148 EFFECTS OF SODIUM CHLORIDE ON GROWTH AND MINERAL NUTRITION OF PURPLETOP VERVAIN

There is a rising demand for salt-tolerant species for landscaping. Purpletop vervain is an excellent landscape plant for gardens and parks, with fragrant lavender to rosepurple flowers. However, little is known concerning the effect of sodium chloride on morphological characteristics, flowering and mineral uptake of purpletop vervain. In this study, carried out in 2013–2014, the plants of purpletop vervain were grown in pots in an unheated plastic tunnel. The plants were watered with 200 mM NaCl solution four times, every seven days. Salinity-exposed plants were characterized by slightly reduced plant height, weight of the aboveground part and visual score. Salt stress caused also an increase in leaf content sodium, chlorine and manganese. Salinity had no effect on earliness of flowering and content in leaves of phosphorus, potassium, magnesium, copper, zinc and iron. Purpletop vervain seems to be plant species able to tolerate salt stress under controlled conditions.


INTRODUCTION
Salinity is one of the most serious environmental problems in cultivation of ornamental plants in urban areas [Wrochna et al., 2010;Cassaniti et al., 2012].It is well-known that salinity causes stunting, chlorosis and necrosis, leading to a reduction in plant quality [Marschner, 1995].Salt stress affects all major processes and plant metabolism such as growth, flowering, photosynthesis, respiration, water potential, enzymatic activity, absorption of minerals and nutrient balance [Cassaniti et al., 2013;Parihar et al., 2015].However, for ornamental plants, visual quality is more important than growth rate [Niu and Cabrera, 2010].There is a rising demand for landscape plants whose aesthetic values are not or are only slightly affected by salt stress [Niu et al., 2013].
Verbena (Verbena L.) is a genus in the Verbenaceae containing 211 species indigenous to Americas and Europe [Erhardt et al., 2014].Of all the verbenas, purpletop vervain (V.bonariensis L.) is among the tallest species and particu-larly effective as a ornamental plant [Armitage and Laushman, 2008].This species is annual or herbaceous perennial, with fragrant lavender to rose-purple flowers in tight clusters located on stems with very long internodes [Grossman et al., 2014].Purpletop vervain is an excellent landscape plant for gardens and parks.However, little is known about the response of purpletop vervain to salt stress.
The objective of this study was to investigate the effect of sodium chloride on the growth, flowering, ecophysiological parameters, visual quality relative, and nutritional status of purpletop vervain.

MATERIALS AND METHODS
The experiment was carried out in the years 2013-2014 at the West Pomeranian University of Technology in Szczecin (53° 25' N, 14° 32' E, 25 m a.s.l.).Seeds of purpletop vervain (Verbena bonariensis L.) were purchased from the W. Legutko Breeding and Seed Company Ltd., Jutrosin, Poland.On 10 March 2013 and 12 March 2014 seeds were sown into boxes filled with substrates.Seeds were germinated under dark conditions in an environmentally controlled glasshouse (21/17 ºC day/night; RH 80-90%).After germination, the seedlings were planted in the pots with a capacity of 0.4 dm 3 containing a deacidified peat (pH 6.5), supplemented with 1 g•dm -3 of a fertilizer Azofoska (Grupa INCO S.A., Poland; containing 8.1% N-NH 4 , 5.5% N-NO 3 , 6.4% P 2 O 5 , 19.1% K 2 O, 4.5% MgO, 23.0%SO 3 , 0.045% B, 0.18% Cu, 0.17% Fe, 0.27% Mn, 0.04% Mo, 0.045% Zn, and Cl >0.4%).On 15 May 2013 and 16 May 2014 one plant was maintained in each PCV pot with a capacity of 5 dm 3 .The pots were filed with a deacidified peat substrate (pH 6.5) with a mixture of fertilizer Azofoska (3 g•dm -3 ).The plants were grown in an unheated plastic tunnel under natural photoperiod.The pots were watered three times a week using tap water.The mean ion concentration (mg•dm -3 ) in tap water were: 1.53 N-NO 3 , 1.5 P, 6.2 K, 97.4 Ca, 16.6 Mg, 24.0 Na, 24.0 Cl, 0.62 Cu, 0.42 Zn, 1.3 Fe, 194 HCO 3 , EC 0.65 mS•cm -1 .The plants exposed to salinity stress were watered with 200 mM NaCl solution four times, every seven days from 1 June 2013 and 2014.Each time one plant in a pot was watered with 500 ml of the salt solution.Control plants were treated with tap water.The beginning of plant flowering was determined by counting the number of days from sowing to the day of opening of the first flower in inflorescence.At the beginning of flowering the ecophysiological parameters were assessed.The relative chlorophyll content was measured with the N-Tester chlorophyll meter (Yara International ASA, Oslo, Norway).The measurements included ten leaves located in the middle of a plant and three readings were taken per each leaf.Stomatal conductance was assessed with SC1 porometer (Dekagon Devices, USA).The measurements were performed only on sunny days between 10 a.m. and 12 p.m., and included six leaves per plant.The plants were cultivated under natural photoperiod until 20 August 2013 and 17 August 2014.Total plant height, number of inflorescences, fresh weight of aboveground part and visual plant quality (on a rating scale 1-5, where 5 was the best ornamental value, and 1 was poor ornamental value) were estimated on the last day of the cultivation.The collected leaves were dried, ground and sent to an accredited laboratory of the Chemical and Agricultural Station in Szczecin for determination of nutrient content according to standard methods [Ostrowska et al., 1991].
Each treatment was replicated four times and each replicated included 10 plants.The pots were arranged in a completely randomized design.The mean values were calculated using the analysis of variance ANOVA using the Statistica 10.0 software (Statsoft, Poland), and the significance of differences between means was tested using Duncan's multiple range test at a P ≤ 0.05.

RESULTS AND DISCUSSION
Statistical analysis of the results showed that NaCl treatment reduced plant height, compared with the control by 6% in the first and 16% in the second year of the study (Table 1).Measurements of plants indicated that the fresh weight of aboveground part was decreased under saline irrigation in both seasons.In comparison with the control, NaCl treatment was reduced fresh weight of plants by 10  Salinity effect on the number of inflorescences per plant varied with years of experiment (Table 1).In 2013 salinity treatment had no effect on the number of inflorescences per plant.In 2014 NaCl treatment significantly reduced the number of inflorescences.According to the results presented in a few studies, the number of inflorescences in ornamental plants may increases under salt stress [Shillo et al. 2002 In 2014, the negative effects of salt on the ecophysiological parameters were observed.The stomatal conductance ranged from 96.8 mmol H 2 O m -2 s -1 in the control to 77.9 mmol H 2 O m -2 s -1 in the NaCl treatment and the value of N-tes-ter ranged from 394 in the control to 374 in the NaCl treatment.However, salinity had no effect on specified parameters in the first year of experiment (Table 1).These results may indicate that in each year there were different environmental conditions of experiment.In addition to being species specific, salt tolerance of ornamental plants used in urban landscaping is the combined result of the complex interaction among climatic condition, type of soil and irrigation method [Wu et  The days to flowering ranged between 114 and 124, and the time was not affected by salinity (Table 1).Similarly, no significant effect of salt treatment was found on flowering time of Zantedeschia aetiopica cultivars [Veatch-Blohm and Morningstar, 2001].All plants in the control had a visual scores of 5.0 in both years of the study (Table 1).The visual quality of plants under salt stress was slightly decreased, with a rating of 4.70 points in 2013 and 4.13 points in 2014, which were considered acceptable for landscape.
Concentration of phosphorus, potassium, magnesium, copper, zinc and iron in leaves remained unaffected by the addition of NaCl (Table 2).Sodium as well as chloride content were increased in leaves of plants on exposure to salinity stress in both years of experiment.Analysis of the mineral composition of the plants grown under saline conditions help to understand the mechanism of salinity tolerance [Niu et al., 2013].Numerous studies have demonstrated that the excessive accumulation of ions, especially sodium and chloride may limit the uptake of other ions [Wrochna et al., 2006 2).Increased manganese leaf content under salt stress was also reported in Ziziphus mauritiana [Bhatt et al., 2008] Chrysanthemum morifolium 'Yellow Blush' [Lee and van Iersel, 2008] and Plectranthus ciliatus [Salachna et al., 2015].Manganese is an essential trace element for plants and it is an activator of a number of different enzymes in plant systems, mostly in oxidation, reduction, decarboxylation and hydrolytic reactions [Marschner, 1995].Shahi and Srivastava [2016] demonstrated that foliar spraying with manganese has a crucial role for increasing tolerance of mungbean plants to salinity stress.Increased manganese concentration in leaves of purpletop vervain treated NaCl may be due to a participation of this nutrient in the process of adaptation to salt stress condition.

CONCLUSION
This study has demonstrated that salinity stress slightly reduced plant height, weight of the aboveground part and visual score in purpletop vervain.Moreover, the leaves of salt-exposed plants contained more sodium, chlorine and manganese.Salinity had no effect on earliness of flowering and content in leaves of phosphorus, potassium, magnesium, copper, zinc and iron.Purpletop vervain seems to be plant species tolerant to salinity and may be uses in urban landscape exposed to salt stress.

Table 1 .
Effect of sodium chloride on plant height, fresh weight of aboveground part, earliness of flowering, number of inflorescences, stomatal conductance, N-tester values, and visual score of purpletop vervain (Verbena bonariensis L.)Values are means (n = 24-40) ± standard deviation.Mean values in each row for each year followed by different lower-case letters were significantly different at P ≤ 0.05 by Duncan's Multiple Range test.

Table 2 .
Effect of sodium chloride on macro and micronutrients concentration in the leaves of purpletop vervain (Verbena bonariensis L.) DW -dry weight.Values are the means of three replicate (± standard deviation).Mean values in each row for each year followed by different lower-case letters were significantly different at P ≤ 0.05 by Duncan's Multiple Range test.