莳萝蒿适应盐渍环境的Na+区域化方式和生理特征研究

Abstract

莳萝蒿是广泛分布在我国北方的一种特殊类型的菊科盐生植物,阐明莳萝蒿特殊的耐盐机制和生理特征有助于丰富植物抗盐性研究的内容.用0、100、200、300、400 mmol/L NaCl处理莳萝蒿7 d后,比较莳萝蒿盐处理植株与对照植株在生长和生理方面的差异,并详细分析了Na⁺在莳萝蒿体内的积累水平和区域化方式.结果显示:莳萝蒿虽然能够耐受400 mmol/L NaCl,但盐处理显著抑制了莳萝蒿的生长,整株鲜重随着盐处理浓度的升高逐渐减小.在水分生理方面,随着盐处理浓度的升高,莳萝蒿叶片细胞的渗透调节能力逐渐增强,其叶片肉质化程度却呈逐渐降低的趋势.分析盐处理对光合作用的影响发现,盐处理后莳萝蒿叶片光合速率与气孔导度显著下降,而其PSⅡ光化学活性并未受到抑制,叶绿素含量甚至逐渐增大,说明盐处理后莳萝蒿叶片光合速率的降低主要是由于气孔因素造成的,而不是由于光合结构被破坏.莳萝蒿体内的Na⁺含量随着盐处理浓度的升高显著增加,400 mmol/L NaCl条件下叶、茎、根中的Na⁺含量分别高达321.4、242.1和182.3 μmol/g鲜重;莳萝蒿体内的Na⁺ 70%以上积累在叶片内,而叶片内98%左右的Na⁺积累在叶片原生质体中,叶片原生质体中的Na⁺平均浓度是质外体1.2-1.8倍,推测其叶片细胞内存在着有效的Na⁺区域化机制.盐处理后莳萝蒿叶片液泡膜V-H⁺-ATPase的质子泵活性比对照增加了30%-50%,液泡膜Na⁺/H⁺逆向转运活性则增加至对照的4-7倍,进一步证实莳萝蒿叶片具有较强的液泡Na⁺区域化能力.随着盐处理浓度的升高,Na⁺在叶片中的分布比例相对减少,V-H⁺-ATPase的质子泵活性和Na⁺/H⁺逆向转运活性增幅也减缓.这种Na⁺区域化能力使莳萝蒿获得了较强的耐盐性,有效保护了其光系统,降低了细胞汁液渗透势.但是盐处理后这种耐盐方式并不能阻止莳萝蒿叶片肉质化程度和光合活性下降,莳萝蒿生长仍然受盐抑制,说明Na⁺区域化是莳萝蒿适应盐渍环境的必要条件而非充分条件.;<em>Artemisia anethifolia </em>(<em>Compositae</em>) is a halophyte that is widely distributed in northern areas of China. Studying the mechanisms by which this plant adapts to high levels of salt will increase our understanding of salt adaptation in vascular plants. <em>A. anethifolia</em> plants were treated with 0, 100, 200, 300, and 400 mmol/L NaCl for 7 d, respectively. Then, the differences in growth and physiology were compared between salt-treated and control <em>A. anethifolia </em>plants. In particular, the Na⁺ accumulation levels and Na⁺ compartmentation patterns in <em>A. anethifolia</em> plants were analyzed in detail. Although the <em>A. anethifolia</em> plants were able to survive under 400 mmol/L NaCl, salt-treated plants showed lower fresh weight as the NaCl concentration increased, indicating that plant growth was inhibited by salt. The leaves of <em>A. anethifolia </em>showed an increased capacity for osmotic adjustment but a decreased degree of leaf succulence with increasing NaCl concentrations. Photosynthetic analyses showed that there was a gradual decline in net photosynthetic rate and stomatal conductance of salt-treated leaves with increasing NaCl concentrations. However, the maximal efficiency of photosystem II photochemistry (<em>F</em>_v/<em>F</em>_m) in <em>A. anethifolia</em> leaves was not inhibited by salt, and the chlorophyll content even increased with increasing salt concentrations. These observations suggested that the decreased photosynthetic rate was due to stomatal factors, rather than damage to the components of the photosynthetic machinery. The Na⁺ content in <em>A. anethifolia</em> plants tended to increase with increasing salt concentrations. The Na⁺ content in leaves, stems, and roots of <em>A. anethifolia</em> was 321.4, 242.1, and 182.3 μmol/g FW, respectively, in the 400 mmol/L NaCl treatment. More than 70% of the Na⁺ absorbed by salt-treated <em>A. anethifolia</em> plants accumulated in their leaves, and approximately 98% of the Na⁺ that accumulated in leaves was localized in leaf protoplasts. The average Na⁺ concentration in protoplasts of <em>A. anethifolia </em>leaf tissue was 1.2-1.8 times that in the apoplast. These results indicate that efficient Na⁺ compartmentation occurred in <em>A. anethifolia</em> leaf cells. The V-H⁺-ATPase proton pump activity of salt-treated leaves was 30%-50% higher than that of control leaves, and the tonoplast Na⁺/H⁺ antiporter activity of salt-treated leaves was 4-7 times that in leaves of control plants. These findings suggested that <em>A. anethifolia</em> plants have a strong ability to compartmentalize Na⁺ in the vacuole. As the concentrations of salt increased, the Na⁺ distribution ratio in leaves decreased. Likewise, the range of increased V-H⁺-ATPase proton pump activity and tonoplast Na⁺/H⁺ antiporter activity decreased as the salt concentration increased. Na⁺compartmentation plays an important role in the salt tolerance of <em>A. anethifolia</em> plants, because it protects their photosystems and results in lower osmotic potential in the leaf cells. However, Na⁺ compartmentation could not prevent the decrease in the degree of leaf succulence and the photosynthetic activity of <em>A. anethifolia </em>leaves under highly saline conditions, and so their growth was inhibited by salt. These results suggest that Na⁺ compartmentation is necessary but not sufficient for adaptation of <em>A. anethifolia</em> to a saline environment.