It is well known that saline soil, which takes up about 25% of the whole land area, is one of the most widely distributed soils on Earth. In China, the saline area is over 34 million hm², including one million hm² coastal saline soil. Soil salinity refrains the development and growth of most plant species, except for few species like halophytes. To understand the mechanism of salt tolerance, many studies have been carried out on the herbaceous halophytes, the tolerant woody plants and the sensitive and tolerant crop plants, but much less is known concerning the salt susceptibility and tolerance of oak trees. Quercus virginiana and Quercus acutissima are two different origin oak trees. Q. virginiana, is considered as a moderately sensitive tree species to soil salinity according to US Salinity Laboratory classification. It is one of the dominant tree species along southeastern coast of the United States and has the extreme tolerance to high concentration of salt fog. Q. virginiana acorn was introduced to China in 2000 for the first time, and its tolerance to salts has been confirmed through the regional trial along the coastline of southeastern China. Q. acutissima is widely distributed in China and few researches are reported about its response on salt stress.
The response of plants to excess salt is complex and involves some changes in morphology, physiology and metabolism. Among these changes, the variations changes in the biomass allocation and root morphological characteristics of plants are two important adaptive mechanisms under salt stress. In this research, the changes of root morphology and biomass accumulation of 1-year-old seedlings of Q. virginiana and Q. acutissima were investigated under higher concentration (150 mmol/L) and lower concentration (50 mmol/L) of sodium chloride solution treatments. The results showed that the growth of shoots of two oak trees were significantly inhibited under higher concentration of salt stress, while no obvious changes were observed for the growth of shoots of Q. virginiana under lower concentrations of NaCl. The ratio of root to shoot for two oak trees increased under both concentration of NaCl, which was considered as a biomass allocation strategy for a certain class of plants under salt stress.
For the two oak trees, salinity did not induce the significant change on the root biomass accumulation. While the root morphological responses under salt stress were different between two oak trees. For Q. virginiana, the total root length, root surface area and root volume were increased in various degrees under both lower and higher concentration of NaCl, and significant increasement was observed under lower concentration of NaCl, especially for the stimulation of fine roots with the diameter less than 2mm. The fine roots are more effective than the thicker roots in nutrient and water absorption. For Q. acutissima, although its root development and expandation were inhibited under salt stress, no significant difference was observed when compared to that of control. It was also observed that the different patterns of accumulation of Na⁺ and Cl^- in roots for two oak trees. Therefore, according to the biomass allocation strategy and root morphological responses and ions accumulation patterns of two oak trees under salt stress, it may be deduced that the both oak trees have the similar biomass allocation strategy when exposed to salinity, but have totally different responses on the root development and salt ions accumulation. Q. virginiana could expand the root scope for more effective nutrient absorption to relieve the toxicity of redundant salt ions, while Q. acutissima restricted the expanding of roots in salinity environment due to the redundant uptake of salt ions, and thus lessens the further damage to roots.