Revisiting the Salinity Tolerance of Crapemyrtles (Lagerstroemia spp.)

RESULTS AND DISCUSSION

The addition of NaCl salt concentrations higher than 6 mM to the irrigation water produced average leachate EC and Cl concentrations in excess of 2.5 dS/m¹ (mmhos/cm) and 20 mM (710 ppm), respectively (Figure 1). These values, representative of those being experienced by the roots, have been reported to significantly and negatively affect the growth of ornamental trees and large shrubs like Arbutus unedo (strawberry tree), Abelia grandiflora (glossy abelia), Cotoneaster congestus (Pyrenees cotoneaster), Mahonia aquifolium (Oregon grape holly), Liriodendron tulipifera (tulip tree); Feijoia sellowiana (pineapple guava), as well as an unnamed Lagerstroemia indica (common crapemyrtle) cultivar (Francois and Clark 1978; Francois 1982).

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Figure 1.

Average electrical conductivity (EC) and chloride concentration (C) in leachates collected from containerized plants of three crapemyrtle (Lagerstroemia) cultivars irrigated with increasing levels of NaCl salinity. Data points are means ± standard errors of 27 leachate samples.

Before presenting the salinity response data of the evaluated cultivars, it should be noted that throughout the study it was readily apparent, the inherent vigor of the two interspecific hybrids ‘Natchez’ and ‘Basham’s Party Pink’ visibly dominated over the biomass and growth responses of ‘Pink Lace.’ In southern U.S. landscapes, ‘Pink Lace’ is considered a “medium” grower (Egolf and Andrick 1978), reaching mature heights of 3–4.5 m (10–15 ft), contrasting to the 6–9 m (20–30 ft) mature heights of ‘Natchez’ and ‘Basham’s Party Pink’ (Egolf and Andrick 1978; Byers 1997; Dirr 1998). Within the two interspecific hybrids the ‘Natchez’ plants had higher leaf areas, total dry weights and growth indices, but ‘Basham’s Party Pink’ had the highest root dry weights and lower shoot to root ratios (data not shown). It should be noted that ‘Basham’s Party Pink’ can reach larger heights and widths than ‘Natchez’ on the milder parts of the southern United States and Gulf of Mexico regions, namely USDA Hardiness Zone 9a, but whose growth and performance significantly declines in colder zones (Byers 1997), commonly freezing to the ground. The present study was conducted in Dallas, TX, which is classified as a colder Hardiness Zone 8a.

If actual plant biomass and growth measurements are used, all the measured response variables show an interaction of cultivar selection and salinity levels (data not shown), with inherently large growth differences likely masking the relative response of the cultivars to increasing salt stress. Therefore, with the objective of having a better assessment of the relative salinity tolerance of the crapemyrtle cultivars without involving their genetically-determined growth differences, the use of relative values for dry weight and plant growth response variables was considered as a suitable method. This comparative approach was based on the classical evaluation of relative yield responses of field (agronomic and horticultural) crops to increasing salinity conditions (Maas 1990).

The interactive effects of cultivar by salt concentration observed when using actual values (data not shown) were effectively lost for total plant dry weight, leaf area and top dry weights when these were expressed on a relative basis (Figure 2, Table 1), and the plants from all three cultivars responded indistinctively, and negatively, to the salinity main effect. While Figure 2a shows only relative total plant dry weight as a function of salt stress, relative leaf area and relative top dry weights (not shown) were similar in their response. Interestingly, while relative root dry weight (Figure 2b) and relative plant growth index (Figure 2d) also responded negatively to the increasing salinity conditions, the rate at which these parameters were reduced in the smaller ‘Pink Lace’ cultivar was less steep than in the larger ‘Natchez’ and ‘Basham’s Party Pink’ cultivars.

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Figure 2.

Relative total (A) and root (B) dry weights, shoot to root ratio (C), relative growth index (D), leaf chlorophyll (SPAD) index (E) and foliage salt burn ratings (F) of three crapemyrtle (Lagerstroemia) cultivars irrigated with increasing levels of NaCl salinity. Data points are means ± standard errors of 6 plants. The parameters for the fitted regression lines are shown in Table 2.

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 actually showed a slightly increasing trend at higher salt concentrations. These overall results indicate that despite the absolute and relative reduction in total plant dry weight with salinity stress observed 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 equilibrium (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) readings 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 regarded as salt tolerant (Percival 2005). It has been reported that low to moderate NaCl salinity stimulates chlorophyll degradation whereas higher salt concentrations more drastically affect chlorophyll synthesis (Santos 2004), and both physiological mechanisms conduce to reduced chlorophyll content and fluorescence (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 concentration (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 selection had a highly significant effect on plant nutrient profile, with ‘Basham’s Party Pink’ having the lowest leaf concentrations 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; Cabrera 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 significantly influenced by the cultivar, with ‘Basham’s Party Pink’ plants having the lowest concentrations. The literature rarely reports 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, essential and beneficial plant mineral nutrients (Marschner 1995).

In an effort to identify the range of Na and Cl concentrations (or accumulation) associated with acceptable crapemyrtle growth and quality, these values were correlated with their respective relative plant dry weights and salt burn ratings (Figure 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 interestingly, relative crapemyrtle dry weight yields showed increases with leaf tissue Cl concentrations up to 1.0 % (10,000 ppm) but were significantly depressed at higher concentrations (Figure 3b). The lattermost observation is remarkable in that it suggests the ‘optimum’ range for crapemyrtle leaf Cl concentration puts it effectively in the macronutrient concentration range. While it has been reported that most salt sensitive woody species, like fruit trees, present toxicity symptoms when leaf Cl exceeds 0.35% (3500 ppm; Marschner 1995), some tree species, like kiwifruit (Actinidia deliciosa), have high Cl requirements with established leaf Cl critical concentrations of about 2.0% (20,000 ppm; Smith et al. 1987).

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Figure 3.

Relationships between relative whole plant dry weights (A, C) and foliage salt burn ratings (B, D) and leaf sodium and chloride concentrations in three crapemyrtle (Lagerstroemia) cultivars irrigated with increasing levels of NaCl salinity. Each data point represents a single plant. The parameters for the fitted regression lines are shown in Table 2.

Correlations of leaf Na and Cl with salt burn ratings revealed differential relationships between cultivars, with ‘Pink Lace’ plants showing the steeper and quickest responses (i.e., slopes). Leaf Na concentrations above 300 mg/kg⁻¹ (ppm) were associated with rapidly increasing salt burn ratings in ‘Pink Lace’ but concentrations above 2,000 mg/kg⁻¹ (ppm) were needed to produce unsatisfactory ratings in the hybrid cultivars (Figure 3c). On the other hand, and similar to plant dry weight response to tissue Cl accumulation, unsatisfactory salt burn ratings and plant appearance were observed when leaf Cl exceeded the 1.0 % (10,000 ppm) mark (Figure 3d). Once again, a steeper relationship between leaf Cl and salt burn was noted for ‘Pink Lace’ plants and was minimal in ‘Basham’s Party Pink’ plants.

Overall result analysis indicates that while on the basis of plant biomass (growth) determinants, modern cultivars of the genus Lagestroemia could certainly retain a categorization of salt-sensitive; aesthetically they have a broader range of tolerance to salinity. While further evaluation is needed in a broader range of crapemyrtle cultivars, the data from the present study and that of Francois (1982), suggests that cultivars of the common crapemyrtle (L. indica) are less tolerant of salt stress than the modern interspecific hybrids (L. indica × fauriei) being widely used by the landscape industry today.

From an ecophysiological perspective, it is contended the continental origin of L. indica, attributed to mainland China (Egolf and Andrick 1978), would have evolutionarily provided less exposure of this species to the salinity stress experienced by an island species like the Japanese L. fauriei (Egolf and Andrick 1978; Byers 1997). Under the premise that natural selection has resulted in developing salt tolerant species, and ecotypes within species, researchers have logically expected best salinity tolerance results when using progeny of plants growing in increasingly saline environments (Nicknam and McComb 2000). It is assumed the relatively higher salt tolerance of hybrids ‘Natchez’ and ‘Basham’s Party Pink’ observed in the present study was inherited from the Japanese parent. It should be noted that ‘Basham’s Party Pink’ was a chance seedling identified and selected in the coastal city of Houston, TX (Egolf and Andrick 1978). The results from the present study pointing ‘Basham’s Party Pink’ as effectively the most salt tolerant cultivar, with the more steady (less changing) shoot to root ratios, leaf chlorophyll indices and salt burn ratings (Figure 2c, Figure 2e, Figure 2f) and lowest accumulation of Na and Cl in leaf tissues (Figure 3c, Figure 3d), supports the contention that island and/or coastal regions would result in the selection of plant materials with higher salinity tolerance. While this selection strategy has been commonly used, the expectation however that these starting materials will show higher degrees of salinity tolerance has not always been met (Allen et al. 1994; Nicknam and McComb 2000). To effectively test this hypothesis in crapemyrtles, it will be necessary to comparatively evaluate the salinity tolerance of a larger number of cultivars representing these and other Lagerstroemia species and their interspecific hybrids, an effort that is currently underway.