Evaluation of subtropical ornamental trees for reclaiming salinity affected lands

Jagreeti Gupta · R. K. Dubey · Nirmaljit Kaur · O. P. Choudhary

Abstract This study was conducted to evaluate the tolerance of 1-year-old seedlings of ten subtropical ornamental tree species against a range of salinity levels of NaCl from May 2015 to October 2015. The levels were further enhanced from November to April 2017 as 100% survival was observed in the initial concentrations for all species.The seedlings were grown during the first week of April 2015 in 10′′ earthen pots containing soil: farmyard manure(2:1), irrigated with tap water for 1 month followed by saline irrigation in May by maintaining electrical conductivity at 0.75,1.00,1.25,1.50,2.25,and 3.00 dS/m for 30,40, 50, 60, 90, and 120 mM NaCl. Every 3 months,heights, relative leaf water content, and percent survival were determined;total soluble sugars of the upper leaves of each tree were quantified. All species exhibited consistent early growth and survival when supplied with 30, 40, 50 and 60 mM of NaCl. Koelreutaria paniculata, Ficus benjamina, Putranjiva roxburghii, Bauhinia purpurea and Millettia ovalifolia were sensitive to elevated salinity levels and did not survive at the highest salt concentrations of 90 and 120 mM.

Keywords Saline irrigation · Ornamental trees · Relative leaf water content · Survival · Osmolytes

Introduction

Salinity is a major abiotic stress affecting crop productivity,and results in an ecological build-up,especially in arid and semiarid regions (Zahran 1999). It leads to the degradation of vegetative cover and hence disturbs ecological balances. Urban expansion and increasing populations have given rise to competition for fresh water among municipal, industrial and agricultural sectors in several regions. The consequences have been reduced freshwater allocation to the agriculture sector(Tilman et al.2002)and salinization is emerging as an alarming threat to sustainable agriculture. One ecologically viable option to address salinization could be the selection of species that can grow well in saline soils. This strategy would remain the most effective option for utilization and greening of lands affected by salinity (Ghoulam et al. 2002). Reforestation has always been the most practical and effective strategy to address the problem of soil salinity. Trees phytoremediate the soils by lowering the saline water table, utilizing underground water and decreasing the rate of water evaporation from the soil surface (Barrett 2002). However, a lack of information or, perhaps, a lack of salt-tolerant tree species pose a constraint for afforestation of saline lands.According toflowers et al. (1977), plants can be grouped as glycophytes and halophytes on the basis of their abilities to grow on soils with different salt concentrations. Halophytes complete their life cycles under high concentrations of salt, e.g., Atriplex, Vesicaria. The majority of terrestrial plants, including agricultural crops, are glycophytic and cannot tolerate high salt concentrations.

Primary response of plants to salinity is a decrease in water potential, which is detrimental to plant water use efficiency (Glenn and Brown 1998). Plants cope with salinity stress by employing different mechanisms and the predominant among these is the accumulation of compatible solutes like proline,soluble sugars and glycine betaine that function as osmolytes (Hasegava et al. 2000).

Saline soil has mineral salts in the form of cations,Na+,Ca2+, Mg2+and K+, and anions, Cl-, SO42-, HCO3-,CO32-and NO3-(Tanji 2002). These ions directly affect plant growth and development, causing either osmotic or ionic effects (Mansour and Salama 2004; Parida and Das 2005 and Lauchli and Grattan 2007).

Acacia auriculiformis A. Cunn. ex Benth. (northern black wattle) belongs to the family Leguminoseae and is native of arid central Australia. Callistemon lanceolatus(Sm.) DC. (crimson bottle brush) belongs to the family Myrtaceae and is especially suited for planting near ponds or lily pools with weeping branches falling over the pool.Casuarina equisetifolia L. belongs to the Casuarinaceae family and is commonly known as coast she-oak. Koelreutaria paniculata Laxm.(goldenrain tree,varnish tree)is a flowering tree in the family Sapindaceae, native to eastern Asia,in China and Korea.Ficus benjamina L.(weeping fig) belongs to the Moraceae family and is native of the Indo-Malaya region. It is a medium to large evergreen species and is planted for its attractive foliage in parks,single or in groups, and alongside roads for shade. Pongamia pinnata syn.P.glabra(L.)Pierre.(Karanj)is in the family Leguminoseae and originated in India. Putranjiva roxburghii Wall.belongs to the family Euphorbiaceae,and is commonly known as child life tree. It is a good avenue tree and can be planted in public places as a shade tree.Cassia fistula L.,commonly known as the Indian laburnum(Amaltas),is in the Leguminoseae family and originated in India. The tree remains leafless on commencement of flowering,and at the end of the flowering season,the leaves start to appear.Bauhinia purpurea L.or B.triandra Roxb.,also originated in India, belongs to the family Fabaceae,and is commonly known as purple Kachnar. It is good for roadside and group planting. Millettia ovalifolia Kurz. is a dwarf tree with small lilac- colored flowers, belonging to the family Fabaceae and is African in origin(Randhava and Mukhopadhyay 1986; Arora 1990; Peter 2008). Screening of these important ornamental trees for tolerance to salinity will enable the reclaiming of salt-affected lands.Hence,the present studies were carried out with the objective to screen subtropical ornamental species for tolerance to salinity on the basis of growth performance, relative leaf water content, total soluble sugars accumulation, and survival.

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Materials and methods

Seeds of A. auriculiformis, C. equisetifolia, K. paniculata,P. pinnata, P. roxburghii, Cassia fistula, B. purpurea, M.ovalifolia,air layering of C. lanceolatus and cuttings of F.benjamina were grown during July 2014. Seedlings of all species were transplanted to 10′′earthen pots with soil:FYM (farmyard manure; 2:1) as the growing medium during the first week of April 2015. Thirty days after transplanting, i.e., in the first week of May 2015, different concentrations of analytical reagent grade NaCl, i.e.,30 mM(1.75 g/L of water/pot),40 mM(2.34 g/L of water/pot),50 mM(2.92 g/L of water/pot)and 60 mM(3.51 g/L of water/pot) were applied with irrigation water. Before starting the experiment,the dosages were standardized and the electrical conductivity (EC) of the soil was maintained at 0.75, 1.00, 1.25, 1.50 dS/m at 30, 40, 50 and 60 mM NaCl and 0.75,1.50,2.25 and 3.00 dS/m at 30,60,90 and 120 mM NaCl respectively, in the irrigation water. The irrigation water volume was determined by adding the leached amount to the water consumed by the plants, i.e.,1 L/pot. The selected concentrations of NaCl were applied up to October 2015, and then further enhanced as 30 mM(1.75 g/L of water/pot), 60 mM (3.51 g/L of water/pot),90 mM (5.26 g/L of water/pot) and 120 mM (7.02 g/L of water/pot). Relative leaf water content (RLWC) and total soluble sugars were measured in the upper leaves using the following formula:

In this study,growth of all species was more in the rainy season and less in summer and winter.Many plants are less tolerant to salinity under hot, dry weather which favours high water uptake and leads to the development of water deficits, as compared to cool and humid conditions (Maas and Hoffman 1977). Extremely cool temperatures also result in cold stress and affect plant growth.

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Mean heights varied significantly among the ten ornamental species(P = 0.0001)over 3 months(P = 0.0001)at five salinity levels (P = 0.009), salt treatment × month interaction (P = 0.0001), tree × salt treatment interaction(P = 0.0001), tree × salt treatment × month interaction(P = 0.0001). The effect of month × salt treatment was insignificant from April to October 2015. As the concentrations were further increased, mean heights varied significantly among all the treatment combinations at P = 0.0001 during January 2016-April 2017.

Results and discussion

Plant heights

The experiment was arranged in a factorial completely randomized design (CRD) of three factors: salt treatment,ornamental trees and months) with three replications and six plants per replication. SAS software version 9.0 was used and the means were compared using Duncan’s New Multiple Range test (DMRT).

Heights of some species, (A. auriculiformis, C. lanceolatus, C. equisetifolia, P. pinnata and C. fistula) were higher (Fig. 1a) under low salt concentrations as reported for salt bush Atriplex nummularia Lindl. (Araujo et al.2006), possibly due to increased water uptake by these species(Munns and Tester 2008).This increase in height in the five species was observed from April to October 2015.Low salt concentrations might have induced osmotic adjustment activity in the plants, which might have improved growth. Similar results have been reported by Kumari et al. (2012) for Azadirachta indica A. Juss.However, heights of the other five species, K. paniculata,F.benjamina,P.roxburghii,B.purpurea and M.ovalifolia decreased(Fig. 1b)as salt concentrations increased.Height of all the species decreased (Fig. 2a and b) after October 2015 due to higher concentrations of NaCl possibly due to the adverse effect on photosynthesis and on carbohydrates,particularly sugar as osmolytes, both of which can inhibit growth (Mazher et al. 2007). Reduction in height may be due to lower cell water potential, which causes the closure of stomata and reduced CO2assimilation.At the end of the experiment in April 2017, the least reduction in growth at the highest salt concentration compared to the control was with C. equisetifolia (13.6%), followed by P. pinnata(17.7%). These species proved to be salt tolerant. Several researchers have suggested that each species may respond to salinity in different ways(Shaybany and Kashirad 1978;Alam et al. 2002; Marosz 2004).

After 6 months when the concentrations of salts were further increased, the level of sugars increased in salt-tolerant species and were either maintained or slightly decreased in salt-sensitive species (Table 2). Under saline stress, the accumulation of sugars along with other compatible solutes contributes to osmotic adjustment (Dubey and Singh 1999). A decrease in sucrose, glucose and total carbohydrate contents was also reported by Roussos et al.(2013)in sour orange plants subjected to NaCl stress.This may be due to the change of leaves from source to sink under adverse conditions (Arbona et al. 2005) or may be due to the reduction in chlorophyll concentration, stomatal closure and/or toxicity induced by salinity (Banuls et al.1997). Salinity causes an increase in the accumulation of sugars in tolerant species that helps to maintain the osmotic potential of cytoplasm of cells, important for the survival of plants under stress (Table 2).

It has been reported that in salt tolerant species, height increased under low salt concentrations but were reduced as concentrations increased (Dantus et al. 2005; Memon et al.2010;Qados 2011).Different climatic conditions may influence plant response to salinity stress, particularly temperature and humidity.

where RLis estimated using the method of Wveatherley(1950), and total soluble sugars were estimated calorimetrically using the phenol and sulphuric acid method(Dubois et al. 1956). Fris fresh weight, Dris dry weight,and Sais saturated weight.

Fig. 1 Effect of different concentrations of NaCl on heights of salt-tolerant and salt-sensitive species.a Salt tolerant trees,b salt sensitive trees

Fig. 2 Effect of different concentrations of NaCl on heights of salt-tolerant and salt-sensitive species.a Salt tolerant trees,b salt sensitive trees

Survival percentage

Salt tolerance is often described on the basis of survival percentage (Marcar et al. 1993 and Sun and Dickinson 1993). In the present study, all species had 100% survival when the plants were irrigated with low levels of salinity,i.e., 30, 40, 50, 60 mM of NaCl up to October 2015.However,when the concentrations were increased(Fig. 3),C.equisetifolia and P.pinnata had 50-60%survival at the end of experiment at the highest salt concentration. A.auriculiformis and C. lanceolatus had 15-35% survival,whereas P. roxburghii only had 5% survival at the highest salt concentration up to April 2017. K. paniculata, F.benjamina,C.fistula,B.purpurea and M.ovalifolia did not survive at the highest salt concentrations beyond December 2016, September 2016, February 2017, August 2016, and February 2017, respectively, which indicates their unsuitability at this concentration (120 mM).

Fig. 3 Survival percentage of salt-tolerant and salt-sensitive species in different salt concentrations. a Salt tolerant trees, b salt sensitive trees

A study by Tomar et al. (2003) on 31 species showed that A. auriculiformis, A. farnesiana (L.)Willd, A. tortilis(Forssk.) Hayne, C. fistula, C. siamea Lam., P. pinnata,Melia azedarach L. and Terminalia arjuna Roxb. ex DC.showed satisfactory early growth and 100% survival after 1 year when supplied with saline irrigation water. They also reported that C. equisetifolia was unable to survive in drought conditions,but in this study,Casuarina was highly tolerant to saline water.

Survival percentage is the most suitable criteria to judge the salinity tolerance of plants. On the basis of this parameter, trees were categorized as salt tolerant and salt sensitive. Salt tolerant trees include: C. equisetifolia, P.pinnata, C. lanceolatus, A. auriculiformis and C. fistula,whereas, salt sensitive trees were P. roxburghii, M. ovalifolia, K. paniculata, F. benjamina and B. purpurea.

Total soluble sugars

The primary product of photosynthesis in higher plants is soluble sugars which play key roles as building blocks of macromolecules controlling growth and development(Gibson 2000; Price et al. 2004). The use of these organic solutes for osmotic adjustment during salt stress will,however, deprive the plant of energy that could otherwise be used for growth.In the present study,total soluble sugar levels varied among species with respect to months and to different concentrations of salts.During the initial stages of the experiment, when low salt concentrations were applied from May to October 2015, in A. auriculiformis, C.lanceolatus, C. equisetifolia, P. pinnata and C. fistula, the total soluble sugar contents decreased with respect to salt treatment (Table 1), as the sugars were utilized for growth and therefore height increased in these species. This is contradictory to earlier reports where sugar concentrations rose with increase in salt levels(Varma 2015;Nisha 2015).This may be due to sugar translocation which is more efficient in these five species,and results in increased sugar glycolysis in the leaves(Strogonov 1962).However,in the other species, sugar levels increase to enable the plants to adjust to the saline environment. In addition, the increase of soluble sugars in the leaves could be associated with decreased growth under salinity.

Table 1 Effect of different concentrations of NaCl on total soluble sugar content(mg/g dry wt.)of salt-tolerant and salt-sensitive species for the period April 2015-October 2015

Table 1 continued

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In intolerant species, low sugar levels or a lack of an increase in sugar levels following the yellowing of leaves,gradually leads to plant death. Singh (2004) reported that an accumulation of sugar lowered the osmotic potential of cells and reduced the loss of turgidity in tolerant species.

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The amount of water that a plant can hold is measured by its RLWC which, with the present results, indicates that RLWC varied with salt treatments, species and months.During the first year when the low salt concentrations were applied, A. auriculiformis, C. lanceolatus, C. equisetifolia,P. pinnata and C. fistula increased RLWC (Fig. 4a) by 5.7%, 4.1, 7.1, 1.4, and 5.7% respectively, whereas in K.paniculata, F. benjamina, P. roxburghii, B. purpurea and M. ovalifolia, RLWC decreased by 26.4, 32.1, 50.5, 13.4 and 18.5% respectively, at the highest salt concentration(60 mM) compared to the control during October 2015.

Table 2 Effect of different concentrations of NaCl on total soluble sugar content(mg/g dry wt.)of salt-tolerant and salt-sensitive species for the period January 2016-April 2017

Table 2 continued

Fig. 4 Effects of different concentrations of NaCl on relative leafV2:Callistemon lanceolatus;V7:Putranjiva roxburghii;V3:Casuarwater content(%)of different species for the period April 2015-Aprilina equisetifolia;V8:Cassia fistula;V4:Koelreutaria paniculata;V9:2017. (Species: V1: Acacia auriculiformis; V6: Pongamia pinnata;Bauhinia purpurea; V5: Ficus benjamina; V10: Millettia ovalifolia)

Relative leaf water content (RLWC)

In the present investigation,growth was more during the rainy season, suggesting that whatever sugar was synthesized was being utilized for growth and hence less sugar accumulated during the rainy season. Optimum temperatures and light intensities during the July rainy season also led to higher photosynthesis and thus more growth.During January, temperatures and light intensities were less than the optimum required for growth, and whatever sugar was synthesized accumulated as osmoprotectants to protect the plant from cold and salt stress. The levels of total soluble sugar were maximum during January, followed by April,July and October.In summer and winter seasons,there was respectively heat and cold stress along with salt stress.Exposure to stresses such as salinity,water deficits and low temperatures leads to increased levels of osmoprotectants(McNeil et al.1999).Machado et al.(2002),while working on citrus, reported that the lowest photosynthetic rates and lowest photo assimilate supplies occurred during the winter when plants have reduced metabolic activity with reduced carbohydrate consumption (Davenport 1990). Under such conditions,an increase in leaf carbohydrate content may be anticipated, this being the result of a low sink demand rather than greater photo assimilate supply (Goldschmidt and Koch 1996).

At the end of the experiment in April 2017,seedlings of K.paniculata,F.benjamina,C.fistula,B.purpurea and M.ovalifolia died earlier at the 120 mM concentration; A.auriculiformis, C. lanceolatus, C. equisetifolia and P.pinnata survived at 120 mM concentration and RLWC decreased by 17.6, 10.8, 10.2, and 9.1% compared to the control (Fig. 4c). This showed that these four ornamental species are salt tolerant up to 120 mM of NaCl. Nonstressed species exhibited higher RLWCs than stressed trees. The reduction of RLWC becomes more severe with the length of time in the stressed plants but may vary with seasons, as in the rainy season, higher RLWCs were observed as compared to summer and winter. Siddique et al. (2000) reported that the cause of higher RLWCs in salt-tolerant cultivars is due to their ability to absorb more water from the soil and compensate for transpiration.

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As determined by survival percentage, salt-tolerant species in this study include:C.equisetifolia,P.pinnata,A.auriculiformis, C. lanceolatus and C. fistula, whereas saltintolerant ones are P. roxburghii, M. ovalifolia, K. paniculata, F. benjamina and B. purpurea. Similar results have been reported for total soluble sugars and RLWC analysis in the leaves.

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Conclusion

The survival ofornamental tree species under salt stress is mainly due to their salt-tolerant character,the maintenance of water potential and the production of osmolytes. In the present investigation, the species differed in their levels of tolerance to salinity.All the species survived under low salt concentrations up to October 2015. Growth, relative leaf water contents, survival,