|Other Abstract||Coastal saline land is an important reserve land resource in China with great potential and economic value. There are some problems in coastal saline-alkali land, such as shallow groundwater level with high salinity, high soil salt content, low fertility of clay and heavy soil, lack of fresh water resources, etc. The shortage of fresh water resources and soil salinization are the main constraints of sustainable agricultural development in coastal saline lands. Brackish water irrigation plays an important role in alleviating the shortage of fresh water resources and improving and utilizing saline land. Based on the above background, four treatments of 0 mm (T1), 40 mm (T2), 80 mm (T3) and 120 mm (T4) were set to carry out the field experiment of brackish water irrigation, and the moderate and severe iso-osmotic salt (150 and 300 mM NaCl) and drought (19.3% and 28% PEG-6000) were used to stress with hydroponic protocol, aiming at explore the spatial and temporal distribution of water and salts and soil nutrients in coastal saline soils, elucidate the mechanism honeysuckle (Lonicera japonica Thunb.) growth and development under brackish water irrigation, clarify the main limiting factors affecting honeysuckle yield in saline soils, reveal the change pattern of honeysuckle quality under brackish water irrigation, deeply dissect its salt adaptability by contrasting photosynthesis, photoinhibition and oxidative damage. On this basis, the evaluation system of honeysuckle planting in coastal saline land was established, and the optimal irrigation amount of brackish water was proposed. The main results are as follows:
(1) With the increase of brackish water irrigation, the water content of 0~20 cm soil increased, while both the soil salt content and sodium sorption ratio showed a downward trend. The soil pH increased significantly with increasing amount of irrigation water, and soil alkalinization exhibited. In the 0-20 cm soil surface layer, there was a significant negative correlation between soil conductivity and soil pH value. The distribution of soil water and salt showed obvious seasonal changes. Irrigation in spring reduced soil salt content, while the better leaching of soil salt driven by precipitation in summer. The influence of irrigation amount on soil environmental variation mainly occurred in 0-20 cm soil layer, but had no obvious influence on deep soil layer. Both total soil carbon and ammonium nitrogen showed a significant increasing trend with increasing the irrigation level, whereas nitrate nitrogen decreased tendency with increasing the irrigation level. Besides, excessive irrigation water caused loss of quick-acting potassium from the soil.
(2) Compared with the T1 treatment, brackish water irrigation significantly improved the growth traits such as plant height, base diameter, single plant leaf area index and photosynthetic characteristics such as leaf chlorophyll fluorescence and photosynthetic rate of individual honeysuckle. After 2 years of brackish water irrigation, the dry matter mass of T2, T3, and T4 treatments increased by 97.33%, 408.87%, and 782.09%, respectively, compared to T1 treatment. The relationship between the amount of brackish water irrigation and photosynthetic rate of individual honeysuckle was conic. Brackish water irrigation changed the distribution pattern of honeysuckle biomass, and the root-shoot ratio and root biomass fraction of T1 were larger. To sum up, compared with T1, the leaves of honeysuckle plants irrigated with brackish water had higher photosynthetic performance and greater photosynthetic rate of individual, which was beneficial to the growth and root system development of honeysuckle plants and facilitated the dry matter accumulation of honeysuckle plants, thus providing material guarantee for the high yield of honeysuckle.
(3) Yield and Na+ accumulation of honeysuckle was significantly increased by brackish water irrigation compared to T1 treatment. The pH was a major factor influencing the yield of honeysuckle per plant, which explained 74% and 64% of the variation in the yield during the two growing seasons, respectively. The high water content, low salinity, high pH and high water potential soil environment created by brackish water irrigation was to some extent favorable for the increase of honeysuckle yield. In this study, the content of phenolic compounds such as chlorogenic acid of honeysuckle under T3 and T1 treatments was higher, while the content of phenolic compounds such as chlorogenic acid of honeysuckle under T2 treatment was the lowest. In conclusion, considering the yield, quality, Na+ accumulation in leaves and long-term water use efficiency of leaves, the evaluation system of honeysuckle planting in coastal saline land was established, and it was proposed that the best brackish water irrigation amount for honeysuckle planting in coastal saline land was 80 mm each time.
(4) Photosynthesis was more susceptible to drought stress than iso-osmotic salt stress in honeysuckle according to drought-induced greater decrease in photosynthetic rate. In contrast to salt-induced mild PSⅡ and PSI photoinhibition, severe photosystem II (PSⅡ) and photosystem I (PSI) photoinhibition arose upon iso-osmotic drought stress, indicated by greater decreased the maximal photochemical efficiency of PSⅡ and PSI and remarkable loss of their reaction center proteins. However, PSⅡ and PSI interaction hardly contributed to salt stability of photosynthetic apparatus because of salt-induced finite restriction on electron flow from PSⅡ to PSI. Consistent with photosystems photoinhibition, leaf lipid peroxidation, H2O2 production and electrolyte leakage were elevated much greater by drought stress than iso-salt stress, confirming drought-induced severe oxidative stress in honeysuckle. Furthermore, the principal components analysis comprehensively showed higher salt adaptability in honeysuckle due to larger cluster separation upon drought stress than iso-osmotic salt stress. As an apparent reason, honeysuckle could prevent drought-induced tremendous leaf water loss upon iso-osmotic salt stress, and had a capacity to dispose accumulated Na+. Therefore, honeysuckle resembles halophytes in this respect and seems appropriate for planting in coastal saline land.|