Testing S. oblata plants had grown for more than five years in soda soil at the trial base in the suburb of Baicheng, China. Control S. oblata plant materials were taken from Zoological and Botanical Garden of Changchun City, China.

The soil samples of the trial base were taken from five evenly distributed sites on the land where testing S. oblata plants were grown at depths of 0 - 10 cm and 20 - 40 cm. The soil of the same depth were fully mixed, air-dried at a shady place, pulverized with a clean glass bottle, and bottled for examine. Control soil samples were taken from the nearby of control plants roots at depths of 0 - 10 cm and 20 - 40 cm, and were pretreated with the above-mentioned methods. Both soil samples were taken on a fine day at the beginning of March, July and November. Soil samples were sent to the Research Center of Analysis and Measurement of Northeast Normal University to measure the pH and electric conductivity of soil solutions. The electric conductivity of soil solutions, in which the ratio of soil to distilled water is 1:5 in weight, was detected with a conductivity meter (SevenGo SG3, METTLER TOLEDO, Switzerland).

Fully expanded leaves of both S. oblata plants were cut into small pieces with a razor blade. These pieces were first fixed in 2.5% glaraldehyde in 0.1 mol/L phosphate buffer, pH 7.2, under vacuum, for more than 24 h at room temperature, and then post-fixed in 1% osmium tetroxide in the same buffer for 2 h at room temperature. After dehydration in a graded series of ethanol solutions and in acetone, leaf pieces were embedded in Epon 812 resin. Ultra-thin sections were cut with glass knives on a Reichert-Jung Ultracut-E ultramicrotome, stained with uranyl acetate and lead citrate, and examined with a HITACHI H-600 transmission electron microscope. The Motic Image Advanced software was used for determining the areas of chloroplasts and storage inclusions and the diameter of the plastoglobules.

According to determination, the soda soil of the trial base in the suburb of Baicheng is a kind alkalinized soil with high pH value, and the soil from Zoological and Botanical Garden in Changchun belongs to neutral soil (Table 1).

The longitudinal section of the chloroplast is oval in shape, with the major axis parallel to that of the cell (Figure 1(A)). The Chloroplast matrix is dense, with an electron density similar to that of thylakoids. Both the granal and stromal membranes are sometimes slightly expanded (Figure 1(B) and Figure 1(C)). Mitochondria of different sizes were observed near chloroplasts (Figure 1(C)).

In contrast with the chloroplasts of general mesophyll cells, which almost contained no starch grains, the chloroplasts of the mesophyll cells near the vascular bundle had big starch grains (Figure 1(D) and Figure 1(E)). The accumulation of starch grains near vascular bundle supports the transport of the products of photosynthesis from leaves to other organs via the vascular tissue.

Large amounts of plastoglobules accumulated in chloroplasts (Figure 1(A) and Figure 1(C)). Although Plastoglobules are primarily round in shape, oval ones were also observed (Figure 1(C)). Inconsiderable difference in electron density between plastoglobules may depend on the thickness of ultrathin sections. There were numerous contacts between plastoglobules, plastoglobules and thylakoids (Figure 1(B) and Figure 1(C)). Cytoplasmic globules with the similar electron densities to plastoglobules were observed near chloroplasts (Figure 1(C)).

Values are mean ± s.d., n = 3. *EC: electric conductivity of soil solution. The ratio of soil to distilled water is 1:5 in weight. A: soil sample from the trial base in the suburb of Baicheng, China. B: soil sample from the Zoological and Botanical Garden in Changchun, China.

Chloroplasts are in the cytoplasm, which was separated from the cell wall (Figure 2(A)). The chloroplasts were smaller than those of the control plants (Table 2). The chloroplast somewhat rounded up, especially of the side toward the vacuole (Figure 2(A)). Sometimes, the envelope of chloroplasts bulged out to form protrusions comprised plastoglobules (Figure 2(B)). Most chloroplasts had well-developed thylakoids system with regular granal stacking (Figure 2(C)); in contrast, some chloroplasts expanded destructively, and their thylakoids system could not be discerned (Figure 2(D) and Figure 2(E)).

*Values are mean ± s.d., n = 30. **For 100 chloroplasts samples.

As compared to S. oblata growing in neutral soil, the starch grains content of chloroplasts considerably increased in S. oblata plants growing in soda soil (Figure 2(B)) (Table 2). Starch grains tended to accumulate at the chloroplast side close to the vacuole; probably due to this fact, the chloroplasts slightly projected towards the tonoplast (Figure 2(A)). Starch grains of different electron densities were observed in different chloroplasts; some of them were almost electron transparent (Figures 2(A)-(D)).

Compared with control plants, the plastoglobules of soda-stressed plants increased in volume, but decreased in number; the total content of plastoglobules did not change notably (Table 2). Plastoglobules with different electron densities were observed in chloroplast matrix (Figure 2(B), Figure 2(E) and Figure 2(F)). Within some dark plastoglobules, there were electron-transparent zones of irregular shape (Figure 2(E)). Contacts between plastoglobules were clear showed (Figure 2(E) and Figure 2(F)).

Smaller chloroplasts could decrease the photosynthesis rate of S. oblata growing in soda soil. The slight expansion of chloroplasts envelopes of soda-stressed S. oblata might be due to the osmotic stress induced by salinity and drought [11] . The maintenance of well-developed thylakoids system, which was necessary for the growth of plants, manifested the adaptability of S. oblata to soda soil after growing in it for a long period. The destructive expansion of some chloroplasts decreased the number of functional chloroplasts, and consequently blocked the plant growth to some extent.

The differences between the soda-stressed and control S. oblata plants were primarily related to the storage inclusions, especially to the content of starch grains. Hydrolysis of starch could produce sugars, which raised the osmotic pressure in the vacuoles and promoted water influx. Under stress conditions, sugars always accumulate as cell protectors, and starch serves as a specific sugar depot, which is important for its osmoprotective function [12] . Therefore, the capacity of S. oblata plant to accumulate large amounts of starch grains in the chloroplasts might promote its survival under soda stress. The inclination of starch grains to accumulate at the chloroplast side facing the tonoplast probably facilitated the entry of sugars into the vacuole. Various electron densities of starch grains were probably due to the dissimilar degrees of starch hydrolysis; and the electron-transparent areas of starch grains might be the cavities left after starch lysis.

As a storage form of the outcome of photosynthesis, starch is an important carbon bank in plant. Although the vast amounts of starch grains accumulated in chloroplasts provided a steady source of sugars for plants under soda stress, they also occupied lots of carbon resources that did not, at least temporarily not, participate in the carbon metabolism, and consequently slowed down the growth of plants to a degree [13] .

Plastoglobules were observed in chloroplasts of both soda-stressed and control S. oblata. Plastoglobules consist mainly of lipids, probably due to which they are primarily round in shape; in addition, plastoglobules also contain proteins, pigments and wax [14] . The well-preserved membranes of thylakoids system in chloroplasts suggest that the accumulation of plastoglobules did not result from the destruction of membranes. Lipids could be synthesized in chloroplasts, and moved into the cytoplasm later [15] . The chloroplast protrusions that consist of plastoglobules probably represented the midway step of the plastoglobules exit from the chloroplast into the cytoplasm. The similar electron density of plastoglobules and cytoplasmic globules might be due to the same origin of them.

It was presumed that the initial plastoglobules occurred at the thylakoid cavities; and when they later moved into the chloroplast matrix, their contacts with the thylakoids were not broken [16] . The contacts between different plastoglobules probably indicated the future fusing of them.

It was reported that salinity and water stress could induce the changes of phospholipid composition [17] [18] . Therefore, we presumed that it was the qualitative changes in plastoglobules that caused the appearance of various plastoglobules with different electron densities under soda stress.