Tight coupling of root-associated nitrogen fixation and plant photosynthesis in the salt marsh grass Spartina alterniflora and carbon dioxide enhancement of nitrogenase activity

The coupling of root-associated nitrogen fixation and plant photosynthesis was examined in the salt marsh grass Spartina alterniflora. In both field experiments and hydroponic assay chambers, nitrogen fixation associated with the roots was rapidly enhanced by stimulating plant photosynthesis. A kinetic analysis of acetylene reduction activity (ARA) showed that a five-to sixfold stimulation occurred within 10 to 60 min after the plant leaves were exposed to light or increased CO2 concentrations (with the light held constant). In field experiments, CO2 enrichment increased plant-associated ARA by 27%. Further evidence of the dependence of ARA on plant photosynthate was obtained when activity in excised roots was shown to decrease after young greenhouse plants were placed in the dark. Seasonal variation in the ARA of excised plant roots from field cores appears to be related to the annual cycle of net photosynthesis in S. alterniflora.     

Evidence for NH4+ switch-off regulation of nitrogenase activity by bacteria in salt marsh sediments and roots of the grass Spartina alterniflora.

The regulatory effect of NH4+ on nitrogen fixation in a Spartina alterniflora salt marsh was examined. Acetylene reduction activity (ARA) measured in situ was only partially inhibited by NH4+ in both the light and dark after 2 h. In vitro analysis of bulk sediment divided into sediment particles, live and dead roots, and rhizomes showed that microbes associated with sediment and dead roots have a great potential for anaerobic C2H2 reduction, but only if amended with a carbon source such as mannose. Only live roots had significant rates of ARA without an added carbon source. In sediment, N2-fixing mannose enrichment cultures could be distinguished from those enriched by lactate in that only the latter were rapidly inhibited by NH4+. Ammonia also inhibited ARA in dead and live roots and in surface-sterilized roots. The rate of this inhibition appeared to be too rapid to be attributed to the repression and subsequent dilution of nitrogenase. The kinetic characteristics of this inhibition and its prevention in root-associated microbes by methionine sulfoximine are consistent with the NH4+ switch-off-switch-on mechanism of nitrogenase regulation.     

Lacunal allocation and gas transport capacity in the salt marsh grass Spartina alterniflora

Summary Lacunal allocation as the fraction of the total cross sectional area of leaves, stem bases, rhizomes, and roots was determined in both tall and short growth forms of Spartina alterniflora collected from natural monospecific stands. The results indicate that in both growth forms lacunal allocation is greater in stem bases and rhizomes than in leaves and roots and that tall form plants allocate more of their stem and rhizome to lacunae than short form plants.Measurements made in natural stands of Spartina alterniflora suggest that total lacunal area of the stem base increases with increasing stem diameter and that stem diameter increases with increasing plant height and above-ground biomass. However, the fraction of cross section allocated to lacunae was relatively constant and increased only with the formation of a central lacuna.Experimental manipulations of surface and subsurface water exchange were carried out to test the influence of flooding regime on aerenchyma formation. No significant differences in lacunal allocation were detected between plants grown in flooded (reduced) and drained (oxidized) sediments in either laboratory or field experiments. While aerenchyma formation in Spartina alterniflora may be an adaptation to soil waterlogging/anoxia, our results suggest that lacunal formation is maximized as a normal part of development with allocation constrained structurally by the size of plants in highly organic New England and Mid-Atlantic marshes.The cross sectional area of aerenchyma for gas transport was found to be related to the growth of Spartina alterniflora with stands of short form Spartina alterniflora exhibiting a lower specific gas transport capacity (lacunal area per unit below ground biomass) than tall form plants despite having a similar below-ground biomass supported by a 10 fold higher culm density. The increased specific gas transport capacity in tall vs. short plants may provide a new mechanism to explain the better aeration, higher nutrient uptake rates and lower frequency of anaerobic respiration in roots of tall vs. short Spartina alterniflora.     

Evidence for NH4+ switch-off regulation of nitrogenase activity by bacteria in salt marsh sediments and roots of the grass Spartina alterniflora

The regulatory effect of NH4+ on nitrogen fixation in a Spartina alterniflora salt marsh was examined. Acetylene reduction activity (ARA) measured in situ was only partially inhibited by NH4+ in both the light and dark after 2 h. In vitro analysis of bulk sediment divided into sediment particles, live and dead roots, and rhizomes showed that microbes associated with sediment and dead roots have a great potential for anaerobic C2H2 reduction, but only if amended with a carbon source such as mannose. Only live roots had significant rates of ARA without an added carbon source. In sediment, N2-fixing mannose enrichment cultures could be distinguished from those enriched by lactate in that only the latter were rapidly inhibited by NH4+. Ammonia also inhibited ARA in dead and live roots and in surface-sterilized roots. The rate of this inhibition appeared to be too rapid to be attributed to the repression and subsequent dilution of nitrogenase. The kinetic characteristics of this inhibition and its prevention in root-associated microbes by methionine sulfoximine are consistent with the NH4+ switch-off-switch-on mechanism of nitrogenase regulation.     

Carbon assimilation in contrasting streamside and inland Spartina alterniflora salt marsh

Carbon assimilation and standing crop biomass of Spartina alterniflora were studied in a contrasting streamside and inland salt marsh in Louisiana Gulf coast, USA. A substantially lower leaf dry weight, leaf area index, and standing crop biomass were recorded for inland plants as compared to streamside plants. Net assimilation rates ranged between 8 to 25 mol m^–2 s^–1 for streamside and between 4 to 19 mol m^–2 s^–1 for inland plants. The average photosynthetic rates were significantly lower for inland plants which were growing in an apparently more stressed environment. In addition, the differences were more profound with progression of the growing season. The reduced photosynthetic activity in the inland marsh was attributed to greater soil waterlogging, increased anaerobic root respiration, plant toxins (sulfide), restricted nutrient uptake or a combination of these factors.Abbreviations E_h = redox potential - g_w = stomatal conductance - LAI = leaf area index - P_n = net photosynthesis - PPFD = photosynthetic photon flux density - T₁ = leaf temperature     

Designing an effective clipping regime for controlling the invasive plant Spartina alterniflora in an estuarine salt marsh

Many invasive plants are highly tolerant of disturbances and can invade various habitats, leading to difficulties in managing them. Most of control techniques need to be performed repeatedly, and the efficiency of the same control method may vary along environmental gradients. To develop a control strategy for an invasive plant in heterogeneous environments, it is important not only to optimize the control regime, but also to consider the efficiency in different habitats. We conducted a field experiment to test how clipping regime (i.e., initial treatment timing and treatment interval) and habitat type (i.e., high and low tidal zones) influence the control efficiency for invasive Spartina alterniflora in an estuarine salt marsh. Among the 12 treatments, double-clipping treatment performed in early July and mid September (9-week-interval) had the highest efficiency, which not only eliminated sexual reproduction of S. alterniflora, but also inhibited its vegetative growth in the current and following years. The clipping efficiency in low tidal zone was higher than that in high tidal zone. Therefore, an optimized control strategy for the invasive plants could be established based on the efficient control regime and habitat types. According to our study, a recommended double-clipping control strategy for S. alterniflora is that the initial clipping treatment is performed in the flowering period and the second one is performed 9 weeks later. In order to effectively reduce proliferation of the invasive plant, we suggest that the clipping regime should be applied for at least 2–3 years in low tidal zone and 3–4 years in high tidal zone.     

Interaction among sediment anaerobiosis, nitrogen uptake and photosynthesis of Spartina alterniflora

Spartina alterniflora Lois. plants were grown under controlled sediment-pH-redox conditions. Uptake of added ¹⁵NH₄-nitrogen and subsequent photosynthetic activity under different redox conditions in the plant root rhizosphere were measured. Data for total plant nitrogen and ¹⁵N concentration indicated that nitrogen allocation was not altered by anaerobic conditions of the sediment. However, average net photosynthesis was reduced by up to 35% for plants under anaerobiosis. The results indicate that anaerobiosis in the root rhizosphere, rather than limiting nitrogen uptake, influences photosynthesis and growth of S. alterniflora under anaerobic conditions.     

Relationship between nitrogen-fixing sulfate reducers and fermenters in salt marsh sediments and roots of Spartina alterniflora.

A combination of inhibitors and carbon substrates was used to determine the relative contribution of sulfate-reducing bacteria (SRB) and fermenting bacteria to nitrogen fixation in a salt marsh sediment and on the roots of Spartina alterniflora. Because a lag period precedes acetylene-reducing activity (ARA) in amended sediments, an extensive analysis was done to be sure that this activity was due to the activation of dormant cells, not simply to cell proliferation. Since ARA was not affected by metabolic inhibitors such as rifampin, nalidixic acid, or methionine sulfoximine, it appeared that cell growth was not responsible for this activity. Instead, dormant cells were being activated by the added energy source. Molybdate inhibition studies with glucose-amended sediment slurries indicated that ARA in the upper 5 cm of the salt marsh was due primarily (70%) to SRB and that below that level (5 to 10 cm) it was due primarily (greater than 90%) to fermenting bacteria. ARA associated with washed roots of intact S. alterniflora plants was not inhibited by molybdate, which indicates that bacteria other than SRB were responsible. However, when the roots were excised from the plant, the activity (per unit of root mass) was 10-fold higher and was severely inhibited by molybdate. While this high activity is probably an artifact, due to the release of oxidizable substrates from the excised roots, it indicates that SRB are present in high numbers on Spartina roots.     

Estimation of primary production using five different methods in a Spartina alterniflora salt marsh

The aboveground production of Spartina alterniflora in a salt marsh in Barataria Bay, Louisiana, USA was estimated using five different harvest methods: peak standing crop (PSC), Milner-Hughes, Smalley, Wiegert-Evans, and Lomnicki et al., and a non-destructive method based on measurement of stem density and longevity. Annual production estimates were 831 ± 41, 831 ± 62, 1231 ± 252, 1873 ± 147 and 1437 ± 96 g dry wt m^–2 for each method, respectively. The average longevity of individually tagged young shoots was 5.2 ± 0.2 months, equivalent to an annual turnover rate of 2.3 crops per year. Among the five methods, Wiegert-Evans and Lomnicki et al. were considered more accurate than the other three because they corrected for mortality losses between sampling times. The Lomnicki et al. method was preferred over the Wiegert-Evans method because of its greater simplicity.     

Accumulation of proline and glycinebetaine in Spartina alterniflora Loisel. in response to NaCl and nitrogen in the marsh

Summary The possible interaction of high soil salinity and low soil nitrogen content in affecting the growth of Spartina alterniflora Loisel in the high and low marshes of the Eastern U.S. was explored. Throughout the whole growing season, the short plants growing in the high marsh, where there was a higher soil salinity and lower available soil nitrogen, contained more proline and glycinebetaine and showed a lower leaf water potential than the tall plants growing in the low marsh. In both short and tall plants, the growing season, with the highest content occurring in spring and fall. In contrast, the glycinebetaine content in both short and tall plants remained fairly constant throughout the growing season, and was consistently at least 10 fold higher than the proline content. It is estimated that 19–30% of the total leaf nitrogen was in the form of proline and glycinebetaine in the short plants, and 14–27% in the tall plants. Ammonium nitrate fertilization in the field resulted in increased growth, higher proline and glycinebetaine contents, and lower water potentials in the short plants, but had little effect on these parameters in the tall plants. We suggest that in the low marsh, the plants can obtain sufficient nitrogen for osmoregulation and other metabolism. In the high marsh with higher soil salinity and lower nitrogen content, the plants have to allocate a even greater proportion of the already limited nitrogen supply for osmoregulation. Thus, nitrogen available for osmoregulation and other nitrogen-requiring metabolism is insufficient, resulting in reduced growth.     

Seasonal patterns of daily net photosynthesis,transpiration and net primary productivity of Juncus roemerianus and Spartina alterniflora in a Georgia salt marsh

Summary Studies of the seasonal CO₂ and water vapor exchange patterns of Juncus roemerianus and Spartina alterniflora were conducted in an undisturbed marsh community on Sapelo Island, Georgia. Daily patterns of net photosynthesis, transpiration, leaf diffusive conductance and water-use efficiency in response to ambient conditions were monitored on intact, in situ plants. Net primary productivity was calculated from the daytime CO₂ fixation totals, nighttime CO₂ loss, leaf standing stock and aboveground to belowground biomass ratios for each plant type.The tall form of S. alterniflora had higher rates of photosynthesis and higher water-use efficiency values which, in conjunction with low respiratory losses and large leaf standing crop, results in high values of net primary productivity. The environmental factors in the marsh which permit these physiological responses occur in less than 10% of the marsh. Overall, the physiological capabilities of the short form of S. alterniflora were reduced in comparison to the tall form, but the combination of environmental factors which determine the physiological responses of this form occur in a much greater portion of the marsh, and the short form of S. alterniflora dominates the Sapelo Island marshes.The response patterns of the C₃ species, J. roemerianus, differed somewhat from the height forms of S. alterniflora. A large, seasonally constant leaf standing crop coupled with moderate rates of photosynthesis resulted in a net primary productivity value which was between the tall and short height forms of S. alterniflora. However, as with the tall S. alterniflora, the environmental conditions under which this high productivity and high water loss rate can be sustained are restricted to specific regions of the environmental gradient in the marsh.Contribution No. 435 from the University of Georgia Marine Institute     

三种盐胁迫对互花米草和芦苇光合作用的影响

互花米草(Spartina alterniflora)的入侵给海岸带盐沼生态系统的结构和功能带来了显著影响。互花米草盐沼中的硫含量高于附近的土著芦苇(Phragmites australis)盐沼。为探讨硫元素对互花米草和芦苇竞争过程的可能影响及其作用机制, 以50 mmol·L^-1的Na₂SO₄和Na₂S对互花米草和芦苇进行处理, 分析处理前后5天内两种植物光合气体交换和叶绿素荧光指标变化的差异, 实验另设等Na⁺浓度的NaCl处理作为比较。研究发现: Na₂S对互花米草和芦苇光合作用影响的差异最大, NaCl次之, Na₂SO₄最小。Na₂S处理后, 互花米草净光合速率(P_n)出现显著上升, 芦苇P_n值大幅度下降。互花米草的光饱和点(I_sat)上升而芦苇的I_sat值无变化。表明Na₂S处理对互花米草的光合能力有促进作用, 但对芦苇的光合能力有抑制作用。NaCl处理后互花米草P_n值也出现小幅上升, 而芦苇P_n值略有下降。Na₂SO₄处理对互花米草和芦苇的P_n值均无显著影响。除Na₂SO₄处理的互花米草外, 不同盐处理后的互花米草和芦苇非光化学淬灭(NPQ)均出现上升趋势。研究结果表明互花米草对环境硫胁迫的适应能力显著高于芦苇, 暗示盐沼高硫环境尤其是硫化物有助于互花米草相对于芦苇的竞争, 也很可能是其形成单一植被的重要原因之一。     

Effect of arbuscular mycorrhizae on soil microbial populations and associated plant performance of the salt marsh grass Spartina patens

The effect of arbuscular mycorrhizae (AM) on soil microbial populations and on growth performance of the high salt marsh plant Spartina patens was investigated in a AM suppression study on field-collected soil cores with S. patens. The application of benomyl resulted in a significant reduction of AM colonization on roots of S. patens, but did not completely suppress AM. Non-treated cores had significantly greater colonization (26 ± 6%) than either benomyl- (12 ± 7%) or benomyl-phosphorus-treated (7 ± 3%) cores at a depth of 2.5 cm. Colonization differences between cores declined with depth (5.0 and 7.5 cm), however, so that at 7.5 cm there was no difference between treatments. This decline was attributed to a reduction in oxygen availability with depth as evidenced by decreasing redox potential. Basic environmental conditions generally resembled those found at the field site. There were no environmental differences between treatments at the depths examined. Cell numbers and specific biomass of DAPI-stained organisms as well as members of the Domain Bacteria were significantly higher when AM colonization was suppressed, while those of the Domains Eucarya and Archaea were not significantly influenced. The increase in both microbial and bacterial population size and biomass in the presence of lower levels of AM colonization is most likely due to increases in carbon exudation to soil and rhizosphere populations that accompany AM suppression. PCR-RFLP analysis of nifH amplicons in bulk soil and rhizosphere at varying depths through the soil cores showed differences in banding patterns between rhizosphere and soil material in the presence of AM. The lack of such strong differences in the benomyl-treated cores suggests that AM colonization more strongly affects the nitrogen-fixing population than do physicochemical conditions (e.g. redox potential) alone. Plant growth performance assessed by analyzing root and leaf biomass, as well as excitation transfer efficiency of open photosynthesis system II (PS II) reaction centers (Fv/Fm) was not significantly influenced by AM. Significant differences were found between treatments for C/N ratios and nitrogen content in leaf tissue, indicating that suppression of AM increased plant nitrogen acquisition.     

三种盐胁迫对互花米草和芦苇光合作用的影响北大核心CSCD

互花米草(Spartina alterniflora)的入侵给海岸带盐沼生态系统的结构和功能带来了显著影响。互花米草盐沼中的硫含量高于附近的土著芦苇(Phragmites australis)盐沼。为探讨硫元素对互花米草和芦苇竞争过程的可能影响及其作用机制,以50mmol·L–1的Na2SO4和Na2S对互花米草和芦苇进行处理,分析处理前后5天内两种植物光合气体交换和叶绿素荧光指标变化的差异,实验另设等Na+浓度的Na Cl处理作为比较。研究发现:Na2S对互花米草和芦苇光合作用影响的差异最大,Na Cl次之,Na2SO4最小。Na2S处理后,互花米草净光合速率(Pn)出现显著上升,芦苇Pn值大幅度下降。互花米草的光饱和点(Isat)上升而芦苇的Isat值无变化。表明Na2S处理对互花米草的光合能力有促进作用,但对芦苇的光合能力有抑制作用。Na Cl处理后互花米草Pn值也出现小幅上升,而芦苇Pn值略有下降。Na2SO4处理对互花米草和芦苇的Pn值均无显著影响。除Na2SO4处理的互花米草外,不同盐处理后的互花米草和芦苇非光化学淬灭(NPQ)均出现上升趋势。研究结果表明互花米草对环境硫胁迫的适应能力显著高于芦苇,暗示盐沼高硫环境尤其是硫化物有助于互花米草相对于芦苇的竞争,也很可能是其形成单一植被的重要原因之一。     

In situ biogenic silica variations in the invasive salt marsh plant, Spartina alterniflora: A possible link with environmental stress

Aims

Higher plants are an understudied component of the global silicon cycle; they absorb silicic acid (dSi) which is stored as biogenic silica (bSiO₂). Si is believed to alleviate physical, chemical, and biological stresses such as storms, high salinity, heavy metal toxicity, grazing, and disease. We investigated a Si-accumulating invasive species growing in the tidal marshes of the Bay of Brest (France), viz., Spartina alterniflora. Our objectives were to determine (1) where and when bSiO₂ accumulates in the plant during its life cycle, (2) whether this accumulation varies with abiotic factors: wave action, estuarine salinity, and duration of immersion, and (3) if the accumulation was limited by dSi availability in marsh porewater.

Methods

A 2?years field survey permitted to sample plants which were analyzed for there bSiO₂ concentrations. Sediment cores were sampled seasonally and the dSi concentrations in the porewater were measured from 0 to 10?cm.

Results

bSiO₂ accumulated more in mature leaves than in other organs. There was a strong linear relationship between bSiO₂ concentration and plant length. bSiO₂ concentrations did not increase, but rather decreased as a function of exposure to the three abiotic factors tested. dSi availability was not significantly different for each of the tested sites and dSi profiles did not exhibit huge losses in the root zone.

Conclusions

Our evidence suggests that dSi availability did not seem to be a limiting factor. bSiO₂ did not increase with increasing abiotic stresses but was strongly correlated with growth. Hence, S. alterniflora is likely to have other adaptive strategies for dealing with environmental stressors but it did not exclude the possible role of Si in alleviating these stresses. If this is the case, there remain intriguing questions about Si uptake, its availability, and its role in silicification and growth.     

互花米草凋落物有机碳在不同发育阶段盐沼中的分解动态

采用分解袋法,在江苏盐城新洋港互花米草(Spartina alterniflora)盐沼,根据盐沼发育阶段,从海向陆设置以下5个样地:建群3年互花米草盐沼(SAF2011)、5年互花米草盐沼(SAF2009)、11年互花米草盐沼(SAF2003)、25年互花米草盐沼(SAF1989)以及互花米草-碱蓬(Suaeda salsa)盐沼(SAF-SS),研究互花米草凋落物分解过程中总有机碳、惰性有机碳及活性有机碳动态变化。结果表明:(1)各样地凋落物经过1年的分解,其总有机碳、惰性有机碳、活性有机碳残留率逐月下降,惰性有机碳残留率高于活性有机碳残留率,有机碳各组分分解残留率随盐沼建群时间增加而增加;(2)各样地凋落物有机碳分解速率有明显季节变化,总有机碳、惰性有机碳和活性有机碳分解速率分别在8、1和6月最高,总有机碳和活性有机碳分解速率在10和2月较低,惰性有机碳分解速率在12和2月较低;活性有机碳分解速率高于惰性有机碳;凋落物总有机碳和活性有机碳分解速率均随着互花米草盐沼建群时间的增加而下降,建群3年盐沼中凋落物惰性有机碳分解速率最高;(3)凋落物分解后,总有机碳中惰性有机碳比重增高,活性有机碳比重降低,总有机碳中活性有机碳的分解比例大于惰性有机碳;(4)各样地凋落物总有机碳年分解量分别为0.3167、0.2632、0.2608、0.2380和0.2583 g·g-1,建群3年的盐沼中凋落物有机碳分解量高于其余样地。盐沼发育程度对凋落物的分解动态有显著影响。     

植物的光合作用与光合氮、碳代谢的耦联及调节

概述了光合作用反应与CO2同化和NO^-3/NO^-2还原的耦联关系,提出了应该从氮,碳代谢整合角度讨论作动和光合作用,以便根据生产目的,调节作物的氮,碳代谢,实现农业生产的高产,优质。     

Effects of salt marsh invasion by Spartina alterniflora on sulfate-reducing bacteria in the Yangtze River estuary,China

To investigate how plant invasion affects sulfate-reducing bacteria (SRB) responsible for sulfate reduction, we conducted a comparative study of diversity and composition of SRB in rhizosphere soils of invasive exotic species (Spartina alterniflora) and two native species (Phragmites australis and Scirpus mariqueter) on Jiuduansha Island located in the Yangtze River estuary, China. Throughout the growing season, profiles of DGGE fingerprints of SRB had distinct variations in relation to phenological stages of these three plant species. The higher richness and abundance of SRB in the rhizospheres of native plants mainly occurred when the plants were in vegetative growth and reproductive stages. However, the higher richness and abundance of SRB also occurred in the late growing season (senescent stage) of S. alterniflora rhizosphere, during which Desulfobulbus, Desulfuromonas, Desulfovibrio, and Firmicutes were dominant. Our results adding to our previous studies suggested that abundant SRB in late stage might have close relationships with decomposition of soil organic matters produced by S. alterniflora.