Low Light Availability Research Articles

Plants in a crowded stand regulate their height growth so as to maintain similar heights to neighbours even when they have potential advantages in height growth

Although being tall is advantageous in light competition, plant height growth is often similar among dominant plants in crowded stands (height convergence). Previous theoretical studies have suggested that plants should not overtop neighbours because greater allocation to supporting tissues is necessary in taller plants, which in turn lowers leaf mass fraction and thus carbon gain. However, this model assumes that a competitor has the same potential of height growth as their neighbours, which does not necessarily account for the fact that height convergence occurs even among individuals with various biomass. Stands of individually potted plants of Chenopodium album were established, where target plants were lifted to overtop neighbours or lowered to be overtopped. Lifted plants were expected to keep overtopping because they intercept more light without increased allocation to stems, or to regulate their height to similar levels of neighbours, saving biomass allocation to the supporting organ. Lowered plants were expected to be suppressed due to the low light availability or to increase height growth so as to have similar height to the neighbours. Lifted plants reduced height growth in spite of the fact that they received higher irradiance than others. Lowered plants, on the other hand, increased the rate of stem elongation despite the reduced irradiance. Consequently, lifted and lowered plants converged to the same height. In contrast to the expectation, lifted plants did not increase allocation to leaf mass despite the decreased stem length. Rather, they allocated more biomass to roots, which might contribute to improvement of mechanical stability or water status. It is suggested that decreased leaf mass fraction is not the sole cost of overtopping neighbours. Wind blowing, which may enhance transpiration and drag force, might constrain growth of overtopping plants. The results show that plants in crowded stands regulate their height growth to maintain similar height to neighbours even when they have potential advantages in height growth. This might contribute to avoidance of stresses caused by wind blowing.

Reversal in competitive dominance of a toxic versus non-toxic cyanobacterium in response to rising CO2.

Climate change scenarios predict a doubling of the atmospheric CO2 concentration by the end of this century. Yet, how rising CO2 will affect the species composition of aquatic microbial communities is still largely an open question. In this study, we develop a resource competition model to investigate competition for dissolved inorganic carbon in dense algal blooms. The model predicts how dynamic changes in carbon chemistry, pH and light conditions during bloom development feed back on competing phytoplankton species. We test the model predictions in chemostat experiments with monocultures and mixtures of a toxic and non-toxic strain of the freshwater cyanobacterium Microcystis aeruginosa. The toxic strain was able to reduce dissolved CO2 to lower concentrations than the non-toxic strain, and became dominant in competition at low CO2 levels. Conversely, the non-toxic strain could grow at lower light levels, and became dominant in competition at high CO2 levels but low light availability. The model captured the observed reversal in competitive dominance, and was quantitatively in good agreement with the results of the competition experiments. To assess whether microcystins might have a role in this reversal of competitive dominance, we performed further competition experiments with the wild-type strain M. aeruginosa PCC 7806 and its mcyB mutant impaired in microcystin production. The microcystin-producing wild type had a strong selective advantage at low CO2 levels but not at high CO2 levels. Our results thus demonstrate both in theory and experiment that rising CO2 levels can alter the community composition and toxicity of harmful algal blooms.

Changes in light and nutrient availabilities do not alter the duration of use of seed reserves in Araucaria angustifolia seedlings

Initial plant growth is entirely dependent on seed reserves, until the photosynthetic organs become a significant net carbon source to the growing plant. The time during which these reserves are used may vary within species, depending on the availability of external resources. We have investigated the duration of use of the seed reserves in the large-seeded tree species Araucaria angustifolia, when growing under different light and nutrient availabilities. We hypothesised that this duration would not vary with growing conditions, because of the high pressure exerted by predators on these seeds and because of the existing possibility of transferring to and restoring part of their starchy reserves in the underground hypocotyl. Plant growth took place for 139 days under shade cloth in outside gardens, under each of the four combinations of high and low light or nutrient availabilities. Plants within each treatment were separated into six different groups, each including seedlings which were separated from their seeds by cotyledon excision at five different time periods after seed sowing (43, 63, 83, 103 and 124 days), plus a Control group of seedlings which were not deprived from their seeds. Final mass accumulation by seedlings that had full access to the seed reserves was compared with that of seedlings which were separated from their seeds at different time periods after seed sowing. Seed mass evaluations at each excision period allowed us to follow the temporal pattern of seed consumption. We found that seedlings growing under low light used seed reserves for growth longer than those under high light, regardless of their nutrient condition, pointing to the importance of a large seed for seedling establishment under deep shade. However, seed consumption followed a very similar pattern in both light treatments. Nutrient availability did not affect the duration of growth dependency on seed reserves or the pace of seed mass depletion. Our initial hypothesis was then accepted, and the possible implications of the reported early growth behaviour of the species in coping with seed predation and shading are discussed.

Lagrangian observation of phytoplankton dynamics at an artificially enriched subsurface water in Sagami Bay, Japan

Phytoplankton dynamics in the lower euphotic zone were observed by tracking a subsurface water released at 20-m depth from Takumi, an artificial upwelling device. Takumi continually discharged seawater pumped up from a depth of 205 m: this water was mixed with 5-m depth water to adjust the density to that of 20-m depth water of Sagami Bay, Japan. The discharged water was pulse-labeled at Takumi with uranine and tracked for 63.9 h with a drifting buoy equipped with a drogue at 20-m depth. We present a simple model to estimate in situ phytoplankton net growth rates from temporal changes in phytoplankton abundance in the discharged water with correction for the influence of water exchange between the discharged water and neighboring layers. Lagrangian observation showed active growth of pico- and nanophytoplankton, especially cryptophytes and Synechococcus (Cyanobacteria), in the subsurface layer. In contrast, diatoms grew little in spite of micromolar concentrations of nutrients. The active growth of pico- and nanophytoplankton was in good agreement with shipboard serial dilution culture experiments. The low growth activity of diatoms was suggested to be related to low light availability in the subsurface layer.

Effects of light on direct and indirect defences against herbivores of young plants of Mallotus japonicus demonstrate a trade-off between two indirect defence traits.

Although most studies on plant defence strategies have focused on a particular defence trait, some plant species develop multiple defence traits. To clarify the effects of light on the development of multiple defence traits, the production of direct and indirect defence traits of young plants of Mallotus japonicus were examined experimentally under different light conditions. The young plants were cultivated under three light conditions in the experimental field for 3 months from May to July. Numbers of ants and pearl bodies on leaves in July were examined. After cultivation, the plants were collected and the developments of trichomes and pellucid dots, and extrafloral nectaries (EFNs) on the leaves were examined. On plants without nectar-collecting insects, the size of EFNs and the volume of extrafloral nectar secreted from the EFNs were examined. Densities of trichomes and pellucid dots did not differ significantly among the plants under the different light conditions, suggesting that the chemical and physical defences function under both high and low light availability. The number of EFNs on the leaves did not differ significantly among the plants under the different light conditions, but there appeared to be a trade-off between the size of EFNs and the number of pearl bodies; the largest EFNs and the smallest number of pearl bodies were found under high light availability. EFN size was significantly correlated with the volume of extrafloral nectar secreted for 24 h. The number of ants on the plants was smaller under low light availability than under high and moderate light availability. These results suggest that direct defence traits function regardless of light conditions, but light conditions affected the development of indirect defence traits.

Plasticity in response to phosphorus and light availability in four forest herbs

The differential ability of forest herbs to colonize secondary forests on former agricultural land is generally attributed to different rates of dispersal. After propagule arrival, however, establishing individuals still have to cope with abiotic soil legacies from former agricultural land use. We focused on the plastic responses of forest herbs to increased phosphorus availability, as phosphorus is commonly found to be persistently bioavailable in post-agricultural forest soils. In a pot experiment performed under field conditions, we applied three P levels to four forest herbs with contrasting colonization capacities: Anemone nemorosa, Primula elatior, Circaea lutetiana and Geum urbanum. To test interactions with light availability, half of the replicas were covered with shade cloths. After two growing seasons, we measured aboveground P uptake as well as vegetative and regenerative performance. We hypothesized that fast-colonizing species respond the most opportunistically to increased P availability, and that a low light availability can mask the effects of P on performance. All species showed a significant increase in P uptake in the aboveground biomass. The addition of P had a positive effect on the vegetative performances of two of the species, although this was unrelated to their colonization capacities. The regenerative performance was affected by light availability (not by P addition) and was related to the species' phenology. Forest herbs can obviously benefit from the increased availability of P in post-agricultural forests, but not all species respond in the same way. Such differential patterns of plasticity may be important in community dynamics, as they affect the interactions among species.

Phytoplankton community structure in five subtropical shallow lakes with different trophic status (Uruguay): a morphology-based approach

Phytoplankton abundance and biomass can be explained as a result of spatial and temporal changes in physical and biological variables, and also by the externally imposed or self-generated spatial segregation. In the present study, we analyzed contrasting-season changes in the phytoplankton communities of five subtropical shallow lakes, covering a nutrient gradient from oligo-mesotrophy to hypereutrophy, using a morphologically based functional approach to cluster the species. Six environmental variables accounted for 46% of the total phytoplankton morphological groups variance, i.e., turbidity (Secchi disk), conductivity, total phosphorus, total nitrogen, total zooplankton abundance, and herbivorous meso:microzooplankton density ratio. The differences in resource availability and zooplankton abundance among the systems were related with important changes in phytoplankton composition and structure. Within phytoplankton assemblages, adaptations to improve both light and phosphorus/nitrogen uptake were important in nutrient-poor systems; while grazing-avoidance mechanisms, such as colonial forms or bigger individuals, seemed relatively important only in eutrophic Lake Blanca, where light was not a limiting factor. However, this was not observed in the nutrient-rich Lake Cisne, where low light availability (due to clay resuspension and dark water color) was identified as the main structuring factor. Our results suggest that the composition of phytoplankton morphologically based functional groups appear to reliably describe the trophic sate of the lakes. However, other factors, such as nonbiological turbid condition, or zooplankton composition, may interact rendering interpretations difficult, and therefore, deserve further studies and evaluation.

Drivers of metabolism and net heterotrophy in contrasting lakes

We investigated the influence of light, nutrients, and organic matter on gross primary production (GPP), ecosystem respiration (R), and net ecosystem production (NEP = GPP ‐ R) in a dystrophic forest lake and an open eutrophic lake. Forest vegetation reduced incoming irradiance (20%) and wind speed (34%) in dystrophic Gribsø, having thermal stratification 1 month longer than in eutrophic Slotssø. While Gribsø had nutrient‐limited phytoplankton during most of the year, Slotssø only experienced nutrient depletion during algal blooms. Colored dissolved organic matter (CDOM) absorbed most light (average 82%) in dystrophic Gribsø, while phytoplankton and other particles absorbed most light (45%) in eutrophic Slotssø. GPP and NEP were positively related to irradiance in both lakes. However, because of higher CDOM absorbance, three times more light was needed to attain autotrophy in Gribsø, being net heterotrophic (NEP < 0) for 79% of all days, compared to 59% in Slotssø. This difference vanished when NEP was scaled to light absorption by pigments, although the eutrophic lake maintained a higher photon yield. Metabolic rates varied much more in Slotssø, where higher light and nutrient availability facilitated occasional phytoplankton blooms, while low light and nutrient availability in Gribsø dampened temporal variability. Both lakes were annually net heterotrophic with similar annual areal rates (NEP, ‐14 mol C m−2). Net heterotrophy in dystrophic Gribsø derives from high import of organic carbon‐rich water, while heterotrophy in eutrophic Slotssø is fueled by degradation of sediment pools of organic matter accumulated under previous hypereutrophic conditions, emphasizing the importance of lake history on the contemporary metabolic state.

Compensatory growth responses to defoliation and light availability in two native Mexican woody plant species

Abstract:Defoliation, often caused by herbivory, is a common cause of biomass loss for plants that can affect current and future growth and reproduction. There are three models that predict contrasting compensatory growth responses of plants to herbivory and resource availability: (1) Growth rate model, (2) Compensatory continuum hypothesis and (3) Limiting resource model. The predictions of these three models were tested on the tree Brosimum alicastrum and the liana Vitis tiliifolia. Seedlings were subjected to three levels of experimental defoliation (0%, 50% and 90% leaf removal) along a light resource gradient (1%, 9% and 65% of full sun). In both species, defoliation significantly increased leaf production rate and relative growth rate of leaf area, but not of biomass. Net assimilation rate was the strongest driver of biomass growth in both species, but leaf area ratio and specific leaf area were also important in B. alicastrum. Compensatory responses of leaf area growth in B. alicastrum were significantly greater in higher than lower light availability, consistent with the compensatory continuum hypothesis predictions, but in contrast to the growth rate model predictions. The limiting resource model offered an explanation for all possible experimental outcomes by directly considering the effects of environmental differences in resource availability.

Comparison of sapling-level daily light capture and carbon gain between a temperate deciduous and a co-occurring evergreen tree species in the growing season and in winter

Light capture efficiency (Ea) and mass-based daily carbon gain (Amass) were compared between saplings of a deciduous tree species, Ficus erecta Thunb. and the co-occurring evergreen broadleaved tree species, Neolitsea aciculata (Bl.) Koidzumi, in a temperate forest in Japan. Using obtained data and an ecophysiological–architectural model, we calculated the Ea and Amass of each study sapling. We also analysed the response of Amass to changes in photosynthetic traits and Ea. Saplings of F. erecta had a higher Amass than N. aciculata, due to the high leaf area : aboveground mass ratio (LAR). The model calculation suggested that changes in photosynthetic traits and Ea changed Amass but did not modify the interspecific difference of Amass. In winter Amass was lower than that in the growing season due to low light availability during the short day lenght, suggesting modest importance of winter carbon gain for the evergreen saplings of N. aciculata. In conclusion, the advantage of this deciduous species for carbon gain over the co-occurring evergreen broadleaved saplings is not modified by acclimative changes in leaf physiology, crown architecture or prolonged photosynthesis period by evergreen broadleaved trees.

Medicinal understory herbaceous species cultivated under different light and soil conditions in maple forests in southern Québec, Canada

Many northern hardwood understory plants have medicinal properties but their cost of production under conventional cultivation practices is fairly high. Therefore wild harvesting continues putting the natural populations at risk. Their cultivation in a forest farming system seems a promising alternative. This study was aimed at assessing the impact of canopy opening and soil fertility on growth and active component production of four medicinal plants: black cohosh (Actaea racemosa), wild ginger (Asarum canadense), blue cohosh (Caulophyllum thalictroides) and bloodroot (Sanguinaria canadensis). Rhizome sections were planted in two sugar maple forests. Mortality and total leaf area were monitored during 3 years, while subplots were harvested at the end of the second year for biomass and active component analyses. Multiple regressions indicated that all species responded more to acidity related elements (pH, Al, Al + H, Fe, Zn) than to soil fertility (Ca, K, Mg, P, C/N, base saturation and cation exchange capacity). This suggests that adjusting the pH by liming could be appropriate. Growth increased with irradiance, except for blue cohosh, indicating that forest openings could also be part of an appropriate forest management plan for their cultivation. While active component concentrations of rhizomes and roots increased in conditions that negatively affected growth, such as low pH, soil fertility or light availability, their total active component content exhibited responses that were similar to those of the growth variables. Thus, it seems that soil and light conditions that favour the growth of these medicinal plants also favour their total yield in active components.

Effectiveness of benthic foraminiferal and coral assemblages as water quality indicators on inshore reefs of the Great Barrier Reef, Australia

Although the debate about coral reef decline focuses on global disturbances (e.g., increasing temperatures and acidification), local stressors (nutrient runoff and overfishing) continue to affect reef health and resilience. The effectiveness of foraminiferal and hard-coral assemblages as indicators of changes in water quality was assessed on 27 inshore reefs along the Great Barrier Reef. Environmental variables (i.e., several water quality and sediment parameters) and the composition of both benthic foraminiferal and hard-coral assemblages differed significantly between four regions (Whitsunday, Burdekin, Fitzroy, and the Wet Tropics). Grain size and organic carbon and nitrogen content of sediments, and a composite water column parameter (based on turbidity and concentrations of particulate matter) explained a significant amount of variation in the data (tested by redundancy analyses) in both assemblages. Heterotrophic species of foraminifera were dominant in sediments with high organic content and in localities with low light availability, whereas symbiont-bearing mixotrophic species were dominant elsewhere. A similar suite of parameters explained 89% of the variation in the FORAM index (a Caribbean coral reef health indicator) and 61% in foraminiferal species richness. Coral richness was not related to environmental setting. Coral assemblages varied in response to environmental variables, but were strongly shaped by acute disturbances (e.g., cyclones, Acanthaster planci outbreaks, and bleaching), thus different coral assemblages may be found at sites with the same environmental conditions. Disturbances also affect foraminiferal assemblages, but they appeared to recover more rapidly than corals. Foraminiferal assemblages are effective bioindicators of turbidity/light regimes and organic enrichment of sediments on coral reefs.

Is distribution of hydraulic constraints within tree crowns reflected in photosynthetic water‐use efficiency? An example of Betula pendula

AbstractSpatial and daily variation in photosynthetic water‐use efficiency was examined in leaves of Betula pendula Roth with respect to distribution of hydraulic conductance within the crown, morphological properties of stomata, and water availability. Intrinsic water‐use efficiency (An/gs) was determined from gas‐exchange measurements performed both in situ in a natural forest stand and on detached shoots under laboratory conditions. In intact foliage, sun leaves demonstrated significantly higher (P < 0.001) An/gs than shade leaves, as photosynthesis in the lower canopy was chronically limited by low light availability. However, this difference reversed in the mid‐day period under sufficient irradiance (I > 800 µmol m−2 s−1): An/gs averaged 28.8 and 24.0 µmol mol−1 (P < 0.01) for shade and sun leaves, respectively. This last finding coincided with the data obtained in laboratory conditions: under equivalent leaf water supply and light, An/gs in shade foliage was greater (P < 0.001) than in sun foliage across a wide range of irradiance. Thus, shade foliage of B. pendula is characterized by inherently higher An/gs than sun foliage, associated with more conservative stomatal behavior, and lower soil‐to‐leaf (KT) and leaf hydraulic conductances. Under unlimited light conditions, a within‐crown trade‐off between An/gs and KT becomes apparent. Differences in stomatal conductance between the detached shoots from sunlit and shaded canopy layers were largely attributable to the variation in stomatal morphology; significant relationships were established with characteristics combining stomatal size and density (relative stomatal surface, stomatal pore area index). Stomatal morphology is very likely involved in long‐term adjustment of photosynthetic WUE.

Bottom-up and top-down effects on algal community dynamics in an aridland river: the Middle Rio Grande, New Mexico*

Abstract Background/Question/Methods In this study, we examined interactive effects of nutrient availability, river flow and food web structure on algal growth and species composition in the Middle Rio Grande, New Mexico. Algal community dynamics can be affected by both bottom-up and top-down factors. Algal production may be limited by low levels of nutrients or low light availability. Conversely, production is expected to increase when nutrient levels are high and light availability is not limited. Top-down factors such as grazing by invertebrates can also alter periphyton growth and community composition. Generally, a reduction in grazers results in periphyton biomass increases and shifts in growth forms (to more upright taxa).Nutrient diffusing substrates (NDS) were used in combination with grazer exclusion to investigate bottom-up and top-down effects on periphyton growth and species composition. Previously, NDS has been used in small, headwater streams and not in large sandy rivers like the Middle Rio Grande. NDS were made from inverted terracotta saucers, with four nutrient treatments – control, N, P and N+P. Large invertebrates were excluded from a subset of the saucers using an electrical field. The saucers were placed in a slow-flowing area of the Middle Rio Grande, and samples were collected weekly for a month. Results/Conclusions River conditions made it challenging to conduct the NDS experiment. Water levels were variable and turbidity levels were extremely high (>4000NTU). Algal growth was extremely low on all treatments, largely due to light limitation caused by consistently high turbidity levels and a series of flooding and drying events which also decimated organismal growth on the substrates. Initial surveys of the diatom communities indicate extremely low densities and chlorophyll biomass was very low (mean ± se = 0.31 ± 0.09mg/m2). While dense periphyton communities did not develop on the NDS saucers, there was evidence of colonization by invertebrate populations. Blackfly larvae (Simuliidae) were most abundant in earlier samples whereas chironomid larvae (Chironomidae) were more abundant later in the experiment. Also, blackfly larvae were significantly more abundant on control saucers than on N+P saucers in the grazer/non-grazer experiment (P = 0.044). However, it was difficult to detect differences in the abundance of invertebrate grazers among nutrient treatments. Relationships among invertebrate population densities and the availability and diversity of periphyton for grazing will be described. These results are not definitive, but there are indications that nutrients, flow and grazers do influence algal community dynamics.

Bottom-up and top-down effects on algal community dynamics in an aridland river: the Middle Rio Grande, New Mexico*

In this study, we examined interactive effects of nutrient availability, river flow and food web structure on algal growth and species composition in the Middle Rio Grande, New Mexico. Algal community dynamics can be affected by both bottom-up and top-down factors. Algal production may be limited by low levels of nutrients or low light availability. Conversely, production is expected to increase when nutrient levels are high and light availability is not limited. Top-down factors such as grazing by invertebrates can also alter periphyton growth and community composition. Generally, a reduction in grazers results in periphyton biomass increases and shifts in growth forms (to more upright taxa).Nutrient diffusing substrates (NDS) were used in combination with grazer exclusion to investigate bottom-up and top-down effects on periphyton growth and species composition. Previously, NDS has been used in small, headwater streams and not in large sandy rivers like the Middle Rio Grande. NDS were made from inverted terracotta saucers, with four nutrient treatments – control, N, P and N+P. Large invertebrates were excluded from a subset of the saucers using an electrical field. The saucers were placed in a slow-flowing area of the Middle Rio Grande, and samples were collected weekly for a month. River conditions made it challenging to conduct the NDS experiment. Water levels were variable and turbidity levels were extremely high (>4000NTU). Algal growth was extremely low on all treatments, largely due to light limitation caused by consistently high turbidity levels and a series of flooding and drying events which also decimated organismal growth on the substrates. Initial surveys of the diatom communities indicate extremely low densities and chlorophyll biomass was very low (mean ± se = 0.31 ± 0.09mg/m2). While dense periphyton communities did not develop on the NDS saucers, there was evidence of colonization by invertebrate populations. Blackfly larvae (Simuliidae) were most abundant in earlier samples whereas chironomid larvae (Chironomidae) were more abundant later in the experiment. Also, blackfly larvae were significantly more abundant on control saucers than on N+P saucers in the grazer/non-grazer experiment (P = 0.044). However, it was difficult to detect differences in the abundance of invertebrate grazers among nutrient treatments. Relationships among invertebrate population densities and the availability and diversity of periphyton for grazing will be described. These results are not definitive, but there are indications that nutrients, flow and grazers do influence algal community dynamics.

Distribution and development of under-ice phytoplankton in 90-m deep water column of Lake Päijänne (Finland) during spring convection

Distribution and development of phytoplankton were studied in the deep and large Lake Paijanne from mid-winter until the disappearance of ice. Diatoms were an important part of the phytoplankton assemblage and, with cryptophytes and chrysophytes, made up 50–80% of the phytoplankton biomass. In mid-winter, chlorophyll a and phytoplankton biomass were uniformly distributed over the whole water column down to a depth of 90 m. Thus, most of the phytoplankton was in virtual darkness and there was negligible growth. Only motile cryptophytes were concentrated in the layers below the ice and were rare in deep water. After the disappearance of snow, convection developed, but at first cryptophytes were able to resist mixing. When convection turned from penetrative to predominantly horizontal, all phytoplankton were generally uniformly distributed in the water column. In spite of the full under-ice overturn with low average availability of light, the phytoplankton biomass doubled in April. The growth of cryptophytes was higher than that of diatoms, suggesting that motile species gained an advantage by being able to maintain themselves in the upper, illuminated layers. The results show that knowledge of the basic physical framework is essential for interpretation of under-ice phytoplankton results.

Limitation of coffee leaf photosynthesis by stomatal conductance and light availability under different shade levels

In agroforestry systems, the effect of shade trees on coffee net photosynthesis (A n) has been the object of debates among coffee scientists. In this study, we undertook over 600 coffee A n “spot” measurements under four different artificial shade levels (100, 72, 45 and 19% of full solar irradiance) and analyzed limitations to A n by low light availability (photon flux density, PFD) and stomatal conductance (g s). These gas exchange measurements were carried out during two consecutive coffee growing seasons in a commercial plantation in the Orosi valley of Costa Rica. Levels of A n were related to PFD and g s in order to calculate envelope functions which were used to establish PFD or g s limitations to A n. Under the growing conditions of the present trial, mean leaf A n remained stable for growth irradiance (GI) as low as 45% of full sun and decreased by ~20% at 19% GI. Limitation to A n due to g s was strong in full sun and decreasing with increasing shade levels. On the other hand, limitation due to PFD remained at a similar level for all shade treatments. These different evolutions of limitations of A n by PFD and g s in response to shade explain the absence of a decrease in coffee leaf A n with a shade level up to 55%. Consequently, these results confirm that Arabica coffee is a shade-adapted plant with leaves that can maintain a high photosynthetic performance under low light availability.

Dynamics of biomass partitioning in two competing meadow plant species

The optimal allocation theory predicts that growth is allocated between the shoot and the roots so that the uptake of the most limiting resource is increased. Allocation is dynamic due to resource depletion, interaction with competitors, and the allometry of growth. We assessed the effects of intra- and inter-specific competition on growth and resource allocation of the meadow species Ranunculus acris and Agrostis capillaris, grown in environments with high (+) or low (−) availability of light (L) and nutrients (N). We took samples twice a week over the 7 weeks experiment, to follow the changes in root-to-shoot ratios in plants of different sizes, and carried out a larger scale harvest at the end of the experiment. Of all the tested factors, availability of nutrients had the largest effect on the growth rate and shoot-to-root allocation in both species, although both competition and light had significant effects as well. The highest root-to-shoot ratios were measured from the L+N− treatment, and the lowest from the L−N+ treatment, as predicted by the optimal allocation theory. Competition changed resource allocation, but not always toward acquiring the resource that is most limiting to growth. We thus conclude that the greatest variation in shoot-to-root allocation was due to the resource availability and the effects of competition were small, probably due to low density of plants in the experiment.

Determinants of fruit removal in Geonoma pauciflora, an understory palm of neotropical forests

AbstractCompetition for seed dispersers is supposedly a selective force that drives the evolution of plant reproductive traits. In the understory of tropical forests, such competition should be especially severe among bird‐dispersed plant species because (i) the production of copious fruit crops is limited by low light availability; (ii) there usually is a high density of fleshy fruited plants, and (iii) understory frugivorous birds are not abundant. In this paper, we took advantage of a high‐density population of Geonoma pauciflora, a bird‐dispersed palm species growing in the understory of the Brazilian Atlantic forest, to investigate the influence of plant traits and its immediate neighborhood on fruit removal. Intrinsic (crop and fruit sizes) and extrinsic traits (related to light availability) affected the fruit removal of G. pauciflora. Crop size had a greater influence than fruit size on the absolute number of fruits removed, whereas none of the investigated traits influenced decisively fruit removal efficiency (i.e., the proportion of an individual's crop removed). The influence of light availability was mostly indirect, through its positive influence on fruit production. A significant positive spatial autocorrelation in removal efficiency occurred among neighboring plants within a 7‐m radius, which is indicative of facilitation among neighboring individuals. The consequence of such positive spatial autocorrelation in removal efficiency for clonal plants such as G. pauciflora is that by attracting a frugivore a given ramet may promote the removal of fruits of other ramets, thus enhancing the reproductive output of the genet as a whole.

Carbon and nitrogen metabolism of an eutrophication tolerative macrophyte, Potamogeton crispus, under NH 4+ stress and low light availability

Submersed macrophytes in eutrophic lakes often experience high NH 4 + concentration and low light availability in the water column. This study found that an NH 4 +–N concentration of 1 mg L −1 in the water column apparently caused physiological stress on the macrophyte Potamogeton crispus L. The plants accumulated free amino acids (FAA) and lost soluble carbohydrates (SC) under NH 4 + stress. These stressful effects of NH 4 + were exacerbated under low light availability. Shading significantly increased NH 4 + and FAA contents and dramatically decreased SC and starch contents in the plant shoots. At an NH 4 +–N concentration of 1 mg L −1 in the water column, neither growth inhibition nor NH 4 + accumulation was observed in the plant tissues of P. crispus under normal light availability. The results showed that 1 mg L −1 NH 4 +–N in the water column was not toxic to P. crispus in a short term. To avoid NH 4 + toxicity, active NH 4 + transportation out of the cell may cost energy and thus result in a decline of carbohydrate. When NH 4 + inescapably accumulates in the plant cell, i.e. under NH 4 + stress and shading, NH 4 + is scavenged by FAA synthesis.