模拟酸雨对小黑杨幼苗生长和光合特性的影响

目的 研究小黑杨(Populus simonii × P. nigra)幼苗生长、生理特性和叶片光系统Ⅱ(PSⅡ)活性对酸雨胁迫的响应情况,为其在酸雨沉降日趋频发的北方地区栽植提供参考。方法 以小黑杨为研究材料,对其连续喷施不同酸度pH为2.5、4.5和7.0(CK)]的模拟酸雨,待小黑杨叶片出现明显病症时,测定其生长、叶片超微结构、抗氧化酶和光系统Ⅱ活性指标。结果 与对照相比,pH 4.5的模拟酸雨处理下小黑杨株高、茎粗、叶片数和叶面积显著增加;叶片颜色变浅绿色,但叶绿体结构紧凑,类囊体排列规则,未见淀粉粒,嗜锇滴数量略增多。而pH 2.5模拟酸雨处理的小黑杨幼苗茎粗和叶面积显著降低;叶片颜色变黄、叶尖和叶脉出现明显斑点,且细胞膜局部破损,叶绿体体积增大,类囊体间隙增大,可见少量淀粉粒,嗜锇滴数量和体积明显增加。模拟酸雨处理的小黑杨叶片中超氧阴离子 {\mathrm{O}}_2^{\stackrel{-}{\underset{·}{\mathrm{}}}}产生速率、过氧化氢(H₂O₂)含量和过氧化氢酶活性较CK显著增加,且酸度越大,增幅越大;而过氧化物酶活性和超氧化物歧化酶活性随酸度的增大表现为先升后降。其中,pH 2.5模拟酸雨处理杨树叶片的可变荧光强度(V_L、V_K、V_J、V_I)和诱导曲线初始斜率(M_O)显著高于CK,但叶绿素荧光曲线面积(Fix Area)和质体醌库的大小(S_m)显著低于CK。pH 4.5模拟酸雨处理中,仅V_L和V_K显著高于CK,而其他荧光参数与CK差异不显著。结论 酸雨对小黑杨生长的影响主要取决于其pH,pH 2.5模拟酸雨处理抑制小黑杨生长,并且叶片出现坏死斑点症状时,叶绿体结构异常,类囊体排列不规整,光系统Ⅱ电子传递受到抑制;而pH 4.5模拟酸雨处理促进小黑杨生长、叶片超微结构表现正常,小黑杨幼苗对pH 4.5酸雨沉降具有一定抗性。

Effects of simulated acid rain on growth and photosynthetic physiological characteristics of Populus simonii ×P. nigra

【Objective】 Populus simonii × P. nigra belongs to the genus Populus, family Salicaceae, and grows in northern China. This hybrid tree grows fast, reproduces easily, develops quickly, and has strong adaptability. It has been reported that P. simonii × P. nigra has biological characteristics that contribute to various types of resistance, such as to drought, barren conditions and alkali salt. This is an important fast-growing and high-yield timber species in China. Acid rain has negative impacts on the vegetation of ecological systems and is widespread in northern and southern China. However, relatively little information is known about the influence of acid rain on the growth and photosynthetic characteristics of P. simonii × P. nigra in China. To elucidate this issue, we studied the responses of P. simonii × P. nigra to simulated acid rain (SAR) at different pH values. The growth indices and leaf ultrastructure, antioxidant enzymes, and photosystem Ⅱ activities of P. simonii × P. nigra were studied in response to SAR stress. The results could provide necessary referenced data for planting P. simonii × P. nigra in northern China, where acid rain has increased. 【Method】 P. simonii × P. nigra stecklings were treated with SAR at three levels. The acid rain concentrate was diluted to simulate severe acid rain (pH 2.5) and moderate acid rain (pH 4.5), and diluted electrolyte liquor was used as a control (CK, pH 7.0). All treatment P. simonii × P. nigra stecklings were sprayed with SAR every day. To account for the stomatal distribution, leaves were sprayed with acid rain on both sides. Meanwhile,the pots were covered with a plastic hood to avoid the indirect effects of SAR from the soil. When apparent symptoms occurred in leaves, we detected and analyzed the effects of acid rain on the index of growth, leaf anatomic structure, physiological indices, OJIP curves, and chlorophyll fluorescence parameters; chlorophyll a fluorescence technology is considered a probe to monitor the photosynthetic characteristics of plants, which is crucial for analyzing the extent of damage to the photosynthetic apparatus under various stress conditions. 【Result】 Compared to CK, the plant height, stem diameter, leaf number and leaf area increased significantly in the SAR treatment at pH 4.5. The chloroplast structure was complete, and the thylakoids were intact, but the leaf color became light green, there were no starch grains, and the number of osmiophilic droplets increased slightly. Moreover, the stem diameter and leaf area in SAR at pH 2.5 were significantly reduced. The leaves became yellow, prominent spots appeared on the blade tips and veins, and there were a small number of starch grains, but the number and volume of osmiophilic droplets increased significantly. The physiological index, the rate of {\mathrm{O}}_2^{\stackrel{-}{\underset{·}{\mathrm{}}}}production, and the content of H₂O₂ and CAT activities were higher than that of CK, and the greater the acidity, the more significant the increase. However, POD and SOD activities were increased in the pH 4.5, but decreased in the pH 2.5. There was no noticeable difference in the treatment of pH 4.5 and CK except for relative variable fluorescence of point L at 0.15 ms (V_L), and relative variable fluorescence of point K at 0.3 ms (V_K). Correspondingly, the parameters of V_L and V_K and relative variable fluorescence of point J at 2 ms (V_J), relative variable fluorescence of point I at 30 ms (V_I), and the relative reduced rate of Q_A (M_O) increased significantly at pH 2.5 treatment. However, the chlorophyll fluorescence curve area (Fix Area) and the size of the plastid quinone pool (S_m) in the pH 2.5 treatment were significantly lower than those in the CK. 【Conclusion】 The effect of simulated acid rain on the growth of P. simonii × P. nigra mainly depends on the pH value. When SAR at pH 2.5 inhibited the growth and leaf physiology of P. simonii × P. nigra and showed symptoms of necrotic spots, the chloroplast structure was abnormal, the thylakoid arrangement was irregular, and the electron transfer of photosystem Ⅱ was inhibited. However, pH 4.5 SAR treatment benefited P. simonii × P. nigra, and the ultrastructure of leaves was normal. This indicates that P. simonii × P. nigra stecklings had a high tolerance to acid rain at pH 4.5.