Phytoremediation mechanism and planting technique of Apocynum in saline soils and arid areas: a review



Apocynum, a genus of plants in the Apocynaceae family, is well recognized for its high level of stress tolerance. It is utilized as a traditional medicine, as a commercial crop, and for the phytoremediation of saline areas that have deteriorated. The cultivation of Apocynum can lower the levels of salt in soils and can also provide livelihood in the form of high-quality fiber in saline areas where arable crop development is almost impossible due to excessive salt toxicity in soil. In order to emphasize the relevance of cultivating Apocynum as an ideal option for vegetation restoration in salty soils in the dry region, this review set out to critically investigate the phytoremediation mechanism and the propagation methods of Apocynum species.

Apocynum has long been known to the peoples of Central Asia and Kazakhstan as a bast fiber plant. Traditional Chinese plants known as luobuma include Apocynum venetum L. and Apocynum pictum Schrenk, which are also used as fiber crops. In Central Asia and Kazakhstan, people used the fibers of wild Apocynum to make fishing lines, rope, knitting, clothing, and other products as early as the 19th century. In some areas, Apocynum is still used for this purpose today. In economic terms, Apocynum is a precious plant. Its stems have bast fibers with high fiber content. The fibers of the plant are of high technological quality. It is much stronger than the fibers of flax, hemp, hemp miscanthus, and other bast crops and does not rot. Apocynum is not only a resource plant with high commercial worth but also an excellent substitute for reforestation and the control of desertification on saline-alkaline land. In brief, it has a phytoremediation mechanism. Additionally, the effective adoption and use of various sexual and asexual propagation strategies also contribute significantly to the cultivation success of Apocynum in saline soils. In conclusion, there are many opportunities for advancing the Apocynum industry through the use of saline-alkali soils and research into Apocynum resource conservation techniques and cultivation technology in dry environments.

Phytoremediation

Phytoremediation (phytoremediation) is the use of plants and associated micro-organisms for environmental clean-up. This technology uses natural processes through which plants and rhizosphere microorganisms degrade and accumulate various pollutants.

Phytoremediation includes absolutely the whole spectrum of metabolic processes on absorption and accumulation of heavy metals. Accordingly, when developing the technology for phytoremediation of soils from heavy metals, the focus is on the selection of plants capable of transforming them through the plant organism, converting them into a less mobile and active form [1].

According to reports, A. venetum accumulated higher concentrations of lithium in its stem and leaves—more than 950 mg/kg and 1800 mg/kg, respectively—without suffering any negative impacts. This was more than what was found for some Asteraceae and Solanaceae families, which are particularly known to accumulate more Li, such as Circum avense and Solanum dulcimer, respectively. Li concentrations in plant roots and soil, known as the bio-concentration factor and the translocation factor, respectively, were found to be higher than 1. Lithium is discovered to be distributed throughout the cell components, with the majority of it being in the vacuole and the remainder in the cell wall, plastids, nucleus, and mitochondrion. The existence of numerous metal ligands, such as proteins, organic acids, and sugars, which have an affinity to bind with metal ions, may not be unrelated to this [2].

Propagation and planting methods

Natural propagation and the germination process

Kendyr is a perennial plant with annual shoots that die off each year. The underground organs are perennial — the rhizome and roots, from the buds of which new shoots grow.

In nature, Kendyr is propagated vegetatively, i.e. by separating shoots from their roots. Rarely, it is propagated by seeds. The plant's small seeds have a feathery calyx and are easily dispersed by wind and water. If the seeds get into a good environment, they will form sprouts and conquer new territories. It seems that with such a high adaptation to seed reproduction, Kendyr should reproduce quickly and colonize the environment. This is not the case under natural conditions, as only a few seeds carried by wind and water reach optimum conditions for germination. Such conditions are moist, fresh sandy, and muddy alluvial deposits and loose, well-aerated, moist soils. Kendyr plants that sprout from seed on alluvial soils reach only a low height of 20–25 cm. But they develop a very strong root system. From the 2nd or 3rd year, the plants begin to bear fruit [2].

Such locations are only accessible along riverbanks following flood disasters. Following germination, seedlings grow slowly above ground while making a significant investment in their root systems, as is usual for phreatophytes. The establishment of the seedlings by accessing constant access to the groundwater presents a bottleneck for generative selection, despite the ease with which the seeds develop. In nature, Kendyr is difficult to establish a specific preference for one soil type or another. It grows on sandy, clayey, willowy marshes, salt marshes, and other soils [2].

From observations of the Kendyr in the wild, we can conclude that the optimum conditions for its growth and development are: increased temperature during the growing season, plenty of sunlight, and increased soil moisture.

The findings showed that, particularly with abiotic stresses, extending the storage period greatly reduced the germination percentage and velocity. A. venetum and A. pictum's embryo germination was unaffected by light in any situation. At 10/25 and 15/30 C, seeds germinated more successfully than when they were incubated in any other setting. Seed germination for each species progressively reduced with rising polyethylene glycol (PEG) and salinity concentrations, particularly for seeds kept for longer than two years. Recently developed seeds' germination percentage was not substantially impacted by low PEG (0–20 %) or salinity concentration (0–200 mM). Nevertheless, throughout the germination stage, long-term storage greatly reduced the aridity and tolerance to salinity of A. venetum and A. pictum [3].

During the 38 days, the concentration of NaCl was increased from 0,2 % to 0,4 % which obviously impact the development of the Venetum plant length. The length decreased from 5.7 cm to 2.7. During the experiment, all the seedlings emerged after 28 days. With a concentration of salt of about 2 % Apocynum Pictum’s germination level is twice higher than Apocynum Venetum which proves the tolerance ability of the Apocynum Pictum to salinity [4].

Cultivation and planting techniques

Direct seeding

Direct sowing is a method for growing Apocynum. In this method of propagation, sandy loam that has minimal salinity and alkali content is appropriate (Table 1). Two essential elements that can affect seed germination and seedling establishment for direct seeding are soil water level and seed longevity. Apocynum seeds remain viable for at least three years, and the most advantageous seed is one that has just finished maturing. For straight seeding, early spring is the ideal time to plant. The ground has been thoroughly prepared, and it is level. The mixture of Apocynum seeds and wet sand was spread at a ratio of 1:10 after the area had been drip-irrigated. Apocynum is typically planted with 6–8 kg/ha of seeds, with a row spacing of 0.8–1.0 m. [5].

Table 3

Key techniques and matters needing attention for planting inland saline land and coastal saline land

Land type	Inland saline land		Coastal saline land
Water suppy	Insufficient water	Sufficient water	Sufficient water
Watering method	Drip irrigation	Flooding	Flooding
Species	Apocynumpictum Schrenk	Apocynumpictum Schrenk or Apocynumvenetum L.	Apocynumvenetum L.
Propagation method	Direct seeding: Freshly matured seeds of A. pictum is the best choice for this method. The survival rate of the seedling is higher than 80 %, and the highest survival rate of the seedling is 95 %, equal to the survival rate of wild seedlings.	Seedling transplanting: The climate is arid, evaporation is intense, and salt loam should be hardened. The effect of direct seeding is poor. Seedling transplantation can be carried out. Seedbed should be loose and fertile loam soil with sufficient base fertilizer.	Seedling transplanting: The seed size of A. venture is small. Seedlings are not easy to emerge from and are not resistant to waterlogging. In coastal areas with heavy salt and alkali, the transplantation of seedlings raised in greenhouses is beneficial to improve the survival rate.
References	Zhang, 2002; Zhang et al., 2005; Jiang Jiang et al., 2018; et al., 2018; Ma et al., Ma et al., 2019 2019		Liu et al., 2010; Chen et al., 2012; Jiang et al., 2018

Seedling transplanting

Young plants can be moved from the nursery to the field using the effective and popular seedling transplantation technique. Production of seedlings and nursery administration are the first steps. A solution of potassium permanganate (0.5 %) is used to sanitize Apocynum seeds for two hours before rinsing them in tap water. Seeds are kept for 12 hours at 20–25 °C. The seeds are then planted in germination plates that have a peat vermiculite blend (3:1 v/v) in them. Each seedling cup can hold two seeds, each sown to a depth of 0.5 centimeters. The ideal relative humidity (RH) for incubation is 45–60 % on average, and the ideal incubation temperature is 15–30 °C. Transplanting seedlings is the next stage Picture 1.

a) b)

Fig. 1. The experimental process of cultivating Apocynum at the Kazakh National Agrarian University. a) planting of the Apocynum in the substrate b) transplanting of the plant to the soil

The best period to transplant is in the spring (March to May) or the fall (September to November). The height for transplanting seedlings is 10–12 centimeters. 6–9 centimeters is the planting depth. After transplanting the seedlings, the third stage is management. No more than 380 m3 ha 1 of water should be provided by drip irrigation at a time. The areas need to have the weeds pulled out. A faster method to increase the cultivation area is by transplanting seedlings, but this has the drawback of taking a lot of time and money [6].

Cutting

The stems are treated using this technique. This approach consists of four stages. Initially, between mid-June and mid-July, pick strong Apocynum branches and cut them into sections (10–15 centimeters). The pieces are then sterilized for 2–3 minutes with 0.1 % corrosive sublimate, rinsed with tap water, and then soaked in 30 °C water for 24 hours. Dip the clippings into the ABT rooting powder when they're just damp. Third, plant cuttings at a depth of 2 to 5 centimeters. Fourth, maintaining slight moisture on the ground. Cuttings are appropriate for large-scale propagation, but their high cost is their primary drawback. The primary process is accidental root formation, and the soil water level is a crucial element [7].

The roots are treated using this technique. First, the roots and aboveground components of the mature Apocynum plants are dug up and removed.

Second, strong Apocynum roots with two to three blooms are chosen for a division. Planting depth and timing is essential for the adoption of this method to be effective. 10 centimeters is the ideal planting depth. The best time to sow is in the early spring or late fall [8].

Conclusion

In conclusion, phytoremediation is the use of plants and associated micro-organisms for environmental clean-up, using natural processes to degrade and accumulate pollutants which we can observe in the Apocynum plant. According to reports, A. venetum accumulated higher concentrations of lithium in its stem and leaves than other Asteraceae and Solanaceae families. Lithium is distributed throughout the cell components, with the majority in the vacuole and the remainder in the cell wall, plastids, nucleus, and mitochondrion, and from the cultivating technics we found that seed propagation is the most efficient and suitable method for mass production. Seedlings are grown in a nursery and planted in plantations.

In the second year, the growth of the plant usually begins in late March or early April. The buds of the previous year begin to grow vigorously, develop quickly, and the shoots penetrate the soil surface and emerge as thin arrows — shoots. But still, in there are several questions on behalf of irrigation as how much water is needed for crop production and also the process of degemination consumes much water which is not available in the areas where we observe water shortage we can say that in this case, it is not enviromentally friendly.

References:

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