Stem respiration of ponderosa pines grown in contrasting climates: implications for global climate change

We examined the effects of climate and allocation patterns on stem respiration in ponderosa pine (Pinus ponderosa) growing on identical substrate in the cool, moist Sierra Nevada mountains and the warm, dry, Great Basin Desert. These environments are representative of current climatic conditions and those predicted to accompany a doubling of atmospheric CO₂, respectively, throughout the range of many western north American conifers. A previous study found that trees growing in the desert allocate proportionally more biomass to sapwood and less to leaf area than montane trees. We tested the hypothesis that respiration rates of sapwood are lower in desert trees than in montane trees due to reduced stem maintenance respiration (physiological acclimation) or reduced construction cost of stem tissue (structural acclimation). Maintenance respiration per unit sapwood volume at 15°C did not differ between populations (desert: 6.39 ± 1.14 SE μmol m⁻³ s⁻¹, montane: 6.54 ± 1.13 SE μmol m⁻³ s⁻¹, P = 0.71) and declined with increasing stem diameter (P = 0.001). The temperature coefficient of respiration (Q ₁₀) varied seasonally within both environments (P = 0.05). Construction cost of stem sapwood was the same in both environments (desert: 1.46 ± 0.009 SE g glucose g⁻¹ sapwood, montane: 1.48 ± 0.009 SE glucose g⁻¹ sapwood, P = 0.14). Annual construction respiration calculated from construction cost, percent carbon and relative growth rate was greater in montane populations due to higher growth rates. These data provide no evidence of respiratory acclimation by desert trees. Estimated yearly stem maintenance respiration was greater in large desert trees than in large montane trees because of higher temperatures in the desert and because of increased allocation of biomass to sapwood. By analogy, these data suggest that under predicted increases in temperature and aridity, potential increases in aboveground carbon gain due to enhanced photosynthetic rates may be partially offset by increases in maintenance respiration in large trees growing in CO₂-enriched atmospheres. Received: 4 November 1996 / Accepted: 23 January 1997     

GENETICS OF PHYSIOLOGICAL AND BEHAVIORAL RESISTANCE TO HOST FURANOCOUMARINS IN THE PARSNIP WEBWORM

Depressaria pastinacella, the parsnip webworm, feeds almost exclusively on the flowers and fruits of Pastinaca sativa, the wild parsnip. Resistance to webworms in wild parsnip populations is largely attributable to genetically based variation in furanocoumarin chemistry; by differentially reducing fruit set among chemical phenotypes, parsnip webworms may act as selective agents on wild parsnip populations. To determine whether wild parsnip chemistry can act as a selective agent on webworm populations, it is necessary to establish that resistance mechanisms in the webworm to furanocoumarins are genetically based. In this study, we estimated the amount of genetic variation in behavioral and physiological responses of webworms to parsnip furanocoumarins. Virtually no variation was found among webworm families for feeding preferences for diets varying as much as fourfold in furanocoumarin content. Nor was significant variation found for mean furanocoumarin intake over the assay period, except in one case, in which maternal effects may account for differences among families. In contrast, substantial familial variation existed for cytochrome P450–mediated metabolism of bergapten and xanthotoxin, two host furanocoumarins. The presence of additive genetic variation in metabolism, and the absence of such variation in discriminative feeding behavior, suggests that adaptation to changes in furanocoumarin chemistry, resulting either from changes in the distribution of chemical phenotypes in parsnip populations or from shifts to new chemically different host plants, is likely to be facilitated by physiological rather than behavioral means.     

Optimal defence, kin conflict and the distribution of furanocoumarins among offspring of wild parsnip

The factors influencing the allocation of chemical defences to plant offspring have largely been unexplored, conceptually and experimentally. Because evolutionary interactions between maternal plants and their progeny can affect resource allocation patterns among sibling offspring, we suggest that kin conflict as well as herbivore–plant interaction theories need to be considered to predict chemical defence allocation patterns. Optimal defence theory predicts that maternal plants should defend more heavily those offspring in which resources have been disproportionately invested. In contrast, kin conflict theory predicts that natural selection will favour genotypes that can compete successfully for maternal defences irrespective of their quality, even at the expense of the fitness of siblings and the maternal plant. Evidence for these defence patterns were evaluated by examining the allocation of furanocoumarins to seeds of the wild parsnip (Pastinaca sativa, Apiaceae). Furanocoumarins are toxins that are localized within the oil tubes of the maternal tissues of seeds. We evaluated the role of offspring investment (endosperm mass) and seed genotype on furanocoumarin allocation by mating an array of pollen donors with pollen recipients. Furanocoumarins were found to be positively correlated with endosperm mass on one side of the seed, a result consistent with optimal defence theory; however, on the other side of the seed, furanocoumarin content was influenced by seed genotype and was unrelated to endosperm mass. These effects varied with maternal plant. Further experiments demonstrated that nearly 80% of furanocoumarin production occurs after pollination, when fertilization products are active. Although the amount of furanocoumarin influenced by the seed genotype is small relative to the total quantity in the seed, these furanocoumarins are the first line of defence against important predators, such as the parsnip webworm, Depressaria pastinacella (Lepidoptera: Oecophoridae). We found that parsnip webworm larvae were able to discriminate among genotypes within an inflorescence. In line with previous studies, these results suggest that a genotype's ability to influence furanocoumarin defence may affect its probability of survival. We conclude that the distribution of defences among plant offspring in wild parsnip is probably influenced by competition among seed genotypes that conflicts with maternal optimal defence. This revised version was published online in July 2006 with corrections to the Cover Date.     

Patterns of furanocoumarin production and insect herbivory in a population of wild parsnip (Pastinaca sativa L.)

Summary Seasonal changes in the distribution and abundance of furanocoumarins in wild parsnip, Pastinaca sativa (Umbelliferae), were examined in a population of plants in Tompkins County, New York. Xanthotoxin, imperatorin and bergapten (linear furanocoumarins) occur in all above-ground parts of the plant; in addition, angelicin and sphondin (angular furanocoumarins) occur in umbels of some individuals. Total furanocoumarin content, as measured by percent dry weight, is greatest in reproductive parts, particularly buds and seeds; variation in concentrations between plants is greatest in vegetative structures (e.g., leaves).Within the plant, the distribution of furanocoumarins is significantly correlated with nitrogen, as opposed to biomass, allocation. In that nitrogen is often a factor limiting the plant growth, furanocoumarins appear to be allocated in proportion to plant tissue value; reproductive structures, obvious contributors to plant fitness, contain over ten times the amount of nitrogen and furanocoumarin contained in vegetative structures such as senescent leaves.Stepwise multiple regression analysis revealed that generalized insect herbivores tend to feed on plants or plant parts low in furanocoumarin content and, correspondingly, low in nitrogen content. Parsnip specialists, notably Depressaria pastinacella (Lepidoptera: Oecophoridae), feed exclusively on umbels, plant parts rich in nitrogen and furanocoumarins; furanocoumarin number and content in fact account for over 60% of the variance in number of umbel feeders. These patterns conform with previous determinations of the toxicological properties of furanocoumarins. Nitrogen is known to affect growth rate, fecundity, longevity and survivorship of insect herbivores; by tolerating or detoxifying furanocoumarins, D. pastinacella can consume plant tissues containing significantly greater amounts of nitrogen than tissues consumed by generalist feeders. That the presence of D. pastinacella on individual plants is correlated with the number of furanocoumarins present is consistent with the hypothesis that parsnip specialists use angular furanocoumarins as host recognition cues.     

Modeling growth and photosynthetic response in <Emphasis Type="Italic">Arthrospira platensis</Emphasis> as function of light intensity and glucose concentration using factorial design

Combined effect of light intensity and glucose concentration on Arthrospira platensis growth and photosynthetic response was evaluated using a 3² factorial design. This design was carried out with light levels of 50, 100, and 150 μmol photons m⁻² s⁻¹ and glucose concentrations of 0.5, 1.5, and 2.5 g L⁻¹. Results from the response surface methodology were that the highest level of light intensity and glucose concentration improved biomass (1.33 g L⁻¹), maximum specific growth rate (0.49 day⁻¹), and net photosynthetic rate (139.89 μmol O₂ mg Chl⁻¹ h⁻¹). Furthermore, the interaction of both factors showed that at low light, glucose had a low effect on maximum biomass and maximal net photosynthetic rate. However, at the highest light levels, the effect of glucose was more sensitive and the increase of glucose concentration increased the levels of all responses. The rates of the instantaneous relative growth, net photosynthesis, and dark respiration of growth cultures showed two different phases in mixotrophic condition. The first was distinguished by the preponderance of the photoautotrophic mode; the second was based mainly on photoheterotrophy.     

Growth, biomass allocation and photosynthesis of invasive and native Hawaiian rainforest species

Growth, biomass allocation, and photosynthetic characteristics of seedlings of five invasive non-indigenous and four native species grown under different light regimes were studied to help explain the success of invasive species in Hawaiian rainforests. Plants were grown under three greenhouse light levels representative of those found in the center and edge of gaps and in the understory of Hawaiian rainforests, and under an additional treatment with unaltered shade. Relative growth rates (RGRs) of invasive species grown in sun and partial shade were significantly higher than those for native species, averaging 0.25 and 0.17 g g⁻¹ week⁻¹, respectively, while native species averaged only 0.09 and 0.06 g g⁻¹ week⁻¹, respectively. The RGR of invasive species under the shade treatment was 40% higher than that of native species. Leaf area ratios (LARs) of sun and partial-shade-grown invasive and native species were similar but the LAR of invasive species in the shade was, on average, 20% higher than that of native species. There were no differences between invasive and native species in biomass allocation to shoots and roots, or in leaf mass per area across light environments. Light-saturated photosynthetic rates (Pmax) were higher for invasive species than for native species in all light treatments. Pmax of invasive species grown in the sun treatment, for example, ranged from 5.5 to 11.9 μmol m⁻² s⁻¹ as compared with 3.0−4.5 μmol m⁻² s⁻¹ for native species grown under similar light conditions. The slope of the linear relationship between Pmax and dark respiration was steeper for invasive than for native species, indicating that invasive species assimilate more CO₂ at a lower respiratory cost than native species. These results suggest that the invasive species may have higher growth rates than the native species as a consequence of higher photosynthetic capacities under sun and partial shade, lower dark respiration under all light treatments, and higher LARs when growing under shade conditions. Overall, invasive species appear to be better suited than native species to capturing and utilizing light resources, particularly in high-light environments such as those characterized by relatively high levels of disturbance. Received: 30 December 1997 / Accepted: 1 September 1998     

Host plant, host plant chemistry and the polyembryonic parasitoid Copidosoma sosares: indirect effects in a tritrophic interaction

Host plant identity and host plant chemistry have often been shown to influence host finding and acceptance by natural enemies but comparatively less attention has been paid to the tritrophic effects of host plant and host plant chemistry on other natural enemy fitness correlates, such as survivorship, clutch size, body size, and sex ratio. Such studies are central to understanding both the selective impact of plants on natural enemies as well as the potential for reciprocal selective impact of natural enemies on plant traits. We examined the effects of host plant and host plant chemistry in a tritrophic system consisting of three apiaceous plants (Pastinaca sativa, Heracleum sphondylium and H. mantegazzianum), the parsnip webworm (Depressaria pastinacella) and the polyembryonic parasitic wasp Copidosoma sosares. All of these plants produce furanocoumarins, known resistance factors for parsnip webworms. Furanocoumarin concentrations were correlated neither with the presence nor the number of webworms on a given plant. Concentrations of two furanocoumarins were negatively associated with C. sosares fitness correlates: isopimpinellin with the likelihood that a given webworm would be parasitized and xanthotoxin with both within‐brood survivorship (of all‐male and mixed‐sex broods) and clutch size. Brood sex ratio and body sizes of individual wasps were not correlated with furanocoumarin chemistry. Because additive genetic variation exists in P. sativa for furanocoumarin chemical traits, these are subject to selection by webworms through herbivory. Third trophic level selective impacts on furanocoumarin traits may include selection for reduced production of those chemicals that affect parasitoid survivorship yet do not influence host plant choice by the herbivore. That such might be the case is suggested by patterns of furanocoumarin production in populations of P. sativa with different histories of infestation; in the Netherlands, where parasitism rates of webworms by C. sosares are high, plants produce lower levels of all linear furanocoumarins and proportionately less isopimpinellin than do midwestern U.S. populations of P. sativa, where natural enemies of the webworm are effectively absent.     

CONSTRAINTS ON CHEMICAL COEVOLUTION: WILD PARSNIPS AND THE PARSNIP WEBWORM

The parsnip webworm (Depressaria pastinacella) and the wild parsnip (Pastinaca sativa) together represent a potentially “coevolved” system in that throughout their ranges the plant has relatively few other herbivores and the insect has virtually no other hosts. Individual wild parsnip plants within a central Illinois population vary in their content and composition of furanocoumarins, secondary compounds with insecticidal properties. Half-sib and parent-offspring regression estimates of the heritability of furanocoumarins demonstrate that this variation is genetically based. Wild parsnip plants also vary in their resistance to damage by the parsnip webworm, which feeds on flowers and developing seeds. In an experimental garden, seed production in the primary umbel ranged from 0 to 1,664 seeds among individuals, and mean seed production of half-sib families ranged from 3.7 seeds to 446.0 seeds. Approximately 75% of the variation in resistance among half-sib families to D. pastinacella was attributable to four furanocoumarin characteristics—resistance is positively related to the proportion of bergapten and the amount of sphondin in seeds, and negatively related to the amount of bergapten and the proportion of sphondin in leaves. Each of the four resistance factors had significant heritability. Thus, resistance in wild parsnip to the parsnip webworm is to a large extent chemically based and genetically controlled. Genetic correlations among fitness and resistance characters, however, tend to limit coevolutionary responses between herbivore and plant. In greenhouse plants protected from herbivory, several of the resistance factors have negative genetic correlations with potential seed production. Ostensibly, highly resistant plants in the absence of herbivory would be at a competitive disadvantage in the field. The selective impact of the herbivore is also limited in this population by a negative genetic correlation among resistance factors. Selection to increase one resistance factor (e.g., the proportion of bergapten in the seed) would at the same time decrease the amount of a second resistance factor (e.g., the amount of sphondin in the seed). The wild parsnip and the parsnip webworm, then, appear to have reached an evolutionary “stalemate” in the coevolutionary arms race.     

Photosynthetic responses of Microstegium vimineum (Trin.) A. Camus, a shade-tolerant, C4 grass, to variable light environments

Microstegium vimineum (Trin.) A. Camus, a shade-tolerant C₄ grass, has spread throughout the eastern United States since its introduction in 1919. This species invades disturbed understory habitats along streambanks and surrounding mesic forests, and has become a major pest in areas such as Great Smoky Mountains National Park. The focus of this study was to characterize the photosynthetic induction responses of M. vimineum, specifically its ability to utilize low light and sunflecks, two factors that may be critical to invasive abilities and survival in the understory. In addition, we were curious about the ability of a grass with the C₄ photosynthetic pathway to respond to sunflecks. Plants were grown under 25% and 50% ambient sunlight, and photosynthetic responses to both steady-state and variable light were determined. Plants grown in both 25% and 50% ambient sun became 90% light saturated between 750–850 μmol m⁻² s⁻¹; however, plants grown in 50% ambient sun had significantly higher maximum steady-state photosynthetic rates (16.09 ± 1.37 μmol m⁻² s⁻¹ vs. 12.71 ± 1.18 μmol m⁻² s⁻¹). Both groups of plants induced to 50% of the steady-state rate in 3–5 min, while it took 10–13 min to reach 90% of maximum rates, under both flashing and steady-state light. For both groups of plants, stomatal conductance during induction reached maximum rates in 6–7 min, after which rates decreased slightly. Upon return to low light, rates of induction loss and stomatal closure were very rapid in both groups of plants, but were more rapid in those grown in high light. Rapid induction and the ability to induce under flashing light may enable this species to invade and dominate mesic understory habitats, while rapid induction loss due to stomatal closure may prevent excess water loss when low light constrains photosynthesis. The C₄ pathway itself does not appear to present an insurmountable barrier to the ability of this grass species to respond to sunflecks in an understory environment. Received: 21 February 1997 / Accepted: 10 October 1997     

Optimized culture conditions for the production of furanocoumarins by micropropagated shoots of Ruta graveolens

Ruta graveolens in vitro cultures are a potential source of secondary metabolites (furanocoumarins) of significant medical interest. Experiments led in our laboratory showed that micropropagated shoots were richer in furanocoumarins than any other plant material. In order to optimize the molecule production by such cultivation systems, several factors related to the culture medium were studied. Effects of medium composition on biomass growth and furanocoumarin content were analysed and optimal conditions were determined for phosphate (300 mg l⁻¹ of NaH₂PO₄), nitrate (2527 mg l⁻¹ of KNO₃), carbon source (10 g l⁻¹ of sucrose) and phytohormones (2,4-dichlorophenoxyacetic acid (2,4-D) 50 μM and benzylaminopurine (BAP) 50 μM). Ruta shoot growth and furanocoumarin production were compared for optimized and standard culture conditions. Specific medium gave better results in terms of growth (t_D equal to 6.9 days against 8.6 for standard conditions) but no significant differences appeared concerning metabolite concentrations. However, the present study opens the way to scale-up studies with bioreactor cultivation systems. This revised version was published online in June 2006 with corrections to the Cover Date.     

Increased levels of adenine nucleotides modify the interaction between starch synthesis and respiration when adenine is supplied to discs from growing potato tubers

To investigate the importance of the overall size of the total adenine nucleotide pool for the regulation of primary metabolism in growing potato tubers, freshly cut discs were provided with zero or 2 mM adenine in the presence of 1 or 100 mM [U-¹⁴C]glucose or 100 mM [U-¹⁴C]sucrose in the presence and absence of 20 mM orthophosphate (Pi). Adenine led to a 150–250% increase of the total adenine nucleotide pool, which included an increase of ADP, a larger increase of ATP and an increase of the ATP:ADP ratio. There was a 50–100% increase of ADP-glucose (ADPGlc), and starch synthesis was stimulated. Respiratory oxygen uptake was stimulated, and the levels of glycerate-3-phosphate, phosphoenolpyruvate and α-ketoglutarate decreased. The response to adenine was not modified by Pi. It is proposed that increased ATP stimulates ADPGlc pyrophosphorylase, leading to a higher rate of starch synthesis. The impact on starch synthesis is constrained, however, because increased ADP can lead to a stimulation of respiration and decline of glycerate-3-phosphate, which will inhibit ADPGlc pyrophosphorylase. The quantitative impact depends on the conditions. In the presence of 1 mM glucose, the levels of phosphorylated intermediates and the rate of starch synthesis were low. Adenine led to a relatively large stimulation of respiration, but only a small stimulation of starch synthesis. In the presence of 100 mM glucose, discs contained high levels of phosphorylated intermediates, low ATP:ADP ratios (<3) and low rates of starch synthesis (<20% of the metabolised glucose). Adenine led to marked increase of ATP and 2- to 4-fold stimulation of starch synthesis. Discs incubated with 100 mM sucrose already had high ATP:ADP ratios (>8) and high rates of starch synthesis (>50% of the metabolised sucrose). Adenine led to a further increase, but the stimulation was less marked than in high glucose. These results have implications for the function of nucleotide cofactors in segregating sucrose mobilisation and respiration, and the need for energy conservation during sugar-starch conversions. Received: 9 February 2000 / Accepted: 9 June 2000     

Furanocoumarin metabolism in Papilio polyxenes: biochemistry,genetic variability,and ecological significance

The ubiquitous occurrence of series of biosynthetically related plant secondary compounds within individual species has given rise to the suggestion that such multiplicity is adaptive; one possible mechanism that would serve to maintain such within-plant diversity is analog synergism. In a series of experiments, we provide evidence that synergism may account for the presence of multiple structurally related furanocoumarins in apiaceous plants. The black swallowtail, Papilio polyxenes, feeds exclusively on plant species containing furanocoumarins. Growth of larvae fed parsley leaves treated with both xanthotoxin and angelicin, two furanocoumarins that co-occur widely in swallowtail hostplants, was significantly slower than that of larvae fed leaves with an equimolar concentration of either xanthotoxin or angelicin. A multivariate combination of growth, food consumption and frass excretion differed significantly between larvae fed leaves treated with both xanthotoxin and angelicin and larvae fed leaves treated with angelicin alone. In addition, we measured rates of in vitro cytochrome P450-mediated metabolism of three furanocoumarins — bergapten, xanthotoxin, and angelicin. While bergapten and xanthotoxin, both linear furanocoumarins, were metabolized at similar rates (8.07 and 9.86 nmoles/min/g fw caterpillar, respectively), angelicin, an angular furanocoumarin, was metabolized more slowly (2.76 nmoles/min/g fw caterpillar). When all three furanocoumarins were assayed together, overall rates of metabolism were significantly reduced, suggesting substrate inhibition. Thus, the pattern of growth of larvae is consistent with the pattern of in vitro metabolism and is evidence in support of analog synergism. In a separate experiment, metabolism of xanthotoxin and angelicin individually and together were compared in six maternal families. Again, angelicin was metabolized more slowly than xanthotoxin and each furanocoumarin inhibited metabolism of the other. That significant family effects were found for rates of metabolism and for the ratio of moles of angelicin metabolized for each mole of xanthotoxin metabolized raises the possibility that genetic variation exists for the rate and specificity of metabolism and suggests that insect herbivores may be able to adapt to analog synergism.     

Effects of salinity on photosynthetic characteristics in photomixotrophic cell-suspension cultures from Alternanthera philoxeroides

We studied the effect of NaCl salinity on the development of cellular photosynthesis using a green, photomixotrophic, cell-suspension culture of Alternanthera philoxeroides (Mart.) Griseb. For these cells, increasing the concentration of sucrose in the media produces a rapid drop in net photosynthetic rate, which recovers as sucrose is depleted from the media. This predictable recovery provides a simple system to examine cellular photosynthetic development. Cells, unadapted to high salinity, were transferred to nutrient media with 30 mM sucrose (Control) or nutrient media with 30 mM sucrose and 100 mM NaCl (Salt). A dramatic increase in the dark respiration rate of Control and Salt cells during the first 6 d of the experiment produced net oxygen consumption in the light. The high dark respiration rates during this period were accompanied by a decline in total Chl and the amounts of two photosynthetic proteins, the light harvesting Chl a/b binding protein of photosystem II (LHCP) and the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco SSU). The dark respiration rate of Salt cells was greater than that of Control cells on days 4–8. After day 4, dark respiration rates decreased and net photosynthesis increased to stable values in both treatments at day 11 after media sucrose concentration reached a minimum. As dark respiration rates decreased and net photosynthetic rates increased, total Chl and the amounts of LHCP and rubisco SSU increased in both Control and Salt cells. The slower development of photosynthetic capacity in salt cells was correlated with a fresh weight that was 20% lower than that of control cells at the end of the experiment.     

The Distribution of Coumarins and Furanocoumarins in Citrus Species Closely Matches Citrus Phylogeny and Reflects the Organization of Biosynthetic Pathways

Citrus plants are able to produce defense compounds such as coumarins and furanocoumarins to cope with herbivorous insects and pathogens. In humans, these chemical compounds are strong photosensitizers and can interact with medications, leading to the “grapefruit juice effect”. Removing coumarins and furanocoumarins from food and cosmetics imply additional costs and might alter product quality. Thus, the selection of Citrus cultivars displaying low coumarin and furanocoumarin contents constitutes a valuable alternative. In this study, we performed ultra-performance liquid chromatography coupled with mass spectrometry analyses to determine the contents of these compounds within the peel and the pulp of 61 Citrus species representative of the genetic diversity all Citrus. Generally, Citrus peel contains larger diversity and higher concentrations of coumarin/furanocoumarin than the pulp of the same fruits. According to the chemotypes found in the peel, Citrus species can be separated into 4 groups that correspond to the 4 ancestral taxa (pummelos, mandarins, citrons and papedas) and extended with their respective secondary species descendants. Three of the 4 ancestral taxa (pummelos, citrons and papedas) synthesize high amounts of these compounds, whereas mandarins appear practically devoid of them. Additionally, all ancestral taxa and their hybrids are logically organized according to the coumarin and furanocoumarin pathways described in the literature. This organization allows hypotheses to be drawn regarding the biosynthetic origin of compounds for which the biogenesis remains unresolved. Determining coumarin and furanocoumarin contents is also helpful for hypothesizing the origin of Citrus species for which the phylogeny is presently not firmly established. Finally, this work also notes favorable hybridization schemes that will lead to low coumarin and furanocoumarin contents, and we propose to select mandarins and Ichang papeda as Citrus varieties for use in creating species devoid of these toxic compounds in future breeding programs.     

Acclimation of the summer annual species, Lolium temulentum, to CO2 enrichment

Lolium temulentum L. Ba 3081 was grown hydroponically in air (350 μmol mol⁻¹ CO₂) and elevated CO₂ (700 μmol mol⁻¹ CO₂) at two irradiances (150 and 500 μmol m⁻² s⁻¹) for 35 days at which point the plants were harvested. Elevated CO₂ did not modify relative growth rate or biomass at either irradiance. Foliar carbon-to-nitrogen ratios were decreased at elevated CO₂ and plants had a greater number of shorter tillers, particularly at the lower growth irradiance. Both light-limited and light-saturated rates of photosynthesis were stimulated. The amount of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) protein was increased at elevated CO₂, but maximum extractable Rubisco activities were not significantly increased. A pronounced decrease in the Rubisco activation state was found with CO₂ enrichment, particularly at the higher growth irradiance. Elevated-CO₂-induced changes in leaf carbohydrate composition were small in comparison to those caused by changes in irradiance. No CO₂-dependent effects on fructan biosynthesis were observed. Leaf respiration rates were increased by 68% in plants grown with CO₂ enrichment and low light. We conclude that high CO₂ will only result in increased biomass if total light input favourably increases the photosynthesis-to-respiration ratio. At low irradiances, biomass is more limited by increased rates of respiration than by CO₂-induced enhancement of photosynthesis. Received: 23 February 1999 / Accepted: 15 June 1999     

Metabolism in slices from growing potato tubers responds differently to addition of sucrose and glucose

The short-term changes in metabolism that occurred after adding glucose or sucrose to freshly cut discs from growing potato (Solanum tuberosum L.) tubers were investigated. (i) When glucose was supplied, there was a marked increase in glycolytic metabolites, and respiration was stimulated. When sucrose was supplied, amounts of glycolytic metabolites including hexose phosphates and 3-phosphoglycerate (3PGA) were similar to or lower than in control discs incubated without sugars, and respiration did not rise initially above that in control discs. This different response to sucrose and glucose was found across the concentration range 5–200 mM. A larger proportion of the metabolised ¹⁴C was converted to starch when [¹⁴C] sucrose was supplied than when [¹⁴C] glucose was supplied. The different effect on metabolite levels, respiration and starch synthesis was largest after 20–30 min, and decreased in longer incubations. (ii) When 5 or 25 mM sucrose was added in the presence of [¹⁴C] glucose, it led to a decrease in hexose phosphates and 3PGA, and a small increase in the rate of starch synthesis compared to discs incubated with glucose in the absence of sucrose. These differences were seen in a 30-min pulse and a 2-h pulse. Whereas ADP-glucose levels after adding sucrose resembled those in control discs, glucose led to a decrease in ADP-glucose. This decrease did not occur when 5 or 25 mM sucrose was added with the glucose. (iii) To check the relevance of these experiments for intact tubers, water or 100 mM mannitol, sucrose or glucose were supplied through the stolon to intact tubers for 24 h. A 0.2 mM solution of [¹⁴C] glucose was then introduced into the tubers, and its metabolism investigated during the next 30 min. Labelling of starch was increased after preincubation with sucrose, and significantly inhibited after preincubation with glucose. (iv) It is concluded that glucose and sucrose have different effects on tuber metabolism. Whereas glucose leads to a preferential stimulation of respiration, sucrose preferentially stimulates starch synthesis via a novel mechanism that allows stimulation of ADP-glucose pyrophosphorylase even though the levels of hexose phosphates and the allosteric activator 3PGA decrease. Received: 9 October 1997 / Accepted: 3 February 1998     

Adaptation and acclimation of growth and photosynthesis of five Antarctic red algae to low temperatures

Temperature requirements for growth, photosynthesis and dark respiration were determined for five Antarctic red algal species. After acclimation, the stenothermal species Gigartina skottsbergii and Ballia callitricha grew at 0 or up to 5 °C, respectively; the eurythermal species Kallymenia antarctica, Gymnogongrus antarcticus and Phyllophora ahnfeltioides grew up to 10 °C. The temperature optima of photosynthesis were between 10 and 15 °C in the stenothermal species and between 15 and 25 °C in the eurythermal species, irrespective of the growth temperature. This shows that the temperature optima for photosynthesis are located well below the optima from species of other biogeographical regions, even from the Arctic. Respiratory rates rose with increasing temperatures. In contrast to photosynthesis, no temperature optimum was evident between 0 and 25 °C. Partial acclimation of photosynthetic capacity to growth temperature was found in two species. B. callitricha and Gymnogongrus antarcticus acclimate to 0 °C, and 5 and 0 °C, respectively. But acclimation did in no case lead to an overall shift in the temperature optimum of photosynthesis. B. callitricha and Gymnogongrus antarcticus showed acclimation of respiration to 5 °C, and P. ahnfeltioides to 5 and 10 °C, resulting in a temperature independence of respiration when measured at growth temperature. With respect to the acclimation potential of the species, no distinction can be made between the stenothermal versus the eurythermal group. (Net)photosynthetic capacity:respiration (P:R) ratios showed in all species highest values at 0 °C and decreased continuously to values lower than 1.0 at 25 °C. In turn, the low P:R ratios at higher temperatures are assumed to determine the upper temperature growth limit of the studied species. Estimated daily carbon balance reached values between 4.1 and 30.7 mg C g⁻¹ FW day⁻¹ at 0 °C, 16:8 h light/dark cycle, 12–40 μmol m⁻² s⁻¹. Received: 4 November 1999 / Accepted: 7 March 2000     

Energy Budget for the Cultured,Zooxanthellate Octocoral <Emphasis Type="Italic">Sinularia flexibilis</Emphasis>

The zooxanthellate octocoral Sinularia flexibilis is a producer of potential pharmaceutically important metabolites such as antimicrobial and cytotoxic substances. Controlled rearing of the coral, as an alternative for commercial exploitation of these compounds, requires the study of species-specific growth requirements. In this study, phototrophic vs. heterotrophic daily energy demands of S. flexibilis was investigated through light and Artemia feeding trials in the laboratory. Rate of photosynthetic oxygen by zooxanthellae in light (≈200 μmol quanta m⁻² s⁻¹) was measured for the coral colonies with and without feeding on Artemia nauplii. Respiratory oxygen was measured in the dark, again with and without Artemia nauplii. Photosynthesis–irradiance curve at light intensities of 0, 50, 100, 200, and 400 μmol quanta m⁻² s⁻¹ showed an increase in photosynthetic oxygen production up to a light intensity between 100 and 200 μmol quanta m⁻² s⁻¹. The photosynthesis to respiration ratio (P/R > 1) confirmed phototrophy of S. flexibilis. Both fed and non-fed colonies in the light showed high carbon contribution by zooxanthellae to animal (host) respiration values of 111–127%. Carbon energy equivalents allocated to the coral growth averaged 6–12% of total photosynthesis energy (mg C g ⁻ 1 buoyant weight day ⁻ 1) and about 0.02% of the total daily radiant energy. “Light utilization efficiency (ε)” estimated an average ε value of 75% 12 h ⁻ 1 for coral practical energetics. This study shows that besides a fundamental role of phototrophy vs. heterotrophy in daily energy budget of S. flexibilis, an efficient fraction of irradiance is converted to useable energy.     

Chemical barriers to adaptation by a specialist herbivore

Summary Depressaria pastinacella, the parsnip webworm (Lepidoptera: Oecophoridae), feeds throughout eastern North America on Pastinaca sativa (wild parsnip) and few other species. The assumption that specialist herbivores such as the parsnip webworm are adapted to hostplant chemistry, and are therefore unaffected by chemical variation in hostplants, was tested. Flower buds from plants grown first in the greenhouse and then in the field were fed to ultimate instar webworms. Plant phenotype had a significant effect on virtually all webworm food utilization parameters. While nutritional factors (i.e., nitrogen content) were correlated with approximate digestibility, two constituents of the flowers — bergapten and xanthotoxin, both linear furanocoumarins — independently accounted for a significant amount of variation in food utilization indicies. The physiological effects of these furanocoumarins were confirmed in artificial diet experiments. Despite the fact that the two most important furanocoumarins in parsnip flowers relative to webworm feeding and growth are isomers, differing only in the positioning of a methoxy substituent, they have different physiological effects; while xanthotoxin in general has no effect on growth, bergapten decreases growth and digestibility of the diet. These results underscore the need in studies of plant-animal interactions to examine individual chemical components rather than classes of compounds.     

Anomalies in the growth kinetics of Saccharomyces cerevisiae strains in aerobic chemostat cultures

Aerobic glucose-limited chemostat cultivations were conducted with Saccharomyces cerevisiae strains NRRL Y132, ATCC 4126 and CBS 8066, using a complex medium. At low dilution rates all three strains utilised glucose oxidatively with high biomass yield coefficients, no ethanol production and very low steady-state residual glucose concentrations in the culture. Above a threshold dilution rate, respiro-fermentative (oxido-reductive) metabolism commenced, with simultaneous respiration and fermentation occurring, which is typical of Crabtree-positive yeasts. However, at high dilution rates the three strains responded differently. At high dilution rates S. cerevisiae CBS 8066 produced 7–8 g ethanol L⁻¹ from 20 g glucose L⁻¹ with concomitant low levels of residual glucose, which increased markedly only close to the wash-out dilution rate. By contrast, in the respiro-fermentative region both S. cerevisiae ATCC 4126 and NRRL Y132 produced much lower levels of ethanol (3–4 g L⁻¹) than S. cerevisiae CBS 8066, concomitant with very high residual sugar concentrations, which was a significant deviation from Monod kinetics and appeared to be associated either with high growth rates or with a fermentative (or respiro-fermentative) metabolism. Supplementation of the cultures with inorganic or organic nutrients failed to improve ethanol production or glucose assimilation. Journal of Industrial Microbiology & Biotechnology (2000) 24, 231–236. Received 09 August 1999/ Accepted in revised form 18 December 1999