菹草茎叶微界面O2时空动态

Micro-optode analysis of the spatio-temporal dynamics of O_2 in the microboundary layer around Potamogeton crispus leaves and stems

采用高分辨率光纤氧微电极测定了富营养化水体中沉水植物菹草(Potamogeton crispus)茎叶微界面(0—2.0 mm)氧(O_2)。菹草叶微界面O_2浓度梯度具明显的时空变化。时间上,菹草叶微界面O_2浓度具有明显的生长阶段变化和昼夜变化。幼苗期和快速生长期微界面O_2浓度增加幅度较小,稳定期叶表O_2浓度梯度增加幅度最大,衰亡期叶微界面O_2浓度受附着物影响具明显的空间梯度。菹草叶微界面O_2表现为昼高夜低的单峰变化模式,主要受光照和温度的影响。空间上,越接近茎叶表面,O_2浓度越高。顶部幼叶微界面O_2浓度梯度增加较平缓,中部成熟叶微界面O_2浓度梯度变化最陡,波动幅度最大,中部茎和基部衰老叶微界面O_2浓度梯度由于受密集附着物的影响,在附着物表面达到最大值,进入附着层后略有下降。结果表明,菹草茎叶微界面O_2时空变化主要受附着物和植物光合放氧能力的影响。光纤微电极是一种分析植物叶微界面氧时空分布的理想工具,对深入研究植物微界面在富营养化水体中养分的迁移转化具有重要意义,可为水生植物生理生态研究提供有力工具。

Submerged macrophytes constitute an important natural component in shallow aquatic ecosystems. The submersed aquatic plants are characterized by leaf ology with a high area-to-volume ratio and a thin cuticula, which facilitates a rapid exchange of gases between the plants and bulk water. The micro-boundary layer around the submerged macrophyte surface plays a significant ecological role in plant growth and nutrient transformation in the aquatic environment. Potamogeton crispus is one of the dominant species in eutrophic shallow lakes in China, and oxygen （ O2 ） is the key parameter shaping the oxidation-reduction heterogeneous microenvironment around submerged plants. Thus, the characterization of the O2 microgradients in the micro-boundary layer around submerged macrophytes is of particular interest. Using micro-optodes, O2in the micro-boundary layer around P. crispus leaves and stems was investigated across different growing stages at various leaf positions relating to diurnal variations. The periphyton and rapid light curves were measured by conventional methods and the pulse amplitude modulated fluorometer （Diving-PAM）, respectively. Results showed that significant spatio-temporal variations in O2 concentration gradients existed in the micro-boundary layer around P. crispus leaves. In the vertical direction from the stem/leaf surface, the O2 concentration in the micro-boundary layer increased markedly with decreasing distance from the surface of leaf/stem and peaked at the leaf/stem surface. At the temporal scale, O2concentration in the micro-boundary layer varied significantly among different growing stages within the entire life cycle. The fluctuation in O2 concentrations in the micro-boundary layer was the lowest during seedling stages （9.08--9.65 mg/L） while its amplitude peaked （9.28--13.16 mg/L） at stable growing stages. However, O2 gradients markedly differentiated spatially at declining stages, and O2concentrations decreased significantly relative to that of stably growing stages. The O5 concentration at the surface of the leaf displayed diurnal variations with a significant unimodal pattern. The O2 concentration increased gradually and reached the maximum of 16.68 mg/L at 15：00, and then decreased with the decreasing light intensity. The O2 concentration dropped rapidly after sunset and reached the minimum of 6.01 mg/L at 05： 10. The O2 concentration was effected mainly by light and water temperature during the diurnal cycle. At the spatial scale, marked differences in the Ozconcentration were observed in the micro-boundary layer of leaves at different parts of individual plants. O2 concentrations in the micro-boundary layer around young leaves at the shoot apex fluctuated slightly, while those of the mature leaves at the middle shoot were steep, with the greatest amplitude of fluctuation. However, O2concentrations in the micro-boundary layer around stems at the middle shoot and around senescent leaves at the basal shoot maximized at the surface of periphyton which possessed higher dense, and declined slightly when entering the periphyton layer. In conclusion, among different growing stages, O2concentration gradients in the micro-boundary layer around P. crispus leaves and stems were mainly affected synergistically by the photosynthetic capability and the attached periphyton. However, among different positions on individual plants, O2 in the micro-boundary layer was mainly affected synergistically by plant physiological characteristics and the periphyton. The micro-optodes are ideal oxygen microsensors for investigating the micro- boundary layer around submerged macrophytes for fine spatial （ 〈50 μm） and temporal （s） resolutions. This study provides methods for better understanding the ecological role of the micro-boundary layer around submersed macrophytes and for verifying the processes within the micro-boundary layer for regulating nutritional cycling in eutrophic waters.