Light Habitats Research Articles

Testing the adaptive plasticity of Iris pumila leaf traits to natural light conditions using phenotypic selection analysis

A multivariate selection analysis has been used to test the adaptiveness of several Iris pumila leaf traits that display plasticity to natural light conditions. Siblings of a synthetic population comprising 31 families of two populations from contrasting light habitats were grown at an open dune site and in the understory of a Pinus nigra stand in order to score variation in phenotypic expression of six leaf traits: number of senescent leaves, number of live leaves, leaf length, leaf width, leaf angle, and specific leaf area. The ambient light conditions affected the values of all traits studied except for specific leaf area. In accordance to ecophysiological expectations for an adaptive response to light, both leaf length and width were significantly greater while the angle between sequential leaves was significantly smaller in the woodland understory than at the exposed dune site. The relationship between leaf traits and vegetative fitness (total leaf area) differed across light habitats as predicted by functional hypotheses. The standardized linear selection gradient (β′) for leaf length and width were positive in sign in both environments, but their magnitude for leaf length was higher in the shade than under full sunlight. Since plasticity of leaf length in the woodland shade has been recognized as adaptive, fitness cost of producing plastic change in leaf length was assessed. In both of the available methods used, the two-step and the multivariate regression procedures, a rather high negative association between the fitness value and the plasticity of leaf length was obtained, indicating a cost of plasticity. The selection gradient for leaf angle was weak and significant only in the woodland understory. Genetic correlations between trait expressions in contrasting light environments were negative in sign and low in magnitude, implying a significant genetic variation for plasticity in these leaf traits. Furthermore, leaf length and leaf width were found to be genetically positively coupled, which indicates that there is a potential for these two traits to evolve toward their optimal phenotypic values even faster than would be expected if they were genetically independent.

Asplenium cimmeriorum, a new fern species from New Zealand

Asplenium cimmeriorum sp. nov. is described from the Waitomo district in the North Island and from the Oparara Valley and north Westland in the South Island. It is an octoploid species which grows in lowland sites on calcareous substrates in high rainfall areas. In the North Island it is largely confined to cave entrances and limestone walls, favouring sites of very low light intensity. In the South Island it extends also into forest, growing on rotting logs, roots, and rock. It is morphologically close to both Asplenium hookerianum and A. bulbiferum subsp. gracillimum, and is known to hybridise with A. bulbiferum. It is suggested that it may be an allopolyploid species with A. hookerianum as one parent. Asplenium hookerianum is apparently absent from the regions now occupied by A. cimmeriorum, and may have been displaced by a species better adapted to high rainfall and low light habitats on calcareous soils. The conservation status of A. cimmeriorum is discussed and it is recommended for classification as a “local” species.

Coral‐Seaweed‐Grazer‐Nutrient Interactions on Temperate Reefs

Why corals dominate tropical reefs but are rare or absent on temperate reefs is one of the more intriguing global—scale questions in marine ecology. Restriction of corals from temperate reefs has been suggested to be due to low temperature, competition with seaweeds, and synergistic interactions of physical and biological factors. However, most studies addressing these hypotheses have been non—experimental and conducted on tropical corals and reefs near the extremes of their distribution rather than in temperate habitats using corals that are physiologically suited to temperate environments. Some corals occur only in temperate regions, but their ecology is largely unstudied. In an attempt to understand how temperate corals function at latitudes where corals are relatively rare and where reefs tend to be dominated by seaweeds, we examined responses of the temperate coral Oculina arbuscula to competition with seaweeds on reefs in North Carolina (USA). We asked how competitive interactions were affected by levels of grazing, nutrients, or interactions of these factors. Sampling of 12 reef habitats ranging from 1 m deep, inshore sites to 25 m deep, offshore sites demonstrated a strong negative relationship between percentage cover of seaweeds and density of Oculina colonies. Seaweeds dominated well—lit habitats (shallow inshore reefs or deep offshore reefs with clear waters), while coral was significantly more abundant in low—light habitats where seaweeds were rare (deep inshore habitats or nearshore reefs with turbid waters). This general among—site pattern also occurred within an individual site as one moved from shallow, well—lit waters to deeper, darker waters. Corals transplanted into seaweed—dominated areas grew well if seaweed canopies were removed but grew poorly, or not at all, if canopies were left intact. Seaweeds also significantly inhibited recruitment of O. arbuscula. At a turbid inshore reef, recruitment was high at a depth of 6 m where seaweeds were rare, but very low at a depth of 1 m where seaweeds were abundant. Removal of seaweeds from shallow plots increased recruitment about 12—fold to levels that did not differ significantly from those at 6 m. In grazer—exclusion cages on offshore reefs, Oculina recruitment varied significantly between sites and years. Almost no recruitment was observed at a well—lit, plant—dominated site while recruitment was higher, to very high, at a turbid site with few seaweeds. At this turbid site, recruitment was facilitated by grazing but could not be related to grazer effects on seaweeds. Facilitation was apparently due to consumers removing barnacles, which dominated this low—light site if grazers were excluded. Cage exclusion of larger herbivores at two offshore sites (primarily fishes) and one inshore site (fishes and urchins) had no significant effect on coral growth. At the well—lit inshore site, herbivores had a large effect on the species composition of the seaweed community, but little effect on the total abundance of seaweeds. Large grazers caused palatable red seaweeds to be replaced by similar amounts of unpalatable brown seaweeds. Therefore, herbivory alone had little impact on total seaweed abundance and the levels of seaweed competition affecting co—occurring O. arbuscula. In contrast, when we conducted a factorial experiment manipulating both herbivory and nutrient levels, exclusion cages significantly reduced coral growth. In this experiment, nutrient addition had no effect on brown seaweeds but significantly increased the percentage cover of red seaweeds in exclusion cages. Nutrient addition also tended to suppress coral growth in herbivore—exclusion cages where red seaweeds were stimulated, but to increase coral growth in open cages where herbivores had removed the red seaweeds. Thus, nutrients and herbivory may have acted synergistically to affect seaweeds, and hence corals, on this temperate reef. Different groups of algae (red vs. brown) experienced differential degrees of nutrient limitation and exerted differential competitive effects on corals at this site. Our findings show that competition with seaweeds plays a large role in excluding Oculina from well—lit temperate reefs, and support the hypothesis that seaweed competition may interact with latitudinal changes in physical parameters to limit coral recruitment, growth, and accumulation at high latitudes, thus suppressing the potential for reef development.

A Phytotron for Plant Stress Research: How Far Can Artificial Lighting Compare to Natural Sunlight?

Plants have adapted very efficiently to their natural light habitat. Artificial plant illumination, therefore, requires careful design. Not only the quantity of radiation per area or volume (intensity) but also the spectral quality has to match seasonal and diurnal variations of natural global radiation as close as possible. The GSF Research Center has developed a phytotron system especially devoted to plant stress research, where these requirements are of particular importance. The phytotron consists of seven closed chambers (4 walk-in size chambers, two medium and one small sun simulator). Our contribution oudines the basic design of the lighting and presents spectral data. A good approximation of terrestrial global radiation is achieved if several commercially available lamp types are combined and adequate filters are applied to reject unwanted infrared and harmful ultraviolet radiation. A programmable switch control for the individual lamp banks allows a variation of both spectrum and intensity of the illumination. Spectroradiometric measurements show that the maximum level of illumination in the small and in the medium size chambers can compete both in spectral distribution and in intensity with outdoor global radiation for solar elevations up to 60°. The maximum light level available inside the large walk-in chambers reaches an irradiance corresponding to solar elevation of 50°. The UV-B: UV-A: PAR ratio, which mirrors the spectral balance of plant lighting, can be adjusted to values following the diurnal variation of natural global radiation.

Effect of canopy gap light environment on evaporative load and stomatal conductance in the temperate forest understory herbAster macrophyllus (Asteraceae)

Aster macrophyllus, a temperate forest understory species of the northeastern United States, inhabits a broad range of light habitats. Plants receiving several minutes of direct sun in canopy gap and forest edge habitats occasionally wilt, a response indicative of water stress. We compared two alterative scenarios for patterns of evaporative load and stomatal conductance for plants in large (0.15 ha) tree canopy gaps and small (3 m2) herbaceous subcanopy gaps: 1) evaporative loads are typically moderate and stomatal conductance is largely governed by light intensity; or 2) evaporative loads are often substantial, mandating stomatal closure to prevent excessive transpiration. In all cases evaporative loads were elevated by light intensity above 25% of full sun. This was accompanied by substantial stomatal closure. Transitions from low to moderate light intensity (<13% full sun) caused little increase in leaf evaporative load, and stimulated increases in stomatal conductance. Very brief periods of high light also stimulated stomatal opening. Light environments in the small herbaceous subcanopy gaps differ greatly in their patterns of evaporative load from day to day.

The Color of Light in Forests and Its Implications

Forests exhibit much variation in light environments, and this can affect communication among animals, communication between animals and plants, photosynthesis, and plant morphogenesis. Light environments are caused by, and can be predicted from, the geometry of the light paths, the weather conditions, and the time of day. The structure of forests leads to four major light habitats when the sun is not blocked by clouds: forest shade, woodland shade, small gaps, and large gaps. These are characterized by yellow—green, blue—gray, reddish, and "white" ambient light spectra, respectively. When the sun is blocked by clouds, the spectra of these four habitats converge on that of large gaps and open areas, so the single light environment during cloudy weather will be called open/cloudy. An additional light environment (early/late) is associated with low sun angles (near dawn or dusk); it is purplish. Each light environment is well defined and was found in forests of Trinidad, Panama, Costa Rica, Australia, California, and Florida. Scattered literature references suggest similar patterns elsewhere in North America, Europe, and Java. Perceived colors of animals, flowers, and fruits depend upon the interaction between ambient light color and the reflectance color of the animal or plant parts. As a result, an animal or plant may have a different appearance in each environment, i.e., a color pattern may be relatively cryptic in some light environments while relatively conspicuous in others. This has strong implications for the joint evolution of visual signals and vision, as well as microhabitat choice. Plant growth and form may also be affected by variation in the color of forest light.

Ammonium and nitrate uptake in gap, generalist and understory species of the genus Piper.

We studied root net uptake of ammonium (NH 4+ ) and nitrate (NO 3- ) in species of the genus Piper (Piperaceae) under high, intermediate and low photosynthetically active photon flux densities (PFD). Plants were grown hydroponically, and then transferred to temperature controlled (25° C) root cuvettes for nutrient uptake determinations. Uptake solutions provided NH 4+ and NO 3- simultaneously (both) or separately (single). In the first experiment, seven species of Piper, from a broad range of rainforest light habitats ranging from gap to understory, were screened for mineral nitrogen preference (100 μM NH 4+ and/or 100 μM NO 3- ) at intermediate PFD (100 μmol m-2 s-1). Preference for NH 4+ relative to NO 3- , defined as the ratio of NH 4+ (both):NO 3- (both) net uptake, was higher in understory species than in gap species. Ammonium repression of NO 3- uptake, defined as the ratio of NO 3- (single): NO 3- (both) net uptake, was also higher in understory species as compared to gap species. In a second set of experiments, we examined the effect of nitrogen concentration (equimolar, 10 to 1000 μM) on NH 4+ preference and NH 4+ repression of NO 3- net uptake at high (500 μmol m-2 s-1) and low (50 μmol m-2 s-1) PFD in a gap (P. auritum), generalist (P. hispidum) and understory species (P. aequale). All species exhibited negligible NH 4+ repression of NO 3- net uptake at high PFD. At low PFD, NH 4+ preference and repression of NO 3- net uptake occurred in all species (understory > generalist > gap), but only at intermediate nitrogen concentrations, i.e. between 10 and 200 μM. Ammonium repression of net NO 3- uptake decreased or increased rapidly (in < 48 h) after transitions from low to high or from high to low PFD respectively. No significant diurnal patterns in NO 3- or NH 4+ net uptake were observed.

Natural History of a Neotropical Gap-Inhabiting Grasshopper

I studied the natural history of a short-horned tropical forest grasshopper, Microtylopteryx hebardi Rehn (Orthoptera: Acrididae: Ommatolampinae) at La Selva Biological Station, Costa Rica. M. hebardi is flightless as an adult, and hence has low dispersal capabilities, yet inhabits light gaps and other temporary habitats. Numbers of adult M. hebardi were relatively constant between gaps and during the seven month study period, and all life stages were found year-round. Host plant range of M. hebardi included 52 species in 16 monocot and dicot families, but the majority of feeding records and all oviposition records occurred in four related monocotyledonous families.

Effects of habitat and size of fruit display on removal of Smilacina stellata (Liliaceae) fruits

The effects of habitat and infructescence size on fruit removal by birds and grazing mammals from artificial infructescences of Smilacina stellata (L.) Desf. were examined. The study site, a coniferous forest in the Blue Mountains of eastern Washington, differed from sites previously examined for bird – fruit interactions in North America in that autumnal censuses for 3 years indicated numbers of avian frugivores were lower. Removal of fruits from infructescences in light gaps was consistently faster than was removal from infructescences placed along forest edge or beneath dense canopy. Larger fruit displays influenced removal along forest edge only. The greatest percentage seed loss to grazing mammals occurred generally in the largest fruit displays and in those infructescences under a closed canopy. The results suggest that levels of seed dispersal and seed predation in understory plants differ in light gaps, forest edges, and habitats beneath forest canopy.

Photosynthetic characteristics of Prochloron sp./ascidian symbioses

The prokaryotic green alga Prochloron sp. (Prochlorophyta) is found in symbiotic association with colonial didemnid ascidians that inhabit warm tropical waters in a broad range of light environments. We sought to determine the light-adaptation features of this alga in relation to the natural light environments in which the symbioses are found, and to characterize the temperature sensitivity of photosynthesis and respiration of Prochloron sp. in order to assess its physiological role in the productivity and distribution of the symbiosis. Colonies of the host ascidian Lissoclinum patella were collected from exposed and shaded habitats in a shallow lagoon in Palau, West Caroline Islands, during February and March, 1983. Some colonies from the two light habitats were maintained under conditions of high light (2 200 μE m−2 s−1) and low light (400 μE m−2 s−1) in running seawater tanks. The environments were characterized in terms of daily light quantum fluxes, daily periods of light-saturated photosynthesis (Hsat), and photon flux density levels. Prochloron sp. cells were isolated from the hosts and examined for their photosynthesis vs irradiance relationships, respiration, pigment content and photosynthetic unit features. In addition, daily P:R ratios, photosynthetic quotients, carbon balances and photosynthetic carbon release were also characterized. It was found that Prochloron sp. cells from low-light colonies possessed lower chlorophyll a/b ratios, larger photosynthetic units sizes based on both reaction I and reaction II, similar numbers of reaction center I and reaction center II per cell, lower respiration levels, and lower Pmax values than cells from high-light colonies. Cells isolated from low-light colonies showed photoinhibition of Pmax at photon flux densities above 800 μE m−2 s−1. However, because the host tissue attenuates about 60 to 80% of the incident irradiance, it is unlikely that these cells are normally photoinhibited in hospite. Collectively, the light-adaptation features of Prochloron sp. were more similar to those of eukaryotic algae and vascular plant chloroplasts than to those of cyanobacteria, and the responses were more sensitive to the daily flux of photosynthetic quantum than to photon flux density per se. Calculation of daily minimum carbon balances indicated that, though high-light cells had daily P:R ratios of 1.0 compared to 4.6 for low-light cells, the cells from the two different light environments showed nearly identical daily carbon gains. Cells isolated from high-light colonies released between 15 and 20% of their photosynthetically-fixed carbon, levels sufficient to be important in the nutrition of the host. Q10 responses of photosynthesis and respiration in Prochloron sp. cells exposed briefly (15–45 min) to temperatures between 15° and 45°C revealed a discontinuity in the photosynthetic response at the ambient growth temperatures. The photosynthetic rates were found to be more than twice as sensitive to temperatures below ambient (Q10=3.47) than to temperatures above ambient (Q10=1.47). The Q10 for respiration was constant (Q10=1.66) over the temperature range examined. It appears that the photosynthetic temperature sensitivity of Prochloron sp. may restrict its distribution to warmer tropical waters. The ecological implications of these findings are discussed in relation to published data on other symbiotic systems and free-living algae.

Possible monocular range-finding mechanisms in stomatopods from different environmental light conditions

1. 1. Stomatopods have double eyes. 2. 2. In the middle band dividing the two halves of the eye ommatidia are perpendicular to the cornea. 3. 3. Rows of ommatidia parallel to the middle band have divergent optical axes. 4. 4. In the columns across the middle band the optical axes of ommatidia near the middle band are convergent between the two halves of the eye. 5. 5. From the fifth to tenth ommatidium towards the sides optical axes on each side are parallel to each other and to those in the middle band. 6. 6. This results in a strong overlap of visual fields between ommatidia with parallel optical axes in each half of the eye and with the fields of the skewed ommatidia from the opposite half of the eye. 7. 7. We postulate this as the morphological basis for a range-finding and motion measuring device along the columns, repeated around the eye along the rows. 8. 8. We hypothesize that this device may measure distances by the amount and pattern of overlap of visual fields at each point in space. 9. 9. Species from bright light habitats show less skewed ommatidia and similar patterns of overlap are shifted to longer distances than in dim light species.

Ultrastructure of a cave-wall cyanophyte-Gloeocapsa NS4

Gloeocapsa strain NS4, a cyanophyte (cyanobacterium) which grows in low light levels inside cave entrances, was studied in the electron microscope by thin sectioning and freeze-etching. The cells are surrounded by a microfibrillar sheath divided by ‘dense lamellae’, which are probably an acidic mucopolysaccharide. Inside this is a typical Gramnegative cell wall. Double-replica freeze-fracture showed that the outer envelope of the wall fractures to give two faces each consisting of densely-packed particles; the particles of the outer leaflet seem to consist of subunits arranged in a hollow cylinder. A structural model of the outer envelope is proposed. The plasma membrane fractures to give a PF face with 3000 ∼ 9 nm particles μm-1 and an EF face with 150–700 11–12 nm particles μm-1. The thylakoids are arranged in a pattern not previously found in a unicellular cyanophyte, parallel arrays which intersect, and may fuse with, the plasma membrane. The thylakoid membranes have 2,850 particles μm-1, mean size 10.9 nm, on the PF face and 560 particles μm-1, mean size 12.3 nm, on the EF face. Phycobilisomes are difficult to see, but may be unusually large. These ultrastructural features may be adaptations to a very low light habitat.

Evolution of visual pigments

Rod visual pigments exhibit a variety of absorption maxima, determined partly by the nature of the prosthetic group, retinal or 3-dehydroretinal, and partly by the apoprotein opsin. Some animal taxa exhibit little diversity. On the other hand, selective pressures arising from the different light habitats of aquatic environments are believed to have produced the correspondingly wide range of λ max in fish visual pigments, which often extends over 80 nm. Opsins appear to have similar molecular weights. Latest estimates of molecular weight suggest that they have approximately 315 amino acid residues. Some species, notably members of the salmonidae, have visual pigments with different λ max yet have diverged only during the past several million years. This would indicate a very high rate of protein evolution in circumstances where we should expect that amino acid substitution would be severely restricted. Certain fish species (e.g. the deepwater sculpin, Myoxocephalus) have mixtures of visual pigments with λ max separated by only 6 nm. The physiological usefulness of such a pair is dubious, but it may indicate polymorphism at the gene locus coding for opsin. The use of retinol and 3-dehydroretinol, the basis of rhodopsin and porphyropsin, may vary between quite closely related species. Extraneous factors such as light and hormones are known to be important in amphibian larvae and fishes, but there is also clear evidence for genetic influences, as revealed by the divergent evolution of the system in various populations of originally anadromous smelt after only 10 000 years of isolation.