Arboriculture & Urban Forestry 35(3): May 2009 131 Table 1. Equations of regressions lines fitted to relative plant growth and quality responses, and their relationships with leaf Na and Cl concentrations, in three crapemyrtle (Lagerstroemia) cultivars subjected to increasing NaCl stress. Data used in the regression procedures are presented in Figure 2 and Figure 3. Response Variable Relative plant DW y Relative root DW Shoot: root ratio Relative growth index ‘Pink Lace’ Y = 76.2 - 1.35X + 0.01X2 Y = 2.61 + 0.02X - 0.003X2 Y = 87.5 - 0.20X - 0.01X2 Chlorophyll index (SPAD) Y = 70.1 - 0.94X - 0.01X2 Salt burn rating Y = 0.465 + 0.166X Relative plant DW Salt burn rating Relative plant DW Salt burn rating dent variables). y R2 R2 R2 R2 R2 ‘Natchez’ For all cultivars, Y = 84.5 - 3.28X + 0.06X2 = 0.92 = 0.94 = 0.96 = 0.99 = 0.99 R2 Y = 0.403 + 0.0018X - 1.8x10-7 = 0.90 Growth and quality responses to applied NaCl z R2 R2 Y = 79.6 - 4.85X + 0.12X2 Y = 3.94 + 0.09X - 0.004X2 Y = 89.6 - 0.98X + 0.01X2 Y = 55.1 - 0.76X - 0.01X2 Y = 0.179 + 0.062X For all cultivars, Y = 81.9 - 0.021X + 2.5x10-6 X2 R2 R2 R2 R2 R2 R2 = 0.89 = 0.98 = 0.75 = 0.81 = 0.96 = 0.99 Relative plant DW and salt burn relationship to leaf Na X2 Y = 0.222 + 0.0004X - 2.2x10-8 = 0.91 For all cultivars, Y = 16.8 + 240.1X - 240.7X2 Y = 0.210 - 1.03X + 1.17X2 = 0.69 R2 R2 = 0.68 X2 Relative plant DW and salt burn relationship to leaf Cl + 83.4X3 - 9.6X4 Y = 0.102 + 0.10X + 0.12X2 = 0.81 R2 Y = 0.017 + 0.0005X - 4.5x10-8 = 0.77 R2 = 0.79 Y = 0.249 - 0.68X + 0.51X2 = 0.65 R2 z For equations: Y corresponds to the listed response (dependent) variable, while X denotes applied NaCl concentration or Leaf Na or Cl concentration (indepen- DW = Dry weight. salinity conditions, the rate at which these parameters were re- duced in the smaller ‘Pink Lace’ cultivar was less steep than in the larger ‘Natchez’ and ‘Basham’s Party Pink’ cultivars. The vigorous shoot (top) growth of ‘Natchez’ resulted in the generation of the largest average shoot to root ratio (S/R) across salinity levels, almost twice as large as that found in ‘Pink Lace,’ which incidentally was also the only cultivar showing the most visible decreases in S/R with increasing salt stress (Figure 2c). While the S/R of ‘Basham’s Party Pink’ were similar to those of ‘Pink Lace’ at the lowest salt concentrations (< 6 mM), they actu- ally showed a slightly increasing trend at higher salt concentra- tions. These overall results indicate that despite the absolute and relative reduction in total plant dry weight with salinity stress ob- served in all cultivars (Figure 2a), the relative growth of the tops (shoots) in ‘Pink Lace’ decreased more than in its roots under increasing salinity, whereas those (S/R) ratios in ‘Natchez’ and ‘Basham’s Party Pink’ tended to maintain an homeostatic equilib- rium (Kozlowski et al. 1991; Harris et al. 1999). This observation suggests a relatively higher degree of physiological adjustment and/or tolerance to salinity in these two latter hybrid cultivars. The overall foliage quality and plant aesthetics of the three crapemyrtles cultivars responded differentially to the increasing salinity stress. While overall leaf chlorophyll index (SPAD) read- ings decreased with salinity stress, these were largely confined to ‘Pink Lace’ and ‘Natchez’ (Figure 2e), which showed similar rates of reduction (i.e., slope) despite their noticeable difference in foliage, “greenness” (‘Pink Lace’ plants were significantly greener). Interestingly, the chlorophyll index of the ‘Basham’s Party Pink’ plants barely changed across the salinity range, with a slight downward slope (in the fitted regression) observed only at the highest NaCl (24 mM) concentration. This observation is remarkable, as leaf chlorophyll content in woody ornamentals is reduced markedly under salt stress, even in those species re- garded as salt tolerant (Percival 2005). It has been reported that low to moderate NaCl salinity stimulates chlorophyll degrada- tion whereas higher salt concentrations more drastically affect chlorophyll synthesis (Santos 2004), and both physiological mechanisms conduce to reduced chlorophyll content and fluo- rescence (photosynthetic activity) (Santos 2004; Percival, 2005). Foliage salt burn, expressed as scorching and necrosis in leaves and subjectively assessed on a 0 to 5 scale, showed that while these symptoms increased linearly with salinity stress (Figure 2f), they occurred at a steeper rate in ‘Pink Lace’ plants than in the hybrids. Plants of ‘Basham’s Party Pink’ actually had the least foliage salt burn damage, and only the plants receiving the highest salt con- centration (24 mM) could be considered aesthetically unpleasant. Regarding plant nutrient status, interactive effects between cultivar and salinity stress were only observed for leaf nitrogen (N), phosphorous (P), calcium (Ca), sodium (Na) and chloride (Table 2). Across all salinity concentrations cultivar selec- tion had a highly significant effect on plant nutrient profile, with ‘Basham’s Party Pink’ having the lowest leaf concentra- tions for all essential nutrients, albeit all of them were within the normal or sufficiency ranges reported in the literature for crapemyrtle cultivars and hybrids (Mills and Jones 1996; Ca- brera and Devereaux 1998; Cabrera and Devereaux 1999). As expected, leaf Na and Cl concentrations increased with salinity stress (Table 2), but the degree of accumulation was sig- nificantly influenced by the cultivar, with ‘Basham’s Party Pink’ plants having the lowest concentrations. The literature rarely re- ports a range of Cl or Na levels considered adequate or optimum for tree growth and quality, but rather, and often, implies that they should be avoided whenever possible. This perception certainly contrasts with the categorization of Cl and Na as, respectively, es- sential and beneficial plant mineral nutrients (Marschner 1995). In an effort to identify the range of Na and Cl concentra- tions (or accumulation) associated with acceptable crapemyrtle growth and quality, these values were correlated with their re- spective relative plant dry weights and salt burn ratings (Fig- ure 3). The regression analysis showed that increasing leaf Na concentrations were associated with rather fast depressions in the growth of all cultivars (Figure 3a). Conversely, and interest- ©2009 International Society of Arboriculture X2 ‘Basham’s Party Pink’ Y= 91.6 - 5.29X + 0.12X2 Y= 2.29 + 0.08X - 0.002X2 Y= 88.3 - 1.98X + 0.05X2 Y= 58.3 + 0.27X - 0.01X2 Y= 0.064 + 0.038X R2 R2 R2 R2 R2 = 0.86 = 0.70 = 0.76 = 0.36 = 0.99
May 2009
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