Malus Seedlings With Different Salt Tolerance Biology Essay

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Plants of Na+ exclusion in barley, maize, rice and wheat were believed to reabsorb Na+ from the xylem and accumulate in the roots Nassery and Baker, 1972; Yeo et al., 1977; Yeo and Flowers, 1982; Lü and Wang, 1993; Yang et al., 2002, meanwhile, the roots could also extrude Na+ to the medium. Since the downward transport of Na+ existed in phloem (Lessani and Marschner, 1987), the upward transport of Na+ in xylem could be re-transported to roots through landscape orientation transport of stem base (Jacoby, 1979) or through downward transport of shoot leaves (Levi, 1970). Split-roots experiment of Phaseolus vulgaris showed that the most of Na were downward transported to the main roots and the most of Na arriving in the main roots were extruded to the medium (Jacoby, 1979). Isotope tracer of 22Na also showed that the roots of wheat had higher Na+ extrusion capability under salt stress, and the Na+ extrusion capability of roots even stronger with the increase of NaCl concentration (Wang et al., 2008).

The recretohalophytes had salt gland or salt bladder to excrete Na+ (Zhou et al., 2001), such as Limonium bicolor had salt gland, Glycine soja and Atriplex spongiosa had salt bladder. Salt excretion played an important role in regulating iron balance, maintaining osmotic presser and improving salt tolerance (Zhang et al., 2003). Although salt-tolerant Citrus reticulata Blanco had no salt gland or salt bladder, it could also excrete Na+ from cell clearance and stoma (Liu and Zhang, 1994). Therefore, salinity in plants could either be extruded from roots or excreted from leaves. In this article, we aimed to study the Na+ extrusion of roots and Na+ excretion of leaves in Malus seedlings to provide the theory basis for Na+ exclusion mechanism of fruit trees.

2. MATERIALS AND METHODS

2.1. Plant material cultivation

Salt-sensitive variety Malus baccata (L.) Borkh., middle salt-tolerant variety Malus xiaojinensis Cheng et Jiang and salt tolerant variety Malus zumi Mats of Malus seedlings were tissue culture seedlings. After roots growing, tissue culture seedlings were cultivated in nutrient solution, which were ventilated continuously. Malus seedlings were placed in the glasshouse, day/night temperature averaged 26 ℃/18 ℃, and day/night relative humidity averaged 50%/65%. Light intensity was about 800 molïƒ-m-2ïƒ-s-1 and was 12 hours photoperiod.

2.2. Split-roots experiment

Roots of Malus seedlings were separated into two similar parts and put into two plastic vessels. One plastic vessel was added with nutrient solution and another plastic vessel was added NaCl solution with concentration of 50, 100 and 150 mmol/L. Both nutrient and NaCl solution were all ventilated continuously. 5 days later, took nutrient solution and roots for standby.

2.3. Na+ excretion experiment

Washing the salt of leaves surface with double distillation water. Concentration of NaCl stress was 50, 100 and 150 mmol/L. 10 days later, took young leaves, mature leaves and old leaves to weigh quickly, then washed the leaves with double distillation water of 100 mL.

2.4. Measurement of Na+ content

Putting dry and grind material (10 mg) of roots and leaves in furnace (500 ℃) for 20 hours, adding 1 to 2 drip(s) of dense nitric acid to dissolve ash, fixing the volume to 50 mL, swaying evenly and measuring Na+ content with "Hitachi Z8000 Atomic Spectrometer". Na+ content of solution was measured with same instrument.

2.5. Calculation of Na+ extrusion capability and Na+ excretion capability

Salt-sensitive variety Malus baccata (L.) Borkh., middle salt-tolerant variety Malus xiaojinensis Cheng et Jiang and salt tolerant variety Malus zumi Mats of Malus seedlings were tissue culture seedlings. After roots growing, tissue culture seedlings were cultivated in nutrient solution, which were ventilated continuously. Malus seedlings were placed in the glasshouse, day/night temperature averaged 26 ℃/18 ℃, and day/night relative humidity averaged 50%/65%. Light intensity was about 800 molïƒ-m-2ïƒ-s-1 and was 12 hours photoperiod.

3. RESULTS AND DISCUSSION

3.1. Effect of NaCl stress on Na+ extrusion capability of roots in Malus seedlings

With the increase of NaCl stress concentration, the Na+ extrusion capability of roots in M. xiaojinensis and M. baccata decreased, and that in M. baccata decreased more obviously, while that in M. zumi obviously increased by degrees (P<0.01). Under the same NaCl stress, the Na+ extrusion capability of roots in M. zumi was obviously higher than that in M. xiaojinensis (P<0.01) and M.xiaojinensis obviously higher than that in M. baccata (P<0.01). The average Na+ extrusion capability in M.xiaojinensis was 1.58 folds as that in M. baccata and M. zumi 2.87 folds as that in M. baccata (Figure 1). It indicated that the higher Na+ extrusion capability of roots in salt-tolerant varieties would efficiently reduce the Na+ content of shoot leaves and more salt tolerant.

Figure 1: Effect of NaCl stress on Na+ extrusion capability of roots in Malus seedlings

Data are mean ± SE (n = 3, *P<0.05, **P<0.01, the same as below)

3.2. Effect of NaCl stress on whole recirculation amount of Na+ in Malus seedlings

Seeing from Figure 2, the whole recirculation amount of Na+ in 3 Malus seedlings all increased by degrees with the increase of NaCl stress concentration, and that in M. zumi increased the most (P<0.01). Under the same NaCl stress, the whole recirculation amount of Na+ in M. zumi was obviously higher than that in M. xiaojinensis (P<0.01) and M.xiaojinensis obviously higher than that in M. baccata (P<0.01). The average whole recirculation amount of Na+ in M. xiaojinensis was 1.35 folds as that in M. baccata and M. zumi 1.94 folds as that in M. baccata. It showed that the whole recirculation amount of Na+ in salt-tolerant varieties were obviously higher than that in salt-sensitive one (Figure 2), which would efficiently reduce the Na+ content of shoot leaves in salt-tolerant varieties.

Figure 2: Effect of NaCl stress on whole recirculation amount of Na+ in Malus seedlings

3.3. Effect of NaCl stress on Na+ extrusion percent of whole recirculation of roots in Malus seedlings

With the increase of NaCl stress concentration, the Na+ extrusion percent of whole recirculation of roots in M. xiaojinensis and M. baccata obviously decreased (P<0.05 and P<0.01), while that in M. zumi slightly increased. Under NaCl stress concentration of 50 mmol/L, the Na+ extrusion percent of whole recirculation of roots in M. zumi had no obvious difference with that in M. xiaojinensis (P>0.05), but that in M. zumi and M. xiaojinensis all obviously higher than that in M. baccata (P<0.05). Under NaCl stress concentration of 100 and 150 mmol/L, the Na+ extrusion percent of whole recirculation of roots in M. zumi was obviously higher than that in M. xiaojinensis (P<0.05 and P<0.01) and M. xiaojinensis obviously higher than that in M. baccata (P<0.05) (Figure 3). In split-roots experiment of Jacoby (1979), about 11% of absorbed sodium was released to the mediums by main roots. This split-roots experiment showed that the average Na+ extrusion percent of whole recirculation of roots in M. zumi, M. xiaojinensis and M. baccata was 42.23%, 34.41% and 30.03% respectively (Figure 3), that in salt-tolerant varieties were obviously higher than that in salt-sensitive one, which made roots of salt-tolerant varieties have enough volume to take in Na+ from landscape orientation transport of stem base and downward transport of shoot leaves.

Figure 3: Effect of NaCl stress on Na+ extrusion percent of whole recirculation of roots in Malus seedlings

3.4. Effect of NaCl stress on Na+ excretion capability of leaves in Malus seedlings

Salt excretion could reduce Na+ content of shoot and improve salt tolerance (Zhang et al., 2003). Seeing from Figure 4, the Na+ excretion capability of leaves all increased with the increase of NaCl stress concentration, and that of young and old leaves increased more than that of mature leaves did. The Na+ excretion capability of young and old leaves was obviously higher than that of mature ones, and it was even obviously under NaCl stress concentration of 100 and 150 mmol/L (P<0.01). Under the same NaCl stress, the Na+ excretion capability of young and old leaves in M. zumi and M. xiaojinensis was obviously higher than that in M. baccata. It indicated that the Na+ excretion capability of shoot leaves in salt-tolerant varieties was obviously higher than that in salt-sensitive one, which would benifit to reduce the Na+ content of shoot leaves in salt-tolerant varieties and more salt tolerant.

Figure 4: Effect of NaCl stress on Na+ excretion capability of leaves in Malus seedlings

3.5. Effect of NaCl stress on whole Na+ excretion capability in Malus seedlings

With the increase of NaCl stress concentration, the whole Na+ excretion capability in M. xiaojinensis and M. baccata all obviously increased by degrees (P<0.05 and P<0.01), and that in M. zumi increased the most (P<0.01). Under the same NaCl stress, the whole Na+ excretion capability in M. zumi was obviously higher than that in M. xiaojinensis (P<0.01) and M. xiaojinensis obviously higher than that in M. baccata (P<0.01) (Figure 5). It showed that the whole Na+ excretion capability in salt-tolerant varieties were obviously higher than that in salt-sensitive one.

Figure 5: Effect of NaCl stress on whole Na+ excretion capability in Malus seedlings

3.6. Effect of NaCl stress on Na+ excretion percent of whole Na+ in Malus seedlings

Seeing from Figure 6, the Na+ excretion percent of whole Na+ of old leaves all obviously increased with the increase of NaCl stress concentration (P<0.05 and P<0.01) and that of young leaves obviously increased under 150 mmol/L NaCl stress (P<0.05 and P<0.01); however, that of mature leaves showed no obvious change with the increase of NaCl stress concentration (P>0.05). The Na+ excretion percent of whole Na+ of young and old leaves was obviously higher than that of mature leaves under NaCl stress concentration of 100 and 150 mmol/L (P<0.01). Under the same NaCl stress, the Na+ excretion percent of whole Na+ of young and old leaves in M. zumi and M. xiaojinensis was obviously higher than that in M. baccata (P<0.01), and that of young leaves was similar with that of old leaves in M. xiaojinensis; however, the Na+ excretion percent of whole Na+ of young leaves was the highest in M. zumi. The average Na+ excretion percent of whole Na+ in M. zumi, M. xiaojinensis and M. baccata was 13.01%, 11.02% and 8.11% respectively, that in salt-tolerant varieties were obviously higher than that in salt-sensitive one.

Figure 6: Effect of NaCl stress on Na+ excretion percent of whole Na+ in Malus seedlings

4. CONCLUSION

The salt-tolerant Malus seedlings had higher Na+ extrusion capability of roots and Na+ excretion capability of leaves than the salt-sensitive one, which made the Na+ content of shoot even less and more salt resistant in salt-tolerant Malus seedlings.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the financial support from National Natural Science Foundation (39740027), Peoples Republic of China, and would like to acknowledge the Special Fund for Agro-scientific Research in the Public Interest (201203075) and Key Laboratory of Biology and Genetic Improvement of Horticultural Crops (Nutrition and Physiology), Ministry of Agricultural, Peoples Republic of China.

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