Arboriculture & Urban Forestry 35(5): September 2009 yield of photochemistry of PSII. In this study, chlorophyll fluo- rescence was used to evaluate the occurrence of water stress within the genus Tilia and within the Acer platanoides species. Presence of stress was identified by changes in F0 the light harvesting complex. Fv/Fm is the maximum quantum yield of the PSII. Fv/F0 indicators of the occurrence of environmental stresses, includ- ing water stress, on PSII of several woody and herbaceous spe- cies (Angelopulos et al.1996; Maxwell and Johnson 2000; Per- cival and Fraser 2001; Percival et al. 2003, Percival 2005; Lazár 2006; Li et al. 2006; Percival et al. 2006; Yamada et al. 2006). F0 , the minimal fluorescence, is a measure of the stability of is an estimate of the maximum primary or decrease in Fv/Fm and Fv/F0 in nonirrigated plants if compared to irri- gated plants of the same species/cultivar. Comparison between mean annual leaf gas exchange of irrigated and nonirrigated plants of the same species were measured to identify the drought tolerance strategy adopted by the different species/cultivars. Statistical Analysis All data were subjected to one- or two-way analysis of vari- ance (ANOVA) using SPSS statistical package for Windows (SPSS Inc., Chicago, IL). The two genera were always analyzed independently. Effects of irrigation and species/cultivar were analyzed with a mixed model two-way ANOVA, where irriga- tion was the fixed effect and species/cultivar the random effect. When no significant interaction between factors was found, dif- ferences among species/cultivars within a genus/species were tested with Duncan’s multiple range test (P ≤ 0.05 and P ≤ 0.01). Parameters which showed significant interaction between fac- tors were plotted separately in order to compare each level of factor A (species/cultivar) for each level of factor B (irrigation) (Chew 1976). Data on leaf gas exchange were analyzed per single sampling date, merged together, and processed again to obtain an average value on annual basis. Data of daily trend of A and E were analyzed with repeated measures General Linear Model. RESULTS AND DISCUSSION Over the three years of the experiment, no plant died for wa- ter stress. Some signs of wilting were found in July 2007 (the driest month in the two years) in nonirrigated T. platyphyl- los. By the end of August 2006, nonirrigated Acer platanoides ‘Deborah’ shed over half of the leaves, while no leaf shed- ding was observed in irrigated Acer platanoides ‘Deborah’ and in the other cultivars of Norway maple and linden species. Effects of Species/Cultivar and Irrigation on Growth, Gas Exchange and Chlorophyll Content Among the linden species, T. cordata and T. × europaea had, re- spectively, the highest shoot growth in 2006 and 2007 (Table 1; Figure 2). In 2006, T. platyphyllos and T. tomentosa were slower growing than both the former species and in 2007 were slower growing than T. × europaea. Trunk diameter growth had a trend similar to shoot growth in 2006, and no difference among the species appeared in 2007 (data not shown). In 2006, T. cordata had the highest mean annual A and WUE (Table 1). High gas exchange was found also in T. tomentosa, while T. platyphyllos had the lowest gas exchange in both years. In 2006 and 2007, T. cordata and T. tomentosa had the highest chlorophyll con- 243 tent respectively, while T. platyphyllos had the lowest. No dif- ference among the species appeared for WUE in 2007. An in- crease in shoot growth and leaf gas exchange between 2006 and 2007 was observed for T. × europaea, which in 2006 had lower A, WUE, and chlorophyll content especially when compared to T. cordata and T. tomentosa, while it ranked very high in 2007. Irrigation increased shoot growth and transpiration in both years (Table 1). Net photosynthesis was increased by irriga- tion only in 2006. WUE and chlorophyll content were unaf- fected by irrigation. Significant interaction between factors was found for shoot extension in 2006, and for E in 2006 and 2007 (Figure 2). In 2006, shoot growth was increased by irri- gation in all species but T. × europaea. In 2006, T. cordata had the highest E both in non irrigated and irrigated environments. Figure 2. Means separation of those parameters [shoot growth in 2006, transpiration (E, mmol s-1 m-2 ) in 2006 and 2007], which were significantly affected by species × irrigation interaction in Tilia. In this case, Duncan’s Multiple Range Test was used to compare the species for each level of irrigation separately. ©2009 International Society of Arboriculture
September 2009
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