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Tolerance of Phyllospadix scouleri seedlings to hydrogen sulfide
Abstract
Phyllospadix scouleri is a common seagrass along the rocky intertidal coast of the Pacific Northwest. Previously we established a correlation between increased sulfide and hydrogen sulfide (H2S) and Zostera marina seedling senescence. While Z. marina grows in soft sediment environments, here we evaluate the possibility that P. scouleri may experience similar decreases in health when exposed to increasing H2S loading. To do this, seedlings were immersed in various concentrations of H2S, in axenic media, and photosynthetic and respiratory output was measured. We found that at high doses (mM) of H2S Photosystem II was inhibited whereas Photosystem I remained active. At lower levels, total photosynthetic output decreased with increasing H2S concentrations. Using these data we produced an LD50 of 430 μM at 48 h and 86 μM at 7 days. Our study confirms that Phyllospadix seedlings are also vulnerable to increasing sulfide loads.
... Under stress from, for example shading (Holmer and Laursen, 2002), the oxygen release from seagrass roots decreases and sulphide production consequently increases. Sulphide can then diffuse via root and rhizome tissues into the photosynthesizing tissues (Hasler-Sheetal and Holmer, 2015;Mascaró et al., 2009), where it can inhibit photosynthesis by, for example, affecting photosystem II (Dooley et al., 2015) or the activity of enzymes such as cytochrome c oxidase in mitochondria (Koch et al., 1990;Lamers et al., 2013), leading to a decline in energy production and negative effects on a range of other metalcontaining enzymes (Hasler-Sheetal et al., 2016). ...
Though seagrass meadows are among the most productive habitats in the world, contributing substantially to long-term carbon storage, studies of the effects of critical disturbances on the fate of carbon sequestered in the sediment and biomass of these meadows are scarce. In a manipulative in situ experiment, we studied the effects of successive loss of seagrass biomass as a result of shading and simulated grazing at two intensity levels on sulphide (H2S) content and methane (CH4) emission in a tropical seagrass meadow in Zanzibar (Tanzania). In all disturbed treatments, we found a several-fold increase in both the sulphide concentration of the sediment pore-water and the methane emissions from the sediment surface (except for CH4 emissions in the low-shading treatment). This could be due to the ongoing degradation of belowground biomass shed by the seagrass plants, supporting the production of both sulphate-reducing bacteria and methanogens, possibly exacerbated by the loss of downwards oxygen transport via seagrass plants. The worldwide rapid loss of seagrass areas due to anthropogenic activities may therefore have significant effects on carbon sink-source relationships within coastal seas.
... marina) is the most widespread species throughout the temperate northern hemisphere of the Atlantic and Pacific (Green & Short, 2003; Olsen et al., 2016). Growth and physiology of adult plants have received considerable amounts of attention over the last three decades (Phillips, 1972; Phillips, McMillan & Bridges, 1983; Hemminga & Carlos, 2000; Touchette & Burkholder, 2000; Phillips, Milchakova & Alexandrov, 2006; Lee, Park & Kim, 2007; Leoni et al., 2008; Collier, Waycott & McKenzie, 2012; Dooley et al., 2015; Kaldy et al., 2015); however, there is by far less information available concerning the key environmental factors, such as salinity and temperature, that influence seed germination, seedling establishment, and seedling growth (Orth & Moore, 1983; Hootsmans, Vermaat & Van Vierssen, 1987; Conacher, Poiner & O'Donohue, 1994; Abe, Kurashima & Maegawa, 2008; Tanner & Parham, 2010; Salo & Pedersen, 2014; Park, Lee & Son, 2014; Kaldy et al., 2015). Moreover, results to date have been inconsistent as to whether or not the salinity significantly effects seed germination, which might be influenced by the design of salinity levels (Orth et al., 2000). ...
Globally, seagrass beds have been recognized as critical yet declining coastal habitats. To mitigate seagrass losses, seagrass restorations have been conducted in worldwide over the past two decades. Seed utilization is considered to be an important approach in seagrass restoration efforts. In this study, we investigated the effects of salinity and temperature on seed germination, seedling establishment, and seedling growth of eelgrass Zostera marina L. (Swan Lake, northern China). We initially tested the effects of salinity (0, 5, 10, 15, 20, 25, 30, 35, and 40 ppt) and water temperature (5, 10, 15, and 20 C) on seed germination to identify optimal levels. To identify levels of salinity that could potentially limit survival and growth, and, consequently, the spatial distribution of seedlings in temperate estuaries, we then examined the effect of freshwater and other salinity levels (10, 20, and 30 ppt) on seedling growth and establishment to confirm suitable conditions for seedling development. Finally, we examined the effect of transferring germinated seeds from freshwater or low salinity levels (1, 5, and 15 ppt) to natural seawater (32 ppt) on seedling establishment rate (SER) at 15 C. In our research, we found that: (1) Mature seeds had a considerably lower moisture content than immature seeds; therefore, moisture content may be a potential indicator of Z. marina seed maturity; (2) Seed germination significantly increased at low salinity (p < 0.001) and high temperature (p < 0.001). Salinity had a much stronger influence on seed germination than temperature. Maximum seed germination (88.67 ± 5.77%) was recorded in freshwater at 15 C; (3) Freshwater and low salinity levels (< 20 ppt) increased germination but had a strong negative effect on seedling morphology (number of leaves per seedling reduced from 2 to 0, and maximum seedling leaf length reduced from 4.48 to 0 cm) and growth (seedling biomass reduced by 46.15–66.67% and maximum seedling length reduced by 21.16–69.50%). However, Z. marina performed almost equally well at salinities of 20 and 30 ppt. Very few germinated seeds completed leaf differentiation and seedling establishment in freshwater or at low salinity, implying that freshwater and low salinity may potentially limit the distribution of this species in coastal and estuarine waters. Therefore, the optimum salinity for Z. marina seedling establishment and How to cite this article Xu et al. (2016), Salinity and temperature significantly influence seed germination, seedling establishment, and seedling growth of eelgrass Zostera marina L..
Sulfur deposition and eutrophication accelerated the production of hydrogen sulfide (H2S) in aquatic ecosystems. However, the understanding of how H2S affects the invasive potential of exotic aquatic plants is inadequate. Here, the exotic Elodea nuttallii (Planch.) H. St. John was exposed to five H2S concentrations (0–1.0 mM) and compared with the native Hydrilla verticillata (L. f.) Royle. Both plants grew well below 0.5 mM H2S, with E. nuttallii showing better performance. E. nuttallii and H. verticillata maintained their height growth rates by reducing the ramets number and the relative growth rate, respectively. This trade-off in morphological traits was for adequate light and oxygen. Furthermore, E. nuttallii exhibited higher chlorophyll and carotenoids content but lower chlorophyll a/b, indicating better utilization of low light in the water body. High concentrations of H2S induced oxidative stress in E. nuttallii, leading to higher superoxide dismutase (SOD), soluble sugars, and starch. The utilization strategies of C, N, P and S by E. nuttallii remained unchanged with varying H2S concentrations, demonstrating higher stoichiometric stability. In conclusion, E. nuttallii showed greater resistance to H2S compared to H. verticillata. The invasive potential of E. nuttallii in H2S-enriched aquatic environments was depending on the H2S concentration of native community.
Hydrogen sulfide (H2S) was initially recognized as a toxic gas and its biological functions in mammalian cells have been gradually discovered during the past decades. In the latest decade, numerous studies have revealed that H2S has versatile functions in plants as well. In this review, we summarize H2S-mediated sulfur metabolic pathways, as well as the progress in the recognition of its biological functions in plant growth and development, particularly its physiological functions in biotic and abiotic stress responses. Besides direct chemical reactions, nitric oxide (NO) and hydrogen peroxide (H2O2) have complex relationships with H2S in plant signaling, both of which mediate protein post-translational modification (PTM) to attack the cysteine residues. We also discuss recent progress in the research on the three types of PTMs and their biological functions in plants. Finally, we propose the relevant issues that need to be addressed in the future research.
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Supplementary information:
The online version contains supplementary material available at 10.1007/s42994-021-00035-4.
When two ecosystem engineers share the same natural environment, the outcome of their interaction will be unclear if they have contrasting habitat-modifying effects (e.g., sediment stabilization vs. sediment destabilization). The outcome of the interaction may depend on local environmental conditions such as season or sediment type, which may affect the extent and type of habitat modification by the ecosystem engineers involved. We mechanistically studied the interaction between the sediment-stabilizing seagrass Zostera noltii and the bioturbating and sediment-destabilizing lugworm Arenicola marina, which sometimes co-occur for prolonged periods. We investigated (1) if the negative sediment destabilization effect of A. marina on Z. noltii might be counteracted by positive biogeochemical effects of bioirrigation (burrow flushing) by A. marina in sulfide-rich sediments, and (2) if previously observed nutrient release by A. marina bioirrigation could affect seagrasses. We tested the individual and combined effects of A. marina presence and high porewater sulfide concentrations (induced by organic matter addition) on seagrass biomass in a full factorial lab experiment. Contrary to our expectations, we did not find an effect of A.
marina on porewater sulfide concentrations. A. marina activities affected the seagrass physically as well as by pumping nutrients, mainly ammonium and phosphate, from the porewater to the surface water, which promoted epiphyte growth on seagrass leaves in our experimental set-up. We conclude that A. marina bioirrigation did not alleviate sulfide stress to seagrasses. Instead, we found synergistic negative effects of the presence of A. marina and high sediment sulfide levels on seagrass biomass.
In wetland soils and underwater sediments of marine, brackish and freshwater systems, the strong phytotoxin sulfide may accumulate as a result of microbial reduction of sulfate during anaerobiosis, its level depending on prevailing edaphic conditions. In this review, we compare an extensive body of literature on phytotoxic effects of this reduced sulfur compound in different ecosystem types, and review the effects of sulfide at multiple ecosystem levels: the ecophysiological functioning of individual plants, plant-microbe associations, and community effects including competition and facilitation interactions. Recent publications on multi-species interactions in the rhizosphere show even more complex mechanisms explaining sulfide resistance. It is concluded that sulfide is a potent phytotoxin, profoundly affecting plant fitness and ecosystem functioning in the full range of wetland types including coastal systems, and at several levels. Traditional toxicity testing including hydroponic approaches generally neglect rhizospheric effects, which makes it difficult to extrapolate results to real ecosystem processes. To explain the differential effects of sulfide at the different organizational levels, profound knowledge about the biogeochemical, plant physiological and ecological rhizosphere processes is vital. This information is even more important, as anthropogenic inputs of sulfur into freshwater ecosystems and organic loads into freshwater and marine systems are still much higher than natural levels, and are steeply increasing in Asia. In addition, higher temperatures as a result of global climate change may lead to higher sulfide production rates in shallow waters.
Seagrasses, a functional group of marine flowering plants rooted in the world's coastal oceans, support marine food webs and provide essential habitat for many coastal species, playing a critical role in the equilibrium of coastal ecosystems and human livelihoods. For the first time, the probability of extinction is determined for the world's seagrass species under the Categories and Criteria of the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species. Several studies have indicated that seagrass habitat is declining worldwide. Our focus is to determine the risk of extinction for individual seagrass species, a 4-year process involving seagrass experts internationally, compilation of data on species' status, populations, and distribution, and review of the biology and ecology of each of the world's seagrass species. Ten seagrass species are at elevated risk of extinction (14% of all seagrass species), with three species qualifying as Endangered. Seagrass species loss and degradation of seagrass biodiversity will have serious repercussions for marine biodiversity and the human populations that depend upon the resources and ecosystem services that seagrasses provide.
Seagrasses, marine flowering plants, have a long evolutionary history but are now challenged with rapid environmental changes as a result of coastal human population pressures. Seagrasses provide key ecological services, including organic carbon production and export, nutrient cycling, sediment stabilization, enhanced biodiversity, and trophic transfers to adjacent habitats in tropical and temperate regions. They also serve as “coastal canaries,” global biological sentinels of increasing anthropogenic influences in coastal ecosystems, with large-scale losses reported worldwide. Multiple stressors, including sediment and nutrient runoff, physical disturbance, invasive species, disease, commercial fishing practices, aquaculture, overgrazing, algal blooms, and global warming, cause seagrass declines at scales of square meters to hundreds of square kilometers. Reported seagrass losses have led to increased awareness of the need for seagrass protection, monitoring, management, and restoration. However, seagrass science, which has rapidly grown, is disconnected from public awareness of seagrasses, which has lagged behind awareness of other coastal ecosystems. There is a critical need for a targeted global conservation effort that includes a reduction of watershed nutrient and sediment inputs to seagrass habitats and a targeted educational program informing regulators and the public of the value of seagrass meadows.
Many natural and human-induced events create disturbances in seagrasses throughout the world, but quantifying losses of habitat is only beginning. Over the last decade, 90000 ha of seagrass loss have been documented although the actual area lost is certainly greater. Seagrasses, an assemblage of marine flowering plant species, are valuable structural and functional components of coastal ecosystems and are currently experiencing worldwide decline. This group of plants is known to support a complex trophic food web and a detritus-based food chain, as well as to provide sediment and nutrient filtration, sediment stabilization, and breeding and nursery areas for finfish and shellfish.
We define disturbance, natural or human-induced, as any event that measurably alters resources available to seagrasses so that a plant response is induced that results in degradation or loss. Applying this definition, we find a common thread in many seemingly unrelated seagrass investigations. We review reports of seagrass loss from both published and ‘grey’ literature and evaluate the types of disturbances that have caused seagrass decline and disappearance. Almost certainly more seagrass has been lost globally than has been documented or even observed, but the lack of comprehensive monitoring and seagrass. mapping makes an assessment of true loss of this resource impossible to determine.
Natural disturbances that are most commonly responsible for seagrass loss include hurricanes, earthquakes, disease, and grazing by herbivores. Human activities most affecting seagrasses are those which alter water quality or clarity: nutrient and sediment loading from runoff and sewage disposal, dredging and filling, pollution, upland development, and certain fishing practices. Seagrasses depend on an adequate degree of water clarity to sustain productivity in their submerged environment. Although natural events have been responsible for both large-scale and local losses of seagrass habitat, our evaluation suggests that human population expansion is now the most serious cause of seagrass habitat loss, and specifically that increasing anthropogenic inputs to the coastal oceans are primarily responsible for the world-wide decline in seagrasses.
Sudden events of seagrass die-off have been suggested to be induced by invasion of the phytotoxin sulphide under environmental stress generating low oxygen supply in seagrass tissues. Laboratory experiments were conducted with eelgrass (Zostera marina L.) to measure intra-plant changes in oxygen and sulphide by means of microelectrodes at different oxygen concentrations in the water column. The objectives were to examine whether sulphide intrusion into seagrass tissues can be induced, to determine the role of plant oxygen status for sulphide intrusion and to determine how fast internal sulphide pools are depleted after internal oxygen supplies have been restored. Under conditions with oxygen partial pressures (pO2) above 7.4 kPa (> 35% of air saturation) within eelgrass rhizomes or meristematic tissues no intrusion of sulphide occurred in spite of high sediment concentrations of gaseous sulphide (> 1000 µm). Lack of sulphide intrusion at high internal pO2 suggested that oxygen release from the roots ensured complete re-oxidation of sulphide in the rhizosphere. Under oxygen stress, however, the experiments clearly demonstrated intrusion of sulphide in eelgrass rhizomes and meristematic tissues. Rates of sulphide intrusion were controlled by internal pO2, which in turn was controlled by water column oxygen concentrations. Maximum internal sulphide content reached 325 µm which by far exceeded the 1–10 µm known to inhibit mitochondrial activity in eukaryotic cells. Sulphide and low levels of oxygen could coexist in the eelgrass tissues reflecting fast internal transport of sulphide and slow rates of sulphide re-oxidation. Upon re-establishment of high internal oxygen concentrations the depletion of the sulphide pool was slow (half-life = 20–30 min) indicating, that sulphide re-oxidation within the eelgrass tissue was not bacterially or enzymatically facilitated but occurred by simple chemical oxidation. The results of this study are consistent with the proposed detrimental role of sulphide intrusion in events of sudden seagrass die-off.
Change analysis of eelgrass distribution in Waquoit Bay demonstrated a rapid decline of eelgrass habitat between 1987 and
1992. Aerial photography and ground-truth assessments of eelgrass distribution in the Waquoit Bay National Estuarine Research
Reserve documented progressive loss in eelgrass acreage and fragmentation of eelgrass beds that we relate to the degree of
housing development and associated nitrogen loading, largelyvia groundwater, within various sub-basins of the estuary. The
sub-basins with greater housing density and higher nitrogen loading rates showed more rapid rates of eelgrass decline. In
eelgrass mesocosm studies at the Jackson Estuarine Laboratory, excessive nitrogen loading stimulated proliferation of algal
competitors (epiphytes, macroalgae, and phytoplankton) that shade and thereby stress eelgrass. We saw domination by each of
these three algal competitors in our field observations of eelgrass decline in Waquoit Bay. Our study is the first to relate
housing development and nitrogen loading rates to eelgrass habitat loss. These results for the Waquoit Bay watershed provide
supporting evidence for management to limit development that results in groundwater nitrogen loading and to initiate remedial
action in order to reverse trends in eelgrass habitat loss.
Direct census of shoots tagged in permanent plots was used to assess the present (2000–2002)Posidonia oceanica population dynamics in 25 meadows along the Spanish Mediterranean Coast. Shoot density ranged from 154±8 to 1,551±454 shoots m−2, absolute shoot mortality from 5±0 to 249±53 shoots m−2 yr−1, and absolute shoot recruitment from −2yr−1. Specific shoot mortality and recruitment rates, which are mathematically and statistically (p>0.05) independent of shoot density, varied from 0.015±0.006 to 0.282±0.138 yr−1 and 0.018±0.005 to 0.302±0.093 yr−1, respectively. Absolute shoot mortality rate was scaled to shoot density (Pearson correlation, r=0.78, pShoot half-life was estimated to range from 2.5 to 60.4 yr and meadow turnover times between 6.7 yr and more than a century, provided current estimates of shoot mortality, recruitment rates, and density remain uniform. There were differences in shoot mortality and recruitment at the regional scale, with the meadows developing along the coast of the Spanish mainland experiencing the highest shoot mortality (Tukey test, p
Seagrasses evolved from terrestrial plants into marine foundation species around 100 million years ago. Their ecological success,
however, remains a mystery because natural organic matter accumulation within the beds should result in toxic sediment sulfide
levels. Using a meta-analysis, a field study, and a laboratory experiment, we reveal how an ancient three-stage symbiosis
between seagrass, lucinid bivalves, and their sulfide-oxidizing gill bacteria reduces sulfide stress for seagrasses. We found
that the bivalve–sulfide-oxidizer symbiosis reduced sulfide levels and enhanced seagrass production as measured in biomass.
In turn, the bivalves and their endosymbionts profit from organic matter accumulation and radial oxygen release from the seagrass
roots. These findings elucidate the long-term success of seagrasses in warm waters and offer new prospects for seagrass ecosystem
conservation.
Drought is one of the major factors limiting barley yields in many developing countries worldwide. The identification of molecular
markers linked to genes controlling drought tolerance in barley is one way to improve breeding efficiency. In this study,
we analyzed the quantitative trait loci (QTL) controlling chlorophyll content and chlorophyll fluorescence in 194 recombinant
inbred lines (RILs) developed from the cross between the cultivar ‘Arta’ and Hordeum spontaneum 41-1. Five traits, chlorophyll content, and four chlorophyll fluorescence parameters, namely initial fluorescence (Fo), maximum fluorescence (Fm), variable fluorescence (Fv), and maximum quantum efficiency of PSII (Fv/Fm) which are related to the activity of the photosynthetic apparatus, were measured under well-watered and drought stress conditions
at post-flowering stage. QTL analysis identified a total of nine and five genomic regions, under well-watered and drought
stress conditions, respectively, that were significantly associated with the expression of the five target traits at post-flowering
stage. No common QTL was detected except one for chlorophyll content, which was identified in both growth conditions, demonstrating
that the genetic control of the expression of the traits related to photosynthesis differed under different water conditions.
A QTL for Fv/Fm, which is related to the drought tolerance of photosynthesis was identified on chromosome 2H at 116cM in the linkage map
under drought stress. This QTL alone explained more than 15% of phenotypic variance of maximum quantum yield of PSII, and
was also associated with the expression of four other traits. In addition, another QTL for Fv/Fm was also located on the same chromosome (2H) but at 135.7cM explaining around 9% of the phenotypic variance under drought
conditions. The result presented here suggest that two major loci, located on chromosome 2H, are involved in the development
of functional chloroplast at post-flowering stage for drought tolerance of photosynthesis in barley under drought stress.
If validated in other populations, chlorophyll fluorescence parameters could be used as selection criteria for drought tolerance.
The surfgrass Phyllospadix japonicus is an abundant seagrass on the exposed rocky shores of the Korean peninsula. Many surfgrass meadows on the coasts of Korea have been adversely affected by anthropogenic activities and natural phenomena such as “rock whitening,” which is caused by overgrowths of crustose coralline algae. Few attempts have been made to develop transplanting techniques for surfgrass, owing to the difficulty of transplant survival on exposed rocky shores. We developed a new Phyllospadix Transplant System (PTS), which was an artificial underwater structure constructed of a 4:5:1 ratio of cement, sand, and water. In January 2005, we transplanted P. japonicus shoots using the newly developed PTS. P. japonicus shoots were tied together at the lower part of the sheaths to iron screws, which were placed in a hollow part of the PTS, with latex elastic. To evaluate the feasibility of this transplantation technique, we compared vegetative and reproductive shoot densities, growth, and morphological characteristics in transplants to those of shoots in nearby natural beds over a 2-year period. Transplant density increased gradually without significant initial shoot loss. Approximately 6 months after transplantation, leaf growth of transplants was similar to that of naturally growing shoots. The shoot heights and sheath lengths of transplants were similar to those of naturally growing shoots at 10–15 months post-planting. Approximately 50 mm of rhizome elongation was detected in P. japonicus transplants over the 2-year period. Since P. japonicus transplants attached to the PTS successfully established at the test area, the transplantation of surfgrass using the PTS may be an effective approach to restoring the habitat of P. japonicus.
Highly reducing sediments are prevalent in seagrass environments. Under anoxic conditions, hydrogen sulfide can accumulate as an end product of anaerobic respiration at levels which may be toxic to halophytes. The photosynthetic response of Zostera marina L. (eelgrass) to manipulations in sediment sulfide concentration and light regimes was examined in Chincoteague Bay in June 1991. Neutral density screens were used in a mesocosm experiment to decrease downwelling irradiance to 50 and 15% of insolation. Sediment sulfide levels were enriched using Na2S and lowered using FeSO4. Photosynthesis vs. irradiance (PI) relationships were determined experimentally at ten light levels throughout the 21 day experiment. Photoadaptation was detected in response to the previous 4 day light history of the plants, as maximum photosynthesis (Pmax) decreased in response to lower daily light levels. Negative impacts of sulfide on eelgrass in this study were observed through reductions in Pmax, increases in the light intensity at which gross photosynthesis equals respiration, and decreases in the initial slope of the PI curve. The effects of eutrophication through reduced light and increased sediment sulfide on Pmax were additive. Elevated sediment sulfide levels may contribute to seagrass loss in stressed areas as the potential for utilization of available light is reduced.
Hydrogen sulphide (H(2)S) is emerging as a potential messenger molecule involved in modulation of physiological processes in animals and plants. In this report, the role of H(2)S in modulating photosynthesis of Spinacia oleracea seedlings was investigated. The main results are as follows. (i) NaHS, a donor of H(2)S, was found to increase the chlorophyll content in leaves. (ii) Seedlings treated with different concentrations of NaHS for 30 d exhibited a significant increase in seedling growth, soluble protein content, and photosynthesis in a dose-dependent manner, with 100 μM NaHS being the optimal concentration. (iii) The number of grana lamellae stacking into the functional chloroplasts was also markedly increased by treatment with the optimal NaHS concentration. (iv) The light saturation point (Lsp), maximum net photosynthetic rate (Pmax), carboxylation efficiency (CE), and maximal photochemical efficiency of photosystem II (F(v)/F(m)) reached their maximal values, whereas the light compensation point (Lcp) and dark respiration (Rd) decreased significantly under the optimal NaHS concentration. (v) The activity of ribulose-1,5-bisphosphate carboxylase (RuBISCO) and the protein expression of the RuBISCO large subunit (RuBISCO LSU) were also significantly enhanced by NaHS. (vi) The total thiol content, glutathione and cysteine levels, internal concentration of H(2)S, and O-acetylserine(thiol)lyase and L-cysteine desulphydrase activities were increased to some extent, suggesting that NaHS also induced the activity of thiol redox modification. (vii) Further studies using quantitative real-time PCR showed that the gene encoding the RuBISCO large subunit (RBCL), small subunit (RBCS), ferredoxin thioredoxin reductase (FTR), ferredoxin (FRX), thioredoxin m (TRX-m), thioredoxin f (TRX-f), NADP-malate dehydrogenase (NADP-MDH), and O-acetylserine(thiol)lyase (OAS) were up-regulated, but genes encoding serine acetyltransferase (SERAT), glycolate oxidase (GYX), and cytochrome oxidase (CCO) were down-regulated after exposure to the optimal concentration of H(2)S. These findings suggest that increases in RuBISCO activity and the function of thiol redox modification may underlie the amelioration of photosynthesis and that H(2)S plays an important role in plant photosynthesis regulation by modulating the expression of genes involved in photosynthesis and thiol redox modification.
Positive feedbacks cause a nonlinear response of ecosystems to environmental change and may even cause bistability. Even though the importance of feedback mechanisms has been demonstrated for many types of ecosystems, their identification and quantification is still difficult. Here, we investigated whether positive feedbacks between seagrasses and light conditions are likely in seagrass ecosystems dominated by the temperate seagrass Zostera marina. We applied a combination of multiple linear regression and structural equation modeling (SEM) on a dataset containing 83 sites scattered across Western Europe. Results confirmed that a positive feedback between sediment conditions, light conditions and seagrass density is likely to exist in seagrass ecosystems. This feedback indicated that seagrasses are able to trap and stabilize suspended sediments, which in turn improves water clarity and seagrass growth conditions. Furthermore, our analyses demonstrated that effects of eutrophication on light conditions, as indicated by surface water total nitrogen, were on average at least as important as sediment conditions. This suggests that in general, eutrophication might be the most important factor controlling seagrasses in sheltered estuaries, while the seagrass-sediment-light feedback is a dominant mechanism in more exposed areas. Our study demonstrates the potentials of SEM to identify and quantify positive feedbacks mechanisms for ecosystems and other complex systems.
Recolonisation of Zostera marina, following complete destruction caused by an anoxic crisis, was studied in the Than lagoon (French Mediterranean Sea) from February 1998 to September 1999. The recolonisation took place surprisingly rapidly as biomasses similar to those from untouched areas were reached only nine months after seed germination. The recolonisation success was partly due to a high seedling survival rate as well as a rapid vegetative recruitment (ranging from 0.012 to 0.042 per day). Two phases of recovery could be observed: a rapid multiplication of shoots during the first 3 months was followed by an increase in biomass due to elongation of leaves. During the first year of recolonisation no flowering shoot was observed whilst reproductive effort was considerable during the second year. In case of two consecutive anoxic crises at the same site, the recovery would have probably been much slower, since the annual seedbank would have been depleted.
Coastal ecosystems and the services they provide are adversely affected by a wide variety of human activities. In particular, seagrass meadows are negatively affected by impacts accruing from the billion or more people who live within 50 km of them. Seagrass meadows provide important ecosystem services, including an estimated $1.9 trillion per year in the form of nutrient cycling; an order of magnitude enhancement of coral reef fish productivity; a habitat for thousands of fish, bird, and invertebrate species; and a major food source for endangered dugong, manatee, and green turtle. Although individual impacts from coastal development, degraded water quality, and climate change have been documented, there has been no quantitative global assessment of seagrass loss until now. Our comprehensive global assessment of 215 studies found that seagrasses have been disappearing at a rate of 110 km(2) yr(-1) since 1980 and that 29% of the known areal extent has disappeared since seagrass areas were initially recorded in 1879. Furthermore, rates of decline have accelerated from a median of 0.9% yr(-1) before 1940 to 7% yr(-1) since 1990. Seagrass loss rates are comparable to those reported for mangroves, coral reefs, and tropical rainforests and place seagrass meadows among the most threatened ecosystems on earth.
Four different types of adaptation to sulfide among cyanobacteria are described based on the differential toxicity to sulfide of photosystems I and II and the capacity for the induction of anoxygenic photosynthesis. Most cyanobacteria are highly sensitive to sulfide toxicity, and brief exposures to low concentrations cause complete and irreversible cessation of CO(2) photoassimilation. Resistance of photosystem II to sulfide toxicity, allowing for oxygenic photosynthesis under sulfide, is found in cyanobacteria exposed to low H(2)S concentrations in various hot springs. When H(2)S levels exceed 200 muM another type of adaptation involving partial induction of anoxygenic photosynthesis, operating in concert with partially inhibited oxygenic photosynthesis, is found in cyanobacterial strains isolated from both hot springs and hypersaline cyanobacterial mats. The fourth type of adaptation to sulfide is found at H(2)S concentrations higher than 1 mM and involves a complete replacement of oxygenic photosynthesis by an effective sulfide-dependent, photosystem II-independent anoxygenic photosynthesis. The ecophysiology of the various sulfide-adapted cyanobacteria may point to their uniqueness within the division of cyanobacteria.
This experiment used gramineous C4 plant vetiver, Vetiveria zizanioides as material to study the changes of photosynthetic characters and physiological indices under constant heat and drought stress. The results suggested that under the whole stress period, all the relative soil water content the relative leaf water content and the chlorophyll content showed decreasing tendencies, MDA content and dissociated proline content showed increasing tendencies, and the enzyme activity of SOD, POD in peroxide enzyme system increased at first and then decreased. Net photosynthetic rate (Pn) of vetiver's leaf and intercellular CO2 concentration(Ci) decreased notably in the early period of heat and drought stress. Then notable decreases of PSII max photochemistry quanta output (Fv/Fm), PSII effective photochemistry quanta output (Fv'/Fm'), photo-chemical quenching (qP), electron transport rate (ETR) and non photo-chemical quenching (qN) were observed as the extent of stress increased. This suggested that the decrease of photosynthetic rate in the early period of heat and drought stress was caused by the closure of stoma, and the changes of indices in the later period of heat and drought stress was caused by the damage of PSII in leaves.
Target discrimination is a key technique in missile defense system, and after analyzing the defense system configuration, an integrated target discrimination mathematical model based on recursive temporal-spatial data fusion is performed, which brings the expert knowledge, environmental information and measure information into the integrated discrimination flow. During spatial fusion process, the reliability of discrimination approach is measured by Analytical Hierarchy Process (AHP), and the concept of fusion weight is introduced to measure the value of discrimination approach for participating in fusion, which is determined by reliability of discrimination approach and consensus among sensors' results. During temporal fusion process, the fusion discrimination result is inherited and updated by Dempster-Shafer theory. Simulation results show that the integrated discrimination model can fuse the local decision from discrimination approaches effectively, restrain the influence of singularity and tolerate the error of discrimination.
Root respiration decreased across the transect from the intertidal site to the 2 subtidal stations of increased sediment anaerobiosis. Data collected in field and laboratory experiments in which the hydrogen sulphide concentration surrounding the roots and rhizomes was enhanced showed physiological adaptations characteristic of tolerance to anaerobiosis. Zostera marina is thus capable of responding to markedly different microenvironments. - from Authors
Plants of saline habitats can encounter anoxic conditions to extents which vary with habitat and life form. Rhizophytes (with roots or rhizoids in a sediment) routinely have anoxia or hypoxia in their roots, rhizoids and rhizomes. Haptophytes (attached to large particles of substrate) and planophytes (unattached) are generally less prone to anoxia or hypoxia, exceptions being ice-encased polar haptophytes and estuarine and muddy shore macroscopic planophytes (pleustophytes) which can become hypoxic or anoxic as a result of burial in sediment or under new growth.
A major difference between anoxia in low-salinity habitats and anoxia in saline habitats is the presence of high sulphate concentrations in most saline habitats including seawater. Use of sulphate as electron acceptor in microbial oxidation or organic carbon produces sulphide, with relatively less production of methane than in anoxic habitats with lower sulphate concentrations. In addition to its role as a toxin. ³⁴S/³²S natural abundance data show that ³⁴S-depleted sulphide is directly or indirectly used as a sulphur source for roots and rhizomes of seagrasses and for the whole organism of emergent salt marsh herbs and mangroves. Granted the presence of hydrogen sulphide in the rhizosphere, its entry by lipid-solution permeation of the plasmalemma is inevitable. The balance of evidence favours the entry of hydrogen sulphide rather than the oxidation of ³⁴S-depleted sulphide to ³⁴S-depleted sulphate in an aerobic rhizosphere with subsequent entry of sulphate.
The study presented here examined the effects of administering hydrogen sulfide (H2S) to several ancient extant plant species to determine the organisms’ response to stress. Even though sulfur is an essential macronutrient required for growth and productivity, there are toxic compounds of this element that exert detrimental effects and produce physiological stress. It is speculated that the accumulation of H2S, a lethal gas, may have been a major contributing factor in past mass extinction events, where the environment was fairly anoxic with fluctuating temperatures. The potential of this toxic compound to exist as an environmental stressor suggests that certain organisms may have adapted to survive these periods of mass extinctions. It is hypothesized that due to the abundant presence of H2S in the past, ancient land plants may have an adaptive advantage that allowed them to survive and thrive. In this study, species of bryophytes and algae were exposed to specific concentrations of aqueous H2S over a seven-day period and measured their photosynthetic capacity at timed intervals using a FluorCam. Studying the effects of this toxic gas on ancient plants is imperative to our understanding of sulfur's varying biological roles, and provides insight on the evolutionary phenotypic variations amongst plants and stress responses in order to survive mass extinctions. Results indicate that Hypnum, Chlamydomonas, and Charophyta are all able to tolerate significant quantities of H2S and show resilience through increased photosynthetic capacity over a period of exposure, indicating a genetic and phenotypic legacy response.
Successful establishment of seedlings in populations of Zostera marina (eelgrass), especially for restoration efforts using stored seeds, depends in part on viability and germination of seeds. Seeds of Z. marina were collected from plants and stored in seawater at 5 °C for up to several years. Seed viability, assessed with the viability stain, tetrazolium chloride, decreased steadily over a four-year period. There was a strong correlation between age and viability; viability of fresh seeds was approximately 77% whereas for four-year old seeds was 32%. However, only 51 of 975 of fresh seeds that germinated (∼5%) developed leaves. The physical structure of the seed was evaluated to understand the effects of aging. It was determined that as seeds age there is an increase in the number of fractures on the seed coat. These data combined with the recent awareness that global seagrass populations are declining present valuable information to help maintain viable seed repositories which may contribute to the conservation and restoration of these wild plants.
Temporal and spatial variation of d34S, total sulfur (TS) concentration, and elemental sulfur concentration (S0) in leaves, roots, and rhizomes of Zostera marina was followed between June 2002 and May 2003 at four locations in Roskilde Fjord and Øresund, Denmark. These were related to temporal changes in sediment sulfide concentrations, sulfur pool size, and sulfur pool d34S. The d34 So fZ. marina was most negative in the roots, followed by rhizomes and leaves, indicating that roots were mostly affected by sulfide. A significant relationship between decreasing d34S and increasing TS in the plant tissues indicated that sulfide accumulated in the plant and, furthermore, a positive relation between TS and S0 in the plant suggests that part of the sulfide is reoxidized to S0. There were marked temporal changes in all variables at all sites, but the pattern of change varied between sites. The temporal and spatial heterogeneity in plant d34S, TS, and S0 depended on a variety of factors, such as sediment sulfide concentrations and the below : aboveground biomass ratio of the plants. This suggests that mechanisms of sulfide invasion are complex, and several factors (plant morphology, environmental variables) acting in concert or against each other need to be considered to successfully predict sulfide invasion in seagrasses.
The effects of pH changes on photosynthesis in three Mediterranean seagrass species were assessed by combining laboratory experiments with field records of pH. The response of photosynthesis to increasing pH was examined under laboratory conditions. Posidonia oceanica and Cymodocea nodosa showed a linear decrease in photosynthetic rates with pH; values at pH 8.8 were 25–80% of those obtained at pH 8.2. Zostera noltii was much less sensitive to pH increase than the other two species, maintaining high photosynthetic rates up to pH 8.8 and showing a significant reduction only at pH 9. Daily changes in pH over the seagrass meadows showed a maximum amplitude of ca. 0.5 pH units. However, the maximum daily values of pH were reached towards the end of the daily period of photosynthesis, and hence the estimated reduction in photosynthesis caused by the rising pH was relatively small (13–17%), in very shallow (i.e. 1 m deep or less) P. oceanica and C. nodosa meadows, and even less (about 4%) in deeper areas.
Survival, metabolism and growth of Zostera marina exposed to hypoxia and sulfides in the water column were examined during a 3-week experiment. Z. marina was collected in the Archipelago of South Fyn, Denmark and kept under controlled laboratory conditions where they were exposed to low oxygen concentrations and two concentrations of sulfides in the water column. Survival of Z. marina was negatively influenced by the presence of sulfides, and photosynthetic activity stopped after 6 days of exposure to high sulfide concentrations (100–1000μM). Rates of photosynthesis also decreased in plants exposed to hypoxia, but the plants remained alive during the 3-week incubation. Rates of leaf elongation (exposed: 0–13.5mmmm−1d−2; control: 24mmmm−1d−2) and number of leaves per shoot (exposed: 3.2–4.2; control: 5.1) decreased in all treatments compared to control plants indicating that hypoxia and sulfides have negative impacts on Z. marina metabolism. The non-structural carbohydrate reserves in roots were reduced by up to 81% compared to the controls, whereas the reserves of starch in the rhizomes remained similar to the controls. The exposure to hypoxia and sulfides resulted in loss of above-ground biomass, most severe in the sulfide treatment (55% decrease in shoot:root ratio), suggesting that both parameters may play important roles during die-back events of seagrasses.
The effect of synergy between sediment organic enrichment and lack of night oxygen renewal in the water column on growth and survival of Zostera marina, and how it is reflected in the sulfur parameters in the plants (δ34S, TS and S0) was studied experimentally. An experiment consisting of Z. marina mesocosms with different levels of organic enrichment and water column aeration was established, and the effects on sediment conditions, sulfide invasion and growth and survival of Z. marina were examined over a 4 week period. Shoots growing in Ambient Organic matter–sediments showed signs of sulfide invasion, as TS increased in all plant compartments and δ34S of the plant tissues decreased during the experiment, but neither growth rate nor survival were significantly affected. The lack of night oxygen renewal had no evident effects in non-enriched sediments as porewater sulfide concentrations, AVS- and CRS-pools were not different from the corresponding 24 h aeration treatment. Plant growth rate and survival were neither different from the corresponding 24 h aeration treatment. On the contrary, shoots growing on High Organic matter-sediments suffered a massive sulfide invasion and it was directly correlated to the observed decrease in growth rates. Even though the lack of night oxygen renewal had no evident effects on sediment variables there were, however, strong indications that the different aeration levels affected plant performance, suggesting a lower sulfide oxidation capacity and confirming that low water column oxygen concentrations reduces the defense capacity of the shoots against sulfide invasion.Although δ34S, TS and S0 concentrations together provided a powerful set of indicators to detect the invasion of sulfide in Z. marina shoots, this study enlightens the need for a deeper investigation of sulfide intrusion in seagrasses and the relation between plant sulfur parameters and sediment conditions.
Seeds of Zostera marina L. are capable of germinating in anaerobic (nitrogen-gassed) seawater (10%) and developing into seedlings with unusual morphology. The axial hypocotyl of seedlings grown in light and dark was characteristically elongated (mean in dark = 32 mm), but the pumule and adventitious roots remained rudimentary and embryonic in appearance. Aerobic seedlings by contrast exhibited little hypocotyl elongation (mean in dark = 5 mm), and plumule nad roots were well developed. The stalk-like blade of the cotyledon elongated in both aerobic and anaerobic cultures, but growth of the cotyledonary sheath surrounding the plumule was most marked in the presence of oxygen. When seedlings were grown anaerobically in the dark for 16 days and then transferred to aerobic-light conditions, elongation of the axial hypocotyl ceased instantly, but plumule and cotyledonary sheath growth was evident within 32 h, and by 96 h after transfer there was pronounced greening of the plumule leaves and swelling of the root primordia. Hypocotyl elongation of seedlings from seeds planted in sediment varied directly with planting depth. Elongation of the hypocotyl pushed the cotyledonary node with rudimentary plumule and root primordia up through the sediment to a position below the sediment surface where hypocotyl elongation stopped and plumule and root growth commenced. The cotyledonary blade which grew concomitantly with the axial hypocotyl continued to elongate after protruding from the sediment into the overlying water. Results from experiments in which the distal part of a seedling cotyledon was exposed to aerated water while the rest of the seedling was maintained in N2-gassed water suggested that the cotyledon may function as a conduit for oxygen diffusion to the plumular region of the seedling.
Combinations of stresses showed an additive effect in comparison to the individual stress responses. It is apparent from these results that thermal, elevated-light or osmotic stress increases the sensitivity of Halophila ovalis to any of the other stress factors. Photosynthetic stress was detected using chlorophyll a fluorescence. Quantum yield was consistently the most effective measure of photosynthetic stress from combination stress exposures. Chlorophyll pigment analysis supported the general decline in photosynthetic capacity as indicated by the chlorophyll fluorescence; however, several anomalies did occur.
The objectives of this study were to determine the distribution and abundance of Zostera marina (eelgrass) in relation to the distribution of the mat forming bacteria Beggiatoa spp., and the levels of sulfide and organic material (wood waste) in the sediment. Underwater videography and intertidal surveys were used to map the distribution and abundance of Z. marina beds and Beggiatoa in the nearshore area of Commencement Bay, WA (USA), a location that has a long history of sawmill activity. Zostera marina occurred from the intertidal to −6 m mean lower low water (MLLW) on sandy substrates in areas with low levels of sulfide (<50 μm) and organic material (<5 % total volatile solids). Areas with high sulfide levels (>200 μm) occurred where there were significant amounts of organic material in the sediments, which was found to be wood waste that had been discarded from sawmills. Zostera marina was absent from the intertidal and occurred at lower densities in areas with high sulfide levels. In contrast, mats of Beggiatoa were only found in areas where the sulfide levels were >1000 μm and there were significant deposits of wood. Thus, the negative correlation between the distribution and abundance of Z. marina and Beggiatoa suggests that the presence of Beggiatoa mats could be used as a biological indicator of inhibiting levels of hydrogen sulfide in the marine environment.
In this study, we examined anatomical adaptations in the seagrass genus Phyllospadix to rocky substrates and surf exposed habitats not generally exploited by seagrasses. It was hypothesized that the genus exhibits anatomical features related to its specific habitat that distinguish it from a closely related sediment-rooted seagrass. Zostera marina L. A corollary to this hypothesis predicted that individual Phyllospadix species show additional specialization, based on observations that three species are distinctly zoned where they occur together.Comparison with Z. marina was used to define anatomical features in Phyllospadix that are considered adaptations to rocky littoral environments. These features include greater hypodermal fiber and root hair development, thickened rhizomes and smaller lacunae. Comparison among Phyllospadix spp. for microhabitat adaptations was less fruitful, although distribution patterns and comparative leaf thickness of the three species indicated that where P. torreyi S. Watson co-occurs with P. serrulatus Rupr. ex Aschers. P. torreyi may out-compete the latter for space, possibly because it is less vulnerable to disturbance.
The effect of temperature on the photosynthesis and growth of seagrasses may be summarized by considering the ways in which temperature alters the characteristics of the photosynthesis-irradiance (P-I) curve of seagrasses. Within the limits of physiological tolerance (∼6–30°C) temperature has little effect on the initial slope of the P-I curve. At 35–40°C the photosynthetic capacity of seagrasses is reduced. Within the limits of physiological tolerance, the rate of photosynthesis at light saturation, the dark respiration rate and the light compensation point more than double as temperature increases. The optimum temperature for photosynthesis decreases from 25–35°C at light saturation to as low as 5°C as irradiance decreases. As a result of these effects of temperature on the P-I curve, growth of seagrasses in high(saturating) light environments increases with temperature, whereas growth of seagrasses in low (near the light compensation point) light environments decreases as temperature increases.
A large-scale mesocosm (sixteen 500 L tanks) experiment was conducted to investigate the effects of hypersalinity (45–65 psu), porewater sulfide (2–6 mM) and nighttime water column hypoxia (5–3 mg L−1) on the tropical seagrass Thalassia testudinum Banks ex König. We examined stressor effects on growth, shoot survival, tissue sulfur (S0, TS, δ34S) and leaf quantum efficiencies, as well as, porewater sulfides (∑TSpw) and mesocosm water column O2 dynamics. Sulfide was injected into intact seagrass cores of T. testudinum exposing below-ground tissues to 2, 4, and 6 mM S2−, but rapid oxidation resulted in ∑TSpw < 1.5 mM. Hypersalinity at 65 psu lowered sulfide oxidation and significantly affected plant growth rates and quantum efficiencies (Fv/Fm < 0.70). The most depleted rhizome δ34S signatures were also observed at 65 psu, suggesting increased sulfide exposure. Hypoxia did not influence ∑TSpw and plant growth, but strengthened the hypersalinity response and decreased rhizome S0, indicating less efficient oxidation of ∑TSpw. Following nighttime hypoxia treatments, ecosystem level metabolism responded to salinity treatments. When O2 levels were reduced to 5 and 4 mg L−1, daytime O2 levels recovered to approximately 6 mg L−1; however, this recovery was more limited when O2 levels were lowered to 3 mg L−1. Subsequent to O2 reductions to 3 mg O2 L−1, nighttime O2 levels rose in the 35 and 45 psu tanks, stayed the same in the 55 psu tanks, and declined in the 65 psu tanks. Thus, hypersalinity at 65 psu affects T. testudinum's oxidizing capacity and places subtle demands on the positive O2 balance at an ecosystem level. This O2 demand may influence T. testudinum die-off events, particularly after periods of high temperature and salinity. We hypothesize that the interaction between hypersalinity and sulfide toxicity in T. testudinum is their synergistic effect on the critical O2 balance of the plant.
Low iron content in tropical carbonate sediments limits the formation of iron–sulfide compounds such as pyrite. Thus, seagrasses in the tropics may be more susceptible to sulfide toxicity. Sediment sulfide levels greater than 2 mM and up to 13 mM have been hypothesized to cause widespread ‘die-back’ of the tropical seagrass Thalassia testudinum in a subtropical lagoon, Florida Bay. Hydroponic chambers were used to determine the effects of sulfide (0.0, 2.0, 4.0, 6.0 and 10.0 mM) on root and leaf adenylate ratios, energy charge (EC), leaf O2 flux, and growth of T. testudinum under light-saturated conditions. T. testudinum did not suffer mortality under short-term (48 h) exposure to sulfide concentrations up to 10 mM, but several metabolic stress responses were observed. Root ATP and energy charge significantly declined (P<0.05) as a function of increasing sulfide concentrations. Root EC was reduced from 0.78 in the control to 0.63–0.59 in the 2.0–6.0 mM treatments, and fell to 0.43 in the 10.0 mM treatment. Leaf elongation rates declined (P<0.05) by an average of 43% in 2.0–6.0 mM sulfide and 67% in 10 mM sulfide. Although root EC, root ATP production and leaf elongation rates significantly declined under root sulfide exposure, sulfide concentrations of 2.0–10.0 mM failed to produce visual signs of acute sulfide toxicity, such as leaf chlorosis, leaf or root necrotic tissue development, or loss of leaf or root turgor. Photosynthesis and leaf EC remained high after sulfide treatments, suggesting a resilience of T. testudinum leaf metabolism to short-term sulfide exposure. Our data do not support the hypothesis that sulfide initiates rapid ‘die-off’ episodes of T. testudinum in Florida Bay, although this phytotoxin may play a critical role as a root carbon drain over long-term exposures.
The present study shows that in the presence of 600 nm light, sulfide acts as a specific inhibitor of photosynthetic electron transport between water and Photosystem II in the cyanobacteria Aphanothece halophytica and Synechococcus 6311 as well as in tobacco chloroplasts. In the presence of 600 nm light sulfied affects the fast fluorescence transients as does a low concentration (10 mM) of hydroxylamine; the fluorescence yield decreases in the presence of either chemical and can be restored by the addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. In chloroplasts, however, NH2OH, an electron donor at high concentrations (40 mM), relieves the sulfide effect. In the dark, sulfide affects the cyanobacterial fluorescence transients through decrease of oxygen tension. The fluorescence yield increases in a similar pattern to that observed under nitrogen flushing. Upon omission of sulfide in A. halophytica, the characteristic aerobic fluorescence transients return, consistent with the ease of alternation between oxygenic and sulfide-dependent anoxygenic photosynthesis in many cyanobacteria.
Chlorophyll fluorescence measurements have a wide range of applications from basic understanding of photosynthesis functioning to plant environmental stress responses and direct assessments of plant health. The measured signal is the fluorescence intensity (expressed in relative units) and the most meaningful data are derived from the time dependent increase in fluorescence intensity achieved upon application of continuous bright light to a previously dark adapted sample. The fluorescence response changes over time and is termed the Kautsky curve or chlorophyll fluorescence transient. Recently, Strasser and Strasser (1995) formulated a group of fluorescence parameters, called the JIP-test, that quantify the stepwise flow of energy through Photosystem II, using input data from the fluorescence transient. The purpose of this study was to establish relationships between the biochemical reactions occurring in PS II and specific JIP-test parameters. This was approached using isolated systems that facilitated the addition of modifying agents, a PS II electron transport inhibitor, an electron acceptor and an uncoupler, whose effects on PS II activity are well documented in the literature. The alteration to PS II activity caused by each of these compounds could then be monitored through the JIP-test parameters and compared and contrasted with the literature. The known alteration in PS II activity of Chenopodium album atrazine resistant and sensitive biotypes was also used to gauge the effectiveness and sensitivity of the JIP-test. The information gained from the in vitro study was successfully applied to an in situ study. This is the first in a series of four papers. It shows that the trapping parameters of the JIP-test were most affected by illumination and that the reduction in trapping had a run-on effect to inhibit electron transport. When irradiance exposure proceeded to photoinhibition, the electron transport probability parameter was greatly reduced and dissipation significantly increased. These results illustrate the advantage of monitoring a number of fluorescence parameters over the use of just one, which is often the case when the F(V)/F(M) ratio is used.