Contrasting Effects of Carbon Dioxide and Irradiance on the Acclimation of Photosynthesis in Developing Soybean Leaves

Leaves developed at high irradiance (I) often have higher photosynthetic capacity than those developed at low I, while leaves developed at elevated CO₂ concentration [CO₂] often have reduced photosynthetic capacity compared with leaves developed at lower [CO₂]. Because both high I and elevated [CO₂] stimulate photosynthesis of developing leaves, their contrasting effects on photosynthetic capacity at maturity suggest that the extra photosynthate may be utilized differently depending on whether I or [CO₂] stimulates photosynthesis. These experiments were designed to test whether relationships between photosynthetic income and the net accumulation of soluble protein in developing leaves, or relationships between soluble protein and photosynthetic capacity at full expansion differed depending on whether I or [CO₂] was varied during leaf development. Soybean plants were grown initially with a photosynthetic photon flux density (PPFD) of 950 µmol m^–2 s^–1 and 350 µmol [CO₂] mol^–1, then exposed to [CO₂] ranging from 135 to 1400 µmol mol^–1 for the last 3 d of expansion of third trifoliolate leaves. These results were compared with experiments in which I was varied at a constant [CO₂] of 350 µmol mol^–1 over the same developmental period. Increases in area and dry mass over the 3 d were determined along with daily photosynthesis and respiration. Photosynthetic CO₂ exchange characteristics and soluble protein content of leaves were determined at the end of the treatment periods. The increase in leaflet mass was about 28 % of the dry mass income from photosynthesis minus respiration, regardless of whether [CO₂] or I was varied, except that very low I or [CO₂] increased this percentage. Leaflet soluble protein per unit of area at full expansion had the same positive linear relationship to photosynthetic income whether [CO₂] or I was varied. For variation in I, photosynthetic capacity varied directly with soluble protein per unit area. This was not the case for variation in [CO₂]. Increasing [CO₂] reduced photosynthetic capacity per unit of soluble protein by up to a factor of 2.5, and photosynthetic capacity exhibited an optimum with respect to growth [CO₂]. Thus CO₂ did not alter the relationship between photosynthetic income and the utilization of photosynthate in the net accumulation of soluble protein, but did alter the relationship between soluble protein content and photosynthetic characteristics in this species.     

Calcium signalling in glial cells

Various electrical, mechanical, and chemical stimuli, including the influences of neurotrasmitters, neuromodulators, and hormones, trigger complex changes in [Ca²⁺] _i in all types of glial cells. Glial [Ca²⁺] _i responses are controlled by coordinated activity of several molecular cascades. The initiation of [Ca²⁺] _i signal in glial cells results from activation of either plasmalemmal, or intracellular Ca²⁺ permeable channels. The interplay of different molecular cascades enables the development of agonist-specific patterns of Ca²⁺ responses. Such agonist specificity may provide the means for intracellular and intercellular information coding. Furthermore, glial [Ca²⁺] _i signals can travel with no decrement within glial networks. These intercellular Ca²⁺ waves can be regarded as a substrate for information exchange between the glial cells. Neuronal activity can trigger [Ca²⁺] _i signals in neighboring glial cells and, moreover, there is some evidence that glial [Ca²⁺] _i waves can activate neuronal electrical and/or [Ca²⁺] _i , responses. Glial Ca²⁺ signalling can be regarded as a form of glial excitability.     

Low soil temperature inhibits the stimulatory effect of elevated [CO2] on height and biomass accumulation of white birch seedlings grown under three non‐limiting phosphorus conditions

White birch (Betula papyrifera Marsh.) seedlings were exposed to ambient or doubled ambient carbon dioxide concentration ([CO₂]), three soil temperatures (T_soil) (low, intermediate, high), and three phosphorus (P) regimes (low, medium, high) in environment‐controlled greenhouses. Height (H), root‐collar diameter (RCD), biomass, and leaf phosphorus concentration (leaf P) were determined four months after initiation of treatments. The low T_soil reduced H, RCD, shoot biomass, root biomass and total seedling biomass whereas the high‐P level and the [CO₂] elevation increased all the growth and biomass parameters. Elevated [CO₂] significantly reduced leaf P. There were significant two‐factor interactions suggesting that the effect of elevated [CO₂] on (1) H, total biomass, biomass of plant components, and leaf P was dependent on T_soil, (2) total biomass was contingent on P regime. For instance, the positive response of H and total biomass to elevated [CO₂] was limited to seedlings raised under the intermediate and high T_soil, respectively. In addition, [CO₂] elevation increased total biomass only at the high‐P regime but not at the low‐ or medium‐P level where the effect of [CO₂] was statistically insignificant. No significant main effect of treatment or interaction was observed for root to shoot biomass ratio.     

P uptake by arbuscular mycorrhizal hyphae: effect of soil temperature and atmospheric CO2 enrichment

Mycorrhizas are ubiquitous symbioses that may have an important role in the movement of C from air to soil. Studies on the effects of climate change factors on mycorrhizas have been concentrated on the effects of atmospheric [CO₂] whereas temperature effects have been neglected. Based on previous results showing no effect of varying atmospheric [CO₂] on the development and P uptake of the arbuscular mycorrhizal fungi (AMF) colonizing plants growing in controlled conditions, we hypothesized that soil temperature would have a higher impact on AMF development and nutrient uptake than the effects of [CO₂] on the host plant. Pea plants were grown in association with either a single isolate of Glomus caledonium or AMF from field soil in factorial combination with the corresponding current (10 °C) or elevated (15 °C) soil temperatures at current (350 p.p.m) or elevated (700 p.p.m) atmospheric [CO₂]. ³³P uptake by extraradical AMF hyphae was measured independently from root P uptake in a root exclusion compartment. Intraradical colonization developed well at both soil temperatures and almost duplicated from 10 to 15 °C. Extraradical mycelium developed only at 15 °C in the root exclusion compartment and hyphal P uptake could therefore be studied at 15 °C only. Hyphal P uptake differed markedly between inoculum types, but was not altered by growing the host plants at two atmospheric [CO₂] levels. No significant [CO₂] × soil temperature interactions were observed. The results suggested that, in the system tested, AMF development and function is likely more influenced by the temperature component of climate change than by its [CO₂] component. We suggest that much more attention should be paid to temperature effects in future studies.     

The growth of soybean under free air [CO<Subscript>2</Subscript>] enrichment (FACE) stimulates photosynthesis while decreasing in vivo Rubisco capacity

Down-regulation of light-saturated photosynthesis (A_sat) at elevated atmospheric CO₂ concentration, [CO₂], has been demonstrated for many C₃ species and is often associated with inability to utilize additional photosynthate and/or nitrogen limitation. In soybean, a nitrogen-fixing species, both limitations are less likely than in crops lacking an N-fixing symbiont. Prior studies have used controlled environment or field enclosures where the artificial environment can modify responses to [CO₂]. A soybean free air [CO₂] enrichment (FACE) facility has provided the first opportunity to analyze the effects of elevated [CO₂] on photosynthesis under fully open-air conditions. Potential ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation (V_c,max) and electron transport through photosystem II (J_max) were determined from the responses of A_sat to intercellular [CO₂] (C_i) throughout two growing seasons. Mesophyll conductance to CO₂ (g_m) was determined from the responses of A_sat and whole chain electron transport (J) to light. Elevated [CO₂] increased A_sat by 15–20% even though there was a small, statistically significant, decrease in V_c,max. This differs from previous studies in that V_c,max/J_max decreased, inferring a shift in resource investment away from Rubisco. This raised the C_i at which the transition from Rubisco-limited to ribulose-1,5-bisphosphate regeneration-limited photosynthesis occurred. The decrease in V_c,max was not the result of a change in g_m, which was unchanged by elevated [CO₂]. This first analysis of limitations to soybean photosynthesis under fully open-air conditions reveals important differences to prior studies that have used enclosures to elevate [CO₂], most significantly a smaller response of A_sat and an apparent shift in resources away from Rubisco relative to capacity for electron transport.Abbreviations FACE Free air [CO₂] enrichment - Rubisco Ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP Ribulose-1,5-bisphosphate - SoyFACE Soybean free air [CO₂] enrichment - VPD Vapor pressure deficit     

The stomatal CO<Subscript>2</Subscript> proxy does not saturate at high atmospheric CO<Subscript>2</Subscript> concentrations: evidence from stomatal index responses of Araucariaceae conifers

The inverse relationship between the number of stomata on a leaf surface and the atmospheric carbon dioxide concentration ([CO₂]) in which the leaf developed allows plants to optimise water-use efficiency (WUE), but it also permits the use of fossil plants as proxies of palaeoatmospheric [CO₂]. The ancient conifer family Araucariaceae is often represented in fossil floras and may act as a suitable proxy of palaeo-[CO₂], yet little is known regarding the stomatal index (SI) responses of extant Araucariaceae to [CO₂]. Four Araucaria species (Araucaria columnaris, A. heterophylla, A. angustifolia and A. bidwillii) and Agathis australis displayed no significant relationship in SI to [CO₂] below current ambient levels (~380 ppm). However, representatives of the three extant genera within the Araucariaceae (A. bidwillii, A. australis and Wollemia nobilis) all exhibited significant reductions in SI when grown in atmospheres of elevated [CO₂] (1,500 ppm). Stomatal conductance was reduced and WUE increased when grown under elevated [CO₂]. Stomatal pore length did not increase alongside reduced stomatal density (SD) and SI in the three araucariacean conifers when grown at elevated [CO₂]. These pronounced SD and SI reductions occur at higher [CO₂] levels than in other species with more recent evolutionary origins, and may reflect an evolutionary legacy of the Araucariaceae in the high [CO₂] world of the Mesozoic Era. Araucariacean conifers may therefore be suitable stomatal proxies of palaeo-[CO₂] during periods of “greenhouse” climates and high [CO₂] in the Earth’s history.     

Acclimation of photosynthesis to elevated CO2 and temperature in five British native species of contrasting functional type

Acclimation of photosynthesis to growth at elevated CO₂ concentration varies markedly between species. Species functionally classified as stress-tolerators (S) and ruderals (R), are thought to be incapable, or the least capable, of responding positively in terms of growth to elevated [CO₂]. Is this pattern of response also apparent in leaf photosynthesis of wild S- and R-strategists? Acclimatory loss of a photosynthetic and growth response to elevated [CO₂] is assumed to reflect limitation on capacity to utilize additional photosynthate. The doubling of pre-industrial global [CO₂] is expected to coincide with a 3 °C increase in mean temperature which could stimulate growth; will photosynthetic capacity at elevated [CO₂] be greater when the concurrent temperature increase is simulated? Five species from natural grassland of NW Europe and of contrasting ecological strategy were grown in hemispherical greenhouses, environmentally controlled to track the external microclimate. Within a replicated design, plants were grown at (i) current ambient [CO₂] and temperature, (ii) elevated [CO₂] (ambient + 340 μmol mol^–1) and ambient temperature, (iii) ambient [CO₂] and elevated temperature (ambient + 3 °C), or (iv) elevated [CO₂] and elevated temperature. After 75–104 days, the CO₂ response of light-saturated rates of photosynthesis (A_sat) was analysed in controlled-environment cuvettes in a field laboratory. There was no acclimatory loss of photosynthetic capacity with growth in elevated [CO₂] or elevated temperature over this period in Poa alpina (S), Bellis perennis (R) or Plantago lanceolata (mixed C-S-R strategist), and a significant (P ? ? bl 0.05) increase in capacity in Helianthemum nummularium (S) and Poa annua (R). Photosynthetic rates of leaves grown and measured in elevated [CO₂] were therefore significantly higher than rates for leaves grown and measured in ambient [CO₂], for all species. With the exception of Poa alpina, stomatal conductance and stomatal limitation on A_sat showed no acclimatory response to growth in elevated [CO₂]. Carboxylation efficiency, determined from the initial slope of the response of A_sat to intercellular CO₂ concentration was significantly increased by elevated [CO₂] and elevated temperature in H.nummularium, implying a possible increase in in vivo RubisCO activity. Increased carboxylation efficiency of this species was also reflected by an increase in the CO₂- and light-saturated rates of photosynthesis, indicating an increased capacity for regeneration of the primary CO₂ acceptor in photosynthesis. The results show that R-strategists and slow-growing S-strategists, are inherently capable of large increases in leaf photosynthetic capacity with growth in elevated [CO₂] in contrast to expectations from growth studies. With the exception of P.annua, where there was a significant negative interaction between CO₂ and temperature, concurrent increase in growth temperature had little effect on this pattern of response.     

Elevated [CO2] does not ameliorate the negative effects of elevated temperature on drought‐induced mortality in Eucalyptus radiata seedlings

It has been reported that elevated temperature accelerates the time‐to‐mortality in plants exposed to prolonged drought, while elevated [CO₂] acts as a mitigating factor because it can reduce stomatal conductance and thereby reduce water loss. We examined the interactive effects of elevated [CO₂] and temperature on the inter‐dependent carbon and hydraulic characteristics associated with drought‐induced mortality in Eucalyptus radiata seedlings grown in two [CO₂] (400 and 640 μL L^?1) and two temperature (ambient and ambient +4 °C) treatments. Seedlings were exposed to two controlled drying and rewatering cycles, and then water was withheld until plants died. The extent of xylem cavitation was assessed as loss of stem hydraulic conductivity. Elevated temperature triggered more rapid mortality than ambient temperature through hydraulic failure, and was associated with larger water use, increased drought sensitivities of gas exchange traits and earlier occurrence of xylem cavitation. Elevated [CO₂] had a negligible effect on seedling response to drought, and did not ameliorate the negative effects of elevated temperature on drought. Our findings suggest that elevated temperature and consequent higher vapour pressure deficit, but not elevated [CO₂], may be the primary contributors to drought‐induced seedling mortality under future climates.     

The Synthesis of New Polymer Derivatives of ATP by Radical Copolymerization and Their Coenzymic Activity

Three polymerizable ATP derivatives, N⁶-[N-(6-methacrylamidohexyl)carbamoylmethyl]-, N⁶-[N -[2-[N -(2-methacrylamidoethyl)carbamoyl]ethyl]carbamoylmethyl]-, and N⁶ -[N -[N -(2-hydroxy- 3-methacrylamidopropyl)carbamoylmethyl]carbamoylmethyl]-ATP, were synthesized and radically copolymerized with comonomers [acrylamide, N -(2-hydroxyethyl)-, N -ethyl-, N, N - diethylacrylamide, acrylic acid, and 6-methacrylamidohexylammonium chloride] to obtain 18 new polymer derivatives of ATP. The molecular weight distributions were controlled by appropriate initiator concentrations. The monomeric and polymeric ATP derivatives were all coenzymically active against both hexokinase and glycerol kinase. The observed coenzymic activities (Km and V_max) are discussed in connection with the structures of the derivatives.     

Altered physiological and growth responses to elevated [CO2] in offspring from holm oak (Quercus ilex L.) mother trees with lifetime exposure to naturally elevated [CO2]

An important question with respect to plant performance in future climatic scenarios is whether the offspring of mature trees that have experienced lifelong exposure to elevated [CO₂] show altered physiological responses to elevated [CO₂] compared with those originating from current ambient CO₂ concentrations. To investigate this question, acorns were collected from two seed sources, denoted as ‘control’ and ‘spring’, from Quercus ilex mother trees grown at ambient (36 Pa) and at about twice ambient CO₂ concentrations, respectively, close to a natural CO₂ spring, Laiatico, central Italy. The seedlings were raised for 8 months under controlled conditions at ambient and elevated [CO₂] in a reciprocal experimental design and were used for the determination of biomass, photosynthesis and foliar carbohydrate concentrations, as well as the accumulation of structural biomass and lignin during leaf maturation. Under ambient [CO₂], biomass and foliar carbon acquisition in control progeny were not significantly different from spring progeny. However, under elevated [CO₂], spring seedlings showed less CO₂ acclimation than control seedlings but no significant differences in non‐structural carbohydrate concentrations and structural biomass per unit leaf dry mass. Developmental lignin accumulation in leaves was delayed under elevated [CO₂] compared with ambient [CO₂], but only in control progeny. Under elevated [CO₂], whole‐plant biomass, leaf area and stem diameter were significantly increased in Quercus ilex seedlings from both seed sources but with a higher stimulation of above‐ground biomass in spring than in control seedlings and a higher stimulation of below‐ground biomass in control seedlings. These results indicate that life history and/or progeny may determine the species‐specific CO₂ response and suggest that positive CO₂ acclimation is possible.     

Biomass allocation in old-field annual species grown in elevated CO2 environments: no evidence for optimal partitioning

Increased atmospheric carbon dioxide supply is predicted to alter plant growth and biomass allocation patterns. It is not clear whether changes in biomass allocation reflect optimal partitioning or whether they are a direct effect of increased growth rates. Plasticity in growth and biomass allocation patterns was investigated at two concentrations of CO₂ ([CO₂]) and at limiting and nonlimiting nutrient levels for four fast‐ growing old‐field annual species. Abutilon theophrasti, Amaranthus retroflexus, Chenopodium album, and Polygonum pensylvanicum were grown from seed in controlled growth chamber conditions at current (350 μmol mol^?1, ambient) and future‐ predicted (700 μmol mol^?1, elevated) CO₂ levels. Frequent harvests were used to determine growth and biomass allocation responses of these plants throughout vegetative development. Under nonlimiting nutrient conditions, whole plant growth was increased greatly under elevated [CO₂] for three C3 species and moderately increased for a C4 species (Amaranthus). No significant increases in whole plant growth were observed under limiting nutrient conditions. Plants grown in elevated [CO₂] had lower or unchanged root:shoot ratios, contrary to what would be expected by optimal partitioning theory. These differences disappeared when allometric plots of the same data were analysed, indicating that CO₂‐induced differences in root:shoot allocation were a consequence of accelerated growth and development rates. Allocation to leaf area was unaffected by atmospheric [CO₂] for these species. The general lack of biomass allocation responses to [CO₂] availability is in stark contrast with known responses of these species to light and nutrient gradients. We conclude that biomass allocation responses to elevated atmospheric [CO₂] are not consistent with optimal partitioning predictions.     

Can variable pH and low oxygen moderate ocean acidification outcomes for mussel larvae?

Natural variation and changing climate in coastal oceans subject meroplanktonic organisms to broad ranges of pH and oxygen ([O₂]) levels. In controlled‐laboratory experiments we explored the interactive effects of pH, [O₂], and semidiurnal pH fluctuations on the survivorship, development, and size of early life stages of two mytilid mussels, Mytilus californianus and M. galloprovincialis. Survivorship of larvae was unaffected by low pH, low [O₂], or semidiurnal fluctuations for both mytilid species. Low pH (<7.6) resulted in delayed transition from the trochophore to veliger stage, but this effect of low pH was absent when incorporating semidiurnal fluctuations in both species. Also at low pH, larval shells were smaller and had greater variance; this effect was absent when semidiurnal fluctuations of 0.3 units were incorporated at low pH for M. galloprovincialis but not for M. californianus. Low [O₂] in combination with low pH had no effect on larval development and size, indicating that early life stages of mytilid mussels are largely tolerant to a broad range of [O₂] reflective of their environment (80–260 μmol kg^?1). The role of pH variability should be recognized as an important feature in coastal oceans that has the capacity to modulate the effects of ocean acidification on biological responses.     

Photosynthesis and fluorescence responses of C<Subscript>4</Subscript> plant <Emphasis Type="Italic">Andropogon gerardii</Emphasis> acclimated to temperature and carbon dioxide

Increase in both atmospheric CO₂ concentration [CO₂] and associated warming are likely to alter Earths’ carbon balance and photosynthetic carbon fixation of dominant plant species in a given biome. An experiment was conducted in sunlit, controlled environment chambers to determine effects of atmospheric [CO₂] and temperature on net photosynthetic rate (P _N) and fluorescence (F) in response to internal CO₂ concentration (C _i) and photosynthetically active radiation (PAR) of the C₄ species, big bluestem (Andropogon gerardii Vitman). Ten treatments were comprised of two [CO₂] of 360 (ambient, AC) and 720 (elevated, EC) μmol mol⁻¹ and five day/night temperature of 20/12, 25/17, 30/22, 35/27 and 40/32 °C. Treatments were imposed from 15 d after sowing (DAS) through 130 DAS. Both F-P _N/C _i and F-P _N/PAR response curves were measured on top most fully expanded leaves between 55 and 75 DAS. Plants grown in EC exhibited significantly higher CO₂-saturated net photosynthesis (P _sat), phosphoenolpyruvate carboxylase (PEPC) efficiency, and electron transport rate (ETR). At a given [CO₂], increase in temperature increased P _sat, PEPC efficiency, and ETR. Plants grown at EC did not differ for dark respiration rate (R _D), but had significantly higher maximum photosynthesis (P _max) than plants grown in AC. Increase in temperature increased Pmax, R _D, and ETR, irrespective of the [CO₂]. The ability of PEPC, ribulose-1,5-bisphosphate carboxylase/oxygenase, and photosystem components, derived from response curves to tolerate higher temperatures (>35 °C), particularly under EC, indicates the ability of C₄ species to sustain photosynthetic capacity in future climates.     

Indole-3-acetic acid is synthesized from L-tryptophan in roots of Arabidopsis thaliana

The promoter of the nit1 gene, encoding the predominantly expressed isoform of the Arabidopsis thaliana (L.) Heynh. nitrilase isoenzyme family, fused to the β-glucuronidase gene (uidA) drives β-glucuronidase expression in the root system of transgenic A. thaliana and tobacco plants. This expression pattern was shown to be controlled developmentally, suggesting that the early differentiation zone of root tips and the tissue surrounding the zone of lateral root primordia formation may constitute sites of auxin biosynthesis in plants. The root system of A. thaliana was shown to express functional nitrilase enzyme. When sterile roots were fed [²H]₅-L-tryptophan, they converted this precusor to [²H]₅-indole-3-acetonitrile and [²H]₅-indole-3-acetic acid. This latter metabolite was further metabolized into base-labile conjugates which were the predominant form of [²H]₅-indole-3-acetic acid extracted from roots. When [1-¹³C]-indole-3-acetonitrile was fed to sterile roots, it was converted to [1-¹³C]-indole-3-acetic acid which was further converted to conjugates. The results prove that the A. thaliana root system is an autonomous site of indole-3-acetic acid biosynthesis from L-tryptophan. Received: 3 February 1998 / Accepted: 17 April 1998     

Temporal dynamics and spatial variability in the enhancement of canopy leaf area under elevated atmospheric CO2

Increased canopy leaf area (L) may lead to higher forest productivity and alter processes such as species dynamics and ecosystem mass and energy fluxes. Few CO₂ enrichment studies have been conducted in closed canopy forests and none have shown a sustained enhancement of L. We reconstructed 8 years (1996–2003) of L at Duke's Free Air CO₂ Enrichment experiment to determine the effects of elevated atmospheric CO₂ concentration ([CO₂]) on L before and after canopy closure in a pine forest with a hardwood component, focusing on interactions with temporal variation in water availability and spatial variation in nitrogen (N) supply. The dynamics of L were reconstructed using data on leaf litterfall mass and specific leaf area for hardwoods, and needle litterfall mass and specific leaf area combined with needle elongation rates, and fascicle and shoot counts for pines. The dynamics of pine L production and senescence were unaffected by elevated [CO₂], although L senescence for hardwoods was slowed. Elevated [CO₂] enhanced pine L and the total canopy L (combined pine and hardwood species; P<0.050); on average, enhancement following canopy closure was ～16% and 14% respectively. However, variation in pine L and its response to elevated [CO₂] was not random. Each year pine L under ambient and elevated [CO₂] was spatially correlated to the variability in site nitrogen availability (e.g. r²=0.94 and 0.87 in 2001, when L was highest before declining due to droughts and storms), with the [CO₂]‐induced enhancement increasing with N (P=0.061). Incorporating data on N beyond the range of native fertility, achieved through N fertilization, indicated that pine L had reached the site maximum under elevated [CO₂] where native N was highest. Thus closed canopy pine forests may be able to increase leaf area under elevated [CO₂] in moderate fertility sites, but are unable to respond to [CO₂] in both infertile sites (insufficient resources) and sites having high levels of fertility (maximum utilization of resources). The total canopy L, representing the combined L of pine and hardwood species, was constant across the N gradient under both ambient and elevated [CO₂], generating a constant enhancement of canopy L. Thus, in mixed species stands, L of canopy hardwoods which developed on lower fertility sites (～3 g N inputs m^?2 yr^?1) may be sufficiently enhanced under elevated [CO₂] to compensate for the lack of response in pine L, and generate an appreciable response of total canopy L (～14%).     

Increased intracellular calcium concentration causes electrical turbulence in guinea pig ventricular myocytes

Dysregulation of intracellular Ca²⁺ homeostasis is associated with various pathological conditions and arrhythmogenesis of the heart. The objective of this study was to investigate the effects of an acute increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) on the electrophysiology of ventricular myocytes by mimicking intracellular Ca²⁺ overload. The [Ca²⁺]_i was clamped to either a controlled (65–100 nmol L⁻¹) or increased (1 μmol L⁻¹) level. The transmembrane action potentials and ionic currents were recorded using whole-cell patch clamp techniques. We found that the acute increase in [Ca²⁺]_i shortened the action potential duration, reduced the action potential amplitude, maximum depolarization velocity and resting membrane potential, caused delayed after-depolarizations (DADs), and triggered activity—compared with these parameters in the control. The increased [Ca²⁺]_i augmented late I _Na in a time-dependent manner, reduced I _CaL and I _K1, and increased I _Kr but not I _Ks. The results of this study can be used to explain calcium overload-induced ventricular arrhythmias.     

Reconciling the apparent difference between mass- and area-based expressions of the photosynthesis-nitrogen relationship

The relationship between photosynthetic carbon assimilation (A _max) and leaf nitrogen content (N _leaf) can be expressed on either a leaf area basis (A _area vs N _area) or a leaf mass basis (A _mass vs N _mass). Dimensional analysis shows that the units for the slope of this relationship are the same for both expressions (μmol [CO₂] g⁻¹ [N] s⁻¹). Thus the slope measures the change in CO₂ assimilation per gram of nitrogen, independent of leaf mass or leaf area. Although they have the same units, large differences between the area and mass-based slopes have been observed over a broad range of taxonomically diverse species. Some authors have claimed that regardless of these differences, the fundamental nature of the A _max-N _leaf relationship is independent of the units of expression. In contrast, other authors have claimed that the area-based A _max-N _leaf relationship is fundamentally different from the mass-based relationship because of interactions between A _max, N _leaf, and leaf mass per area (LMA, g [leaf] m⁻² [leaf]). In this study we consider the mathematical relationships involved in the transformation from mass- to area-based expressions (and vice versa), and the implications this transformation has for the slope of the A _max-N _leaf relationship. We then show that the slope of the relationship is independent of the units of expression when the effect of LMA is controlled statistically using a multiple regression. The validity of this hypothesis is demonstrated using 13 taxonomically and functionally diverse C₃ species. This analysis shows that the slope of the A _max-N _leaf relationship is similar for the mass- and area-based expressions and that significant errors in the estimate of the slope can arise when the effect of LMA is not controlled. Received: 7 May 1998 / Accepted: 19 October 1998     

Gas chromatographic–mass spectrometric determination of α-ketoisocaproic acid and [H7]α-ketoisocaproic acid in plasma after derivatization with N-phenyl-1,2-phenylenediamine

A method for determination of α-ketoisocaproic acid (KIC) and [4,5,5,5,6,6,6-²H₇]α-ketoisocaproic acid ([²H₇]KIC) in rat plasma was developed using gas chromatography–mass spectrometry-selected ion monitoring (GC–MS-SIM). [5,5,5-²H₃]α-Ketoisocaproic acid ([²H₃]KIC) was used as an analytical internal standard to account for losses associated with the extraction, derivatization and chromatography. The keto acids were extracted by cation-exchange chromatography using BondElut SCX cartridge and derivatized with N-phenyl-1,2-phenylenediamine to form N-phenylquinoxalinone derivatives. Quantitation was performed by SIM of the respective molecular ions at m/z 278, 281 and 285 for the derivatives of KIC, [²H₃]KIC and [²H₇]KIC on the electron impact method. The limit of detection was found to be 70 fmol per injection (S/N=3) and the limit of quantitation for [²H₇]KIC was around 50 nM in rat plasma. Endogenous KIC concentrations in 50 μl of rat plasma were measured with relative intra- and inter-day precision of 4.0% and 3.3%, respectively. The intra- and inter-day precision for [²H₇]KIC spiked to rat plasma in the range of 0.1 to 10 μM gave good reproducibility with relative standard deviation (RSD) of 6.5% and 5.4%, respectively. The intra- and inter-day relative errors (RE) for [²H₇]KIC were less than 6.4% and 3.8%, respectively. The method was applied to determine the plasma concentration of [²H₇]KIC after an intravenous administration of [²H₇]KIC in rat.     

Maintenance costs of serotiny do not explain weak serotiny

Considerable variation in the duration of serotiny exists among species of both Australian and South African Proteaceae. ‘Weak’ serotiny (pre‐fire loss after <3 years) could be dictated by the costs (water or carbon) of cone/fruit retention or by benefits accruing from pre‐fire seed establishment. We determined that cones/fruits of a range of Australian and south western Cape Proteaceae species (Leucadendron xanthoconus, Aulax umbellata, L. linifolium, L. gandogeri, Hakea drupacea, H. sericea) are not sealed dead wood, but that they continuously lose H₂O and CO₂. Water loss from cones/fruits was poorly controlled, occurring in both light and dark. The rates of both H₂O and CO₂ loss from mature cones/fruits were negatively correlated with the degree of serotiny (r² = 0.59 and 0.18, respectively, P < 0.001 both). However, the amounts of H₂O and CO₂ lost per weight were small relative to the fluxes from leaves (13–29% for H₂O and 3–10% for CO₂). The [N] and [P] in the cones/fruits and seeds was substantial. Despite 25% of N and 38% of P being recovered from the cones/fruits following maturation, the loss of the cones/fruits and seeds would still incur a substantial nutrient cost. The seed [P] was positively correlated with the degree of serotiny (r² = 0.24, P = 0.001). We suggest that maintenance costs (water and carbon) of serotiny, although exceeding those of soil stored seeds, are relatively low. The correlation between the degree of serotiny and seed [P] indicates that stronger serotiny is required, much like sclerophylly, for survival under low nutrient availability in frequently burnt vegetation.     

Interactive direct and plant‐mediated effects of elevated atmospheric [CO2] and temperature on a eucalypt‐feeding insect herbivore

Understanding the direct and indirect effects of elevated [CO₂] and temperature on insect herbivores and how these factors interact are essential to predict ecosystem‐level responses to climate change scenarios. In three concurrent glasshouse experiments, we measured both the individual and interactive effects of elevated [CO₂] and temperature on foliar quality. We also assessed the interactions between their direct and plant‐mediated effects on the development of an insect herbivore of eucalypts. Eucalyptus tereticornis saplings were grown at ambient or elevated [CO₂] (400 and 650 μmol mol^?1 respectively) and ambient or elevated ( + 4 °C) temperature for 10 months. Doratifera quadriguttata (Lepidoptera: Limacodidae) larvae were feeding directly on these trees, on their excised leaves in a separate glasshouse, or on excised field‐grown leaves within the temperature and [CO₂] controlled glasshouse. To allow insect gender to be determined and to ensure that any sex‐specific developmental differences could be distinguished from treatment effects, insect development time and consumption were measured from egg hatch to pupation. No direct [CO₂] effects on insects were observed. Elevated temperature accelerated larval development, but did not affect leaf consumption. Elevated [CO₂] and temperature independently reduced foliar quality, slowing larval development and increasing consumption. Simultaneously increasing both [CO₂] and temperature reduced these shifts in foliar quality, and negative effects on larval performance were subsequently ameliorated. Negative nutritional effects of elevated [CO₂] and temperature were also independently outweighed by the direct positive effect of elevated temperature on larvae. Rising [CO₂] and temperature are thus predicted to have interactive effects on foliar quality that affect eucalypt‐feeding insects. However, the ecological consequences of these interactions will depend on the magnitude of concurrent temperature rise and its direct effects on insect physiology and feeding behaviour.