Table 1.
Comparative summary for different methods to measure salt secretion in recretohalophyte.
| Method | Principle of the method | Advantages | Disadvantages | Secreted ions and concentration | Possible driving forces | Applications in recretohalophytes |
|---|---|---|---|---|---|---|
| Leaf disks secretion covered with oil | The method was first developed by Faraday et al. (1986) and Dschida et al. (1992), and recently improved by the use of oil (Yuan et al., 2013). The secreted ion solution by salt glands on single leaf can be isolated in oil droplets. The accurate ion concentration can be obtained in the further steps. | Simple method without a need of special equipment, and usually costs 12–24 h. Resolution is at the level of whole plant or the single leaf. | Cannot provide direct evidence of salt secretion by a single salt gland. | Na+ 20–200 mM, K+ 5–20 mM, Mg2+ 4–11 mM, Ca2+ 0–10 mM, Cl– 20–200 mM, et al. | HKT1, CNGC, NSCC, H+/Cl– symporter | Limonium bicolor (Faraday et al., 1986; Yuan et al., 2013); Avicennia germinans (Dschida et al., 1992) |
| X-ray microanalysis | In X-ray analysis, the ratio of intensity of a target element of unknown concentration to that of an internal standard of known concentration is related to the concentration of the target element. The relative elemental sensitivity of the spectrometer was determined by the analysis of a multi-elemental aqueous standard solution containing V, Co, Zn, Se, and Sr (Sobrado and Greaves, 2000). | The method allows determinations in both materials from plants and animal species and is a useful tool because it provides multi-elemental analysis simultaneously. | Requires special equipment and strict procedure. | Na+ 20–200 mM, K+ 5–10 mM, Ca2+ 1–3 mM, Mg2+ 2–6 mM, SO42- 2–6 mM, et al. | HKT1, CNGC, NSCC | Porteresia coarctata (Flowers et al., 1990); Tamarix aphylla (Storey and Thomson, 1994); Avicennia marina (Sobrado and Greaves, 2000; Chen et al., 2010) |
| Method combined scanning electron microscopy (SEM) with X-ray | The method is based on SEM and X-ray analysis. SEM can give the clear component cells of salt glands and the secreted salts on the salt gland. In addition, X-ray can measure the ions by salt secretion in the specified locations. The structure–function relationships can be explored in-deep. | The method can give the intuitive secreted salt drops without any need for sectioning. Besides, this combined means may give the real-time secretion status of a salt gland. This is the most precise and accurate means to observe salt secretion and measure secreted ions. | Due to the complicated sample preparation procedures, this method required more accurate and efficient operations in order to measure salt secretion rapidly and precisely. | Na+ 20–200 mM, K+ 5–20 mM, et al. | HKT1, CNGC | Chloris gayana (Oi et al., 2013); Cynodon dactylon (Parthasarathy et al., 2015); Limoniastrum guyonianum (Zouhaier et al., 2015) |
| Non-invasive Microsensing System (NMS)-BIO-001A (Younger USA Sci.&Tech) | The ions can be measured by moving an electrode repeatedly between two positions in a predefined excursion (5–30 μm) at a programmable frequency in the range 0.01–10.00 Hz with a range of 0.3–0.5 Hz being typical for many types of electrodes. | The measurement of salt secretion of single salt gland is realized by this method. The salt secretion can be detected real-time. Most of inorganic ions can be measured, usually 5–20 min to measure one ion with simple operation. | NMS detects the net flux of an ion of a salt gland rather than ion efflux. Most of the organic acid anions cannot measure. Salt gland may be destroyed during peeling the epidermis. | Na+ 20–200 mM, K+ 5–20 mM, Cl– 20–200 mM, et al. | HKT1, H+/Cl– symporter | Avicennia marina (Chen et al., 2010); Limonium bicolor (Feng et al., 2014) |
| Nanoscale secondary ion mass spectrometry (SIMS) | This method is operated combined with high-pressure freezing (HPF) and freeze substitution (FS). The bombardment of ions results in the ejection of charged atomic and molecular species from the surface layers of the sample. These secondary ions are then separated on the basis of their mass-to-charge ratio using a high-performance mass spectrometer, and are correlated with their spatial origin to form a chemical image (Smart et al., 2010). | The ions can be measured with spatial resolutions of better than 100 nm. The accurate ion distribution and concentration are showed on a chemical image based on TEM analysis. This method resolved the problems of ion position, distribution and content in situ. | More applications need to be attempted in salt secretion of salt glands. The complicated and strict operating procedures may limit the wide applications. | Na+ 20–200 mM, K+ 10–300 mM in nucleus of salt gland. | HKT1, CNGC | Limonium bicolor (Feng et al., 2015) |
The distinction between the methods is partially arbitrary. The contents are partially referred to Volkov (2015). HKT1, high-affinity K+ transporter 1; CNGC, cyclic nucleotide-gated cation channel; NSCC, non-selective cationic channel.