer pla- tanoides during the establishment phase; 2) evaluate wheth- er differences in drought tolerance during the establishment phase existed within Tilia genus; and 3) evaluate whether dif- ferences in drought tolerance during the establishment phase existed among three Acer platanoides cultivars. This infor- mation can be useful to urban foresters and arborists in se- lecting the right species and cultivars within a desired genus. MATERIALS AND METHODS Plant Material and Growing Conditions In winter 2004–2005, 168 uniform, 2.5 to 3 m (8.3 to 9.9 ft) tall, 3 to 4 cm (1.2 to 1.6 in) diameter [measured at 1.3 m (4.3 ft) height], balled and burlapped [size of the root ball was ap- proximately 30 cm (12 in) in diameter] Tilia platyphyllos, T. cordata, T. × europaea, T. tomentosa, Acer platanoides ‘Sum- mershade’, A. platanoides ‘Deborah’, and A. platanoides ‘Em- erald Queen’ were planted in an experimental plot located at the Fondazione Minoprio (Como, Italy, 45°44’N, 9°04’E). Mean annual temperature and rainfall of the experimental site, calcu- lated over the last 20 years, are 12.99°C (55.38°F) and 1086 mm (47.76 in) respectively. Despite of relatively high rainfall, some drought spells are likely to occur in the study site in summer and especially in July, which is the driest and the warmest month. Trees were planted in a loam soil (51% sand, 40% silt, 9% clay). Planting holes were two times the width of the root-ball and deep enough to position the root flare exactly at the soil level. Trees of the different species or cultivars were planted in a randomized complete block with three replicates. Each block was made up of four plants per species/cv. and irrigation treatment. All plants were irrigated in 2005 to avoid transplant shock. Starting in 2006, 12 plants per species/cultivar (irrigated; 84 plants for the whole experiment) were irrigated with a drip irrigation system (4 l/h) in order to maintain constant soil moisture and for 12 plants per species/cv. (nonirrigated) irrigation was withheld throughout the experiment. Irrigation was provided to irrigated plants when evapotranspiration exceeded precipitation, which occurred from May to the end of August 2006, and from June 20 to the end of September 2007 (Figure 1). Effective evapotranspiration (ETE) can be calculated as the product of reference evapotranspiration (ET0 A pan evaporimeter. Kc was estimated to be 0.5 to 0.8 for many landscape trees and shrubs including maple and linden (Daugherty 2002; Pardossi et al. 2004). Since we only mattered that irrigated plants experienced no water stress, we overestimated Kc and we considered a Kc = 1 for effective evapotranspiration determina- tion as proposed by Pardossi et al. (2004). The difference between rainfall and ETE gave the water balance of the trees. Two irriga- tion events per week were performed on irrigated plants. Irriga- tion time was the time needed to recover to 0 (when negative) the difference between rainfall and ETE. Time needed to achieve this goal varied, generally, from 3 (May) to 15 (July) hours per week. ) and a crop coefficient (Kc). ET0 ©2009 International Society of Arboriculture was measured with a Class Figure 1. Net balance between rainfall and ETo measured weekly from the first of June to the 20th of September 2007. Data Collection Shoot elongation was measured on 20 randomly chosen shoots on first order lateral branches per plant on two plants per spe- cies/cultivar per irrigation treatment per replicate at the end of the growing season in 2006 and 2007. Trunk diameter was measured on all plants at 1.3 m (4.3 ft) above the root flare. Leaf gas exchange (net photosynthesis, A, µmol m-2 spiration, E, mmol m-2 s-1 s-1 s-1) was measured in 2006 and 2007 from June to September us- ; stomatal conductance, gs irradiance (1300 µmol m−2 s−1 ing the CIRAS-2 portable infrared gas analyzer (PP Systems, Hertfordshire, UK). The readings were taken between 8.00 h and 11.00 h (maple), and from 14.00 h to 17.00 h (linden) on sunny days under fixed CO2 concentration (360 ppm) and saturating , provided by a built-in red LED radiation source) on two plants per species/cultivar per irriga- tion treatment and replicate. Five fully expanded leaves per plant were selected from the outer portion of the crown and at different heights and checked for gas exchange. On July 20 and 27, 2007, leaf gas exchange was measured continuously for the entire day, to assess the daily trend of gas exchange. Data are presented in Tables 5 and 6 as the average between the two sampling dates. Water Use Efficiency (WUE) was calculated as the ratio between A and E as reported in a previous work (Ferrini et al. 2008). Leaf greenness index was measured with a chlorophyll meter (SPAD-502, Minolta, Sakai, Osaka, Japan) on six leaves per species per treatment and replicate at the end of July in 2006 and at the beginning of September in 2007. A recent work on three tree species shows that SPAD val- ues of leaf chlorophyll content can also be a good indica- tor of leaf N content and leaf carotenoids (Percival 2008). Chlorophyll fluorescence was measured on six leaves per spe- cies per treatment and replicate with a Plant Efficiency Analyser Chlorophyll Fluorimeter (Handy Pea, Hansatech Ins., King’s Lynn, Norfolk, U.K.) in July 2006 and in June, July, and Au- gust 2007. Fluorescence values were obtained by placing leaves in darkness for 30 minutes by attaching light-exclusion clips to the leaf surface of whole trees. F0 Fm and Fv/F0 were measured after exposing the leaf to a satu- rating light (3000 µmol m-2 s-1 (minimal fluorescence); Fv/ ). These parameters are reliable ; tran- , mmol m-2 Ferrini et al.: Withholding Irrigation During Establishment Phase
September 2009
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