CHLOROPHYLL A VERSUS ADENOSINE TRIPHOSPHATE AS ALGAL BIOMASS INDICATORS IN LAKES1

The relation between phyloplankton chlorophyll a and adenosine triphosphate (ATP) was investigated during a yearly cycle in Lake Tahoe, California–Nevada, and a variety of North American and New Zealand lakes of ranging trophic state. Since cellular concentrations of ATP have been shown as acceptable indicators of live biomass among natural microbial populations, the ratio of chlorophyll a to cellular ATP reveals the extent of pigment production per unit biomass under diverse environmental conditions. In general, mixed systems, i.e., epilimnion of most lakes sampled, showed good consistency of cellular chlorophyll a:ATP ratios. Under thermally stratified conditions at Lake Tahoe extensive modifications of the ratio occurred, leading to a 10-fold increase in the ratio at the bottom of the euphotic zone by late summer. Thus, the applicability of chlorophyll a as a quantitative algal biomass indicator is greatly restricted in lakes having thermally stratified euphotic zones.     

Sun/shade adaptations of the photosynthetic apparatus of <Emphasis Type="Italic">Hoya carnosa</Emphasis>, an epiphytic CAM vine,in a subtropical rain forest in northeastern Taiwan

Most past work on the ecophysiology of the Crassulacean acid metabolism (CAM) plant, Hoya carnosa, in the lab and in situ in Australia indicates that this epiphytic vine is better adapted to shaded, not exposed, locations, although a recent study of this species in Taiwan presents findings that run counter to this conclusion. Thus, photosynthetic characteristics of shaded and exposed individuals of H. carnosa were compared in situ in a subtropical rain forest in northeastern Taiwan in order to determine whether this CAM epiphyte is better adapted to the shade or the sun. Although leaves of shade plants had much greater chlorophyll concentrations than did those of sun plants, chlorophyll a/b ratios did not differ between the two groups of plants. Fluorescence measurements revealed some ability of leaves to acclimate to both shade and sun, although some evidence for photoinhibition (photoprotection) was observed in more exposed plants. Despite the latter, both exposed and shaded plants exhibited CAM, measured as diel fluctuations in leaf acidity, and CAM was more consistently found in the exposed plants. Furthermore, some evidence for more CAM at higher light availabilities was found. Overall, the results of this investigation reveal that H. carnosa in this subtropical rain forest in Taiwan exhibits adaptations to both high and low light levels, which should prove adaptive for an epiphytic vine with leaves on the same individual exposed to a wide range of exposure and shade in the host tree canopy.     

The induction of sun and shade leaves of the European beech (Fagus sylvatica L.): anatomical studies

Summary Primordia from buds of sun and shade twigs of European beech (Fagus sylvatica L.) were collected six times a year for anatomical investigations. Differentiation into sun-leaf and shade-leaf primordia was first observed in early August. Sun-leaf primordia had five, and shade-leaf primordia four layers of mesophyll meristem cells. With potted graft unions of beeches possible structural changes of leaf primordia were investigated. Trees adapted to shade develop sun-leaf primordia when put into full daylight, provided the transfer happened before July. Trees adapted to full daylight developed leaf primordia which remained structurally sun-leaf primordia when the plant was kept under shade conditions. Shadeleaf branches of young beech trees cut in February in order to expose the shade buds to full daylight developed either shade leaves or intermediate shade/sun leaves. These experiments show that the subtending leaf may provide the developing axillary bud with photoassimilates, but its character, whether sun or shade leaf, has no influence on the character of the developing leaf primordia.     

Increased cyanophage infection at the bottom of the euphotic zone,especially in the fall

Picocyanobacteria contribute greatly to offshore primary production with cells extending through the deep euphotic zone. Literature indicates high viral infection of cyanobacteria in ocean transition zones. We postulate that the bottom of the euphotic zone is a transition zone, where communities transition from phototrophic to aphotic processes. We use single-copy core genes to examine cyanophage to cyanobacteria ratios in cellular metagenomes in the subtropical North Atlantic and Pacific. Cyanophage to cyanobacteria terL/rpoB ratios generally increase to >10 in the deep euphotic zone. As light levels decrease in the fall, Prochlorococcus in the deep euphotic zone experience reduced light levels. We find clear differences between spring (Geotraces GA02) and fall (GA03) in the North Atlantic, with terL/rpoB ratios increasing to >40 in the fall. When examining 23 months of the North Pacific Hawaii Ocean Timeseries, the depth of elevated cyanophage to cyanobacteria ratios in cellular metagenomes negatively correlated with surface photosynthetic radiation (PAR), particularly with the change in PAR, which reflected the season. In fall, all picocyanobacteria ecotypes were found at depths enriched with viruses, while in summer, only low light ecotypes were affected. Thus, we find high cyanophage infection both in the deep euphotic zone and during seasonal transitions.     

Adaptation of the Photosynthetic Apparatus of Scenedesmus obliquus to Strong and Weak Light Conditions

Homocontinuous cultures of the unicellular green alga Scenedesmus obliquus were grown under strong (28 W/m²～28,000 lux) and weak (5 W/m²～5000 lux) light conditions to simulate the conditions of ‘sun’ and ‘shade’ plants. As in higher plants the cells adapted to strong light had less chlorophyll but demonstrated a higher photosynthetic capacity and a higher respiration rate, so that their compensation point was reached at three times higher energy than in the cells grown under low light intensities. The CO₂ fixation rate and the RuDP carboxylase activity under saturating light intensities were both higher in the cells grown in strong light. In spite of the differences in the pigment content and in the light saturated photosynthetic capacities for both cultures, the quantum yields of photosynthetic oxygen evolution were equal. As documented for some species of higher plants Scenedesmus is not genetically determined to be either a ‘sun’ or ‘shade’ organism but can adapt its photosynthetic apparatus to the different light intensities.     

Summer depth selection in crustacean zooplankton in nutrient‐poor boreal lakes is affected by recent residential development

1. Strong vertical gradients in light, water temperature, oxygen, algal concentration and predator encounters during summer in stratified lakes may influence patterns of depth selection in crustacean zooplankton, especially Daphnia species. 2. To test how crustacean depth selection varies among lakes along a gradient of catchment disturbance by recent residential development and land use change, we calculated the weighted mean depth distribution of the biomass of crustaceans by day and night in eight nutrient‐poor boreal lakes. 3. Generally, the greatest biomass of crustaceans was located at the metalimnion or at the lower boundary of the euphotic zone during thermal stratification in July. The crustacean zooplankton avoided warm surface layers and tended to stay in colder deep waters by both day and night. They also remained at greater depths in lakes with a more extensive euphotic zone. 4. There was some evidence of upward nocturnal migrations of large Daphnia and copepods in some lakes, and one case of downward migration in a lake inhabited by chaoborid larvae. 5. Multivariate regression trees (MRT) were used to cluster crustaceans and Daphnia species in homogeneous groups based on lake natural and disturbance factors. For crustaceans, the depth of the euphotic zone, the sampling depth (epilimnion, metalimnion and hypolimnion), time (day or night) of sampling and the biomass of chlorophyll a were the main driving factors. For Daphnia species, the drainage area, the sampling depth, the cleared land surface area within the catchment and the concentration of total dissolved phosphorus were the main factors.     

Effects of antimycin A and n-propyl gallate on photosynthesis in sun and shade plants

Sun (Chrysanthemum morifolium) and shade (Spathiphyllum wallisii) plants were used to study the effects of the inhibitors antimycin A and n-propyl gallate on photosynthesis, using chlorophyll fluorescence imaging. Photochemical efficiency in sun and shade plants was not inhibited by antimycin A under low light intensity, but the effective PS II quantum yield was inhibited under high illumination and heat, more so in sun than in shade species. The inhibitor n-propyl gallate had no or little effect on photochemical efficiency under low or high illumination and heat in control plants of both species. However, in stressed plants the effect of n-propyl gallate under high illumination and heat increased, strongly inhibiting the effective PS II quantum yield in S. wallisii. In addition to the effects produced by the inhibitors, changes were also observed by immunoblot analysis in the plastidial NADH DH complex, PTOX and PGR5. The quantities of PTOX and NDH-H subunit of the thylakoidal NADH DH complex and the NADH DH activity in the thylakoid membranes were similar in control plants of both species and increased in stressed plants, particularly in S. wallisii. However, the level of PGR5 polypeptide increased in C. morifolium but remained similar to control in S. wallisii. The results indicate that under high illumination and heat the cyclic electron flow around the PSI antimycin A-sensitive is important for the photosynthetic function in the sun species while another pathway involving the NADH DH complex and PTOX might be more important in the shade species.     

Contribution of intercellular reflectance to photosynthesis in shade leaves

The potential contribution of intercellular light reflectance to photosynthesis was investigated by infiltrating shade leaves with mineral oil. Infiltration of leaves of Hydrophyllum canadense and Asarum canadense with mineral oil decreased adaxial leaf reflectance but increased transmittance. As a result of the large increase in transmittance, infiltration caused a decrease in absorptance of 25% and 30% at 550 and 750 nm, respectively. Thus, intercellular reflectance increased absorptance in these species by this amount. In a comparison of sun and shade leaves of Acer saccharum and Parthenocissus quinquefolia, oil infiltration decreased absorptance more in shade than in sun leaves. This difference suggests that the higher proportion of spongy mesophyll in shade leaves may increase internal light scattering and thus absorptance. The importance of the spongy mesophyll in increasing internal reflectance was also evident in comparisons of the optics of Populus leaves and in the fluorescence yield of oil-infiltrated leaves of several sun and shade species. Oil infiltration decreased the quantum yield of fluorescence (F_o) by 39–52% for shade leaves but only 21–25% for sun leaves. We conclude that the greater proportion of spongy parenchyma in shade leaves increased intercellular light scattering and thus absorptance. Direct measurements with fibre-optic light probes of the distribution of light inside leaves of Hydrophyllum canadense confirmed that oil infiltration decreased the amount of back-scattered light and that most of the light scattering for this species occurred from the middle of the palisade layer to the middle of the spongy mesophyll. We were not, however, able to assess the potential contribution of reflectance from the internal abaxial epidermis to total internal light scattering in these experiments. Using a mathematical model to compare the response of net photosynthesis (O₂, flux) to incident irradiance for control leaves of H. canadense and theoretical leaves with no intercellular reflectance, we calculated that intercellular reflectance caused a 1.97-fold increase in photosynthesis at 20 μmol m^?2s^?1 (incident photon flux density). This enhancement of absorption and photosynthesis by inter-cellular reflectance, without additional production and maintenance of photosynthetic pigments, may maintain shade leaves above the photosynthetic light compensation point between sunflecks and maintain the light induction state during protracted periods of low diffuse light.     

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.     

Carotenoid composition and metabolism in green and blue-green algal lichens in the field

The carotenoid composition of 33 species of green algal lichens and 5 species of blue-green algal lichens was examined and compared with that of the leaves of higher plants. As in higher plants, green algal lichen species which were found in both shade and full sunlight exhibited higher levels of the carotenoids involved in photoprotective thermal energy dissipation (zeaxanthin as well as the total xanthophyll cycle pool) in the sun than in the shade. This was particularly true when thalli were moist during exposure to high light, or presumably became desiccated in full sunlight. However, the reverse trend in the carotenoid composition of green algal lichens was also observed in those species which were found predominantly either in the shade or in full sunlight. In this case sun-exposed lichens often possessed lower levels of zeaxanthin and of the components of the xanthophyll cycle than lichens which were found in the shade. In contrast to higher plants, the lichens from all habitats exhibited a relatively high ratio of carotenoids to chlorophylls (more characteristic of sun leaves), very low levels of α-carotene (similar to that found in sun leaves), and a level of β-carotene similar to that found in shade leaves. Zeaxanthin, but not the expoxides of the xanthophyll cycle, was also frequently found in blue-green algal lichens. A trend for increasing levels of zeaxanthin with increasing growth light regime was observed inPeltigera rufescens, the species which was found to occur over the widest range of light environments. The level of zeaxanthin per chlorophylla in these blue-green algal lichens was in a range similar to that per chlorophylla+b in green algal lichens. However, zeaxanthin was also absent in one species,Collema cristatum, in full sunlight. Thus, the zeaxanthin content of the blue-green algal lichens can be similar to that of higher plants, or it can be rather dissimilar, as was also the case in the green algal lichen species. The presence of large amounts of ketocarotenoids in blue-green algal lichens is also noteworthy.     

Natural selection on light response curve parameters in the herbaceous annual, <Emphasis Type="Italic">Impatiens capensis</Emphasis>

We tested for genetic variation in light response curves and their acclimation to sun versus shade in recombinant inbred lines (RILs) of the annual species Impatiens capensis derived from a cross between sun and shade populations. We exposed replicates of 49 RILs to experimentally manipulated light levels (open versus shade) in a greenhouse and measured photosynthetic light response curves, height, biomass, and reproduction. Plants were taller in the shade treatment, but we were unable to detect differences between light treatments (i.e., acclimation) in the maximal rate of photosynthesis, the light compensation point, or the quantum efficiency of photosynthesis. Genotypic selection analyses indicated that higher maximal rates of carbon assimilation and higher light compensation points (typical of sun-acclimated light curves) were favored by natural selection in both light treatments. Thus, it appears that the pattern of selection on photosynthetic parameters may not depend on light environment in this species.M. Shane Heschel and John R. Stinchcombe contributed equally to the paper.     

Carbohydrate storage,survival, and growth of two evergreen <Emphasis Type="Italic">Nothofagus</Emphasis> species in two contrasting light environments

A number of traits have been attributed important roles in tolerance of shade by plants. Some explanations emphasize traits enhancing net carbon gain; others emphasize energy conservation traits such as storage of non-structural carbohydrates (NSC). To date, cross-species studies have provided mixed support for the role of NSC storage in low-light survival. We examined NSC status, survival, biomass, and growth of large seedlings of two evergreen species of differing shade tolerance (Nothofagus nitida and N. dombeyi) grown in deep shade and 50% light for two growing seasons. We expected to find higher NSC concentration in the more shade-tolerant N. nitida and since allocation to storage involves sacrificing growth, higher growth rate in the shade-intolerant N. dombeyi. NSC concentration of both species was >twofold higher in 50% light than in deep shade, and in roots and stems did not differ significantly between species in either environment. NSC contents per plant were also similar between dead and living plants in deep shade. N. dombeyi outgrew N. nitida in 50% light, while this pattern was reversed in deep shade. Survival in deep shade was not correlated with NSC concentration. Leaf mass fraction was similar between species in 50% light, but lower in N. dombeyi in deep shade. Results provide little evidence of a link between carbohydrate storage and low-light survival in Nothofagus species, and support the view that understorey survival is primarily a function of net carbon gain. Patterns of variation in NSC concentration of the temperate species we studied are likely dominated by more important influences than adaptation to shade, such as limitation of growth or adaptation to cold stress.     

Characterization of pigment apparatus in winter-green and summer-green leaves of a shade-tolerant plant <Emphasis Type="Italic">Ajuga reptans</Emphasis>

Comparative study was performed to assess the content and proportions of photosynthetic pigments and the violaxanthin cycle (VXC) activity in winter-green and summer-green leaves of bugleweed (Ajuga reptans L.) plants grown in shaded (photosynthetically active radiation, PAR 150 μmol/(m² s)) and sunny (PAR 1200 μmol/(m² s)) habitats in the Botanic Garden of Jagiellonian University (Krakow, Poland). In overwintered and newly formed leaves of shade plants, the content of green and yellow pigments was two times higher than in leaves of sun plants. The shade plants were distinguished by accumulation of β-carotene, while lutein was predominant in leaves of sun plants. Under the action of strong light (2000 μmol/(m²s)), the level of violaxanthin deepoxidation in winter-green leaves of shade and sun plants increased five- to sixfold, whereas it changed insignificantly in summer-green leaves of shade plants. It is concluded that, in a shadetolerant species A. reptans, the photosynthetic apparatus of winter-green leaves in sun and shade plants and of summer-green leaves in sun plants is protected against excess insolation by high activity of VXC. The carotenoids of summer-green leaves in shade plants are supposed to function mainly as light-harvesting pigments.     

Symbiotic association of the ciliate Ophrydium naumanni with Chlorella causing a deep chlorophyll a maximum in an oligotrophic South Andes lake

Vertical profiles of temperature, light and chlorophyll a concentration were examined in Lake Moreno Oeste, an oligotrophic South Andean lake (Argentina), during the warmest period of the year (November-April), when thermal stratification is characteristic. Concurrent samples for the enumeration of phytoplankton and green ciliates were taken, and the different contribution of these fractions to total chlorophyll a concentration was analysed. The development of a distinctive deep chlorophyll maximum was observed during summer months. The deep chlorophyll maximum was situated near the limit of the euphotic zone and just below the upper limit of the metalimnion. The results showed that the green ciliate Ophrydium naumanni with endosymbiotic Chlorella dominated the metalimnion causing the deep chlorophyll maximum. Additional laboratory experiments revealed a strong dependence of O.naumanni on light. Therefore, the symbiotic association appears to be an effective exploitation of the water column in poor-nutrient-high-light ecosystems like large Andean lakes.      

Dynamic acclimation to sunlight in an alpine plant,Soldanella alpina L.

In the French Alps, Soldanella alpina (S. alpina) grow under shade and sun conditions during the vegetation period. This species was investigated as a model for the dynamic acclimation of shade leaves to the sun under natural alpine conditions, in terms of photosynthesis and leaf anatomy. Photosynthetic activity in sun leaves was only slightly higher than in shade leaves. The leaf thickness, the stomatal density and the epidermal flavonoid content were markedly higher, and the chlorophyll/flavonoid ratio was significantly lower in sun than in shade leaves. Sun leaves also had a more oxidised plastoquinone pool, their PSII efficiency in light was higher and their non-photochemical quenching (NPQ) capacity was higher than that of shade leaves. Shade-sun transferred leaves increased their leaf thickness, stomatal density and epidermal flavonoid content, while their photosynthetic activity and chlorophyll/flavonoid ratio declined compared to shade leaves. Parameters indicating protection against high light and oxidative stress, such as NPQ and ascorbate peroxidase, increased in shade-sun transferred leaves and leaf mortality increased. We conclude that the dynamic acclimation of S. alpina leaves to high light under alpine conditions mainly concerns anatomical features and epidermal flavonoid acclimation, as well as an increase in antioxidative protection. However, this increase is not large enough to prevent damage under stress conditions and to replace damaged leaves.     

Survey of Thermal Energy Dissipation and Pigment Composition in Sun and Shade Leaves

A survey was conducted of the magnitude of energy dissipationin photosystem II (expressed as nonphotochemi-cal quenchingof chlorophyll fluorescence, NPQ) as well as leaf carotenoidcomposition of a wide range of different plant species growingin deep shade and/or full sun. Consistently higher levels ofthe reversible component of NPQ as well as higher degrees ofrapidly attainable de-epoxida-tion of the xanthophyll cycle(VAZ) pool were observed in sun leaves compared to deep shadeleaves. It is concluded that these altered features of the xanthophyllcycle allowed sun leaves to dissipate excess energy more effectivelyover the short term. In addition to the rapid increase in reversibleNPQ, shade leaves exhibited a slow further, and sustained, increasein NPQ. In contrast to these deep shade leaves experimentallyexposed to high PFDs, understory leaves experiencing highlyvariable PFD in their natural environment appeared to be ableto dissipate excess excitation energy adequately via xanthophyllcycle-dependent thermal dissipation. Furthermore, very consistenttrends across plant species were observed for changes in carotenoidcomposition (pools of carotenes, VAZ, and other xantho-phylls)in response to light environment, as long as it is assumed thatin some species rß-carotene can be replaced by     

Photosynthetic responses to light in seedlings of selected Amazonian and Australian rainforest tree species

Summary Seedlings of the Caesalpinoids Hymenaea courbaril, H. parvifolia and Copaifera venezuelana, emergent trees of Amazonian rainforest canopies, and of the Araucarian conifers Agathis microstachya and A. robusta, important elements in tropical Australian rainforests, were grown at 6% (shade) and 100% full sunlight (sun) in glasshouses. All species produced more leaves in full sunlight than in shade and leaves of sun plants contained more nitrogen and less chlorophyll per unit leaf area, and had a higher specific leaf weight than leaves of shade plants. The photosynthetic response curves as a function of photon flux density for leaves of shade-grown seedlings showed lower compensation points, higher quantum yields and lower respiration rates per unit leaf area than those of sun-grown seedlings. However, except for A. robusta, photosynthetic acclimation between sun and shade was not observed; the light saturated rates of assimilation were not significantly different. Intercellular CO₂ partial pressure was similar in leaves of sun and shade-grown plants, and assimilation was limited more by intrinsic mesophyll factors than by stomata. Comparison of assimilation as a function of intercellular CO₂ partial pressure in sun- and shade-grown Agathis spp. showed a higher initial slope in leaves of sun plants, which was correlated with higher leaf nitrogen content. Assimilation was reduced at high transpiration rates and substantial photoinhibition was observed when seedlings were transferred from shade to sun. However, after transfer, newly formed leaves in A. robusta showed the same light responses as leaves of sun-grown seedlings. These observations on the limited potential for acclimation to high light in leaves of seedlings of rainforest trees are discussed in relation to regeneration following formation of gaps in the canopy.     

Respiratory metabolism in pacifastacus leniusculus (Dana) (crustacea: decapoda) as related to its ecology

Summary Respiratory metabolism in the high-altitude crayfish, Pacifastacus leniusculus from Lake Tahoe (California-Nevada, U.S.A.) was investigated in a series of laboratory experiments. In general, oxygen consumption was found to be directly proportional to size when expressed per individual, and inversely proportional to size when expressed per ash-free gram dry weight per unit time. No significant variations in respiratory rates were observed between males and females. There was little variation in uptake over a temperature range of 5°C to 12.5°C, while the lowest and highest uptake values were observed at 0°C and 15°C respectively. An increase in temperature above 15°C, however, resulted in a decrease in respiratory activity. These results closely correlate with patterns of ventilatory activity, oxygen extraction efficiency, and seasonal feeding and reproductive activity in Pacifastacus as related to the annual temperature profile for their Lake Tahoe habitat. These results, as well as the extremely low tolerance of this species for low oxygen levels, suggest temperature as the possible critical environmental factor in the determination of the nature of seasonal metabolic cycles. It is concluded that Pacifastacus shows little tolerance for fluctuations in environmental oxygen and temperature and is well adapted to the narrow range of condition present in its Lake Tahoe habitat.     

Variability in host-plant quality for the larvae of a polyphagous insect folivore in midseason: the impact of light on three deciduous sapling species

We examined the effects of light availability on the suitability of foliage from red maple (Acer rubrum L.) (Aceraceae), black cherry (Prunus serotina Ehrhart) (Rosaceae), and sassafras [Sassafras albidum (Nuttall) Nees (Lauraceae)] for the larvae of the promethea moth, Callosamia promethea Drury (Lepidoptera: Saturniidae), in midseason. We identified replicate sun- and shade-grown saplings of red maple, black cherry, and sassafras from naturally growing populations in the field. Foliage collected from the experimental saplings was bioassayed using early instars of the promethea moth and assayed for nitrogen and carbon content. Promethea moth-larval performance and survivorship was highest on sassafras, intermediate on black cherry, and lowest on red maple. Larvae feeding on foliage from plants grown in the sun performed better than from those grown in the shade; the effect of light did not depend on sapling species. Foliar nitrogen content varied among the sapling species and was higher, overall, in foliage from plants grown in the sun. Nitrogen concentration related strongly and positively with larval performance and accounted for a great deal of the variation in performance both among the sapling species and between the sun and shade treatments. During midseason, foliar nitrogen content is determined by light availability, it varies among sapling species, and it is likely the primary constituent determining host quality for folivores on these sapling species.     

Carotenoid composition in sun and shade leaves of plants with different life forms

The carotenoid composition of sun leaves of nine species of annual crop plants (some with several varieties) was compared with sun and shade leaves of several other groups of plants, among those sun and shade leaves of several species of perennial shrubs and vines and deep-shade leaves of seven rainforest species. All sun leaves contained considerably greater amounts of the components of the xanthophyll cycle violaxanthin, antheraxanthin and zeaxanthin as well as of β-carotene than the shade leaves, as had previously been reported for a variety of other species by Thayer & Björkman (Photosynthesis Research, 1990, 23, 331–343). Therefore, high light specifically stimulated β,β-carotenoid synthesis. The sun leaves of these crop species did not contain α-carotene which was, however, present in large amounts in all shade leaves and in smaller amounts in sun leaves of three of the four species of perennial shrubs and vines. There was no difference in neoxanthin content on a chlorophyll basis between sun and shade leaves, and there was no consistent general difference in the lutein content between all sun and all shade leaves. The zeaxanthin (and antheraxanthin) content at peak irradiance and the xanthophyll cycle pool size were compared for sun leaves from the different groups of plants with different life forms and different metabolic activities. When growing in full sunlight the annual crop species and a perennial mesophyte had high rates of photosynthesis whereas the perennial shrubs and vines had relatively low photosynthesis rates. More zeaxanthin (and antheraxanthin) were accumulated at noon in full sunlight in those species with the lower photosynthesis rates. However, it was not such that those species also possessed the larger pools of violaxanthin + antheraxanthin + zeaxanthin. Instead, the xanthophyll cycle pools of sun leaves of the annual crop species and the perennial mesophyte were not smaller, and were even possibly larger, than those of sun leaves of the perennial shrubs and vines with low photosynthesis rates. This was so in spite of the fact that the crop species experienced much lesser degrees of excessive light at full sun than the shrubs and vines. Thus, many of the crop species converted only about 30–50% of their xanthophyll cycle pool to zeaxanthin at noon, whereas the shrubs and vines typically converted more than 80% of their pool into zeaxanthin. The crop species also had larger pools of β-carotene than the shrubs and vines but smaller pools of lutein than the majority of the latter species.