珍稀濒危植物毛果木莲幼苗光合特性及对遮阴处理的响应

【目的】光是植物生长发育过程中重要的环境因子,可通过光敏色素等途径来调节植物的生长、发育。毛果木莲(Manglietia ventii)为中国特有种,是国家Ⅱ级重点保护植物。笔者采取人工遮阴方式,对毛果木莲幼苗的光合及叶绿素荧光参数等指标进行测定,探讨毛果木莲幼苗对光环境变化的生理响应和适应性,为开展其保护生物学研究提供理论依据,同时为濒危植物的保护及繁衍提供科学指导。【方法】利用黑色遮阴网进行人工遮阴处理,使用照度计反复测量以准确建立相对光照率(RI)分别为8%、30%和100%(全光照)的生境。待毛果木莲幼苗在3种光照生境中生长6个月后,应用Li-6400 XT型便携式光合-荧光测定仪(Licor-6400, Lincoln, USA),随机选取不同遮阴条件下日照充足的连体叶(每个生境梯度5株,每株2叶片,共30个重复,取其平均值)。于8:30—11:30进行光合-光响应曲线的测定。日变化前,暗适应30 min后进行3种生境下幼苗的PSⅡ最大光化学效率(F_v/ F_m)测定。天气晴朗时连续3 d,从7:00—19:00每隔2 h进行日变化的测定,测定净光合速率(P_n)、蒸腾速率(T_r)、气孔导度(G_s)、光合有效辐射(PAR)等光合参数;光化学淬灭系数(q_P)、非光化学淬灭系数(q_N)、PS Ⅱ有效光化学量子产量( F_v'/F_m')、电子传递速率(ETR)等荧光参数。【结果】在限定条件下毛果木莲幼苗P_n对PAR的响应过程在不同遮阴程度下大致相同,且毛果木莲幼苗最大净光合速率(P_{n, max})、光饱和点(P_LSP)和光补偿点(P_LCP)随遮阴程度的增大而降低,表明毛果木莲幼苗具有一定的耐阴性,能够通过降低P_{n, max}、P_LSP和P_LCP来增强对弱光的利用能力。光合速率日变化呈现单峰型曲线,P_n、T_r、G_s均随遮阴程度增大而降低,且均无“光合午休”现象,说明毛果木莲幼苗在水分充足的条件下能适应多光环境。q_N、F_v'/F_m'和ETR日变化皆呈现单峰型曲线,均随遮阴程度增大而降低;但q_P的日变化随遮阴程度的增大而增加,说明遮阴促进光合作用与增强光能利用率和刺激叶肉细胞的光化学活性有关,主要通过增加PSⅡ反应中心的开放比例和降热耗散比例等途径适应弱光生境。而F_v/F_m随遮阴程度增加而增大,且RI为100%生境下其值低于正常值,具有强光抑制现象。说明长时间强光处理导致毛果木莲幼苗的光合机构受损,光化学活性降低。【结论】RI为100%时F_v/F_m显著低于其他两个遮阴处理(RI为30%、8%)(P<0.05,F=18.979, df=2),具有强光抑制现象,适当遮阴更利于毛果木莲幼苗生长,以RI为30%生境光强最佳。

Response of photosynthetic fluorescence of the endangered plant Manglietia ventii seedlings to shade treatment

【Objective】Light is an important environmental factor in the growth and development of plants. It effects plant growth and development by regulating the biosynthesis of phytochrome and other metabolic pathways. Manglietia ventii, is an endemic plant distributed in western China, and listed in the second-class national list of key conservative plants. The seedlings of M. ventii were cultivated under different light levels. The photosynthetic characteristics and chlorophyll fluorescence parameters of the seedlings were measured to investigate the adaptation and physiological responses and photo-protective mechanisms to the different light environment. Results may provide insights into the conservation and propagation of the endangered species. 【Method】Ten seedlings with similar size were randomly selected into 3 groups. Then each group was cultivated in the following relative illumination rate (RI) levels (100%, 30% and 8% created with different layers of black shading net) respectively for 6 months. During the growth period, the seedlings were watered and provided with nutrients properly. The photosynthetic characteristics of 5 healthy seedlings randomly chosen from RI 100%, 30% and 8%, respectively, were measured with the Li-6400 XT portable photosynthetic-fluorescence analyzer (Licor-6400, Lincoln, USA). The photosynthesis value of each seedling was recorded by averaging the data from 2 matured and healthy leaves. The photosynthetic-photoresponse curve was measured at 8:30-11:30 am Before the diurnal variation, the maximum photochemical efficiency (F_v/F_m) of the PSⅡ photosystem Ⅱ (PSⅡ) of the seedlings under the three light conditions were measured after dark adaptation for 30 minutes. The diurnal variation of photosynthetic parameters, including net photosynthetic rate (P_n), transpiration rate (T_r), stomatal conductance (G_s) were measured from 7:00 am to 19:00, for 3 days without clouds. At the same time, chlorophyll fluorescence parameters were measured, including photochemical quenching coefficient (q_P) and non-photochemical quenching coefficient (q_N), The effective photochemical quantum yield (F_v'/F_m') and electron transport rate (ETR) were measured with the same leaves for the measurement of photosynthesis. 【Result】Results indicated the response of P_n of seedlings under different levels of shade to photosynthetic active radiation (PAR) is similar; however, maximum net photosynthetic rate (P_{n, max}), light saturation point (P_LSP) and light compensation point (P_LCP) decreased with the increase in the levels of shade levels, which implicated that seedlings of M. ventii can tolerate shady environment. The diurnal variation of photosynthetic rate of all the seedlings under different light levels showed single peak curves, andP_n, T_r and G_s decreased with the increase in levels of shade. There is no midday depression of photosynthesis, which showed the seedlings of M. ventii can adapt to the light variation under the growth condition of sufficient water and nutrients. The diurnal variation of q_N, F_v'/F_m', and ETR decreased with the increase in shading levels and showed single peak curves too. However, the diurnal variation of q_P increased with the increase in shading levels, which indicated that shading promoted photosynthesis and enhanced the utilization of light energy and stimulated the photochemical activity of mesophyll cells. Leaves may mainly adapt to the low light habitat by increasing the open ratio of PSⅡ reaction center and the proportion of heat dissipation. Meanwhile,F_v/F_m was raised with the decrease of light levels. The value of F_v/F_m of leaves exposed to RI 100% is lower than 0.8 (that is normal for most plants in nonstress condition), which showed the occurrence of strong light inhibition. It implicated that long-term strong light treatment could damage the photosynthetic components of M. ventii seedlings and reduce its photochemical activity. 【Conclusion】F_v/F_m in full light (RI 100%) was significantly lower than that in the shading treatments (RI 30%, RI 8%) (P<0.05,F = 18.979, df = 2), which illustrated that the shading treatments effectively weakened light suppression of the M. ventii seedlings. Conclusively, shading treatment is more favorable to the growth of M. ventii seedlings and the best growth of M. ventii seedlings was expected under RI 30%.