Low Light Availability Research Articles

Site Soil-Fertility and Light Availability Influence Plant-Soil Feedback

Negative plant-soil feedback (PSF) , where plant performance is reduced in soils conditioned by conspecifics, are widely documented in plant communities. However, the strength and sometimes direction of PSF can vary widely, presumably due to environmental context. We hypothesized that soil fertility and light availability influence the direction and strength of plant-soil feedback experienced by tree seedlings. We conducted a 10-week greenhouse experiment and assessed survivorship of black cherry (Prunus serotina) in low (~1% full sun) vs. high (~18% full sun) light availability and in non-sterile vs. sterile soils collected under the canopy of conspecific vs. heterospecific adult trees at five sites that vary in nutrient availability in Manistee National Forest, Michigan, USA. Northern red oak (Quercus rubra) also was included in the experiment, but no seedlings died and thus there are no survivorship responses to report. Prunus serotina seedlings experienced microbe-mediated negative plant-soil feedback in low fertility sites (higher C:N ratios and lower NH4+) but positive feedback at high fertility sites, but these feedbacks occurred only under high light availability. Consistent with these results, microbe-mediated negative PSF increased with soil Fe3+ and C:N ratios and decreased with NH4+. Thus lower fertility soils, characterized by high C:N and Fe3+availability and low NH4+, are associated with stronger negative conspecific feedback for P. serotina seedlings. Conversely, higher fertility soils flip the feedback to positive. Our results demonstrate the important role of environmental context, specifically light and soil nutrient availability, on the magnitude and direction of conspecific plant soil feedback. Thus, PSF is likely an important driver in plant community dynamics that may be either destabilizing (promote species coexistence) or stabilizing (promote monodominance) depending upon environmental context.

Seasonal and spatial patterns of primary production in a high-latitude fjord affected by Greenland Ice Sheet run-off

Abstract. Primary production on the coast and in Greenland fjords sustains important local and sustenance fisheries. However, unprecedented melting of the Greenland Ice Sheet (GrIS) is impacting the coastal ocean, and its effects on fjord ecology remain understudied. It has been suggested that as glaciers retreat, primary production regimes may be altered, rendering fjords less productive. Here we investigate patterns of primary productivity in a northeast Greenland fjord (Young Sound, 74∘ N), which receives run-off from the GrIS via land-terminating glaciers. We measured size fractioned primary production during the ice- free season along a spatial gradient of meltwater influence. We found that, apart from a brief under-ice bloom during summer, primary production remains low (between 50 and 200 mg C m−2 d−1) but steady throughout the ice-free season, even into the fall. Low productivity is due to freshwater run-off from land-terminating glaciers causing low light availability and strong vertical stratification limiting nutrient availability. The former is caused by turbid river inputs in the summer restricting phytoplankton biomass to the surface and away from the nitracline. In the outer fjord where turbidity plays less of a role in light limitation, phytoplankton biomass moves higher in the water column in the fall due to the short day length as the sun angle decreases. Despite this, plankton communities in this study were shown to be well adapted to low-light conditions, as evidenced by the low values of saturating irradiance for primary production (5.8–67 µmol photons m−2 s−1). With its low but consistent production across the growing season, Young Sound offers an alternative picture to other more productive fjords which have highly productive spring and late summer blooms and limited fall production. However, patterns of primary productivity observed in Young Sound are not only due to the influence from land-terminating glaciers but are also consequences of the nutrient-depleted coastal boundary currents and the shallow entrance sill, features which should also be considered when generalizing about how primary production will be affected by glacier retreat in the future.

Canopy height explains species richness in the largest clade of Neotropical lianas

AbstractAimTall and structurally complex forests can provide ample habitat and niche space for climbing plants, supporting high liana species richness. We test to what extent canopy height (as a proxy of 3‐D habitat structure), climate and soil interact to determine species richness in the largest clade of Neotropical lianas. We expect that the effect of canopy height on species richness is higher for lianas from closed tropical rain forests compared to riparian and savanna habitats.LocationNeotropics.Time periodPresent.Major taxa studiedTribe Bignonieae (Bignoniaceae).MethodsWe used structural equation models to evaluate direct and indirect effects of canopy height, climate (temperature, precipitation and precipitation seasonality), and soil (cation exchange capacity and soil types) on overall Bignonieae species richness (339 liana species), as well as on species richness of lianas from forest, riparian and savanna habitats, respectively. We further performed multiple regression models with Moran's eigenvector maps to account for spatial autocorrelation.ResultsCanopy height was a key driver of liana species richness, in addition to climate and soil. Species richness of forest lianas showed a strong positive relationship with canopy height whereas the relationship was less pronounced for riparian species. Richness of savanna species decreased with increasing canopy height. Climate also explained a substantial proportion of variation in liana species richness whereas soil variables showed little explanatory power.Main conclusionsThe relationship between canopy height and liana species richness differs among habitats. While forest and riparian lianas benefit from tall and complex habitats that provide physical support to reach the canopy to escape low light availability in the understorey, high light availability in open habitats and an increased risk of embolism of conductive vessels for lianas with long stems living in areas with high seasonality might explain the inverse relationship between species richness and canopy height in savannas.

Factors Controlling the Lack of Phytoplankton Biomass in Naturally Iron Fertilized Waters Near Heard and McDonald Islands in the Southern Ocean

The Kerguelen Plateau is one of the regions in the Southern Ocean where spatially large algal blooms occur annually due to natural iron fertilization. The analysis of ocean colour data as well as in situ samples collected during the Heard Earth-Ocean-Biosphere Interactions (HEOBI) voyage in January and February 2016, surprisingly revealed that chlorophyll a concentrations in waters located close to Heard and McDonald islands were much lower than those on the central Kerguelen Plateau. This occurs despite high levels of both glacial and volcanic iron supply from these islands. The analysis of pigment and optical data also indicated a shift in the phytoplankton size structure in this region, from a microphytoplankton to nanophytoplankton dominated community. Possible explanations for this high nutrient, high iron (Fe), low chlorophyll (HNHFeLC) phenomenon were explored. Low light availability due to deep mixing and shading by re-suspended sediment particles and augmented by dilution with surrounding low chlorophyll waters in the Antarctic Circumpolar Current was shown to be an important mechanism shaping phytoplankton communities. The competing dynamics between stimulation and limitation illustrate the complexity of short-term responses to our changing climate and cryosphere.

Functional traits associated to photosynthetic plasticity of young Brazil nut (Bertholletia excelsa) plants

The Brazil nut shows the ability to establish itself in contrasting irradiance conditions. However, physiological strategies with respect to acclimatization to different irradiance still remain unknown. Here we investigated the leaf traits variations related to photosynthetic plasticity of Bertholletia excelsa. In this study, B. excelsa saplings were acclimated during 90 days (PPFD = 30 μmol m−2 s−1) and after the acclimatization, the saplings were subjected to two irradiance conditions: planting in full sun (100% PPFD) and planting in forests (1.5% PPFD). Following this, the specific leaf mass (SLM), gas exchange, maximum yield of photochemistry (FV/FM), chlorophyll and carbohydrate contents were evaluated. Net photosynthesis per unit area (Pn) of saplings in full sun was 2 times greater than in shade and photosynthesis in full sun was positively related to stomatal condutance (gs), dark respiration (Rd), carbohydrates and SLM. On the other hand, in shade, we observed an increase in the apparent quantum yield (α) and a reduction of SLM. Although B. excelsa reduced FV/FM by 30% in full sun, after 7 days it was able to recover from stress by the increase in irradiance. The traits that most contributed to explaining the photosynthetic plasticity were SLM (8.7%), FV/FM (7.7%), Rd (5.9%), gs (5.3%) and sugar (5.2%). Therefore, the fast recovery of photosynthesis during light stress and adjustments of morpho-functional traits in leaves to irradiance in B. excelsa are strong candidates to explain why this species establishes both in environments with low and high light availability.

Effects of ammonium pulse on the growth of three submerged macrophytes.

Ammonium pulse attributed to runoff of urban surface and agriculture following heavy rain is common in inland aquatic systems and can cause profoundly effects on the growth of macrophytes, especially when combined with low light. In this study, three patterns of NH4-N pulse (differing in magnitude and frequency) were applied to examine their effects on the growth of three submersed macrophytes, namely, Myriophyllum spicatum, Potamogeton maackianus, and Vallisneria natans, in terms of biomass, height, branch/ramet number, root length, leaf number, and total branch length under high and low light. Results showed that NH4-N pulse caused negative effects on the biomass of the submerged macrphytes even on the 13th day after releasing NH4-N pulse. The negative effects on M. spicatum were significantly greater than that on V. natans and P. maackianus. The effects of NH4-N pulse on specific species depended on the ammonium loading patterns. The negative effects of NH4-N pulse on P. maackianus were the strongest at high loading with low frequency, and on V. natans at moderate loading with moderate frequency. For M. spicatum, no significant differences were found among the three NH4-N pulse patterns. Low light availability did not significantly aggregate the negative effects of NH4-N pulse on the growth of the submersed macrophytes. Our study contributes to revealing the roles of NH4-N pulse on the growth of aquatic plants and its species specific effects on the dynamics of submerged macrophytes in lakes.

Natural regeneration and recruitment of native Quercus robur and introduced Q. rubra in European oak-pine mixed forests

The North-American Northern red oak (Quercus rubra L.) introduced to European forests over 200 years ago, spreads spontaneously in various types of forest ecosystems. At the same time the retreat of native pedunculate oak (Q. robur L.) has been observed. We aimed to recognize the extent of natural regeneration and recruitment of pedunculate (PO) and red oak (RO) in managed forests (central Poland) on sites typical of oak-pine mixed forest, as well as to identify habitat conditions which favour the occurrence of native oak and spread of the alien oak. The abundance and heights of juveniles of both oaks, as well as the impacts of the canopy, nurse and burial effects, on distribution and occurrence of oak specimens were studied on 1000 study plots (1 m2 each), in two transects crossing the Scots pine-pedunculate oak forest (POF) and the Scots pine-northern red oak forest (ROF). We revealed an advantage of introduced RO over native PO in natural regeneration, recruitment and spread. However, we did not find direct (interspecific) competition between PO and RO recruits. RO predominance over PO resulted from more effective colonization of empty niches by the alien oak in POF (suitable for both species), and the abundant establishment of RO seedlings and saplings within ROF. In contrast, PO was not able to re-colonize ROF. The regeneration of RO was microsite-limited in ROF gaps in the regeneration phase (probably due to site moisture deficiency) and under closed RO canopy in the recruitment phase (due to low light availability), and microsite-favoured under mature tree crowns due to the burial effect of the litter layer (and higher light availability under heterogeneous canopy). In POF, RO regeneration was microsite-favoured due to the burial effect of a dense moss layer and the nurse effect of shrubs, including conspecifics. PO regeneration was also microsite-favoured due to the burial effect of the moss layer, but its recruitment was herbivore-limited there (due to browsing). The co-occurrence of PO and RO specimens, both in POF and ROF, resulted from forest community structure, oak ecology and behaviour of acorn dispersers. Because the native PO is at a disadvantage in its native range caused by a lower share of mature trees in forested areas, strong impacts of fungal pathogens and high pressure by insects and large herbivores (also noted in our study), the introduced oak is able to become a serious competitor.

Algal Community Dynamics Within the Savannah River Estuary, Georgia Under Anthropogenic Stress

Estuaries are naturally dynamic ecosystems mixing fresh and marine waters, while consistently varying in depth and discharge due to diurnal tide shifts. Anthropogenic alterations impacting the Savannah River are expected to change the hydrology of the ecosystem and effect algal community structure and composition. The Savannah Harbor Expansion Project began recently with the aim of widening and deepening the river by more than a meter. It is anticipated that tidal height, influx of salt water, and sedimentation rates will be affected, which will disturb algal communities upstream. Baseline data from the Savannah River mudflats were obtained in 2011 as part of a biodiversity assessment. Algal community indices were also examined and related to species alterations in the community. Living algal species were classified as freshwater, marine, or brackish in addition to planktonic, epipelic, and epipsammic. Live diatoms were replaced by filamentous cyanobacteria representatives like Phormidium and green filamentous algae. There was a decrease in abundance of live chain-forming marine plankton species potentially due to sediment accumulation. Species richness within the diatom communities analyzed in clean material decreased (mean = 40 ± 6.6 standard deviation to 26 ± 4.6) and the 95% confidence interval narrowed when before and during dredging was compared. Cymatosira belgica Grunow remained as the dominant species with an average of 38% relative abundance at low tide and 33% relative abundance at high tide. With exposure to dredging and increased sediment deposition diatom communities showed significant change due to potential increase in turbidity and lower light availability.

Stomatal anatomy and closing ability is affected by supplementary light intensity in rose (Rosa hybrida L.)

Increasing the light level in protected cultivation of ornamental crops via supplementary lighting is critical to enhance both production and external quality especially during the periods of low light availability. Despite wide applications the effects of light intensities were not previously addressed on water loss pathways. In this study rose plants were cultivated at 100, 200 or 400 μmol/(m2·s) photosynthetic photon flux density (PPFD). The stomatal responsiveness to desiccation, stomatal anatomical features and cuticular transpiration were determined. Plant biomass as well as photosynthesis response to light and CO2 were also assessed. Increasing growth PPFD led to a considerable increase in plant biomass (85 and 57% for 100 to 200 and 200 to 400 μmol/(m2·s) respectively). Photosynthesis was marginally affected by increasing growth PPFD from 100 to 200 μmol/(m2·s) while a further rise to 400 μmol/(m2·s) considerably increased photosynthetic rate at high light intensities. Higher PPFD during cultivation generally led to larger stomata with bigger pores. A PPFD increase from 100 to 200 μmol/(m2·s) had a small negative effect on stomatal closing ability whereas a further rise to 400 μmol/(m2·s) had a substantial stimulatory effect. Cultivation at a PPFD higher than 100 μmol/(m2·s) led to lower rates of cuticular transpiration. In conclusion, high growth PPFD (> 200 μmol/(m2·s)) enchanced both photosynthetic and stomatal anatomical traits. High light intensity (> 200 μmol/(m2·s)) also led to a better control of water loss due to more responsive stomata and decreased cuticular permeability.

Herbaceous climbers in herbaceous systems are shade‐tolerant and magnesium‐demanding

AbstractQuestionsClimbers do not invest in self‐supporting architecture, allowing them to grow quickly and search for light and nutrients in canopy gaps. The climbing strategy requires both disturbance and external support; however, in herbaceous systems, disturbances often homogeneously remove external support. As a result, there should be a negative relationship between light availability and the presence of external support. Here, we asked if the distribution of herbaceous climbers is positively or negatively associated with disturbance, light availability and plant‐available nutrients. Further, we tested if climbers differ in traits compared to co‐occurring herbs.LocationCzech Republic.MethodsWe used observations from a phytosociological database, species‐rich grasslands and arable fields. First, we examined the presence of climbers in response to disturbance variables (frequency, severity and regime) and canopy cover (external support and light availability). Second, we examined the distribution of climbers along gradients of plant‐available nutrients in species‐rich grasslands. Finally, we compared seven functional traits between 31 climbers and 1,138 co‐occurring herbs growing in 18 Central European herbaceous habitats.ResultsWe found no relationship between any disturbance variables and the occurrence of climbers; indeed, climbers tended to occur more frequently in sites with denser canopies. In species‐rich grasslands, the presence of climbers was positively related to plant‐available magnesium. Climbers tended to be taller, with greater seed mass and specific leaf area (SLA) than co‐occurring species.ConclusionsHerbaceous climbers grow in plots with low light availability but plenty of external support and require magnesium for the formation of flexible stems. Compared to co‐occurring herbs, climbers allocate resources saved from their lack of self‐support to both shade‐avoidance (canopy height) and shade‐tolerance traits (seed mass, SLA). Compared to lianas, the distribution of herbaceous climbers is not affected by disturbance but is dependent on the availability of external support and magnesium.

Low litter cover, high light availability and rock cover favour the establishment of Ailanthus altissima in forests in southern Switzerland

Future forest composition is uncertain in many areas due to climate change. The spread of non-native species adds to these uncertainties, particularly in forests recently colonised by novel tree species. To anticipate future forest composition, and thus the provision of ecosystem services, a thorough understanding of the factors influencing the establishment of non-native tree species is essential. We studied the presence and abundance of regeneration of Ailanthusaltissima (Mill.) Swingle in 89 plots on a regular grid in three sites in southern Switzerland to determine the main drivers of its establishment. All sites are located in abandoned, i.e. currently unmanaged stands of Castaneasativa Mill. Propagule pressure is caused by single mature A.altissima that colonised the area ca 40 years ago. We found high rock cover, low litter cover and high light availability to be the most important predictors for the presence of A.altissima regeneration, whereas its abundance was positively influenced by high light availability, low litter cover and high browsing on regeneration of competing species. However, the presence models performed much better than the abundance models. Interestingly, the most important factors favouring the establishment of A.altissima in recently undisturbed sites were found to be similar in a nearby site after a severe forest fire, which suggests a similar establishment strategy after a disturbance as in recently undisturbed forests. Based on our results we expect a further expansion of the species in lowland forests currently dominated by C.sativa, likely controlled primarily by light availability.

Three phylogenetically distant shade-tolerant temperate forest herbs have similar seed germination syndromes

Plants growing in the same environment typically share ecophysiological characteristics enabling them to survive and reproduce when exposed to a common selection pressure. As such, seedlings establishing in temperate forest habitats cope with similar issues of low light availability and climate seasonality. We examined similarities in the germination syndromes in three shade-tolerant temperate forest herbs, Allium ursinum (Amaryllidaceae), Mercurialis perennis (Euphorbiaceae) and Dioscorea communis (Dioscoreaceae) with a distant phylogenetic affiliation. Seeds of M. perennis and A. ursinum are dispersed in spring and those of D. communis in autumn. Experiments on phenology of seed germination and seedling emergence in natural conditions revealed that seeds of all three species germinate in the following autumn. Seedlings do not emerge immediately but rather remain below the soil surface until late winter or early spring. Experiments in controlled laboratory conditions showed that seeds germinated best at intermediate autumn or spring temperatures (between 10 and 15°C) following a period at high summer temperatures (around 20°C). Seed germination of Allium ursinum is strongly photoinhibited, while moderate photoinhibition and no photoinhibition at all was observed in Dioscorea communis and Mercurialis perennis, respectively. Although seeds of all three species are endospermic at dispersal, no embryo growth prior to germination was observed. The cotyledons functioned as a haustorium, recuperating the nutrient reserves in the endosperm post-germination. It can be concluded that although phylogenetically unrelated, the three species studied show a remarkable similarity in germination and seedling emergence strategy that is commonly observed in plants adapted to growing in shady conditions.

The movement responses of three libellulid dragonfly species to open and closed landscape cover

AbstractThe land cover between habitats (i.e. matrix environment) can affect connectivity by impacting organismal movement. Many animals, however, have preferences for specific matrix environments, which can affect their movement through the landscape.We examined how different terrestrial matrix environments impacted the fine‐scale movement of adult dragonflies. Based on previous studies of adult dragonfly dispersal and larval distributions, we hypothesised that dragonflies would prefer to enter fields rather than forests and that forests would be a barrier to dragonfly movement, due to forests’ structural complexity, low understory light availability, and lower air temperatures.To test how adult dragonflies responded to various terrestrial environments, we released 108Leucorrhinia intacta, a mixture of 108Sympetrum rubicundulumandobtrusum/rubicundulumhybrids, and 108Sympetrum vicinum, at field‐forest ecotones and assessed their preferences for fields or forests. Individual behavioural responses were recorded, including their probability of taking flight, their direction of movement with respect to the two matrix types, and flight time.The likelihood of adult dragonflies taking flight was species‐specific in response to release location. Adults moved more frequently towards fields than forests when released at a forest edge. Individuals released within forests had shorter flight times, but again this response was species‐specific.The presence of an open matrix (field or meadow) is likely important for facilitating movement in dragonflies; however, forests are not movement barriers for all dragonfly species. Integrating assays of matrix and habitat preferences can provide insight into how landscape connectivity can be maintained for actively dispersing species.

After The Deluge: Plant Revival Post-Flooding.

Increasing flooding events have detrimentally impacted food security amid a growing global population. Complete submergence of plants represents the most severe flooding stress and studies have identified underwater responses to low oxygen and light availability. However, knowledge on plant responses during the post-submergence phase is limited. It is important to consider how plants can resume vegetative growth after enduring submergence and post-submergence stress. This review highlights current knowledge on physiological and molecular adaptations following desubmergence. Interplays of reactive oxygen species (ROS), energy depletion, photoinhibition, desiccation stress, and hormonal signaling have been characterized as components of the post-submergence stress response. Active elucidation of key genes and traits enhancing post-submergence adaptations is highly relevant for the improvement of submergence tolerance and ultimately crop yield.

Mixotrophic Growth Under Micro-Oxic Conditions in the Purple Sulfur Bacterium "Thiodictyon syntrophicum".

The microbial ecosystem of the meromictic Lake Cadagno (Ticino, Swiss Alps) has been studied intensively in order to understand structure and functioning of the anoxygenic phototrophic sulfur bacteria community living in the chemocline. It has been found that the purple sulfur bacterium “Thiodictyon syntrophicum” strain Cad16T, belonging to the Chromatiaceae, fixes around 26% of all bulk inorganic carbon in the chemocline, both during day and night. With this study, we elucidated for the first time the mode of carbon fixation of str. Cad16T under micro-oxic conditions with a combination of long-term monitoring of key physicochemical parameters with CTD, 14C-incorporation experiments and quantitative proteomics using in-situ dialysis bag incubations of str. Cad16T cultures. Regular vertical CTD profiling during the study period in summer 2017 revealed that the chemocline sank from 12 to 14 m which was accompanied by a bloom of cyanobacteria and the subsequent oxygenation of the deeper water column. Sampling was performed both day and night. CO2 assimilation rates were higher during the light period compared to those in the dark, both in the chemocline population and in the incubated cultures. The relative change in the proteome between day and night (663 quantified proteins) comprised only 1% of all proteins encoded in str. Cad16T. Oxidative respiration pathways were upregulated at light, whereas stress-related mechanisms prevailed during the night. These results indicate that low light availability and the co-occurring oxygenation of the chemocline induced mixotrophic growth in str. Cad16T. Our study thereby helps to further understand the consequences micro-oxic conditions for phototrophic sulfur oxidizing bacteria. The complete proteome data have been deposited to the ProteomeXchange database with identifier PXD010641.

The Effects of Leaf Litter on Germination in The Serpentine Endemic Boechera constancei

Leaf litter accumulation may negatively impact seedling germination by altering key ecosystem properties, such as light availability and soil moisture. The impacts of litter depth may be particularly strong for species in dry environments with low vegetative cover and high light availability. The rare species Boechera constancei (Rollins) Al-Shehbaz (Brassicaceae) is endemic to the serpentine outcrops of Plumas and Sierra Counties, where litter accumulation is highly variable and B. constancei inhabits a range of litter depths (0–40 mm). In this study, we tested whether increased litter depth impedes germination, and whether populations vary in tolerance to litter depth. Specifically, we quantified the impacts of treatments representing the observed range of litter depth occupied by B. constancei on the germination of seed collected from nine populations. Differences in germination were analyzed using a two-way ANOVA. While both source population and litter treatment influenced germination, the interaction term was not significant. Post-hoc Tukey's honest significance tests revealed that germination under both median and high litter treatments was significantly lower than under no litter. The median and high litter treatments did not differ significantly. Differences in germination were observed during dark, cold stratification, suggesting that light availability was not a factor. These results indicate that early life stages of this species may benefit from litter reduction treatments, as both median and high litter treatments reduced germination.

New ultra-flat photobioreactor for intensive microalgal production: The effect of light irradiance

One of the main bottlenecks for the exploitation of microalgae is the low biomass concentration of the cultures: high harvesting costs and large cultivation area are always required. This bottleneck is partly due to a low light availability along the optical path of photobioreactors. An ultra-thin flat photobioreactor (UFP) (3 mm thickness) was proposed to increase both biomass concentration and productivity. The performance of the UFP was investigated: the effects of incident light intensity - from 50 and 1000 μmolPhotons m−2 s−1 - on cell growth, photosynthesis rate, and biochemical composition of Chlorella sorokiniana were characterized. The maximum microalgal concentration and the maximum areal productivity were 24 kg m−3 and 1.34 g m−2 h−1, respectively. The cell specific growth rate reached 0.1 h−1 at 1000 μmol m−2 s−1. The biochemical composition of the microalgal biomass changed with the light irradiance. Protein content increased from 35 up to 53% of DW with increasing the light intensity. The concentration of storage compounds, such as starch and lipids, decreased from 30 to 16% and from 30 to 10%, respectively, with increasing the light intensity. A limit in the maximum biomass concentration achievable was identified. Several hypotheses have been discussed. A light transfer model was applied to assess the presence of light limitation. Other hypotheses were analyzed in depth and the most feasible explanations were found to be a) the damage to the photosystem when exposed for long period to continuous and high light irradiances, b) nutrient limitation due to salt precipitation or c) gas-liquid transfer of the CO2. Finally, benefits and drawbacks of the ultra-thin culture system were discussed.

Provenance difference in growth traits and photosynthetic characteristics of Acer catalpifolium seedlings under different shading conditions.

Acer catalpifolium is a unique species and wild endangered one in China. Currently, no research has addressed its endangered status from the perspective of photosynthetic characters. The photosynthesis characteristics and growth rate of A. catalpifolium seedlings from four provenances (Dujiangyan, Dayi, Emeishan and Leibo) under two shading conditions (50% and 80% of full shade) were examined to explore the light response mechanism. The results showed that the net photosynthetic rate (Pn), stomatal conductance (gs), transpiration rate (Tr) and water use efficiency (WUE) of A. catalpifolium seedlings from four provenances sharply decreased with the decreases of light intensity. When the light intensity was reduced, the light compensation point (LCP) and light saturation point (LSP) of A. catalpifolium seedlings from Dujiangyan and Emei-shan were all decreased, while those of the seedlings from Dayi increased, and those of the seedlings from Leibo showed no significant changes. Net photosynthetic rate and water use efficiency were positively correlated with plant height and crown growth. The effects of different light intensities on photosynthetic characteristics and growth rate of A. catalpifolium were significantly different. Low light availability under canopy might be the limiting factor for natural regeneration of A. catalpifolium populations from Dayi. Compared with other provenances, A. catalpifolium seedlings from Leibo were more shade requiring. A. catalpifolium had better light adaptability and growth performance from Dujiangyan and Emeishan, which could be used as high-quality source for transplant protection.

Influence of light availability on the specific density, size and sinking loss of Anabaena flos-aquae and Scenedesmus obliquus

Harmful algal blooms in eutrophic waters pose a serious threat to freshwater ecosystems and human health. In-situ light availability control is one of the most commonly used technologies to suppress algae in lakes and reservoirs. To develop a better understanding of the effects of light on algal growth, specific density, colony size and sinking loss, Anabaena flos-aquae (cyanobacteria) and Scenedesmus obliquus (green algae) were evaluated in varying light scenarios. The results showed that the specific density and colony size of these two species varied during growth, and there were obvious differences among the light scenarios. At the end of exponential phase, S. obliquus incubated under light-limited condition maintained a higher specific density and formed larger aggregates, whereas A. flos-aquae formed a longer filament length. Both species exhibited higher sinking loss rates with lower light availability. These results implied that the sinking loss rate was not always constant but should be considered as a variable response to the change of light availability, and in-situ light availability control might result in a significant increase of the sinking loss of algae due to the change of size and specific density, thereby further affecting the algal biomass in the water column.

Functional traits of coffee along a shade and fertility gradient in coffee agroforestry systems

Intraspecific variation in leaf functional traits has gained widespread attention as a means to evaluate, predict, and manage plant responses to environmental conditions, however there are considerable uncertainties regarding the extent and drivers of intraspecific trait variation (ITV) in domesticated plants. In a coffee (Coffea arabica) agroforestry system, we quantified ITV in seven leaf traits [i.e. area (LA), mass per area (LMA), dry matter content (LDMC), thickness (Lth), nitrogen concentrations (LNC), maximum photosynthetic rate on area and mass bases (Asat, Amass, respectively)] across managed gradients of soil fertility and light availability. Leaf physiological traits (Asat, Amass), as well as LA, showed the greatest extent of variation within coffee, while morphological traits (LMA, LDMC, Lth) and leaf N were less variable. All traits differed significantly as a function of light and fertilization treatment, however light was more influential in driving ITV in coffee leaves. Low light availability resulted in greater ITV for physiological leaf traits (Asat and Amass), while high light constrained ITV in most morphological-(LA, LMA, LDMC), physiological-(Asat, Amass) and chemical-(LNC) traits. Fertilization treatments did not induce systematic shifts in the extent of ITV. In addition, shade management treatments explained 9.2% of the variation in multivariate trait syndromes, while nutrient management regimes explained only 2.9%. Our results indicate that highly heterogeneous aboveground resource environments such those created by agroforestry, results in greater ITV for key crop physiological parameters. Based on ecological theory, such patterns indicate that management systems promoting resource heterogeneity should promote higher rates of resource partitioning, and greater resource-use efficiency in agroecosystems.