| SOIL MANAGEMENT,FERTILIZATION, AND IRRIGATION
HORTSCIENCE 47(9):1351–1355. 2012. shoot and root growth rates, decreased leaf or shoot number (Munns, 2002), decreased gas exchange rates, foliar salt damage, and even Response of Selected Wildflower death as salinity increases (Munns and Tester, 2008; Niu and Cabrera, 2010). The degree of Species to Saline Water Irrigation these negative responses depends on species 1 and the level of the salinity. Many researchers Genhua Niu and Denise S. Rodriguez worldwide have conducted studies on salt Texas AgriLife Research Center at El Paso, Texas A&M University System, tolerance of landscape plants in the past years 1380 A&M Circle, El Paso, TX 79927 (e.g., Fox et al., 2005; Gori et al., 2000; Jordan et al., 2001; Marosz, 2004; Niu and Cabrera, Cynthia McKenney 2010; Niu and Rodriguez, 2006a, 2006b; Tanji Department of Plant and Soil Science, Texas Tech University, Lubbock, et al., 2008; Wu et al., 2001; Zollinger et al., TX 79409 2007). These studies indicate a wide range of salt tolerance existing among different species Additional index words. Hooker’s evening primrose, landscape irrigation, mealy cup sage, and cultivars within the same species. plains zinnia, salt tolerance, water reuse, water-wise landscape Wildflowers are popular plants in water- wise, low-maintenance landscapes. Planting Abstract. Wildflowers are good candidates for water-wise landscapes because many of wildflowers in landscapes could reduce mow- them are drought-tolerant after establishment. Little information is available regarding ing costs and improve soil erosion and soil whether these herbaceous wildflowers are tolerant to salt stress. Container experiments stabilization (Bretzel et al., 2009). Planting were carried out in a greenhouse and a shadehouse under semiarid climate conditions to wildflowers in landscapes also increases aes- investigate the salt tolerance of six native wildflowers: Salvia farinacea (mealy cup sage), thetic appearance by increasing diversity in Berlandiera lyrata (chocolate daisy), Ratibida columnaris (Mexican hat), Oenothera elata colors and vegetation. Herbaceous wildflowers (Hooker’s evening primrose), Zinnia grandiflora (plains zinnia), and Monarda citriodora dominate meadows in arid regions of Australia (lemon horsemint). In the greenhouse experiment, mealy cup sage, Hooker’s evening andthewesternUnitedStates(Beranetal., primrose, and plains zinnia were irrigated with a saline solution with an electrical 1999; Kjelgren et al., 2009; Pe´rez et al., 2010). conductivity (EC) of 1.5 (control, nutrient solution), 2.8, 4.1, 5.1, or 7.3 dS·mL1 for L1 However, little information is available on the 45 days. All plants survived except for plains zinnia at EC of 7.3 dS·m . Shoot dry salt tolerance of these herbaceous wildflowers. weights decreased as EC of irrigation water increased for all three species. In the To introduce wildflowers in landscapes shadehouse experiment (second year), plants of all species (plains zinnia was not where poor-quality water with high salinity included) were irrigated with saline solutions at EC of 0.8 (control, tap water), 2.8, 3.9, L1 may be used for irrigation, this study aimed to 5.5, or 7.3 dS·m for 35 days. Plants were fertilized with slow-release fertilizer in the examine the growth and physiological [os- shadehouse experiment. After 5 weeks of treatment, all plants of lemon horsemint in the motic potential (yS) and ion uptake] responses elevated salinity treatments, regardless of EC levels, were dead. The visual foliar salt of six native wildflowers to a range of salinity damage rating was lowest for lemon horsemint. Chocolate daisy had low survival levels in both greenhouse and shadehouse percentages and low foliar ratings at EC of 5.5 dS·mL1 and 7.3 dS·mL1. For the other L1 environments under semiarid conditions. The three species, survival percentages were 80% and 90% at EC of 7.3 dS·m . Hooker’s selected wildflowers included were Salvia evening primrose and mealy cup sage had similar low foliar visual ratings at EC of L1 farinacea (mealy cup sage), Berlandiera 7.3 dS·m , whereas Mexican hat plants had high foliar visual ratings regardless of lyrata (chocolate daisy), Ratibida columnaris salinity treatment. All species had similar high uptake of Na+ in shoots, whereas Hooker’s L (Mexican hat), Oenothera elata (Hooker’s evening primrose had slightly higher Cl concentrations compared with other species. evening primrose), Zinnia grandiflora (plains Based on these results, lemon horsemint was most sensitive to salinity stress followed zinnia), and Monarda citriodora (lemon horse- by chocolate daisy. Hooker’s evening primrose and mealy cup sage were moderately L1 mint). All these species are native to North tolerant and may be irrigated with low salinity water at EC of less than 3.9 dS·m . America and thrive in well-drained soils with Mexican hat was the most tolerant among the six species. full sun conditions in southwestern United States and northern Mexico (Stubbendieck et al., 2003). Water quantity and quality are critical the only water source that increases with global issues. As the urban population in- population growth (Qian et al., 2005). Many Materials and Methods creases, the competition for high-quality regions with water shortage problems have water among agriculture, industry, and do- started to use municipal reclaimed water Greenhouse experiment. Seeds of selected mestic water users is becoming progressively (also called recycled water) to irrigate golf wildflowers were sown in mid-Jan. 2009. intense. Water consumption in urban land- courses, school yards, and landscapes (Fox Because three species (chocolate daisy, Mex- scape irrigation increases with urban popula- et al., 2005; Gori et al., 2000; Jordan et al., ican hat, and lemon horsemint) did not germi- tion expansion (Kjelgren et al., 2000; Qian 2001; Wu et al., 2001) and for agricultural and nate with an insufficient number of seedlings, et al., 2005). Using alternative water sources horticultural crop production (Dobrowolski they were dropped from the greenhouse ex- such as municipal reclaimed water to irrigate et al., 2008; Safi et al., 2007; Shillo et al., periment. Uniform seedlings of the other three urban landscapes can significantly conserve 2002). However, reclaimed water frequently wildflower species, mealy cup sage, Hooker’s potable water. Municipal reclaimed water is contains high salt levels that may cause evening primrose, and plains zinnia, were damage or even death to sensitive plants if transplanted on 14 Apr. to 2.6-L containers not managed properly. Therefore, screening filled with Sunshine Mix No. 4 (SunGro Hort., and identifying salt-tolerant landscape plants Bellevue, WA). A week after transplanting, Received for publication 14 May 2012. Accepted is urgently needed to expand the use of al- saline water irrigation treatment was initiated. for publication 27 July 2012. ternative and reclaimed water for landscape Saline solution treatments (greenhouse We gratefully acknowledge the financial support irrigation and nursery production. experiment). Saline solutions at EC of 1.5 from Cooperative State Research, Education and Soil salinity is typically high in arid and (nutrient solution, control), 2.8, 4.1, 5.1, and Extension Service, U.S. Department of Agriculture –1 under Agreement No. 2005-34461-15661, and Texas semiarid regions where temperatures are high 7.3 dSÁm were created by adding calculated AgriLife Research. and rainfall is low. Irrigation with poor-quality amounts of sodium chloride (NaCl), magne- 1 To whom reprint requests should be addressed; water exacerbates the soil salinity. Typical sium sulfate (MgSO4Á7H2O), and calcium e-mail [email protected]. plant responses to soil salinity include reduced chloride (CaCl2) at 87:8:5 (weight ratio) to
HORTSCIENCE VOL. 47(9) SEPTEMBER 2012 1351 the nutrient solution. The nutrient solution was emergence occurred. Seedlings were trans- (2010). Specifically, leaves were sampled prepared by adding 0.5 gÁL–1 of 20N–8P–16K planted to larger cells (vol. 26 mL) on 26 from the middle section of the shoots in the of Peter’s 20-20-20 (The Scotts Company Mar. and 9 Apr., depending on species, and early morning at the end of the experiment, LLC, Allentown, PA) to tap water. The EC grown in the greenhouse. On 29 Apr., seed- washed in deionized water and dried with of tap water was 0.8 dSÁm–1 and the major ions lings were transplanted to 2.6-L containers a paper towel, sealed in a plastic bag, and in the tap water were Na+,Ca2+,Mg2+,Cl–,and filled with Sunshine Mix No. 4 (SunGro immediately stored in a freezer at –20 C 2– SO4 at 184, 52.0, 7.5, 223.6, and 105.6 Hort.). On 6 May, a slow-release fertilizer, until analysis. Frozen leaves were thawed in mgÁL–1, respectively. The composition of the Osmocote 14.0N–6.1P–11.6K (4-month re- a plastic bag at room temperature before sap treatment saline solutions was similar to the lease time; Scotts-Sierra Hort. Products, was pressed out with a Markhart leaf press reclaimed municipal effluent of the local water Marysville, OH), was applied to all plants (LP-27; Wescor, Logan, UT) and analyzed utilities. These EC levels were chosen based at 13 g per pot and Marathon (OHP, Inc., using a vapor pressure osmometer (Vapro on the assumption the salt tolerance of the Mainland, PA) was applied at 1 teaspoon Model 5520; Wescor). wildflower plants differed and their EC thresh- per pot. On 7 May, plants were moved to a Survival percentage was calculated as olds, which led to significant growth reduction shadehouse with 25% light exclusion and follows: number of surviving plants at the and foliar salt damage, were within the se- were well irrigated with tap water until saline end of the experiment/total number of lected treatment salinity range. The EC levels solution treatments were initiated on 20 May. plants 3 100%. Shoot DW, leachate EC, of reclaimed water, varied with locations and At treatment initiation, chocolate daisy and and shoot Na+ and Cl– were determined with water sources, ranges from 1.0 to 1.9 dSÁm–1. Mexican hat had an average of 14 leaves, the same methods as described in the green- Saline solutions were prepared in 100-L tanks Hooker’s evening primrose had 20 leaves, house experiment. with confirmed EC levels for each treatment. and mealy cup sage and lemon horsemint Experimental design and statistical analysis. Plants were irrigated with nutrient (control) or were 20 to 30 cm in height. Both experiments followed a split-plot design one of the saline solutions at 1 L per container Saline solution treatments (shadehouse with salinity of irrigation water as the main plot whenever the substrate surface started to dry. experiment). Saline solutions with an EC of and species subplots with 10 replications. All Irrigation frequency depended on species (bio- 0.8 (tap water, control), 2.8, 3.9, 5.5, and 7.3 data were analyzed by a two-way analysis of mass) and climatic conditions. Saline solution dSÁm–1 were prepared by adding calculated variance using PROC GLM. When the main irrigation started on 21 Apr. and ended on 3 amounts of sodium chloride (NaCl), magne- effect was significant, linear regression was June (45 d). At treatment initiation, plain sium sulfate (MgSO4Á7H2O), and calcium performed using PROC REG. To determine zinnia had an average of six nodes; Hooker’s chloride (CaCl2) at 87:8:5 (weight ratio) to the differences among salinity levels on plant evening primrose had 10 leaves, whereas tap water. The preparation of saline solutions growth,Student-Newman-Keulsmultiplecom- mealy cup sage was 20 in height. The and irrigation method were similar to those in parisons were performed. All statistical anal- temperatures in the greenhouse were main- greenhouse experiment except for the base yses were performed using SAS software tained at 25.3 ± 1.8 C(mean±SD) during solution in which tap water was used in this (Version 9.1.3; SAS Institute Inc., Cary, NC). the day and 22.0 ± 2.0 C at night. The daily experiment because a fertilizer injector was light integral (photosynthetically active ra- not available in the shadehouse. The outdoor Results diation) was 15.8 ± 2.6 molÁm–2Ád–1. climatic conditions during the shadehouse Measurement (greenhouse experiment). experiment period were: average daily air Greenhouse experiment. Shoot DW of On termination, shoots were harvested and temperatures at 28.5 ± 2.0 C, relative hu- mealy cup sage and Hooker’s evening prim- dry weight (DW) was determined after being midity at 19.8% ± 7.3%, solar radiation at rose decreased at elevated salinity levels oven-dried at 70 C until constant weight was 29.5 ± 2.6 MJÁm–2Ád–1, and four instances of compared with the control; however, no reached. To quantify the salt accumulation, rainfall with a total of 68 mm recorded by an differences were found among the elevated leachate was collected twice, 3 and 5 weeks on-site weather station. EC treatments except for mealy cup sage at after the treatment, and the EC of leachate Measurements (shadehouse experiment). EC of 7.3 dSÁm–1 (Table 1). For plains zinnia, was determined using an EC meter (Model B- Foliar salt damage was rated by giving plants grew slowly and no substantial differ- 173; Horiba, Ltd., Japan). To analyze tissue a visual score based on a criterion referenced ences in shoot DW among treatments were Na+ and Cl– concentrations, four shoot sam- scale from 0 to 5, where 0 = dead, 1 = over observed. At an EC of 7.3 dSÁm–1, all plains ples per treatment were randomly collected, 90% foliar damage (salt damage: burning, zinnia were dead by the end of the experi- washed three times with deionized water, and necrosis, and discoloration), 2 = moderate ment. Leachate EC, collected 3 and 5 weeks oven-dried at 70 C. Dried tissue was ground (50% to 90%) foliar damage, 3 = slight (less after the initiation of the treatment (aver- to pass a 40-mesh screen with a stainless than 50%) foliar damage, 4 = good quality aged), were 3.53 dSÁm–1, 7.43 dSÁm–1, 10.40 Wiley mill (Thomas Scientific, Swedesboro, with minimal foliar damage, and 5 = excel- dSÁm–1, 12.64 dSÁm–1, and 16.63 dSÁm–1 for NJ) and the samples were submitted to the lent with no foliar damage. The foliar salt treatments of control, EC 2.8, EC 4.1, EC 5.1, Soil, Water, and Air Testing Laboratory of damage rating did not consider the plant size. and EC 7.3, respectively. New Mexico State University (Las Cruces, For example, a score of 5 was given to a plant No interactive effect of treatment and NM) for Na+ and Cl– analyses. Na+ concen- if no salt damage was visible, although the species on shoot Na+ concentration was found trations were determined by EPA method plant was more compact compared with the (Table 2). No differences in shoot Na+ concen- 200.7 [U.S. Environmental Protection Agency control. trations among species were found regardless (EPA), 1983] and analyzed using an Induc- Leaf yS was determined as described in of EC level of the irrigation water. How- tively Coupled Plasma/Atomic Emission Niu and Rodriguez (2006a) and Niu et al. ever, both species and treatment interactively Spectrophotometer Trace Analyzer (Thermo Jarrell Ash, Franklin, MA). Cl– was deter- mined by EPA method 300.0 (U.S. EPA, Table 1. Dry weight of shoots of three wildflower species irrigated with saline solution at electrical 1983) and analyzed using an Ion Chromato- conductivity (EC) of 1.5 (control, nutrient solution), 2.8, 4.1, 5.1, or 7.3 dSÁm–1 for 45 d (greenhouse graph (Dionex, Sunnyvale, CA). experiment). Shadehouse experiment. Seeds of wild- Shoot dry wt (g) flowers were sown on 2 Feb. 2010 into 406- Species EC = 1.5 EC = 2.8 EC = 4.1 EC = 5.1 EC = 7.3 cell flats filled with Sunshine Mix No. 5 Mealy cup sage 19.4 Az 12.7 B 12.5 B 10.0 BC 6.7 C (SunGro Hort.). To improve the germination, Hooker’s evening primrose 35.5 A 12.9 B 12.4 B 11.1 B 7.5 B flats were covered with aluminum foil and Plains zinnia 3.5 A 3.3 AB 3.0 AB 1.9 B — y placed into cold storage at 5 C for 3 to 6 zMeans with same capitalized letters in the same row (among treatments) were not different tested by weeks depending on species and were placed Student-Newman-Keuls multiple comparisons at P = 0.05. back on greenhouse benches when seedling yNot measured (dead or not enough replicates).
1352 HORTSCIENCE VOL. 47(9) SEPTEMBER 2012 affected shoot Cl– concentration. Shoot Na+ Table 2. Shoot Na+ and Cl– concentration of three wildflower species irrigated with saline solution at and Cl– concentrations increased with increas- electrical conductivity (EC) of 1.5 (control, nutrient solution), 2.8, 4.1, 5.1, or 7.3 dSÁm–1 for 45 d ing EC in the irrigation water in all species. (greenhouse experiment). For example, in mealy cup sage, Na+ increased Shoot Na+ concentrations (mgÁg–1) from 2.2 mgÁg–1 in control to 24.5 mgÁg–1 at Species EC = 1.5 EC = 2.8 EC = 4.1 EC = 5.1 EC = 7.3 EC of 7.3 dSÁm–1,whereasCl– increased from Mealy cup sage 2.2 D az 7.4 C a 11.1 C a 15.2 B a 24.5 A a 21.4 mgÁg–1 to 60.4 mgÁg–1 in control to EC of Hooker’s evening primrose 1.2 C a 4.9 C a 9.7 B a 11.3 B a 25.3 A a 7.3 dSÁm–1. Compared with the other species, Plains zinnia 2.4 C a 3.7 C a 10.5 B a 16.4 A a — y – Hooker’s evening primrose had higher Cl Shoot Cl– concentrations (mgÁg–1) concentrations at elevated EC levels. Mealy cup sage 21.4 D a 30.6 C b 41.9 B b 52.3 A a 60.4 A b Shadehouse experiment. Among the five Hooker’s evening primrose 21.0 D a 44.4 C a 57.2 B a 62.3 B a 82.5 A a species, lemon horsemint did not survive at Plains zinnia 7.1 C b 15.1 B c 27.1 A c 32.1 A b — all elevated salinity levels by the end of the zMeans with same capitalized letters in the same row (among treatments) were not different; means with 5-week treatment (Table 3). Plains zinnia was the same small letters in the same column (among species) were not different tested by Student-Newman- excluded as a result of an insufficient number Keuls multiple comparisons at P = 0.05. of seedlings. Eighteen days after treatments, yNot measured (dead or not enough replicates). the survival percentages were 60%, 30%, 20%, and 20% for plants irrigated at EC of –1 2.8, 3.9, 5.5, and 7.3 dSÁm , respectively. For Table 3. Survival percentage (%) of five wildflower species irrigated with saline solution at electrical chocolate daisy, the survival percentages were conductivity (EC) of 0.8 (control, tap water), 2.8, 3.9, 5.5, or 7.3 dSÁm–1 for 5 weeks (shadehouse 30% at EC of 5.5 dSÁm–1 and 7.3 dSÁm–1 on experiment). Day 18. Foliar visual ratings were lowest for Survival percent lemon horsemint. Chocolate daisy plants had Species EC = 0.8 EC = 2.8 EC = 3.9 EC = 5.5 EC = 7.3 low visual ratings at EC of 5.5 dSÁm–1 and 7.3 –1 Chocolate daisy 90 90 100 20 30 dSÁm . For the other three species, survival Lemon horsemint 100 0 0 0 0 percentages were 80% and 90% at the highest Hooker’s evening primrose 100 100 100 100 80 salinity level (Table 3). Hooker’s evening Mexican hat 90 100 100 90 90 primrose and mealy cup sage had similar low Mealy cup sage 100 100 90 100 80 visual ratings at EC of 7.3 dSÁm–1,whereas Mexican hat plants had high ratings regardless of salinity treatment (Table 4). Table 4. Visual quality (score) of five wildflower species irrigated with saline solution at electrical Shoot DW of all surviving species de- conductivity (EC) of 0.8 (control, tap water), 2.8, 3.9, 5.5, or 7.3 dSÁm–1 for 5 weeks (shadehouse creased linearly as salinity of irrigation water experiment).z increased, except for chocolate daisy, which was unaffected by salinity (Fig. 1). Chocolate Visual score daisy grew slowly during the experimental Species EC = 0.8 EC = 2.8 EC = 3.9 EC = 5.5 EC = 7.3 period with large variations among individual Chocolate daisy 4.8 4.4 4.8 0.6 1.5 plants and, therefore, no significant differ- Lemon horsemint 3.5 0.0 0.0 0.0 0.0 Hooker’s evening primrose 4.6 4.6 4.4 4.5 3.1 ences were observed among the treatments. Mexican hat 4.9 5.0 4.5 4.7 4.9 For mealy cup sage, shoot DW was not Mealy cup sage 5.0 4.9 4.4 4.5 3.3 different among control, EC of 2.8, 3.9, zScore of 0 = dead, 5 = excellent (for detail, refer to text). and 5.5 dSÁm–1. For Mexican hat and Hooker’s evening primrose, no differences were found in shoot DW between the control and EC of 2.8 dSÁm–1. According to the linear regression in Figure 1, shoot DW decreased 0.96 g, 1.0 g, and 1.34 g in Mexican hat, mealy cup sage, and Hooker’s evening primrose, respectively, as the salinity of irrigation water increased by 1.0 dSÁm–1. Leachate salinities were pooled from species because species did not affect leach- ate EC. The average EC of leachate measured in the middle of the experiments were 3.8, 7.1, 11.8, 14.6, and 16.2 dSÁm–1 for treat- ments of EC 0.8 (control), 2.8, 3.9, 5.5, and 7.3 dSÁm–1, respectively. Leaf yS in the control was highest in lemon horsemint among the five species, indicating this species had the lowest in osmotic adjustment (Table 5). Because no plants of lemon horsemint survived, there was no data of yS to compare with other species. Generally, leaf yS decreased at higher salinity treatments (EC 5.5 and 7.3 dSÁm–1) in Mexican hat, mealy cup sage, and Hooker’s evening primrose. In the surviving plants of chocolate daisy, Mexi- can hat, and mealy cup sage, no differences Fig. 1. Shoot dry weight (DW) of five wildflower species irrigated with saline solution at electrical –1 in yS were found among control, EC of 2.8, conductivity (EC) of 0.8 (control, tap water), 2.8, 3.9, 5.5, or 7.3 dSÁm (shadehouse experiment). –1 and 3.9 dSÁm . Vertical bars represent SEs.
HORTSCIENCE VOL. 47(9) SEPTEMBER 2012 1353 Shoot Na+ and Cl– concentrations in all et al., 2007). According to these criteria, (Niu et al., 2010; Wright, 1986). For exam- surviving species increased with increasing lemon horsemint was no doubt the most ple, salinity of the leachate was lowest in EC of the irrigation water (Table 6). At EC of sensitive species among the six followed by perlite and highest in peat-based potting mix 5.5 dSÁm–1 and 7.3 dSÁm–1, Hooker’s evening chocolate daisy. Mexican hat was the most (Niu et al., 2007). Our results in both exper- primrose, Mexican hat, and mealy cup sage tolerant, and Hooker’s evening primrose and iments indicated salt accumulation. Similar had similar shoot Na+ and Cl– concentrations. mealy cup sage performed similarly. Al- to a container situation, salts accumulate in Among the species in the control, shoot Na+ though not included in the shadehouse ex- field studies (Marosz, 2004; Wu et al., 2001), was higher in chocolate daisy compared with periment, plains zinnia did not survive when and the degree of salt accumulation depends those in lemon horsemint and Hooker’s even- irrigated with saline solution at EC of 7.3 on soil type. Soil property influences salt ing primrose, whereas no differences were dSÁm–1 in the greenhouse experiment, which accumulation and distribution in the root found in Cl– in the control among species. All may indicate less tolerance to salinity com- zone, which results in differences in plant species had similar shoot Cl– concentrations pared with mealy cup sage and Hooker’s responses to salinity (Shannon et al., 1994). except at EC of 2.8 dSÁm–1 and 3.9 dSÁm–1 in evening primrose. It is recommended to Therefore, soil and substrate properties must which Hooker’s evening primrose had higher further confirm the tolerance of this species. be considered when determining the salinity Cl– concentrations compared with other spe- In a separate study, several cultivars of Z. thresholds of irrigation water when making cies, which agreed with the results in the marylandica and Z. maritima, which per- irrigation guidelines with low-quality water greenhouse experiment. formed well in landscapes under semiarid and recommendation of landscape plants climate conditions (high heat and drought based on their salt tolerance. Discussion stresses), were not tolerant to salinity (Niu Ion exclusion from shoots is one of the et al., 2012). Interestingly, the most tolerant most important mechanisms in tolerating salt Shannon and Grieve (1999) defined the Mexican hat and the most sensitive lemon stress in plants (Munns and Tester, 2008). salt tolerance of crops as the inherent ability horsemint have similar native habitats: prairie, There were no statistical differences in shoot of plants to withstand the effects of high salt plains, meadows, pastures, savannahs, and Cl– concentrations among the five species in concentrations in the root zone or on the roadsides, according to the Native Plant control in the shadehouse experiment. How- leaves without a significant adverse effect. Database (2012). ever, it is not known if the lemon horsemint For ornamental plants, tolerance of salt stress The salinity threshold of irrigation water plants might have accumulated excessive Na+ can be assessed based on survival rate, with is usually different from the soil salinity and/or Cl– in the early stage of the treatment, or without foliar salt damage or the degree of threshold. This is because salts accumulate causing the high mortality rate. The shoot foliar salt damage, and growth. Aesthetic in the root zone, which depends on irrigation Na+ and Cl– concentrations of these wild- appearance is more important in ornamental leaching fraction, salinity of the irrigation flowers were relatively high among most plants than maximum growth and researchers water, irrigation frequency and amount, and glycophytes, defined as plants sensitive to used visual ratings to compare relative salt substrate or soil property. In the situation of low concentrations of salts (Shannon et al., tolerance among tested species (Cameron a container study, salt accumulation can be 1994). For example, rose rootstocks irrigated et al., 2004; Fox et al., 2005; Zollinger quantified by monitoring leachate salinity with saline solution at 8.0 dSÁm–1 had much lower shoot Na+ (less than 2.0 mgÁg–1) and Cl– concentrations (less than 30 mgÁg–1) with some foliar injuries (Niu and Rodriguez, Table 5. Leaf osmotic potential (y ) measured at the end of the experiment of five wildflower species S 2008). Azalea (Rhododendron) hybrids had irrigated with saline solution at electrical conductivity (EC) of 0.8 (control, tap water), 2.8, 3.9, 5.5, or + – 7.3 dSÁm–1 for 5 weeks (shadehouse experiment). much lower Na and Cl accumulation com- pared with wildflowers in this study (Cabrera, Leaf yS (MPa) 2003). These results indicate these wild- Species EC = 0.8 EC = 2.8 EC = 3.9 EC = 5.5 EC = 7.3 z y flower species did not have the high ability Chocolate daisy –1.59 A b –1.71 A a –1.43 A a — — to exclude Na+ and Cl– but tolerated high Lemon horsemint –1.29 a — — — — + – Hooker’s evening primrose –1.81 AB bc –1.76 A a –2.25 DC c –2.15 BC a –2.50 D a concentrations of Na and Cl in shoots to Mexican hat –1.66 A bc –1.76 A a –1.95 AB b –2.17 B a –2.51 C a some degree. Mealy cup sage –1.92 A c –2.12 AB b –2.21 ABC c –2.33 BC a –2.53 C a Osmotic adjustment is another mecha- zMeans with same capitalized letters in the same row (among treatments) were not different; means with nism in tolerating salt stress as well as other the same small letters in the same column (among species) were not different tested by Student-Newman- abiotic stresses such as drought and heat Keuls multiple comparisons at P = 0.05. stresses. Plants are able to tolerate salinity y Not measured (dead or not enough replicates). by reducing the cellular yS as a consequence of a net increase in inorganic and solute accumulation (Hasegawa et al., 2000). Lemon Table 6. Shoot Na+ and Cl– concentration of five wildflower species irrigated with saline solution at horsemint had the highest yS (least negative) electrical conductivity (EC) of 0.8 (control, tap water), 2.8, 3.9, 5.5, or 7.3 dSÁm–1 for 5 weeks compared with other species in the shade- (shadehouse experiment). house experiment, which may indicate its lower osmotic adjustment. At higher salin- Shoot Na+ concentrations (mgÁg–1) ity levels, Hooker’s evening primrose, Mex- Species EC = 0.8 EC = 2.8 EC = 3.9 EC = 5.5 EC = 7.3 z ican hat, and mealy cup sage all had similar Chocolate daisy 8.4 B a 12.8 A a 12.8 A a — — low , indicating similar osmotic adjust- Lemon horsemint 3.2 b — — — — yS Hooker’s evening primrose 2.0 B b 2.2 B c 8.9 AB a 11.3 A a 11.6 A a ment ability. Mexican hat 3.8 B ab 8.3 B ab 7.4 B a 19.2 A a 13.4 A a In summary, lemon horsemint was most Mealy cup sage 4.3 B ab 7.8 B b 17.3 A a 17.9 A a 19.6 A a sensitive to salinity stress and could not tolerate any elevated salinity in irrigation – –1 Shoot Cl concentrations (mgÁg ) water. Chocolate daisy was moderately sensi- Chocolate daisy 14.4 B a 25.3 A b 25.9 A b tive and should only be used in situations with Lemon horsemint 23.2 a Hooker’s evening primrose 17.2 C a 35.5 B a 44.3 A a 46.1 A a 47.7 A a limited salinity. Hooker’s evening primrose Mexican hat 17.0 C a 25.2 B b 24.4 B b 34.2 A a 43.0 A a and mealy cup sage were moderately tolerant Mealy cup sage 11.8 B a 27.2 A b 31.2 A b 36.1 A a 36.7 A a and may be irrigated with low salinity water –1 zMeans with same capitalized letters in the same row (among treatments) were not different; means with with an EC less than 3.9 dSÁm .Mexicanhat the same small letters in the same column (among species) were not different tested by Student-Newman- was the most tolerant among the selected Keuls multiple comparisons at P = 0.05. species.
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