Altitudinal Effects on Osmotic Regulation and Plasma Membrane Permeability of Lonicera caerulea Leaves on the Northern Slope of Changbai Mountain, China

Background: Lonicera caerulea is a perennial deciduous shrub of medical and edible value that is widely distributed on the northern slope of Changbai Mountain. Soil properties and climate parameters at different elevations affect plant growth, but thus far no studies have been conducted on Lonicera caerulea in different elevation gradients on the northern slope of Changbai Mountain. Here, the leaves of Lonicera caerulea, collected from different elevations (800–1800 m) on the northern slope of Changbai Mountain in China, were used as test materials. The aim was to determine the changes in the leaves’ soluble protein content, soluble sugar content, malondialdehyde (MDA) content, superoxide anion content, and plasma membrane permeability along the altitudinal gradient. The leaf-level data were statistically analyzed with respect to various environmental factors (soil properties and climate parameters) to explore the physiological and biochemical mechanisms of their adaptation to mountainous habitats. Results: The soluble protein and soluble sugar contents initially increased and then decreased with greater altitude. The soluble protein content reached its maximum value of 1.84 mg/g at 1400 m while soluble sugar content peaked at 37.40 mg/g at 1600 m. The soluble protein content of the leaves was mainly affected by the total K content of the soil, while the total K content of the soil, total P content of the soil, and organic matter were the main factors explaining their soluble sugar content. The MDA content increased at first, then decreased, and then increased with altitude, for which the lowest values of 5.76 mol/g and 6.29 mol/g occurred at 1000 m and 1600 m, respectively; the MDA content was mainly influenced by hydrolysis N in soil. The superoxide anion content initially decreased and then increased with greater elevation, for which the minimum value of 26 μg/g occurred at 1200 m. The superoxide anion content was mainly driven by the total K and total P contents. The leaves’ plasma membrane permeability initially decreased and then increased with greater elevation; values were lowest at the intermediate elevations, and the minimum value of 0.30% occurred at 1000 m. Plasma membrane permeability was mainly affected by the total K content. Based on their relationships with the environmental factors (soil properties and climate parameters), the soluble protein, soluble sugar, malondialdehyde (MDA), and superoxide anion contents as well as the plasma membrane permeability of Lonicera caerulea leaves are very sensitive to environmental changes. For plants established at differing elevations, the leaf traits appear able to correspondingly adapt to the local habitat. Conclusions: In general, the growth of Lonicera caerulea at low and high altitude areas on the northern slope of Changbai Mountain is easily restricted by various environmental factors, resulting in its poor growth condition there. By contrast, this shrub grows well at mid-elevations; thus, the planting area of Lonicera caerulea should be expanded to those areas to increase this shrub’s fruit yield and quality.