濒危植物银缕梅(Parrotia subaequalis)不同径级个体的光合能力差异与更新限制

Differences in photosynthetic capacity among different diameter-classes of Parrotia subaequalis populations and their implications to regeneration limitation

银缕梅(Parrotia subaequalis)是我国华东地区特有种,属国家Ⅰ级濒危保护植物。分别于5月份与8月份采用Li-6400光合系统测定了江苏宜兴龙池大垅西芥银缕梅种群内大(胸径>10 cm)、中(2 cm<胸径≤10 cm)、幼(胸径≤2 cm)3个径级个体的光合生理参数、光合-光强响应曲线及光环境因子,探讨了不同径级个体的光合能力差异及其对种群更新的影响。结果表明:幼树最大净光合速率(Pnmax)、光饱和点(LSP)、光补偿点(LCP)和暗呼吸速率(RD)显著低于大树与中树,但比叶面积(SLA)显著高于大树及中树;低LCP、低RD与高SLA是银缕梅幼树对弱光环境做出的适应性反应。中树LSP仅为大树的二分之一,而其LCP与大树无明显差异,且显著高出幼树;中树LSP与LCP之间的光能利用区间极为狭窄;中树全天平均接受的RAR稍高于LCP,但远低于其LSP;同时,中树的SLA显著低于幼树与大树,致使中树全天碳同化总量严重不足。基于净光合速率(Pn)与光环境参数建立的逐步回归方程进一步证实了光合有效辐射(PAR)对Pn的贡献最大,是影响银缕梅光合碳同化能力的关键因子。由于幼树与中树受到大树遮荫,叶片上方的光合有效辐射均显著降低。尽管银缕梅幼苗在林下表现出较强的弱光适应能力,但在后期建成为中树的过程中,叶片形态与光合生理特性均表现出对遮萌环境的不适应,光资源不足构成银缕梅种群更新的主要限制因子,导致种群的中等径级个体急剧减少。人为干预以形成适宜的林窗环境,有利于银缕梅幼树的后期生长,并促进银缕梅种群的顺利更新。

Parrotia subaequhlis, an endangered species endemic to eastern China, is on the national list of first-class key conservative plant. We investigated the difference of photosynthetic capacity among three diameter-classes of P. subaequalis trees, i.e. sapling, mid-sized tree and matured tree in Yixin, Jiangsu Province. The photosynthesis-related parameters and light response curves were measured using portable Li-6400 photosynthesis measuring system （Licor, USA）. Our results showed that photosynthetic parameters of saplings, i.e. the maximal net photosynthesis rate （P ）, light saturation point （LSP）, light compensation point （LCP） and dark respiration rate （RD） , were significantly lower than those of mid-sized trees and matured trees. However, the specific leaf area （SLA） of saplings significantly was significantly greater. Photosynthetic physiological and morphological characteristics of saplings＇ leaves, including low LCP, RD and high S/A, demonstrated their great ability to tolerate shade. LSP of mid-sized trees was only half of that of matured trees, LCP of mid- sized trees was significantly higher than that of sapl＇ings, but not different from matured trees; Ranges of light utilization between LSP and LCP of mid-sized trees were extremely narrow. Furthermore, photosynthetically active radiation （PAR） above leaves of mid-sized trees was slightly higher than LCP but far below LSP; SLA of mid-sized trees was significantly lower than that of saplings and even slightly lower than that of matured trees. Based on the stepwise regression equation established between the net photosynthetic rate （P,/） and the environmental parameters, we found that PAR had the greatest contribution to P,, and thus was the key limiting factors affecting photosynthetic capacity of P. subaequalis. PAR and air temperature （Ta ） above leaves of saplings and mid-sized trees, which were under the shade of matured trees, were significantly lower than those above the leaves of matured trees. We concluded that saplings of P. subaequalis have flexible light-adaptation strategy in initial growing stage in shade. However, leaves of mid-sized trees show weak shade-tolerance in; low light conditions, which may contribute to the low numbers of mid-sized trees. Human intervention, such as selective removal and cutting of woods to create suitable canopy gaps may improve light condition and promote population regeneration of P. subaequalis.