沉水植物光合作用测定的电化学方法

光合作用速率测定是植物光合作用研究的重要基础,沉水植物由于所处生境的特殊性,难以应用在陆生植物光合作用研究中发展起来的速率测定方法.在对水体中二氧化碳的基本特征进行详细描述的基础上,介绍了在沉水植物光合作用研究中广泛应用的电化学方法--pH-stat法和pH-drift法的原理,并给出了应用实例.这两种测定水生植物光合作用的电化学方法所依据的溶液无机碳浓度的变化可以利用Gran滴定法方便地计算出来.     

高山植物光合机构耐受胁迫的适应机制

植物的光合作用是易受环境影响的重要生理过程之一.高山植物作为生长在特定极端环境(低温/强辐射)下的植物群体,其光合器官在形态结构和生理功能上形成了抵御强辐射和低温胁迫的特殊适应机制.但由于较高的生境异质性,高山植物的光保护适应机制存在较大的差异.光保护适应机制与光合作用密切关联,影响植物的碳同化能力和生物量的形成能力.本文对近年来国内外有关高山植物光合器官叶绿体的形态、超微解剖结构及光合机构光保护适应机理的研究进展进行了综述,并提出了今后高山植物光合作用生理适应性研究的方向.     

水体无机碳条件对常见沉水植物生长和生理的影响

为了解水华引起的水体无机碳变化对沉水植物生长的影响,对8种沉水植物:金鱼藻、穗花狐尾藻、篦齿眼子菜、光叶眼子菜、微齿眼子菜、伊乐藻、菹草和黑藻在不同无机碳浓度下的生物量、株高、叶绿素以及光合和呼吸速率进行了比较研究.结果表明8种沉水植物均能利用HCO3-作为光合无机碳源,在1.5 mmoL/L外源HCO3-浓度下能促进金鱼藻、菹草和伊乐藻的生长,提高其光合速率;在2.5 mmol/L外源HCO3-浓度下能促进狐尾藻、光叶眼子菜、黑藻、微齿眼子菜和蓖齿眼子菜的生长,提高其光合速率.在CO32-为优势碳源时,8种沉水植物表现出不同的适应性,发现微齿眼子菜、篦齿眼子菜和黑藻在整个实验范围内生长未受抑制,且在不同浓度下表现生长和光合速率的促进,说明这三种沉水植物对[HCO3-]/[CO32-]比值和pH具有较广适应范围.而当CO32-成为优势碳源时,金鱼藻和伊乐藻的生长受到抑制,狐尾藻、菹草和光叶眼子菜均死亡,表明[HCO3-]/[CO32-]比值和pH是这5种沉水植物生长的重要限制因子.     

辽东栎冠层光合生理特性的空间异质性

冠层作为林木与环境因子相互作用最为直接的部分,研究冠层光合作用是分析森林生产力的基础。以北京东灵山辽东栎为对象,利用Li-6400便携式光合仪测定了不同冠层不同方向部位叶片的光合速率和光响应曲线,研究了叶片光合生理特性在冠层空间上的变化。结果表明：在不同冠层和不同方向上,饱和光合速率、光补偿点、光下暗呼吸和表观量子效率均存在差异,随着冠层下降以及从南至北,大多数光合生理特性指标表现出递减趋势。进一步的通径分析结果得出,光强、水气压亏缺、温度是影响不同层次光合速率的主要因子。冠层光合特性的空间异质性研究,对于在冠层水平上揭示植物固碳能力和估算植物生产力具有很重要的意义。     

贵州喀斯特森林三种植物对不同坡位环境的光合生理响应

该研究以贵州普定喀斯特森林中、下坡位生长的构树( Broussonetia papyrifera)、朴树( Celtis sinensis)和光滑悬钩子( Rubus tsangii)为材料,通过对碳酸酐酶( CA)活性、光合作用日变化、净光合速率对CO2与光的响应曲线、叶绿素荧光特性以及稳定碳同位素组成等指标的测定,进而对比分析三种植物不同的光合生理响应特性。结果表明：构树光合作用过程的无机碳源既可来自大气中的CO2,也可以在气孔部分闭合的情况下利用细胞内的HCO3－,下坡位的构树较高的CA活性使其利用HCO3－的效率会更高,并能在较低光强下具有较高的光能利用效率。这可能与下坡位的构树具有较高的CA活性有关,对下坡位具有更好的适应性。朴树光合无机碳的同化能力最低,且光合无机碳源较单一,主要利用大气CO2,其较慢的生长速率使其对无机碳的需求最低,且能保持较稳定的无机碳同化速率。相对来说,中坡位的朴树具有相对较高的净光合速率和光能利用效率,对中坡位表现出较好的适应性。光滑悬钩子主要利用大气中的CO2进行光合作用。中坡位的光滑悬钩子具有较强的光能利用效率,并表现出较高的净光合速率,光滑悬钩子对中坡位同样表现出较好的适应性。该研究结果为喀斯特生态脆弱区植被重建过程中树种的选择及合理配置提供了科学依据。     

5种沉水植物的光合特征

以碘量法测定水中溶解氧的变化作为衡量沉水植物光合作用和呼吸作用变化的指标 ,研究了狐尾藻、金鱼藻、苦草、菹草和黑藻光合作用对光照的响应 ,比较了它们的光合能力及光合特征。不同光照强度下 ,供试材料的光合速率高低排列各不相同。 5种沉水植物的光合作用都表现出强光抑制现象 ,不同种类在高光强下光合速率下降的程度差异较大。狐尾藻等植株的下部可形成没有叶片的茎 (非光合茎 )。这些茎不进行光合作用或光合作用很弱 ,其暗呼吸也只是光合部分的一半左右。具有非光合茎的种类可在较深的水体中生存。 5种沉水植物中 ,苦草对光的需求最低 ,适于在低光照条件的水下生长 ,不耐强光 ;狐尾藻和金鱼藻对光的需求最高 ,在上层有较强的竞争能力 ;菹草和黑藻对光的需求介于中间 ,最大光合产量出现在中层 ,可在水体中层形成优势。     

沉水植物对重金属的吸收净化和受害机理研究进展

沉水植物对重金属的积累净化和受害机理研究主要集中在4个方面：（1）沉水植物对重金属（包括放射性物质）的吸收、积累和净化作用;（2）沉水植物对重金属的抗性强弱和机制;（3）沉水植物用于监测水体的重金属污染;（4）沉水植物的重金属胁迫机制,包括重金属对植物形态和显微结构的损伤,对植物抗氧化酶系统的影响,对植物的叶绿素、蛋白质以及光合与呼吸作用等生理生化指标的影响,植物对重金属的吸附和转运动力学,以及Zn对Cd毒害的拮抗等。     

沉水植物对重金属的吸收净化和受害机理研究进展

沉水植物对重金属的积累净化和受害机理研究主要集中在4个方面: (1)沉水植物对重金属(包括放射性物质)的吸收、积累和净化作用; (2)沉水植物对重金属的抗性强弱和机制; (3)沉水植物用于监测水体的重金属污染; (4)沉水植物的重金属胁迫机制, 包括重金属对植物形态和显微结构的损伤, 对植物抗氧化酶系统的影响, 对植物的叶绿素、蛋白质以及光合与呼吸作用等生理生化指标的影响, 植物对重金属的吸附和转运动力学, 以及Zn对Cd毒害的拮抗等。     

叶绿体功能的扩展

光合细胞器叶绿体的功能不仅仅是进行光合碳代谢,还应包括与光合作用的光反应密切相关的硝酸光合作用和硫酸盐还原等多种生理生化反应。本文简要阐述了叶绿体在硝酸还原和硫酸还原中的作用。     

赤潮藻中肋骨条藻的光合作用对海水pH和N变化的响应

为探讨赤潮发生时中肋骨条藻 (Skeletonemacoatatum)的光合作用生理变化 ,研究了不同无机氮 (N)水平上 ,海水pH值升高对其胞外碳酸酐酶 (CA)和光合生理特性的影响。海水pH从 8.2升至 8.7时 ,中肋骨条藻胞外CA被诱导 ,细胞对无机碳的亲和力 (1/Km)提高 ;在pH8 7时 ,高N条件下的胞外CA活性是低N条件下的 3倍 ,1/Km 值也提高了 80 %。单位叶绿素a的最大净光合能力 (Pam)在不同pH和N水平上没有显著差异 ;但单位细胞的最大净光合能力 (Pcm)提高了 10 0 %。这些结果表明 ,赤潮发生时 ,中肋骨条藻通过启动无机碳浓缩机制 (CCM) ,提高细胞对无机碳利用效率 ,使其在低CO2 (高pH)环境下维持光合机构正常运行 ;充足的N源有利于提高CCM的效率 ,从而提高CO2 环境下的光合固碳能力。     

Diurnal carbon restrictions on the photosynthesis of dense stands of Elodea nuttallii (Planch.) St. John

In slow-moving and static eutrophic waters, submerged macrophytes growing in dense stands produce a highly structured environment, with reduced internal water flow. An afternoon lull in the net photosynthesis of such stands has been reported from a number of previous studies. This has been attributed to increased photorespiration caused by an accumulation of photosynthetically-derived, dissolved oxygen in the surrounding water. Results here demonstrate that even in a water quite rich in dissolved inorganic carbon (2.5 mmol l^–1), limitations on the supply of inorganic carbon will normally be more important in curtailing photosynthesis, with photorespiration playing only a minor role.     

Effects of phytoplankton on the distribution of submerged macrophytes in a small canal

Submerged macrophytes have been disappearing from the Kanto Plain, Japan since the 1960s. This disappearance is usually attributable to the interaction between macrophytes and phytoplankton. Phytoplankton contributes to shading of the available light and changes the availability of inorganic carbon from free CO₂ to HCO ₃ ^? for use in photosynthesis. However, limited information is available about the interaction between carbon fraction and submerged macrophytes through phytoplankton abundance. In this short note, we observe the distribution of submerged macrophytes and phytoplankton in a small canal. We found that, despite high photosynthetically active radiation (PAR) in the downstream region, low free CO₂ concentration through phytoplankton abundance can deplete free CO₂ for submerged macrophytes. In contrast, the upstream region exhibited macrophytes in an environment with high free CO₂ concentration. The stable carbon isotope ratio of submerged macrophytes follows this pattern, with more positive values occurring in the downstream region and more negative values in the upstream region. It has been reported that phytoplankton limits light availability for submerged macrophytes, but carbon availability could also be a factor in the distribution of submerged macrophytes. Because the source of water for submerged macrophytes is groundwater, its preservation possibly plays a key role for the restoration of submerged macrophytes.     

The effect of environmental variables on release of extracellular organic carbon by freshwater macrophytes

The¹⁴C-technique was used to investigate the effect of oxygen, inorganic carbon and light intensity on the release of extracellular organic carbon (EOC) by three submerged macrophytes. At low CO₂ availability and low light intensities the release expressed as a percentage of the total carbon fixation was higher than at more optimal photosynthetic conditions. Oxygen did not influence the release of EOC. As the rates of release generally followed the rates of photosynthesis, it was concluded that release is closely coupled to photosynthesis. Using gel-filtration to separate EOC it was shown that the tested environmental variables did not influence the molecular weight distribution. The release of EOC was low in all species (0.1 to 4% of the total carbon fixation). Due to the low values it is suggested that release of EOC is of no quantitative importance to the carbon budget of the plants.     

Inorganic carbon uptake kinetics of the stream macrophyte Callitriche cophocarpa Sendt.

Photosynthetic carbon uptake of Callitriche cophocarpa Sendt. was examined in plants collected from six Danish streams and in plants grown under variable inorganic carbon conditions in the laboratory. Both field and laboratory plants showed a low affinity for inorganic carbon (CO₂ compensation points ranging from 0.7 to 22 μM, and K_0.5(CO₂) from 51 to 121 μM), consistent with C-3 photosynthesis and use of CO₂ alone. Variation in inorganic carbon uptake characteristics was low in both groups of plants. Only in laboratory-grown plants was a coupling found between carbon uptake and the inorganic carbon regime of the medium. The carbon extraction capacity, expressed as a percentage of the initial amount of dissolved inorganic carbon (DIC) assimilated in PH-drift experiments, increased from −1.4 to 11.8% with declining external carbon availability, and the initial slope of the CO₂ response curve increased from 6.4 to 15.3 g⁻¹ h⁻¹ dm³. The plasticity of the inorganic carbon uptake system of C. cophocarpa was very low compared to the plasticity observed for submerged macrophytes with accessory carbon uptake systems (i.e. HCO₃⁻ use or C-4 photosynthesis), suggesting that the plasticity of the C-3 photosynthetic apparatus as such is restricted. The low carbon affinity of C. cophocarpa indicates that this species depends on CO₂ oversaturation for a sufficient supply of CO₂ for photosynthesis and growth.     

Plasticity in carbon acquisition of the heterophyllous Luronium natans: An endangered freshwater species in Europe

Luronium natans (L.) Raf. (Floating Water-plantain) is an endangered amphibious freshwater species endemic to Europe. We examined the plasticity in carbon acquisition and photosynthesis in L. natans to assess if lack of plasticity could contribute to explain the low competitive ability of the species. The plasticity of photosynthesis in submerged leaves towards inorganic carbon availability was examined and the photosynthesis of submerged, floating and aerial leaves was contrasted. L. natans was shown to be plastic in inorganic carbon uptake, as it was able to effectively acclimate to changed concentrations of free-CO₂. The photosynthetic apparatus was down-regulated in plants grown at high CO₂. Chlorophyll concentration, Rubisco activity and maximum photosynthesis were significantly lower in submerged leaves of plants grown at high CO₂ (200 μM free-CO₂) compared to plants grown at low CO₂ (18 μM free-CO₂). Furthermore, bicarbonate utilization was down-regulated in response to high CO₂. Carbon acquisition of submerged, floating and aerial leaves of L. natans differed significantly. The aerial leaves were superior in photosynthesising in air and, surprisingly, the floating leaves had the highest rates of photosynthesis in water. The study did not support the hypothesis that the low competitive ability of L. natans is caused by inefficient photosynthesis or a lack of plasticity in photosynthesis. However, the somewhat low photosynthetic performance of the submerged leaves may be a contributing factor.     

Photosynthetic use of light and inorganic carbon in air and water by the intertidal alga Petalonia fascia (Scytosiphonales, Phaeophyta)

The photosynthetic performance of the intertidal alga Petalonia fascia (0. F. Muller) Kuntze (Scytosiphona-ceae, Phaeophyta) has been investigated, both in air and water, by analyzing the relationship between apparent photosynthesis rate and photon irradiance and inorganic carbon. In relation to the use of photon irradiance, it was found that the net photosynthetic capacity in water was 5.7 times that in air (fully hydrated thallus). The light compensation point was achieved at 5.9 and 3.0 μmol photons m^?2 s^?1 in air and water, respectively. The light onset-saturation parameter and the photosynthetic efficiency were 77% and 25% greater in water than in air, respectively. The dark respiration rate was one-third greater when emersed in comparison to submersion conditions. These data suggest that the photosynthetic response to irradiance in P. fascia is similar to that in infralittoral species rather than the intertidal species. This assessment can be explained by the winter seasonality of the bladed stage of growth, when storms and waves permit a permanent hydrated status of P. fascia that in the intertidal zone. Moreover, the minimum tissue water content that permitted active photosynthesis in the alga was around 20%. The net photosynthetic capacity as a function of inorganic carbon (C) concentration in water was 1.5 times that in air. Photosynthesis was saturated in both media with respect to the availability of inorganic C in natural conditions. The affinity to inorganic C, and the carbon conductance, were two orders of magnitude higher in air than in water. However, the higher photosynthetic capacity when submerged in comparison to emersion conditions suggests that P. fascia can assimilate the external HCO₃,– or the occurrence of a CO₂ concentrating mechanism in this species.     

The Underwater Life of Secondarily Aquatic Plants: Some Problems and Solutions

The freshwater secondarily aquatic plants, most of which are higher plants, are those returned to the water environment after spending a period of time living on land. The readaptation to living underwater has made it necessary for these plants to put in place morphological and functional strategies to cope with some major problems due to features of the aquatic environment, but also deriving from the specialized organization of their “terrestrial” bodies. The poor O₂ availability underwater accounted for the evolution of wide aerenchyma tissues throughout the plant organs to improve the photosynthetic O₂ flux from the shoot to the roots buried in anoxic sediments and to the neighboring rhizosphere. This favors sediment oxygenation, sustains the aerobic metabolism of roots, and improves the availability and uptake of mineral nutrients, whose delivery to the entire plants, without a transpirational flux, is ensured by an acropetal mass transport depending on root pressure, guttation from hydathodes and channeling by apoplast closure around the vascular tissues. A great expansion of leaf surfaces and an enhanced surface:volume ratio of chloroplast-rich photosynthetic cells help to contact the water medium and to increase the cell/environment exchanges to gain inorganic carbon. Furthermore, different physiological mechanisms operate to cope with the scarce availability of CO₂ and the prevalence of HCO₃ ^? as inorganic carbon form in water. Some of them, like cell wall acidification through H⁺ extrusion by a light-dependent APTase or activation of an apoplastic carbonic anhydrase, operate outside the cells, leading to a conversion of HCO₃ ^? to CO₂, which then diffuses into the cells. Others, on the contrary, act inside the cells to load the active site of Rubisco with CO₂, thus favoring photosynthesis and lowering photorespiration. Aquatic macrophytes with isoetid life form, moreover, can obtain most ot the fixed CO₂ from sediments. In submerged species, in additin to the C₃ cycle, the C₄ and CAM-like photosynthetic metabolisms can also operate, and are modulated by the environmental inorganic carbon availability and the plant photosynthetic demand. Interestingly, in the aquatic plants the C₄ pathway, which can be concomitant with the C₃ one, does not depend on the Kranz anatomy of leaves, but relies on the intracellular compartmentation of carboxylative and decarboxylative enzymes. The CAM-like pathway, defined AAM, which also coexists with the C₃, allows the submerged plants to fix CO₂ in the dark, thus exploiting the higher CO₂ availability in the water medium during the night, and extending to 24?h the period of inorganic carbon assimilation. In almost all the aquatic macrophytes the AAM is only expressed in the submersion state, whereas it is quickly inactivated in emerging leaves in a cell by cell way.     

A COMPARISON OF AIR AND WATER AS ENVIRONMENTS FOR PHOTOSYNTHESIS BY THE INTERTIDAL ALGA FUCUS SPIRALIS (PHAEOPHYTA)1

The response of photosynthesis and respiration of the intertidal brown alga Fucus spiralis L. to light and temperature at ambient and elevated concentrations of inorganic carbon was investigated. The light-saturated rate of photosynthesis was greater in air at 15° C and 20° C, but greater in water at 10° C. Light compensation point and I_k was about 50% lower under submerged relative to emerged conditions, whereas the initial slope of photosynthesis versus irradiance was higher, except at 20° C. Under both submerged and emerged conditions light-saturated photosynthesis was limited to a similar degree (78%, and 65%, respectively) by the availability of inorganic carbon at naturally occuring concentrations. In air, slight desiccation at tissue water contents of about 96% to 92% caused a stimulation in the rate of net photosynthesis to 110–148% of fully hydrated fronds. At lower water contents the rate of net photosynthesis declined linearly with decreasing water content and became zero at a water content of about 15%. Dark respiration declined linearly with tissue water content and remained positive to a water content of 8%. Upon reimmersion the fronds showed a complete recovery within 35 min following desiccation to a water content of 20–30%. Thus F. spiralis seems to be very tolerant to desiccation. Since F. spiralis photosynthesizes effectively in air, even at a higher rate than in water as long as it has not lost a large proportion of its water in desiccation, the alternating exposure to air may be beneficial by increasing the daily carbon gain compared to a fully submerged situation.     

Alterations in Rubisco activity and in stomatal behavior induce a daily rhythm in photosynthesis of aerial leaves in the amphibious-plant Nuphar lutea

Nuphar lutea is an amphibious plant with submerged and aerial foliage, which raises the question how do both leaf types perform photosynthetically in two different environments. We found that the aerial leaves function like terrestrial sun-leaves in that their photosynthetic capability was high and saturated under high irradiance (ca. 1,500 μmol photons m⁻² s⁻¹). We show that stomatal opening and Rubisco activity in these leaves co-limited photosynthesis at saturating irradiance fluctuating in a daily rhythm. In the morning, sunlight stimulated stomatal opening, Rubisco synthesis, and the neutralization of a night-accumulated Rubisco inhibitor. Consequently, the light-saturated quantum efficiency and rate of photosynthesis increased 10-fold by midday. During the afternoon, gradual closure of the stomata and a decrease in Rubisco content reduced the light-saturated photosynthetic rate. However, at limited irradiance, stomatal behavior and Rubisco content had only a marginal effect on the photosynthetic rate, which did not change during the day. In contrast to the aerial leaves, the photosynthesis rate of the submerged leaves, adapted to a shaded environment, was saturated under lower irradiance. The light-saturated quantum efficiency of these leaves was much lower and did not change during the day. Due to their low photosynthetic affinity for CO₂ (35 μM) and inability to utilize other inorganic carbon species, their photosynthetic rate at air-equilibrated water was CO₂-limited. These results reveal differences in the photosynthetic performance of the two types of Nuphar leaves and unravel how photosynthetic daily rhythm in the aerial leaves is controlled.