Effects of salt and phosphorus stress on seed germination and seedling growth of Casuarina equisetifolia and Acacia confusa

Artificial coastal protection forests are difficult to manage because of nutrient stress in harsh environment. To explore the effects of salt and phosphorus stress on the seed germination and seedling growth of Casuarina equisetifolia and Acacia confusa, we respectively watered the seeds and seedlings of the two kinds of plants with the solutions of NaCl (salt) and KH2PO4 (phosphorus) in different concentrations, and then determined their growth indicators, respectively. The results were as follows: (1) High salt concentration solutions significantly inhibited seed germination and had a certain effect on the growth of seedlings, however, the effects of salt stress on seed gemination and growth were different. The salt tolerance of Acacia confusa seed germination was higher than that of Casuarina equisetifolia. The biggest relative salt damage rate of Acacia confusa was 23.03% and that of Casuarina equisetifolia was 89.15%. As the concentration of NaCl increased, the germination rate, germination potential, germination index and vitality index of Casuarina equisetifolia and Acacia confusa seeds decreased. And the maximums of the four germination indexes of Casuarina equisetifolia seed were 38.70%, 34.67%, 18.70 and 0.055, and of Acacia confusa seeds were 76.67%, 62.22%, 48.46 and 6.11, respectively. (2) The height and root length of the plants decreased with the increase of salt concentration. The plant heights of Casuarina equisetifolia and Acacia confusa were 12.29 to 6.01 mm and 48.27 to 17.33 mm, and the root lengths were 8.57 to 1.45 mm and 33.41 to 5.88 mm, respectively. The biomass of root, stem and leaf as well as root shoot ratio of Acacia confusa gradually decreased with the increase of salt concentration, but the differences between the treatments of Casuarina equisetifolia were not obvious. 0.6% and 0.4% salt stress were the thresholds for seed germination and seedling growth of the two plants, respectively. (3)The seed and seedling of Acacia confusa were more tolerant to low-phosphorus environment than Casuarina equisetifolia, and there was a difference in the optimum phosphorus concentration between the two. When the phosphorus concentration was 0.1 mmol·L-1, the seed germination rate(49.33%), germination index (23.12%) and vitality index(0.093) of Casuarina equisetifolia, as well as the biomass of root, stem and leaf, all reached the maximum. In the 5 mmol·L-1 treatment, the maximums of germination rate(81.11%), germination potential(62.22%), gemination index(38.23), vitality index(5.07), plant height (54.48 mm) and root length (37.16 mm) of Acacia confusa seeds all appeared, while their germination was inhibited in the treatment of 10 mmol·L-1. Therefore, when sowing the seed of coastal defense forest or planting its seedling, the sowing or planting mode must be selected according to the soil salt and phosphorus contents.