Growth, physiological characteristics and ion distribution of NaCl stressed Alhagi sparsifolia...

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Growth, physiological characteristics and ion distribu-tion of NaCl stressed Alhagi sparsifolia seedlings

ZENG Jie1,2,3, ZENG FanJiang1,3†, ARNDT S. K.4, GUO HaiFeng1,2,3, YAN HaiLong1,2,3, XING WenJuan1,2,3 & LIU Bo1,2,3 1 Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China; 2 Graduate University of the Chinese Academy of Sciences, Beijing 100049, China; 3 Qira National Field Station of Desert-Oasis Ecosystem, Qira 848300, China; 4 School of Forest and Ecosystem Science, the University of Melbourne, Water Street Creswick, Vic. 3363, Australia

Alhagi sparsifolia is a leguminous perennial desert species that is plays an important role in dune sta-bilization and revegetation of degraded desert ecosystems. We investigated the effects of three differ-ent levels of salinity (50, 150, 250 mmol/L NaCl) on the growth, shoot photosynthetic parameters and salt distribution amongst different plant organs in one-year-old A. sparsifolia seedlings in a pot ex-periment over a 50 d period. The minimum (predawn) and maximum (midday) water potentials of A. sparsifolia seedlings decreased with the increase of external NaCl concentrations as a consequence of the osmotic or water deficit effect of saline solutions outside the roots. Salinity also reduced gas ex-change parameters in A. sparsifolia, with seedlings subjected to salinity having lower photosynthesis rates and reduced stomatal conductances compared to the control. The reductions in photosynthetic rates in high salinity treatments of the A. sparsifolia seedlings were mainly caused by stomatal limita-tion. Consequently plants growing at greater external NaCl concentrations had significantly lower biomass accumulation compared to the control grown at 50 mmol/L. However, plants exposed to higher salinity were able to maintain growth throughout the experiment but allocated a greater proportion of biomass belowground. Plants exposed to higher external salinity levels had increased concentrations of Na+ and Cl− ions in shoots and roots, suggesting that A. sparsifolia seedlings were utilizing Na+ and Cl− as osmolytes to increase the cellular osmolality and decrease their water potential. We observed the greatest NaCl concentrations in the plants treated with 150 mmol/L NaCl indicating that there may be a threshold level of NaCl that can be tolerated by the plants. In conclusion our results indicate that A. sparsifolia seedlings are moderately salt tolerant. Photosynthetic gas exchange parameters were re-duced by greater external salinity but the seedlings maintained substantial photosynthetic rates even under high salinity stress, were able to maintain growth over the 50 d experimental period and showed no signs of salinity toxicity or damage.

Taklamakan Desert, NaCl salinity, Alhagi sparsifolia, growth, physiological indexes

Salinity is a major environmental factor to restrict plant growth and productivity[1], and the mechanisms of plants to tolerate salinity have been intensively studied in the last decades[2–4]. Munns[5] pointed out the concept of a two-phase growth response of plants to salinity. In the first phase the growth of a plant is reduced because of the osmotic effect of the saline solution outside the roots where high external salt concentrations can cause water

stress. The second phase of growth reduction is caused by the accumulation of salts in transpiring leaves and is

Received September 2, 2007; accepted June 2, 2008 doi: 10.1007/s11434-008-6020-5 †Corresponding author (email: [email protected]) Supported by National Natural Science Foundation of China (Grant No. 30670386), Station Foundation of Chinese Academy of Sciences, Science and Technology Key Project of Xinjiang (Grant No. 200633130) and Technology Key Project of Xinjiang (Grant No. 200733144-2)

170 ZENG Jie et al. Chinese Science Bulletin | December 2008 | vol. 53 | Supp. II | 169-176

therefore a salt specific or ion-excess effect of salinity. The balance of the ions in the tissues of plant can be impaired with accumulating salt ions. The excessive uptake of Na+ or Cl− can limit the uptake of other nutri-tional ions such as K+, Ca2+, NO−

3 and H2PO−4

[6]. Excess salt in cellular tissues can also cause changes in the structure of plasma membranes[7] which in turn can lead to damages of the cell metabolism[8]. This can cause de-creases in the activity of photosynthetic enzymes such as PEP-carboxylase and Rubisco, inhibit protein synthesis and therefore negatively affect plant growth[9].

Despite the advances in our understanding of plant salt tolerance there still remain open questions[10]. Many investigations indicated that the water potential decrease in response to greater external salt concentrations causes a stomatal limitation but also non-stomatal limitation that affects photosynthesis in salt stressed plants[11]. However, Bethke et al.[12] pointed out that responses of the water potential and the photosynthetic rates are not always uniform, and that the decreases of water potential might not cause the closure of stomates. Osmotic ad-justment has also been frequently observed in response to salt stress[13,14], resulting in plants to accumulate or-ganic solutes, such as proline, glycine betaine and solu-ble sugars, when growing in saline environments. How-ever, Liu et al.[15] showed that proline accumulation is not always an indicator of salt tolerance but in some plants also a symptom of stress injury.

Alhagi sparsifolia SHAP. is a herbaceous perennial leguminous plant growing in arid and semi-arid regions. Plant commol/Lunities of A. sparsifolia are an important part of the indigenous vegetation in the transition zone between oases and desert in the southern fringe of Ta-kalamakan desert in Western China. These A. sparsifolia commol/Lunities play a significant role in stabilizing sand dunes and supporting the fragile ecosystem in the area[16] and are therefore important in the re-vegetation and restoration of degraded and desertified landscapes. A. sparsifolia has attracted considerable research atten-tion in the last few years[17–19]. In characterizing the ecophysiological characteristics of A. sparsifolia re-search has focused in particular on the water rela-tions[20,21], whilst growth and physiological responses of A. sparsifolia to salinity have not been studied in detail. Many landscapes in the Taklamakan desert are charac-terized by saline soils or a saline ground water table[22] making detailed investigations of the responses of A. sparsifolia to salinity important. Large parts of the A.

sparsifolia vegetation in the foreland of desert oases in the Taklamakan have been destroyed by overuse through local animals and unconscionable utilization, making the oasis susceptible to sand drift of shifting sand dunes. Therefore, there is an urgent need to restore A. sparsifo-lia vegetation and sustain the security of oasis ecosys-tem in this area[16]. Since seedlings are normally used for artificially restoration of vegetation, it is important to characterize the physiological responses of A. sparsifolia seedlings to different levels salinity. The objective of this study was therefore to investigate the effects of dif-ferent levels of salinity on the growth, shoot photosyn-thetic parameters and salt distribution amongst different plant organs in one-year-old A. sparsifolia seedlings in order to assess the level of salt tolerance of the species.

1 Materials and methods 1.1 Study site

The experimental site was located at the foreland of Qira oasis in the southern fringe of Taklimakan desert (37°00′N, 80°43′E). The annual average temperature in the area is 11.9℃, The extreme maximum and minimum temperature is 41.9℃ and −23.9℃. Rainfall is highly erratic and mainly occurs between May and July, with an annual average precipitation of 35.1 mm and an evaporation of 2595.3 mm. The annual average wind speed is 119 m s–1 and high winds frequently cause sand storms and sand drift. There are nine seasonal rivers in Qira oasis, and the water flux of the rivers is subjected to great seasonal oscillation. The surface near ground-water is found in a depth of 1―8 m and is moderately saline (Na+ concentration: 40―50 mmol L−1) in the foreland of the oasis[22]. The vegetation in the Qira oasis desert foreland is sparse with a vegetation cover of 5%― 20% and is dominated by shrubby and sub- shrubby species such as A. sparsifolia and Tamarix ramosissima.

1.2 Experimental methods

(i) Treatment. The study was preformed at the expri-mental site of Qira National Field Station of De-sert-Oasis Ecosystem, CAS. A. sparsifolia seedlings were grown from seeds that were collected in the desert forland in the nursery garden of Qira National Field Sta-tion. In April 2006, one-year-old A. sparsifolia seedlings were planted in 30 plastic pots (50 cm upper diameter, 45 cm lower diameter, 55 cm height). Soil was collected

ZENG Jie et al. Chinese Science Bulletin | December 2008 | vol. 53 | Supp. II | 169-176 171

in the desert foreland nearby the field station and used as substrate for the experiment. The seedlings were well watered after replanting and received 5 L groundwater per pot every 10 d. The salt treatment started on 10 Au-gust, 2006 and plants were grown for 50 days. The groundwater in the study area has a salinity of approxi-mately 40―50 mmol/L Na+ and is usually used for the local restoration and irrigation. Consequently, the treat-ment of the salinity of 50 mmol/L NaCl was regarded as control in the study. We had three salinity treatments: 50 mmol/L, 150 mmol/L and 300 mmol/L NaCl and seed-lings were irrigated with 5 L solution per pot every 10 d with ten replicates per treatment.

(ii) Growth analysis. Destructive harvests were car-ried out after 30 and 50 d of exposure to the salinity treatment and three seedlings per treatment were har-vested at each sample time. The aboveground parts and roots were thoroughly washed with deionised water, all sampled materials (leaf, stem, branches and roots) were oven-dried (80°C for 48 h) and dry weight was recorded. Dried samples were ground into powder and stored for mineral analysis. The root/shoot ratio of the seedlings was calculated as: Dry weight of aboveground biomass / dry weight of belowground biomass.

(iii) Ions analysis. Ground plant material was ex-tracted with hot water (4% w/v; 85℃ for 1 h). Na+ ions were analyzed using an inductively coupled plasma atomic emission spectrometer (ICP-AES, Varian Vista Pro Axial, Varian Australia Pty Ltd, Mulgrave Australia). Cl− ions were analyzed using chemically suppressed ion-chromatography (DX 2500 Dionex Pty Ltd, Lane Cove, Australia)1).

(iv) Water potential. Small shoots that were sampled in the top 25 cm of A. sparsifolia seedings were used to measure predawn water potential and diurnal changes in water potential with a pressure chamber (PMS Instru-ment Company, Albany, Oregon, USA). Diurnal changes of water potential were measured between 08:00 (dawn) and 20:00 (dusk) every two hours using four replicates per treatment. The measurements were made after 10, 30 and 50 d of salt treatment.

(v) Photosynthesis. Photosynthetic shoots (leaves and twigs) in the top 25 cm A. sparsifolia seedlings were selected to measure leaf gas exchange parameters using a portable infrared gas analyzer (LI-6400 LiCor Biosci-

ences, Lincoln, USA). Diurnal changes in leaf gas ex-change parameters were measured between 08:00 (dawn) to 20:00 (dusk) every two hours using three replicates per treatment and were synchronized with the water re-lation measurements. Photosynthetic shoots were har-vested after the measurement to determine the assimila-tive area with a digital scanner (HP Scan Jet 3770) and delta-T scan software (Cambrigde, UK). Shoot gas ex-change parameters were corrected for the determined assimilative area (net photosynthetic rates (Pn), stomatal conductance (gs) and intercellular CO2 concentrations (Ci)). Stomatal limitation values (Ls) were calculated by the way of Berry[23]: Ls=1−Ci /Ca (Ca = CO2 concentra-tion of atmospheric air). All gas exchange measurements were carried out under ambient light intensities and temperatures, with amplitudes of photosynthetic active radiation (PAR) being 0―1400 μmol m−2 s−1, air tem-perature 15―42℃ and ambient atmospheric CO2 con-centration between 360―375 μmol mol−1. There were no statistically significant differences between the measurements at different times and only data measured at 30 d are presented.

(vi) Data analysis. Analysis of variance (One-Way ANOVA) was performed on all measurements with SPSS 13.0. Significant differences between means were determined using the least significant difference test at P<0.05.

2 Results 2.1 Biomass accumulation and distribution

A. sparsifolia seedlings growing at higher salinity had significantly (P<0.05) lower biomass of shoots and roots with the effect more pronounced at higher salt concen-trations (Figure 1). Root to shoot ratio was not influ-enced by salinity after 30 d of treatment, however, at the completion of the experiment plants exposed to greater salinity showed more pronounced reductions in shoot growth, resulting in an increased root : shoot ratio (Fig-ure 2).

2.2 Water relations

Figure 3 shows that the predawn water potential of A. sparsifolia seedlings was influenced by the salinity of the irrigation solution. Even after 10 days of treatment plants treated with 150 or 250 mmol/L NaCl had sig-

1) The data from University of Melbourne.


Figure 1 Aboveground (left) and belowground (right) biomass accumulation in A. sparsifolia seedlings subjected to three different salinity treatments measured 30 and 50 d after start of the treatment; the different letters indicate significant differences in different treatments (LSD, P<0.05); n=3; error bars indicate s.e.

Figure 2 Root to shoot ratio in A. sparsifolia seedlings subjected to three different salinity treatments measured 30 and 50 d after start of the treatment; the different letters indicate significant differences in different treatments (LSD, P<0.05); n=3; error bars indicate s.e.

Figure 3 Predawn water potential of A. sparsifolia seedlings subjected to three different salinity treatments measured 10 d, 30 d and 50 d after start of the treatment; the different letters indicate significant differences in different treatments (LSD, P<0.05); n=4; error bars indicate s.e.

nificantly lower predawn water potentials compared to the 50 mmol/L control, with plants exposed to the highest salinity displaying the most negative water potential. This trend remained consistent during the experimental period with small changes between the individual ob-servation dates.

The same trend was also observed during the diurnal changes of water potential at all times of observation (Figure 4). As a result the plants that were treated with the highest NaCl concentration had the most negative water potentials during the day with minimum water potentials of around −3.5 MPa during midday. In com-parison the control plants had minimum water potentials of around −2.0 MPa during the day and this trend was consistent throughout the experiment.

2.3 Photosynthetic characteristics

Figure 5 shows the diurnal changes of some of the at-mospheric environmental factors observed at day 30 of the experiment that are typical for the experimental pe-riod. The photosynthetically active radiation showed a typical bell-shaped curve with an irradiance maximum at midday. Air temperatures were at a minimum early in the morning and also peaked during the middle of the day. Relative humidity showed an opposing trend com-pared to temperature with the highest RH observed early morning and minimums recorded at midday.

Salinity generally reduced gas exchange parameters in A. sparsifolia seedlings (Figure 6). Seedlings sub-jected to salinity had lower photosynthesis rates and re-duced stomatal conductance compared to the control. This effect was most pronounced during the middle of the day and was more prominent for stomatal conduc-tance. Salinity treated seedlings also had lower internal CO2 concentrations than the control and displayed a greater stomatal limitation. The shoot gas exchange pa-rameters of A. sparsifolia seedlings after 10 and 50 d of treatment showed a similar pattern compared to the one at 30 d (data not shown). However, the net photosyn-thetic rates of A. sparsifolia seedlings tended to decrease


Figure 5 Diurnal changes of irradiance, relative humidity and tempera-ture measured at the experimental site at 30 d.

in the high salinity treatment with the treatment duration. In the 250 mmol/L treatment, the daily average value of net photosynthetic rates of A. sparsifolia seedlings were 5.56 ± 0.47 μmol CO2 m−2 s−1

after 10 d, 5.47 ± 0.19 μmol CO2 m−2 s−1 after 30 d, and 3.92 ± 0.32 μmol CO2

m−2 s−1 after 50 d.

2.4 Distribution of Na+ and Cl− ions

The concentration of Na+ and Cl− ions in different organs of A. sparsifolia seedlings were also influenced by salinity treatments, with seedlings exposed to greater salinity generally displaying a greater concentration of Na+ and Cl− in their organs (Figure 7). The plants accumulated more Na+ and Cl− in aboveground tissues compared to roots in all treatments. In roots of A. sparsifolia seed-lings tended to accumulate more NaCl with increasing salinity whilst in the aboveground tissues plants exposed to the 150 mmol/L NaCl treatment had the greatest con-centration of Na+ and Cl− and the control the lowest.

3 Discussion and conclusion 3.1 Biomass accumulation and distribution

The biomass accumulation of plants subjected to salinity can be used as an integrated parameter for the responses of plants to salinity[24]. Our results demonstrate that greater salinity levels caused significant decreases in

Figure 4 Diurnal changes in water potential of A. spar-sifolia seedlings subjected to three different salinity treat-ments measured 10 d (left), 30 d (middle) and 50 d (right) after start of the treatment; n = 4; error bars indicate s.e.


Figure 6 Photosynthetic assimilation (Pn, top left), stomatal conductance (Gs top right), internal CO2 concentration (Ci bottom left) and stomatal limita-tion (Ls) in A. sparsifolia seedlings exposed to three different salinity concentrations after 30 d of treatment. Statistically significant differences between the treatments are indicated by an asterisk at the top of the graph; LSD, P<0.05; n = 4, error bars indicate s.e.

Figure 7 Concentration (mg ion L−1 plant water) of Na+ (left panel) and Cl− (right panel) in leaves, stems, branches and roots of A. sparsifolia seedlings subjected to three different salinity concentrations for 50 d. biomass accumulation in A. sparsifolia compared to the control grown at 50 mmol/L. However, despite the re-ductions in overall growth there were nonetheless in-creases in biomass in the salinity exposed plants over the study period. Shoot growth was more strongly influ-enced by salinity than root growth which resulted in an increase of the root/shoot ratio of the salinity exposed plants. This indicates a shift in biomass accumulation to enhanced root growth which has often been interpreted as an adaptation of plants to an environmental stress that

allows the plant to have greater to water and nutrient resources[25,26]. However, this reduction in shoot growth and to a lesser extent root growth has been observed in other plant species and was interpreted as an osmotic effect of the saline solution in the soil[8].

3.2 Water relations

Water potential is an important indicator to estimate the water status of plants[27]. Romero et al.[28] suggested that osmotic stress can decrease plant water potential in sa-


line environments. In our experiment the water potential of A. sparsifolia seedlings also decreased with the in-crease of external NaCl concentrations and this was ob-served for minimum (predawn) and maximum (midday) water potentials. Predawn water potential is often used to estimate the degree of the plant water deficit[27]. The more negative water potential in the salinity exposed A. sparsifolia seedlings indicated that the water supply of the plants was impacted by salinity of NaCl. This is a consequence of the osmotic or water deficit effect of saline solutions outside the roots. The greater concentra-tion of osmotically active Na+ and Cl− ions reduces the water potential of the soils and the more negative water potentials in the plants are an adaptation that allows wa-ter uptake from the soil.

3.3 Photosynthetic characteristics

The increase of external NaCl concentrations led to de-creases of all measured shoot gas exchange parameters in A. sparsifolia. According to Matthews et al.[29], os-motic stress caused by high external salt concentrations is the major factor to impact on plant growth in the first phase of salt stress. However, decreases of plant photo-synthetic rates can be caused by stomatal limitation and non-stomatal limitation under osmotic stress. Stomatal limitation is the decrease in stomatal conductance that causes a decrease of intercellular CO2 concentrations, thereby resulting in a reduction in the supply of CO2 in the chloroplast[30]. Our data indicate that the reductions in photosynthetic rates in high salinity treatments of the A. sparsifolia seedlings were mainly caused by stomatal limitation. The reductions in stomatal conductance in the 150 mmol/L and 250 mmol/L NaCl treated seedlings were greater than the observed reductions in photosyn-thetic rates, resulting in increases of stomatal limitation values and increases in water-use efficiency in the salt treated seedlings. The decrease of photosynthetic rates in the high salinity treatment with prolonged duration of the experiment is consistent with the results in other species[31–33].

3.4 Distribution of salt ions

Plants growing in saline environments have different

mechanisms of taking up and distributing salt ions[34,35]. Arndt et al.[36] suggested that A. sparsifolia showed a high degree of ion selectivity by excluding Na+ to main-tain low salt concentrations in leaves. The A. sparsifolia seedlings in our experiment also maintained lower con-centrations of Na+ ions compared to Cl− in most organs and treatments. However, plants exposed to higher ex-ternal salinity levels increased their internal Na+ and Cl− concentrations. Consequently, A. sparsifolia seedlings were utilizing Na+ and Cl− as osmolytes to increase the cellular osmolality and decrease their water potential. The fact that we observed the greatest NaCl concentra-tions in the plants treated with 150 mmol/L NaCl indi-cates that there may be a threshold level of NaCl that can be tolerated by the plants. However, we can con-clude that A. sparsifolia seedlings were able to tolerate the greater outside salinity levels because the differences in photosynthetic indicators and also growth of the seed-lings in the two treatments with greater salinity were small and we did not observe any death or visible dam-ages to seedling tissues. Jin[37] reported that the content of Na+ in the organs of natural perennial A. sparsifolia followed the order of root>stem>leaf. Our study showed that after the 50 d experimental period the concentration of Na+ in the organs of A. sparsifolia seedlings mainly followed the order of branch>stem>leaf> root. It is pos-sible that the roots of mature field grown perennial A. sparsifolia plants have a greater ability to restrict Na+ uptake than the roots of one-year-old seedlings. How-ever, field grown A. sparsifolia usually form large clones of plants with interconnected root systems. I.e. excess salt can be diluted by growing new shoots which was not possible in our experiment.

In conclusion our results indicate that A. sparsifolia seedlings are moderately salt tolerant. Photosynthetic gas exchange parameters were reduced by greater ex-ternal salinity but the seedlings maintained substantial photosynthetic rates even under high salinity stress and were able to maintain growth over the 50 d experimental period. Salinity caused shifts in biomass allocation with more biomass allocated to root growth in the high salin-ity treatments.

The authors thank Prof. Zhang Ximing for his valuable help.

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