The effect of salt stress on physiological indicators, essential oil content and composition of Melastomataceae
In recent years, due to the increasingly serious problem of soil salinization, there is an excess of Na+or CL – in the soil. More than 20% of arable land worldwide is affected by salt stress. Soil salinization can affect the germination and growth of plant seeds, photosynthesis, ion balance, and nutrient balance, leading to the inability of plants to grow normally. Therefore, planting salt tolerant plants is not only an effective way to improve the ecological environment of saline alkali land, but also can promote the development of animal husbandry and improve the economic benefits of saline alkali land. Yao et al. found that a certain concentration of compound salt formula can increase the essential oil production of Ocimum basilicum L., but has no significant effect on the six main essential oil components. Feng et al. found that although salt stress can lead to a decrease in the content of aroma components in the essential oil of Rosa rugosa Thunb., it can also increase the amount of aroma components in the essential oil. It can be seen that appropriate salt stress treatment can have varying degrees of impact on the essential oil content and composition of aromatic plants.
Melaleuca bracteata F. Muell belongs to the family Myrtle and is an evergreen tree in the genus Melaleuca. It not only has a wide range of survival adaptability, drought resistance, and salt alkali tolerance, but can also be planted from Hainan Province to areas south or even north of the Yangtze River Basin. It also has high ornamental value and is often used for urban road greening in coastal areas. Melaleuca bracteata leaves are rich in plant essential oils, with a strong aroma. The main components include ethers, esters, hydrocarbons, phenols, alcohols, aldehydes, etc. The highest content of essential oil is methyl eugenol, which has significant essential oil activity and can kill mites and attract insects of the genus Drosophila. It also has the function of attracting Drosophila, Staphylococcus aureus, Pseudomonas, etc. All seven pathogens have significant inhibitory effects. At present, there have been relevant studies both domestically and internationally on the salt alkali stress, cutting seedling cultivation, water stress, and other aspects of Qiancengjin. Dunn et al. investigated the planting performance of tree species in saline alkali land in southeastern Queensland, Australia. The study found that Qiancengjin has high salt tolerance, with a tolerance of up to 1.5 dS/m salinity. Qiu et al.’s research shows that when the cutting substrate is vermiculite, the cutting effect of Qiancengjin is best when the Qiancengjin branches are treated with 200mg/L rooting agent for 3 hours. Hou et al. found that Qiancengjin has strong drought tolerance, which can alleviate drought damage by increasing chlorophyll content under drought stress, and plants will not die under extreme drought. However, there are few reports on the ecological adaptability of Qiancengjin planted in saline alkali land, and the impact of salt stress on the content and composition of Qiancengjin essential oil is not yet clear.
To investigate the growth of Melastomataceae in saline alkali land, this study conducted salt stress treatment on Melastomataceae by setting different salt concentration gradients. The aim was to gain a deeper understanding of the physiological indicators, essential oil content, and composition of Melastomataceae plants under salt stress, and to provide reference for the planting and development of Melastomataceae in saline alkali land.

The area of saline alkali land in China is about 99 million hectares. When a large amount of salt accumulates in the soil, it will cause a series of physical deterioration of the soil, thereby damaging the ecological environment. Excessive salt ions in soil can damage plant cell structure, leading to osmotic stress effects. Some ions can directly poison plants, causing changes in plant morphology and structure, resulting in abnormal plant growth. Under normal growth conditions, due to the accumulation of harmful substances in the plant body and the maintenance of a relatively balanced state between the defense system, the plant grows more robust.
In this experiment, with the increase of NaCl concentration, the trend of proline content in the leaves of Lonicera japonica was consistent with that of the salt tolerant plant Rhizophora stylosa under different concentrations of salt stress. Therefore, it is speculated that appropriate salt content is actually beneficial for the growth of layered gold. However, under low concentration NaCl treatment (100-200mmol/L), the content of malondialdehyde did not show significant changes in the leaves of Lonicera japonica, indicating that at this concentration, the leaves were less damaged by membrane lipid peroxidation and had strong stress resistance. When the NaCl concentration was 300mmol/L, the content of malondialdehyde decreased, which may be related to the regulation of the antioxidant enzyme system. Subsequently, with the increase of NaCl concentration, the content of malondialdehyde continued to rise, indicating that high concentrations of NaCl (400-500 mmol/L) gradually increased the degree of damage to the cell membrane of Lonicera japonica leaves, and also indicating that Lonicera japonica has reached its tolerance limit under salt stress at this concentration. The activity changes of antioxidant enzymes under salt stress are important indicators of plant salt resistance, and POD and CAT are important contents for studying plant stress resistance. The activities of POD and CAT in the leaves of Qiancengjin were higher than those in the control group (CK) when treated with 100-400 mmol/L NaCl, and reached their maximum values under 300 mmol/L NaCl treatment. It can be seen that Qiancengjin is resistant to the damage caused by salt stress and will adjust the two antioxidant enzymes according to the concentration of different salt stress. Overall, it is shown that under salt stress conditions, Qiancengjin can self regulate and exhibit good salt tolerance and antioxidant properties.
Plant essential oils have various biological activities such as antibacterial, antioxidant, and anti-inflammatory properties, and are important secondary metabolites in some plants. Research has shown that rose (Rosa rugosa Thunb.) plants have the highest amount of aromatic components in their essential oils when treated with 50mmol/L NaCl; Basil plants can increase their essential oil content after being treated with appropriate compound salt solution. It can be seen that appropriate salt stress can indeed induce changes in the composition and content of aromatic plant essential oils. This study shows that different concentrations of NaCl treatment have a significant impact on the accumulation of essential oil in the leaves of Lonicera japonica. Under salt concentration treatment of 100-300mmol/L, the essential oil content significantly increases. When the salt stress concentration exceeds the tolerance limit of Lonicera japonica, the growth and development of plants are inhibited, resulting in a decrease in the content of essential oil shed from Lonicera japonica leaves. Therefore, the production of essential oil in Qiancengjin plants can be increased by treating them with 100-300mmol/L NaCl. It is interesting that although the essential oil content in the leaves of Lonicera japonica increased by 22.34%, 22.14%, and 57.83% compared to CK under NaC1 treatment at concentrations of 100-300mmol/L, the components in its leaf essential oil decreased with increasing salt stress concentration, while the content of the main component methyl eugenol in the essential oil did not show significant changes. It can be seen that although salt stress reduced the components in the essential oil of Lonicera japonica japonica leaves, it did not affect its essential oil quality.
In summary, under the treatment of 100-300mmol/L NaCl, Qiancengjin exhibits strong salt tolerance and antioxidant activity. Not only are all physiological indicators normal, but the essential oil content in the leaves also increases with concentration. Therefore, Qiancengjin is expected to become a salt tolerant plant for planting in saline soil, which can not only beautify the environment and improve the ecological environment of saline soil, but also enhance its essential oil production and economic value.