Plant materials and growing conditions.

Rooted cuttings of the 10 species of Asteraceae mentioned above were received from Southwest Perennials (Dallas, TX) on 18 Sept. 2014. Uniform plants were transplanted into 3.9-L pots with equal amount of Metro-Mix 360 (Sun Gro Horticulture, Bellevue, WA) on 6 Oct. and placed in the greenhouse in El Paso, TX (31°45′31′′N/106°29′11′′W). Two weeks after transplanting, saline water irrigation treatment was initiated. Before treatment, all plants were well irrigated with a nutrient solution with an EC of 1.2 dS·m⁻¹, which was made by adding 1 g·L⁻¹ 15N–2.2P–12.5K (Peters 15-5-15 Ca-Mg Special; Scotts, Marysville, OH) to reverse osmosis water. The average air temperature in the greenhouse was 26.8 °C/21.5 °C (day/night). The average daily light integral was 12.2 mol·m⁻²·d⁻¹, and the average relative humidity was 40.4% during the experiment period.

Experimental design and treatments.

The experiment used a split-plot design with the salinity treatment as the main plot and 10 species as the subplot with 10 replications per treatment per species. Saline solutions at EC of 5.0 dS·m⁻¹ (EC 5) and 10.0 dS·m⁻¹ (EC 10) were made by adding 1.20 g·L⁻¹ sodium chloride (NaCl) and 1.16 g·L⁻¹ calcium chloride (CaCl₂), and 2.80 g·L⁻¹ and 2.67 g·L⁻¹ CaCl₂, respectively, to the nutrient solution mentioned above. pH of the solutions was adjusted to 6.6 to 6.8. Plants in the control were irrigated with nutrient solution. Both nutrient and saline solutions were prepared in 100-L tanks with confirmed EC using an EC meter (Model B173; Horiba, Ltd., Kyoto, Japan) before irrigation. From 20 Oct. to 17 Nov., irrigation treatments were applied once a week, five times in total. Between the treatment solution irrigations, plants were irrigated with nutrient solution whenever substrate surface became dry. Irrigation frequency varied with environmental condition and treatment. For example, water use of plants at high salinity was less and irrigated less often compared with the plants in the control. At each irrigation, plants were irrigated with 1 L treatment solution per plant, resulting in a leaching fraction of ≈21%. From 17 Nov. until 12 Dec. (termination of the experiment), all plants were irrigated with the nutrient solution because the salinity in the root zone already reached or exceeded the salinity of the treatment solutions.

Leachate EC and survival percentage.

The leachate EC was determined following the pour-through methods according to Cavins et al. (2008). Three plants per treatment per species were chosen for measurement each time after saline solution treatment (weekly). The substrate EC were determined using saturated paste extract according to Gavlak et al. (1994) at the end of the experiment. Three pots per treatment per species were measured. Before harvest, the number of dead plants was recorded and survival percentage was calculated.

Growth parameters.

Plant height (centimeters), from the juncture point of stem and root to the top growing point (usually to the tallest flower), was recorded at the end of the experiment. Crown diameter (centimeters) at perpendicular directions was also measured. At the end of the experiment, the number of shoots was counted for all species, with the exception that the number of crowns instead of number of shoots in shasta daisy and four-nerve daisy was counted due to their rosette growth habit. The criteria for number of shoots for the eight species were made according to their morphological characteristics (>10 cm for gregg’s mistflower; >5 cm for damianita, blackfoot daisy, aromatic aster, and skeleton-leaf goldeneye; >3 cm for lavender cotton; and >2 cm for zexmenia). The first and second branches >5 cm were counted for copper canyon daisy. Flowering characteristics were also quantified by counting the number of flower buds, open flowers, and faded flowers (Niu et al., 2013). Thereafter, all aboveground parts (including stems, leaves, and flowers) were harvested and DW was determined after oven-drying at 70 °C for 4 d.

Foliar salt damage evaluation.

Foliar salt damage was rated by giving a visual score based on a reference scale from 0 to 5, where 0 = dead; 1 = over 90% foliar damage (salt damage: burning, necrosis, and discoloration); 2 = moderate (50% to 90%) foliar damage; 3 = slight (<50%) foliar damage; 4 = good quality with minimal foliar damage; and 5 = excellent with no foliar damage (Cai et al., 2014b). The foliar salt damage rating did not consider the plant size.

Leaf greenness.

Leaf greenness (or relative chlorophyll content, SPAD reading) was measured using a handheld chlorophyll meter (measured as the optical density; Minolta Camera Co., Osaka, Japan) at the end of the experiment (on 3 Dec.). Healthy and fully expanded leaves in the middle of the shoot of surviving plants were chosen for measurements. Damianita was excluded due to its small leaves.

Chlorophyll fluorescence.

Minimal florescence (F₀), maximum fluorescence (F_m), the maximal photochemical efficiency of photosystem II (F_v/F_m, F_v = F_m − F₀), and performance index (P_i) were measured using the Pocket PEA chlorophyll fluorescence system (Hansatech, Norfolk, UK) to examine the effect of elevated salinity on leaf photosynthetic apparatus for three species, i.e., gregg’s mistflower, shasta daisy, and aromatic aster, on 11 Dec. Five plants per treatment were measured randomly. Other species were not measured because of small size of leaves.

Mineral analysis.

To analyze shoot and leaf sodium (Na), chloride (Cl), potassium (K), and calcium (Ca) concentrations, four samples per treatment per species were randomly selected from the 10 samples. Dried tissue was ground to pass a 40-mesh screen with a stainless Wiley mill (Thomas Scientific, Swedesboro, NJ). Dried tissue samples were extracted with 2% acetic acid (EM Science, Gibbstown, HJ) to determine Cl using the method described in Gavlak et al. (1994). The concentration of Cl was determined with a M926 Chloride Analyzer (Cole Parmer Instrument Company, Vernon Hills, IL). Dried tissue samples were submitted to the Soil, Water and Forage Testing Laboratory at Texas A&M University (College Station, TX) for analyses of the foliar Na, Ca, and K. In brief, dried tissue samples were digested in nitric acid following the protocol described by Havlin and Soltanpour (1989). Na, K, and Ca in digested samples were analyzed by inductively coupled plasma optical emission spectrometry (SPECTRO Analytical Instruments Inc., Mahwah, NJ) and reported on a dry plant basis as described by Isaac and Johnson (1975).

Statistical analysis.

A two-way analysis of variance was used to test the effects of soil salinity and species on plant growth. Means separation among treatments was conducted using Tukey’s honest significant difference multiple comparison. Relative shoot DW was calculated for each plant in EC 5 or EC 10 treatment as:

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Similarly, relative values for height, crown diameter, number of shoots and flowers, and DW were calculated. These relative values were used as salt tolerance indexes for hierarchical cluster analysis (Zeng et al., 2002). The dendrogram of the 10 aster species was obtained based on Ward linkage method and squared Euclidian distance on the means of the salt tolerance indices for six multivariate parameters including all relative growth data. All statistical analyses were performed using JMP (Version 12; SAS Institute Inc., Cary, NC).

Leachate EC, substrate EC, and survival percentage.

Species did not affect leachate EC and data were pooled across species. The average leachate EC of control treatment (nutrient solution at EC of 1.2 dS·m⁻¹) ranged from 3.1 to 5.4 dS·m⁻¹ during the experiment (Fig. 1). For EC 5 and EC 10, the leachate EC increased from 7.1 to 11.4 dS·m⁻¹, and from 9.5 to 18.1 dS·m⁻¹, respectively. From 18 Nov. (the fifth treatment) to the end of the experiment, all the plants were watered with nutrient solution (EC = 1.2 dS·m⁻¹), which resulted in the decrease of leachate EC in both EC 5 and EC 10 (Fig. 1). The substrate ECs were also unaffected by species, so the data of all species per treatment were pooled. The substrate ECs of control, EC 5, and EC 10 were 2.0 ± 0.1, 4.3 ± 0.2, and 6.9 ± 0.5 dS·m⁻¹, respectively. The substrate EC increased as the salinity of irrigation water increased, indicating that more salts were accumulated in the rhizosphere.

Variation of weekly leachate electrical conductivity (EC) during the experimental period. Control (CNT) represents EC at 1.2 dS·m⁻¹; EC 5 represents EC at 5.0 dS·m⁻¹; and EC 10 represents EC at 10.0 dS·m⁻¹. All plants were watered with the nutrient solution (EC = 1.2 dS·m⁻¹) after the fifth treatment on 17 Nov. till the harvest on 12 Dec. Data were pooled across all species since species did not affect leachate EC. Vertical bars represent ses.

Citation: HortScience 51, 2; 10.21273/HORTSCI.51.2.197

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Variation of weekly leachate electrical conductivity (EC) during the experimental period. Control (CNT) represents EC at 1.2 dS·m⁻¹; EC 5 represents EC at 5.0 dS·m⁻¹; and EC 10 represents EC at 10.0 dS·m⁻¹. All plants were watered with the nutrient solution (EC = 1.2 dS·m⁻¹) after the fifth treatment on 17 Nov. till the harvest on 12 Dec. Data were pooled across all species since species did not affect leachate EC. Vertical bars represent ses.

Citation: HortScience 51, 2; 10.21273/HORTSCI.51.2.197

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Variation of weekly leachate electrical conductivity (EC) during the experimental period. Control (CNT) represents EC at 1.2 dS·m⁻¹; EC 5 represents EC at 5.0 dS·m⁻¹; and EC 10 represents EC at 10.0 dS·m⁻¹. All plants were watered with the nutrient solution (EC = 1.2 dS·m⁻¹) after the fifth treatment on 17 Nov. till the harvest on 12 Dec. Data were pooled across all species since species did not affect leachate EC. Vertical bars represent ses.

Citation: HortScience 51, 2; 10.21273/HORTSCI.51.2.197

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Salinity at EC of 5.0 and 10.0 dS·m⁻¹ had no effect on the survival of damianita, gregg’s mistflower, shasta daisy, lavender cotton, and skeleton-leaf goldeneye (Table 1). The survival percentages of aromatic aster, blackfoot daisy, copper canyon daisy, and zexmenia in EC 10 were 90%, 80%, 60%, and 60%, respectively. Four-nerve daisy had a survival percentage of 40% in EC 5 and 10% in EC 10. Salinity can impair plant function, growth, and developmental process and can also reduce survival as extreme (Taiz and Zeiger, 2015).

Table 1.

Survival percentage of 10 aster species irrigated with nutrient solution [electrical conductivity (EC) = 1.2 dS·m⁻¹; control] or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] in the greenhouse.

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Plant height and crown diameter.

Height of all species decreased as the EC of irrigation water increased except for blackfoot daisy and skeleton-leaf goldeneye (Table 2). EC 5 only reduced the height of damianita, shasta daisy, and lavender cotton (Table 2). The three most severe reductions in height were found in shasta daisy in EC 10 by 64%, four-nerve daisy by 37%, and damianita by 31%, compared with control.

Table 2.

Height and crown diameter of 10 aster species irrigated with nutrient solution [electrical conductivity (EC) = 1.2 dS·m⁻¹; control] or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] in the greenhouse.

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Saline water irrigation affected the crown diameter of nine species, with only gregg’s mistflower growing unaffected by saline treatment (Table 2). Compared with their respective control, reductions of 31%, 25%, and 23% were found in crown diameter for damianita, shasta daisy, and copper canyon daisy plants in EC 5, respectively. Crown diameters of the other seven species were not different between EC 5 and control. Crown diameter of four-nerve daisy, copper canyon daisy, and damianita had greater reduction in EC 10 compared with other species.

Number of shoots, number of flowers, and shoot DW.

Except gregg’s mistflower and aromatic aster, the number of shoots in the other seven species in EC 5 was reduced by 21% to 71% (Table 3). In addition, the number of shoots of all perennials in EC 10 was decreased except for four-nerve daisy. The highest reductions in the number of shoots were observed in EC 10 in damianita, copper canyon daisy, and blackfoot daisy by 91%, 63%, and 61%, respectively, compared with their respective control (Table 3).

Table 3.

Number of shoots and flowers, and dry weight (DW) of shoots per plant of 10 aster species irrigated with nutrient solution [electrical conductivity (EC) = 1.2 dS·m⁻¹; control] or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] in the greenhouse.

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Shasta daisy and lavender cotton did not flower throughout the period of experiment. Damianita, copper canyon daisy, and four-nerve daisy in EC 5 had the greatest reductions in number of flowers by 91%, 51%, and 50%, respectively, compared with their respective control (Table 3). No difference in number of flowers among treatments was found in blackfoot daisy and zexmenia. The number of flowers in damianita, copper canyon daisy, and four-nerve daisy decreased the most. Among all tested species, number of flowers of damianita was the most affected by increased salinity. Although number of flowers in gregg’s mistflower was reduced significantly by elevated EC, plants in EC 10 still had more than 100 flowers and looked attractive. Niu et al. (2013) and Cai et al. (2014b) reported that salinity reduced the number of flowers and buds and flower DW of different Rosa spp. and cultivars. Elevated salinity reduced flower size and number in seven cultivars of Zinnia marylandica (Niu et al., 2012b).

Elevated salinity reduced DW of shoots in all species except for four-nerve daisy (Table 3). No difference in DW of shoots was found in gregg’s mistflower, aromatic aster, and zexmenia between EC 5 and control. The DW of all the other species were all reduced in EC 5 and EC 10 compared with control. DW of shoots in copper canyon daisy, damianita, and shasta daisy in EC 10 was reduced by 66%, 64%, and 59%, respectively, compared with control. DW of shoots in lavender cotton, gregg’s mistflower, zeximenia, skeletion-leaf goldeneye, and aromatic aster plants in EC 10 was reduced between 21% and 41%. It has been reported that increased salinity resulted in a reduction in shoot and leaf biomass in a variety of ornamental species (Cai et al., 2014b; Niu and Rodriguez, 2006; Niu et al., 2012a, 2012b). Based on the reduction of DW of shoots, copper canyon daisy and damianita were the most salt-sensitive species among all tested plants, and lavender cotton and gregg’s mistflower were more salt tolerant than others.

Foliar salt damage evaluation and salt tolerance cluster.

Visual quality is an important parameter for ornamental plants. High soil salinity may cause deleterious effects such as necrosis and leaf edge burn on plant leaves (Wahome et al., 2001). Salt-tolerant plants usually have less foliar salt injury and less growth reduction at elevated salinity (Grieve et al., 2008). Gregg’s mistflower and lavender cotton plants had good visual quality and minimal foliar damage in EC 5 and EC 10 treatments, and their averaged visual scores were above 4.5 (Table 4). Skeleton-leaf daisy plants had minimal foliar damage in EC 5 treatment and slight foliar salt damage in EC 10 treatment. Damianita and shasta daisy plants in EC 5 and EC 10 treatments exhibited slight foliar salt damage with averaged visual scores ranging from 2.9 to 3.5. Aromatic aster and copper canyon daisy plants in EC 5 experienced slight foliar salt damage. However, those in EC 10 had moderate to severe foliar salt damage with the averaged visual scores of 2.4 and 1.5, respectively. Zexmenia and four-nerve daisy plants displayed moderate to severe foliar salt damage in EC 5 treatment, and almost all four-nerve daisy plants died in EC 10 treatment.

Table 4.

Visual score and relative chlorophyll content (SPAD) of 10 aster species irrigated with nutrient solution [electrical conductivity (EC) = 1.2 dS·m⁻¹; control] or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] in the greenhouse.

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Hierarchical cluster analysis of the 10 aster species was conducted using multivariate parameters including the percentages of reductions in visual scores and growth parameters with the exclusion of number of flowers (Zeng et al., 2002). The dendrogram showed three distinguishable clusters in both EC 5 and EC 10 treatments (Fig. 2). Gregg’s mistflower, skeleton-leaf goldeneye, lavender cotton, aromatic aster, and zexmenia were consistently clustered together in both EC 5 and EC 10 treatments and considered as the salt-tolerant group. In EC 5, blackfoot daisy, copper canyon daisy, and four-nerve daisy were clustered in the same group and considered to have moderate salt tolerance, whereas in EC 10 shasta daisy and four-nerve daisy were in the moderate salt-tolerant group. Damianita ranked in the salt-sensitive group in both salt treatments, whereas shasta daisy was also considered to be salt sensitive in EC 5 cluster, and blackfoot daisy and copper canyon daisy were classified as salt-sensitive species in EC 10 cluster.

Cluster analysis on the salt tolerance of 10 aster species according to the reduction of height, crown diameter, number of shoots, dry weight, and visual score as affected by elevated salinity at electrical conductivity of (A) 5.0 dS·m⁻¹ and (B) 10.0 dS·m⁻¹.

Citation: HortScience 51, 2; 10.21273/HORTSCI.51.2.197

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Cluster analysis on the salt tolerance of 10 aster species according to the reduction of height, crown diameter, number of shoots, dry weight, and visual score as affected by elevated salinity at electrical conductivity of (A) 5.0 dS·m⁻¹ and (B) 10.0 dS·m⁻¹.

Citation: HortScience 51, 2; 10.21273/HORTSCI.51.2.197

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Cluster analysis on the salt tolerance of 10 aster species according to the reduction of height, crown diameter, number of shoots, dry weight, and visual score as affected by elevated salinity at electrical conductivity of (A) 5.0 dS·m⁻¹ and (B) 10.0 dS·m⁻¹.

Citation: HortScience 51, 2; 10.21273/HORTSCI.51.2.197

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SPAD readings and chlorophyll fluorescence.

All gregg’s mistflower, lavender cotton, aromatic aster, four-nerve daisy, and skeleton-leaf goldeneye plants in EC 5 and EC 10 had similar SPAD readings to those in control, indicating that salt treatment did not affect their chlorophyll content in per unit (Table 4). EC 5 did not impact the SPAD readings of blackfoot daisy, copper canyon daisy, and zexmenia, but EC 10 reduced their SPAD readings by 22%, 47%, and 32%, respectively, compared with control. Interestingly, SPAD readings of shasta daisy increased significantly with EC of irrigation solution.

Mineral analysis.

The concentration of Na for blackfoot daisy, copper canyon daisy, skeleton-leaf goldeneye, and zexmenia in control was similar to that in EC 5 (Table 5). EC 5 significantly increased the Na concentrations of the other six species. Damianita and four-nerve daisy accumulated the most Na, which was 28 and 31 times higher than those in control. EC 10 further increased the concentration of Na up to 59 and 62 times higher than those in control. Gregg’s mistflower, shasta daisy, lavender cotton, aromatic aster, and zexmenia plants in EC 10 had 8 to 31 times higher Na than their respective control. The highest Na concentration (35.6 mg·g⁻¹ DW) was found in four-nerve daisy in EC 10.

Table 5.

Na, Cl, K, and Ca concentrations of 10 aster species irrigated with nutrient solution [electrical conductivity (EC) = 1.2 dS·m⁻¹; control] or saline solution [EC = 5.0 dS·m⁻¹ (EC 5) or 10.0 dS·m⁻¹ (EC 10)] in the greenhouse.

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Salt treatment also increased leaf Cl concentrations (Table 5). EC 5 increased the leaf Cl concentration of all tested plants by one to nine times, compared with their respective control, whereas EC 10 resulted in 3 to 21 times more Cl concentration in their leaves. The highest Cl concentration (139.4 mg·g⁻¹ DW) was found in four-nerve daisy in EC 10. High concentrations of Na and/or Cl accumulated in leaf tissue may damage the chloroplasts and thus inhibit photosynthesis (Taiz and Zeiger, 2015). Excessive Na and Cl uptake also competes with uptake of other nutrient ions such as K, Ca, or nitrogen (N), resulting in nutritional disorders and reduce plant quality (Grattan and Grieve, 1999). These might be the reasons why most of four-nerve daisy died and had poor visual quality in both EC 5 and EC 10 treatments.

Potassium plays an important role in the regulation of the osmotic potential of plant cell and also activates many enzymes involved in respiration and photosynthesis (Taiz and Zeiger, 2015). High accumulation of Na will subsequently cause the reduction of K contents (Hasegawa et al., 2000). Leaf K concentration decreased significantly with increasing EC in all species except in blackfoot daisy, aromatic aster, copper canyon daisy, and zexmenia (Table 5). Compared with control, damianita, four-nerve daisy, lavender cotton, shasta daisy, and skeleton-leaf goldeneye accumulated 16% to 28% less K in their leaves in EC 5, whereas 29% to 56% in EC 10.

Salt solution prepared with NaCl and CaCl₂ also increased the leaf Ca concentration of all species except for gregg’s mistflower. EC 5 treatment did affect the leaf Ca concentration of damianita, lavender cotton, and zexmenia. Four-nerve daisy and blackfoot daisy in EC 5 had the most increase in leaf Ca concentration of 104% and 99%, whereas those in EC 10 accumulated three and two times more Ca concentration in their leaves.

There are three distinct mechanisms of action for plants to adapt to salinity: osmotic stress tolerance, Na⁺ or Cl⁻ exclusion, and the tolerance of tissue to accumulated Na⁺ or Cl⁻ (Munns and Tester, 2008). In many cases, salt-tolerant species have lower shoot Na⁺ and/or Cl⁻ concentrations because they have the ability to restrict Na⁺ and/or Cl⁻ transport to shoots. However, some tolerant species can tolerate high internal Na⁺ or Cl⁻ concentrations such as lavender cotton in this study with high Na⁺ and/or Cl⁻ concentrations. Apparently, there were substantial differences in their mechanisms in dealing with salinity among the tested species.