SUN-SHADE ECOTYPES OF A BLUEGREEN ALGA IN A HOT SPRING1,2

Clone cultures of the thermophilic alga Plectonema notatum Schmidle were established from cells collected from the high and low light intensity regions of the algal mat which developed in Jerry Johnson Hot Spring, Idaho. Clones isolated from cither high, or low light intensify zones were grown at light intensities of 8000 and 400 ft-c. The existence of specialized and genetically fixed sun or shade ecotypes was evidenced, by the ability of low light intensity clones to synthesize more light harvesting chlorophyll a when grown under low light conditions than the high light clones. High light clones showed light saturation of photosynthesis at higher light intensities with higher carboxylating enzyme activities and less chlorophyll a than low light clones when both were cultured at 8000 ft-c. These clones displayed infraspecific variation along the light intensity gradient and therefore exist as a mosaic of light, intensity ecotypes.     

Physiological differences of five Holm oak (Quercus ilex L.) ecotypes growing under common growth conditions were related to native local climate

An increase in the frequency and intensity of extreme climatic conditions is expected for the Mediterranean area as a response to climate change. As a consequence, the ability of Mediterranean plant species to adapt to complex and stressful environmental conditions plays an important role in driving their future distribution. The adaption of plant species may be expressed by ecotypes already adapted to local climate. Our goal was to analyse the seasonal physiological behaviour of five Quercus ilex ecotypes coming from different Italian geographical areas (from the north to the south) in order to test if ecotypes maintained their physiological traits when grown in the same environmental conditions. Measurements of gas exchange, biochemistry and chlorophyll fluorescence carried out during winter, spring and summer underlined that the response of the considered ecotypes reflected the climate of the original provenances, particularly under suboptimal conditions. The ecotypes from the northernmost and the southernmost limits were the most sensitive to high and low temperatures, respectively. Our results can be used to advance hypotheses about the respone of Q. ilex to climate change.     

Further Studies on Differentiation of Photosynthetic Properties in Sun and Shade Ecotypes of Solidago virgaurea

Measurements of the fraction of the incident light absorbed by diverse Solidago leaves revealed that differences in light harvesting capacity cannot explain the differences in efficiency of utilization of weak light in photosynthesis that have previously been shown to exist between sun and shade ecotypes when these have been grown in strong light and between identical clones of shade ecotypes when grown at different light intensities. Photosynthesis measurements at low and normal oxygen concentrations, provided no evidence that a different degree of inhibition of photo-synthetic CO₂ uptake by atmospheric oxygen is responsible for the observed differences in photosynthetic efficiency, at low or high light intensities. These results support the conclusion that the markedly less efficient use of weak light by shaded habitat clones grown in strong as compared with weak light is caused primarily by damage to the photosystems, or to a site close to them. Measurements of Emerson enhancement and of light-induced absorbance changes provide some evidence that photoreaction II is more affected than I. Enzyme extracts prepared from clones native to an exposed habitat were found to contain considerably higher activities of carboxydismutase (ribulosc-l,5-diphos-phate carboxylase) than from clones native to a shaded habitat when the plants were previously grown at a moderately high light intensity. Exposed habitat clones apparently have a genetically determined, higher capacity to produce the carboxyla-tion enzyme than shaded habitat clones. The high degree of correlation found when the light-saturated rate of CO₂ uptake in vivo of a number of individual Solidago leaves is plotted against the carboxydismutase activities found in the extracts of these same leaves suggests that low carboxydismutase activity is one of the intrinsic properties responsible for the low capacity for light-saturated photosynthesis of clones from shaded habitats. It is concluded from this and other investigations that differentiation between plants from habitats with contrasting light intensities, whether unrelated species or ecotypos of the same species, probably involves the capacity of several component steps of the photosynthetic process.     

The effect of temperature on reproduction in the summer and winter annual Arabidopsis thaliana ecotypes Bur and Cvi

Background and Aims

Seed yield and dormancy status are key components of species fitness that are influenced by the maternal environment, in particular temperature. Responses to environmental conditions can differ between ecotypes of the same species. Therefore, to investigate the effect of maternal environment on seed production, this study compared two contrasting Arabidopsis thaliana ecotypes, Cape Verdi Isle (Cvi) and Burren (Bur). Cvi is adapted to a hot dry climate and Bur to a cool damp climate, and they exhibit winter and summer annual phenotypes, respectively.

Methods

Bur and Cvi plants were grown in reciprocal controlled environments that simulated their native environments. Reproductive development, seed production and subsequent germination behaviour were investigated. Measurements included: pollen viability, the development of floral structure, and germination at 10 and 25 °C in the light to determine dormancy status. Floral development was further investigated by applying gibberellins (GAs) to alter the pistil:stamen ratio.

Key Results

Temperature during seed development determined seed dormancy status. In addition, seed yield was greatly reduced by higher temperature, especially in Bur (>90 %) compared with Cvi (approx. 50 %). The reproductive organs (i.e. stamens) of Bur plants were very sensitive to high temperature during early flowering. Viability of pollen was unaffected, but limited filament extension relative to that of the pistils resulted in failure to pollinate. Thus GA applied to flowers to enhance filament extension largely overcame the effect of high temperature on yield.

Conclusions

High temperature in the maternal environment reduced dormancy and negatively affected the final seed yield of both ecotypes; however, the extent of these responses differed, demonstrating natural variation. Reduced seed yield in Bur resulted from altered floral development not reduced pollen viability. Future higher temperatures will impact on seed performance, but the consequences may differ significantly between ecotypes of the same species.     

Differential effects of temperature change and human impact on European Late Quaternary mammalian extinctions

Species that inhabited Europe during the Late Quaternary were impacted by temperature changes and early humans, resulting in the disappearance of half of the European large mammals. However, quantifying the relative importance that each factor had in the extinction risk of species has been challenging, mostly due to the spatio‐temporal biases of fossil records, which complicate the calibration of realistic and accurate ecological niche modeling. Here, we overcome this problem by using ecotypes, and not real species, to run our models. We created 40 ecotypes with different temperature requirements (mean temperature from ?20 °C to 25 °C and temperature range from 10 °C to 40 °C) and used them to quantify the effect of climate change and human impact. Our results show that cold‐adapted ecotypes would have been highly affected by past temperature changes in Europe, whereas temperate and warm‐adapted ecotypes would have been positively affected by temperature change. Human impact affected all ecotypes negatively, and temperate ecotypes suffered the greatest impacts. Based on these results, the extinction of cold‐adapted species like Mammuthus primigenius may be related to temperature change, while the extinction of temperate species, like Crocuta crocuta, may be related to human impact. Our results suggest that temperature change and human impact affected different ecotypes in distinct ways, and that the interaction of both impacts may have shaped species extinctions in Europe.     

Influence of krummholz mat microclimate on needle physiology and survival

Summary Microclimate and photosynthesis of krummholz mat growth forms of Picea engelmanii (Parry) and Abies lasiocarpa [Hook.] Nutt. were investigated to determine structural features which may aid survival in alpine environments. The structure of krummholz mats was described in terms of the vertical distribution of leaf area index and leaf area density, which exceeded 50 m^-1 (based on total leaf surface area) near the canopy surface and approached zero below 30 cm from the surface in both species. Photosynthetic photon flux density (PPFD, 0.4–0.7 m wavelengths) and wind decreased by an average of 6 and 50-fold, respectively, between 1 m above and 10 cm below mat surfaces in both species. Needle temperatures on a P. engelmannii krummholz mat during July averaged about 2°C above air temperature during the day, with a maximum overtemperature of greater than 20°C above T _air during one sunlit period. At night, needle temperatures averaged 3–4°C below T _air.Net photosynthesis in year-old P. engelmannii shoots reached a maximum at 15–20°C during July and August. Surface shoots were light saturated at near 1200 moles m^-2s^-1 PPFD, and had higher photosynthetic rates than subsurface, predominantly shaded shoots above 800 moles m^-2s^-1. Shade shoots had higher photosynthetic rates when PPFD was below 600 moles m^-2s^-1, and at 250 moles m^-2s^-1 shade shoots maintained about 50% of the net photosynthetic rate of sun shoots at light saturation. Shade shoots appeared capable of benefitting photosynthetically from elevated temperatures within krummholz mats despite relatively low light levels. Especially rapid photosynthesis may occur when canopy needles are illuminated by sunflecks and needle temperatures rise by 10° C or more.Snow cover appears crucial for the survival of needles during winter. Snow accumulated within krummholz needle canopies before the sub-canopy zone of unfoliated branches became filled. The concentrated needle growth in the krummholz canopy captured snow in early autumn without support from ground-level snowpack. Early snow cover in both species prevented cuticle abrasion and resulted in high winter needle water contents and viabilities for subsurface compared to surface needles which became abraded, severely dehydrated, and had high mortality between December and February, especially on windward sides of shoots.Extremely high concentrations of needles within krummholz mat canopies created an aerodynamic structure which elevated needle temperatures to more optimal photosynthetic levels in summer and resulted in more efficient snow accumulation in winter. These factors appear crucial for winter needle survival. Thus, krummholz mats appear to be an important adaptation in growth form which provides survival benefits in both summer and winter.     

Light climate and phytoplankton photosynthesis in maritime Antarctic lakes

The responses of phytoplankton populations to seasonal changes in radiation flux in two Antarctic lakes with extensive winter ice-cover are described. A phytoplankton capable of photosynthesis was found throughout the year in both systems. During winter, low incident radiation combined with thick layers of snow and ice prevented in situ photosynthesis becoming detectable. The beginning of spring was marked by a reduction in snow cover which resulted in a considerable increase in surface penetrating radiation. Planktonic algae rapidly adapted to utilise these increased levels efficiently, though they still showed characteristics of strong shade adaptation.Loss of ice cover at the start of the short open water period further increased the radiation levels and a summer population developed which was much less shade adapted. Saturation and photoinhibition effects were widespread during this period as the algae proved unable to utilise high radiation levels efficiently. They were however effective at the radiation fluxes prevalent in the lower part of the rapidly circulating water columns.     

Photosynthetic Adaptation to Light Intensity in Plants Native to Shaded and Exposed Habitats

Photosynthetic adaptation to light intensity has been studied in clones of populations from shaded and exposed habitats of Rumex acetosa and Geum rivale. Clones of the shade species Lamium galeobdolon and the sun species Plantago lanceolata were also included for comparison. The plants were grown under controlled conditions at a high and a low light intensity. The capacity of photosynthetic carbon dioxide uptake at low as well as at saturating light intensities was determined on single attached leaves. As was previously demonstrated in Solidago virgaurea, clones of populations native to shaded and to exposed environments show differences in the photosynthetic response to light intensity during growth. The data provide evidence that populations of the same species native to habitats with contrasting light intensities differ in their photosynthetic properties in an adaptive manner Ln a similar mode as sun and shade species.     

Application of Esterase Isozymes for Garlic Ecotype Identification

Expression of esterase isozymes from cloves of bolting and non-bolting garlic clones belonging to winter and summer ecotypes were analyzed. The isozyme patterns were obtained by means of vertical block electrophoresis in polyacrylamide gels. Quantitative and qualitative differences between loci of winter and summer garlic ecotype were found. This fact substantiates application of the isoesterases for identification of garlic clones.     

FINE STRUCTURE OF A TERRESTRIAL CYANOBACTERIAL MAT FROM ANTARCTICA1

The fine structure of a cyanobacterial mat collected from the fellfield ecosystems of Signy Island, Antarctica, was examined using some novel light and scanning electron microscope techniques. The mat was up to 5 mm thick and was distinctly layered. The surface of the mat consisted of nonliving material above a zone of Phormidium autumnale (Ag.) Gom. filaments. We suggest that the surface layer protects the cyanobacterium from the effects of desiccation or high irradiance. Lower layers were less structured than the upper layer and included other taxa of cynobacteria and eukaryotic algae, although still dominated by Phormidium. The lowest layers consisted of dead organic material. The mat bound large amounts of inorganic material within and between the subsurface layers.     

Diatom ecology in the phyllosphere of the common duckweed (Lemna minor L.)

Epiphyton associated with thick, floating mats of the common duckweed (Lemna minor L.) was studied at four sites in western Canada between 1985 and 1988. Maximum epiphyton abundance generally occurred in spring as biomass of the duckweed mat was increasing. Epiphytic biomass was low during summer and increased at some sites in autumn with mat decomposition. The community was composed mostly of diatoms and, during summer, photosynthetic bacteria. Species richness of the diatom flora was low, suggesting that duckweed mats are environments to which few species are adapted. Photosynthesis - irradiance curves indicated that duckweed epiphyton was not adapted to low light levels that occurred in the mat (< 1 % of ambient), suggesting they may survive via other means of nutrition. The mat phyllosphere was also characterized by wide spatial and temporal variation in temperature, and sharp vertical profiles of dissolved oxygen and nutrients.     

Annual cycle of phytoplankton in Ace Lake,an ice covered,saline meromictic lake

The annual cycle of phytoplankton in saline, meromictic Ace Lake (68°2S.4S, 78°11.1E) in the Vestfold Hills, Antarctica, was studied from January, 1979 to January 1980. Ace Lake has permanent gradients of temperature, salinity, dissolved oxygen, and hydrogen sulphide, and is ice covered with up to 2 m of ice for 10–12 months each year. The phytoplankton community had low diversity, consisting of only four species, all flagellates — a prasinophyte Pyramimonas gelidicola McFadden et al., a cryptophyte of the genus Cryptomonas; an unidentified colourless microflagellate, and an unarmoured dinoflagellate. These were restricted to the oxic zone of the lake from the surface to 10 m.The phytoplankton had a cycle of seven months of active growth over spring and summer. Low numbers of cells survived in the water column over winter. Spring growth was initiated below the ice by increased light penetration through the ice into the lake, enhanced at the time by the removal of surface snow which accumulated on the ice over winter. Peak phytoplankton biomass production was by the shade adapted P. gelidicola and occurred at the interface of the oxic and anoxic zones where substantial available nitrogen as ammonia is found.The three dominant phytoplankton species displayed distinct vertical stratification over the oxic zone. This stratification was not static and developed over spring as the flagellates migrated to preferred light climate zones. Mean cell volume of two of the flagellates varied significantly over the year. Minimum volumes were recorded in winter and volume increased progressively over spring to reach maximum mean cell volume in summer. Mean cell volume was positively correlated with light intensity (maximum ambient PAR at the respective depth for date of sample). Low cell volume in winter may be related to winter utilization of carbohydrate reserves by slow respiration, and may represent a survival mechanism.     

REACTION NORMS OF MORPHOLOGICAL AND LIFE-HISTORY TRAITS TO LIGHT AVAILABILITY IN IMPATIENS CAPENSIS

For plants, light availability is an important environmental factor that varies both within and between populations. Although the existence of sun and shade “ecotypes” is controversial, it is often assumed that trade-offs may exist between performance in sun and in shade. This study therefore investigated variation in reaction norms to light availability within and between two neighboring natural populations of the annual Impatiens capensis, one in full sun and the other in a forest understory. Seedlings were collected randomly from both populations and grown to maturity in a greenhouse under two light conditions: full light and 18% of full light. Selfed full-sib seed families were collected from plants from both populations grown in both parental light environments. To characterize family reaction norms, seedlings from each family were divided into the same two light treatments and individuals were scored for a variety of morphological and life-history traits. The maternal light environment had little impact on progeny reaction norms. However, the two study populations differed both qualitatively and quantitatively in plastic response to light availability (indicated by significant population x environment interactions in mixed-model ANCOVA). Much of this difference was attributable to population differences in light sensitivity of axillary meristem allocation patterns, which produced concurrent differences in reaction norms for a suite of developmentally linked traits. Within each population, different sets of traits displayed significant variation in plasticity (indicated by significant family x environment interactions). Thus, the genetic potential for evolutionary response to selection in heterogeneous light environments may differ dramatically between neighboring plant populations. Between-environment genetic correlations were largely positive in the woods population and positive or nonsignificant in the sun population; there was no evidence for performance trade-offs across environments or sun or shade “specialist” genotypes within either population. There was little evidence that population differences represented adaptive differentiation for sun or shade; rather, the results suggested the hypothesis of differential selection on patterns of meristem allocation caused by population differences in timing of mortality and intensity of competition.     

The functional ecology of shoot architecture in sun and shade plants of Heteromeles arbutifolia M. Roem., a Californian chaparral shrub

The functional roles of the contrasting morphologies of sun and shade shoots of the evergreen shrub Heteromeles arbutifolia were investigated in chaparral and understory habitats by applying a three-dimensional plant architecture simulation model, YPLANT. The simulations were shown to accurately predict the measured frequency distribution of photosynthetic photon flux density (PFD) on both the leaves and a horizontal surface in the open, and gave reasonably good agreement for the more complex light environment in the shade. The sun shoot architecture was orthotropic and characterized by steeply inclined (mean = 71^o) leaves in a spiral phyllotaxy with short internodes. This architecture resulted in relatively low light absorption efficiencies (E _A) for both diffuse and direct PFD, especially during the summer when solar elevation angles were high. Shade shoots were more plagiotropic with longer internodes and a pseudo-distichous phyllotaxis caused by bending of the petioles that positioned the leaves in a nearly horizontal plane (mean = 5^o). This shade-shoot architecture resulted in higher E _A values for both direct and diffuse PFD as compared to those of the sun shoots. Differences in E _A between sun and shade shoots and between summer and winter were related to differences in projection efficiencies as determined by leaf and solar angles, and by differences in self shading resulting from leaf overlap. The leaves exhibited photosynthetic acclimation to the sun and the shade, with the sun leaves having higher photosynthetic capacities per unit area, higher leaf mass per unit area and lower respiration rates per unit area than shade leaves. Despite having 7 times greater available PFD, sun shoots absorbed only 3 times more and had daily carbon gains only double of those of shade shoots. Simulations showed that sun and shade plants performed similarly in the open light environment, but that shade shoots substantially outperformed sun shoots in the shade light environment. The shoot architecture observed in sun plants appears to achieve an efficient compromise between maximizing carbon gain while minimizing the time that the leaf surfaces are exposed to PFDs in excess of those required for light saturation of photosynthesis and therefore potentially photoinhibitory. Received: 8 June 1997 / Accepted: 2 November 1997     

Hydrography and characteristics of the phytoplankton in shelf and oceanic waters off southeastern Brazil during winter (July/August 1982) and summer (February/March 1984)

The physical and chemical environment, and the phytoplankton primary production of southeastern Brazil were studied in relation to the general oceanographic structure during two research cruises (winter and summer). In each cruise, a total of 91 stations were occupied. Data were collected on the spatial distribution of nutrients, phytoplankton biomass and photosynthetic capacity over the coastal, shelf and oceanic areas off São Paulo, Paraná and Santa Catarina States.During wintertime, the mixing processes between tropical warm waters of the Brazil Current and subantarctic waters of the Malvinas Current formed strong environmental gradients. The drainings of Rio de La Plata and Lagoa dos Patos are transported northwards by coastal currents, enriching the shelf waters off Santa Catarina State with inorganic nutrients and consequently increasing the chlorophyll a to the highest concentrations (> 3.5 mg m ^–3) measured during the two cruises. In slope waters chlorophyll values were always low (0.05–0.45 mg m ^–3). The chlorophyll within the euphotic layer varied from 8.8–36.7 and 1.2–18.5 mg m^–2 during winter and summer, respectively.The surface photosynthetic rates during winter and summer cruises ranged respectively from 0.21–9.17 and 0.66–19.60 mgC/mgChl.a/h. The mean rates were higher in nearshore waters and decreased seaward.The thermal structure of the water column affected the vertical distribution of chlorophyll a and photosynthesis within the euphotic zone; During unstratified periods (winter) they were uniformly distributed but the occurrence of subsurface peaks of chlorophyll and strong photosynthetic inhibition of low light adapted cells in deeper layers are associated to the seasonal thermocline. Occasionally, upwelling of deep waters from shelf break enriched the deeper euphotic layers in offshore areas. Intensive upwelling was observed off Paranagua Bay (Parana State) and the mechanisms of its formation are discussed.     

Light Intensity Affects the Coloration and Structure of Chimeric Leaves of <i>Ananas comosus</i> var<i>. bracteatus</i>

Ananas comosus var. bracteatus is an important ornamental plant because of its green/white chimeric leaves. The accumulation of anthocyanin makes the leaf turn to red especially in the marginal part. However, the red fades away in summer and winter. Light intensity is one of the most important factors affecting leaf color along the seasons. In order to understand the effects of light intensity on the growth and coloration of the chimeric leaves, Ananas comosus var. bracteatus was grown under full sunlight, 50% shade and 75% shade for 75 days to evaluate the concentration of pigments, the color parameters (values L*, a*, b*) and the morpho-anatomical variations of chimeric leaves. The results showed that a high irradiance was beneficial to keep the chimeric leaves red. However, prolonged exposure to high irradiance caused a damage, some of the leaves wrinkled and even burned. Shading instead decreased the concentration of anthocyanin and increased the concentration of chlorophyll, especially in the white marginal part of the leaves. Numerous chloroplasts were observed in the mesophyll cells of the white marginal part of the chimeric leaves under shading for 75 days. The increase in chlorophyll concentration resulted in a better growth of plants. In order to balance the growth and coloration of the leaves, approximately 50% shade is suggested to be the optimum light irradiance condition for Ananas comosus var. bracteatus in summer.     

Sun-shade adaptability of the red mangrove,Rhizophora mangle (Rhizophoraceae): Changes through ontogeny at several levels of biological organization

Rhizophora mangle L., the predominant neotropical mangrove species, occupies a gradient from low intertidal swamp margins with high insolation, to shaded sites at highest high water. Across a light gradient, R. mangle shows properties of both “light-demanding” and “shade-tolerant” species, and defies designation according to existing successional paradigms for rain forest trees. The mode and magnitude of its adaptability to light also change through ontogeny as it grows into the canopy. We characterized and compared phenotypic flexibility of R. mangle seedlings, saplings, and tree modules across changing light environments, from the level of leaf anatomy and photosynthesis, through stem and whole-plant architecture. We also examined growth and mortality differences among sun and shade populations of seedlings over 3 yr. Sun and shade seedling populations diverged in terms of four of six leaf anatomy traits (relative thickness of tissue layers and stomatal density), as well as leaf size and shape, specific leaf area (SLA), leaf internode distances, disparity in blade–petiole angles, canopy spread: height ratios, standing leaf numbers, summer (July) photosynthetic light curve shapes, and growth rates. Saplings showed significant sun/shade differences in fewer characters: leaf thickness, SLA, leaf overlap, disparity in bladepetiole angles, standing leaf numbers, stem volume and branching angle (first-order branches only), and summer photosynthesis. In trees, leaf anatomy was insensitive to light environment, but leaf length, width, and SLA, disparities in bladepetiole angles, and summer maximal photosynthetic rates varied among sun and shade leaf populations. Seedling and sapling photosynthetic rates were significantly depressed in winter (December), while photosynthetic rates in tree leaves did not differ in winter and summer. Seasonal and ontogenetic changes in response to light environment are apparent at several levels of biological organization in R. mangle, within constraints of its architectural baiiplan. Such variation has implications for models of stand carbon gain, and suggest that response flexibility may change with plant age.     

SHADE ADAPTED BENTHIC DIATOMS BENEATH ANTARCTIC SEA ICE1

A dense community of shade adapted microalgae dominated by the diatom Trachyneis aspera is associated with a siliceous sponge spicule mat in McMurdo Sound, Antarctica. Diatoms at a depth of 20 to 30 m were found attached to spicule surfaces and in the interstitial water between spicules. Ambient irradiance was less than 0.6 μE · m^?2· s^?1 due to light attenuation by surface snow, sea ice, ice algae, and the water column. Photosynthesis-irradiance relationships determined by the uptake of NaH¹⁴CO₃ revealed that benthic diatoms beneath annual sea ice were light-saturated at only 11 μE·m^?2·s^?1, putting them among the most shade adapted microalgae reported. Unlike most shade adapted microalgae, however, they were not photoinhibited even at irradiances of 300 μE·m^?2·s^?1. Although in situ primary production by benthic diatoms was low, it may provide a source of fixed carbon to the abundant benthic invertebrates when phytoplankton or ice algal carbon is unavailable.     

Seasonal differences in xanthophyll cycle characteristics and antioxidants in Mahonia repens growing in different light environments

We investigated differences between summer and winter in photosynthesis, xanthophyll cycle-dependent energy dissipation, and antioxidant systems in populations of Mahonia repens (Lindley) Don growing in the eastern foothills of the Colorado Rocky Mountains in deep shade, full exposure, and under a single-layered canopy of Pinus ponderosa (partially shaded). In summer, increasing growth irradiance (from deep shade to partial shade to full exposure) was associated with increased xanthophyll cycle-dependent energy dissipation in PSII and an increased capacity to detoxify reactive reduced oxygen species, as measured by increases in the activities of ascorbate peroxidase, superoxide scavenging, glutathione reductase, and monodehydroascorbate reductase, as well as increases in leaf ascorbate and glutathione content. Leaves of exposed and partially shaded plants exhibited decreased capacities for photosynthetic O₂ evolution in winter compared to summer, while in the deeply shaded plants this parameter did not differ seasonally. Seasonal differences in the levels of antioxidants generally exhibited an inverse response to photosynthesis, being higher in winter compared to summer in the exposed and partially shaded populations, but remaining unchanged in the deeply shaded population. In addition, total pool size and conversion state of the xanthophyll cycle were higher in winter than in summer in all populations. These trends suggest that both xanthophyll cycle-dependent energy dissipation in PSII and the capacity to detoxify reactive reduced oxygen species responded to the level of excess light absorption. Received: 23 October 1997 / Accepted: 23 March 1998     

AUTECOLOGY OF AN ULTRAPLANKTONIC SHADE ALGA IN LAKE TAHOE2

The ultraplanktonic green alga Monoraphidium contortum Korm. in Lake Tahoe (California-Nevada) demonstrated several ecological and physiological attributes of a genetically adapted shade species. Monoraphidium contortum achieved maximum biomass during deep mixing in winter when light availability was at a minimum. During stratification it was found in maximum abundance in the deep euphotic region, 100–150 m. This species was also distributed through the deep aphotic zone where, despite prolonged darkness, it remained capable of immediate photosynthesis when re-exposed to light levels in the euphotic zone. The spirally twisted cells were grazed by two calanoid copepods in Lake Tahoe as readily as much larger-celled phytoplankton species of less complex morphology. Slow growth rates in combination with high susceptibility to copepod grazing may effectively exclude M. contortum from the upper 75 m, where it was rarely recorded. In culture it showed a marked incapacity to adjust to ‘sun’ conditions but was well adapted to low light regimes. Under a wide range of irradiances, photochemical capacity, photosynthetic capacity and growth rates were low, but cellular pigment content remained high. The ratio of P_max to respiration was at the lower end of the range for shade plants. Genetically distinct sun and shade populations of phytoplankton may play a determining role in major shifts of community structure over depth and time in Lake Tahoe.