Does lateral gas diffusion in leaves matter?

Photosynthesis depends on the diffusion of gaseous CO(2) inside the leaf spaces from the stomatal entry point to the mesophyll cell walls. Although most research considers only the vertical diffusion from stomata on upper and/or leaf lower surfaces, some of the gas will diffuse in the lateral (paradermal) direction. The importance of lateral CO(2) diffusion is reviewed, and the anatomical characteristics of leaves, including the variation of air space volume between species and conditions are discussed. The contribution of the air space conductance to the limitation of photosynthesis by the overall CO(2) diffusion pathway is usually ignored. However, the need to consider three-dimensional diffusion at the small scale of a few stomata is emphasized because stomata are discrete, and separated by 20-300 microm. At the large scale of 100s of micrometres, there may be barriers to CO(2) caused by the vascular tissue, particularly if there are bundle sheath extensions. The possible extent and controls on CO(2) lateral and vertical diffusion in different species and conditions are illustrated using chlorophyll a fluorescence imaging techniques. It is clear that there is a range of effective lateral permeabilities depending on the particular vascular patterns and cell arrangements, and that species cannot be simply divided into homobaric and heterobaric anatomies. Lateral diffusion in more permeable leaves can be sufficient to affect measurements of leaf gas exchange, particularly when fluxes are low, although its contribution to leaf photosynthesis in natural conditions needs clarification.     

Ethylene production in habituated and auxin-requiring tobacco callus cultures. Does ethylene play a role in the habituation?

Ethylene production of habituated and auxin-requiring tobacco ( Nicotiana tabacum L. cv. Xanthi) callus cultures were compared. More ethylene was produced by auxinrequiring i.e. auxin-heterotrophic cultures than by habituated ones. Treatment with 2,4-dichlorophenoxyacetic acid increased the ethylene evolution of habituated cultures over the range 10⁻⁷ to 10⁻⁴ M , which suggests that the higher ethylene production of auxin-dependent callus is caused by the 2,4-D in the medium. The IAA levels depended on the age of both types of callus cultures.     

Does heparin inhibit renin activity?

Although heparin was reported in the 1960s to inhibit renin activity, this has not always been confirmed by other investigators. Hence, we re-examined whether heparin really inhibits renin or not. Renin activities were determined by radioimmunoassay of angiotensin I generated at pH 7.4. (i) No significant difference was found between the two kinds of plasma samples obtained with heparin and with EDTA as anticoagulant, in ARC (renin activity with addition of sheep renin substrate), TRC (ARC after activation of inactive renin by trypsin), or PRA (plasma renin activity without additional substrate). (ii) Even in higher concentrations of heparin up to 500 U/mL, neither PRA, ARC, nor TRC of plasma was affected significantly. (iii) Heparin, in concentrations up to 500 U/mL, exerted no significant effect on TRC of the media of human vascular smooth muscle cell culture. In conclusion, heparin does not exert any significant inhibitory effect on human renin nor does it affect activation of inactive renin by trypsin in the range of concentration of practical use, under the conditions employed in this study.     

Does abscisic acid influence proline accumulation in stressed leaves?

Root treatments of barley (Hordeum distichum L.) plants with 10^-7 to 10^-4 M abscisic acid (ABA) caused an increase in proline content, especially at higher concentrations, within 2–3 h. Even 3 h after the removal of ABA from the medium the plants continued to accumulate proline. The higher the concentration of the ABA, the higher was the proline level at 6 h. When the highest ABA concentration, 10^-4 M, was tested with polyethylene glycol (PEG) (-5.0 bars) in the medium, the ABA treatment resulted in a higher proline content than in control plants. The treatments PEG alone and PEG + ABA resulted in heavy accumulation of proline, especially, 3 h after releasing the plants from the stress. The proline content in PEG+ABA-treated plants was always higher than plants treated with PGE or ABA alone. In peas (Pisum sativum L. cv Alaska) the same trend occurred although to a lesser degree. These findings indicate an influence of ABA on proline accumulation in water-stressed plants.Abbreviations ABA abscisic acid - PEG polyethylene glycol - RWC relative water content     

Is light quality involved in the regulation of the photosynthetic apparatus in attached rice leaves?

The regulatory effect of light quality on the photosynthetic apparatus in attached leaves of rice plants was investigated by keeping rice plants under natural light, in complete darkness, or under illumination with light of different colors. When leaves were left in darkness and far-red (FR)-light conditions for 6 days at 30°C, there was an initial lag in chlorophyll (Chl) content, Chl a/b ratio, and maximum photosystem (PS) II photochemistry that lasted until the second day; these then rapidly decreased on the fourth day. In contrast, ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) rapidly disappeared with no lag under low or zero light conditions. By using spectrophotometric quantitation, it was determined that the PSII and PSI reaction centers were regulated by light quality, but cytochrome (Cyt) f was regulated by light intensity. However, the PSII heterogeneity was also strongly modified by the light intensity; PSIIα with the large antenna decreased markedly both in content and in antenna size. Consequently, the PSIIα/PSI ratio declined under FR-light because the low intensity of FR-light dominated over its quality in the modulation of the PSIIα/PSI ratio. An imbalance between them induced the generation of reactive oxygen species (ROS), although the ROS were scavenged by stromal enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX), and glutathione reductase (GR). The activities of these stromal enzymes are also regulated by light quality. Thus, although the photosynthetic apparatus is regulated differently depending on light quality, light quality may play an important role in the regulation of the photosynthetic apparatus.     

Does tree diversity increase wood production in pine forests?

Recent experimental advances on the positive effect of species richness on ecosystem productivity highlight the need to explore this relationship in communities other than grasslands and using non-synthetic experiments. We investigated whether wood production in forests dominated by Aleppo pine (Pinus halepensis) and Pyrenean Scots pine (Pinus sylvestris) differed between monospecific and mixed forests (2-5 species) using the Ecological and Forest Inventory of Catalonia (IEFC) database which contains biotic and environmental characteristics for 10,644 field plots distributed within a 31,944 km(2) area in Catalonia (NE Spain). We found that in Pyrenean Scots pine forests wood production was not significantly different between monospecific and mixed plots. In contrast, in Aleppo pine forests wood production was greater in mixed plots than in monospecific plots. However, when climate, bedrock types, radiation and successional stage per plot were included in the analysis, species richness was no longer a significant factor. Aleppo pine forests had the highest productivity in plots located in humid climates and on marls and sandstone bedrocks. Climate did not influence wood production in Pyrenean Scots pine forests, but it was highest on sandstone and consolidated alluvial materials. For both pine forests wood production was negatively correlated with successional stage. Radiation did not influence wood production. Our analysis emphasizes the influence of macroenvironmental factors and temporal variation on tree productivity at the regional scale. Well-conducted forest surveys are an excellent source of data to test for the association between diversity and productivity driven by large-scale environmental factors.     

Shoot inversion-induced ethylene production: a general phenomenon?

Shoot inversion induction of ethylene production was found in inverted shoots of corn, peas, soybean, sunflower, tomato, andPharbitis nil. The increases in ethylene production were found to range from two- to threefold in soybean to eightfold in corn and sunflower. The occurrence of peaks of ethylene production ranged from 16 h following shoot inversion in corn to 72 h inPharbitis. That the enhanced ethylene production was due to activation of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase is supported by the finding of increased ACC content in inverted shoots of all species tested. Shoot inversion inhibition of elongation was found in inverted shoots of pea, soybean, sunflower, tomato, andPharbitis nil. This inhibition is thought to be mediated via increased ethylene production in the inverted shoots. That shoot inversion induction of ethylene is not a persistent effect is supported by the finding that ethylene synthesis could be terminated by reorientation of shoots to the upright position and could be reinitiated by the subsequent inversion of the shoots. The effects of shoot inversion on the enhancement of ethylene production and on the inhibition of elongation of the inverted shoot appear to be general phenomena.     

Does 2,3-butanedione monoxime inhibit nonmuscle myosin?

Summary. BDM (2,3-butanedione monoxime) has been used extensively to inhibit nonmuscle myosin. However, recent articles raise the question of what BDM actually does, because of experiments in which BDM did not affect the actin-activated ATPase of nonmuscle myosins. We describe results that indicate that BDM indeed inhibits motility due to nonmuscle myosins: in many different cells BDM has the same effects as anti-actin agents and/or as other anti-myosin agents, and BDM slows or stops the sliding between actin filaments and myosin in vitro. We discuss how the two sets of apparently contradictory results might be resolved, and we suggest possible experiments that might clarify the contradictory interpretations. Correspondence and reprints: Biology Department, York University, 4700 Keele Street, Toronto, ON M3J 1P3, Canada.     

Does amiodarone inhibit T3 binding to solubilized nuclear receptors in vitro?

One of the proposed mechanisms of action of amiodarone is by interfering with T3-receptor interactions. However, reports in the literature on this matter are contradictory and, in an attempt to resolve these contradictions, the effects of amiodarone on T3 binding to solubilized nuclear receptors from rat liver were investigated. A drug concentration-related competitive inhibition of T3 binding occurred in the presence of considerable amiodarone precipitation. Measurement of the proportion of soluble amiodarone over the range where significant inhibition of T3 binding was observed, revealed that the soluble amiodarone decreased over this range. Since the amiodarone preparation was free of any contaminant, these findings suggested that a product generated from amiodarone during incubation was responsible for the inhibition. HPLC analysis of the soluble amiodarone fraction indicated that, during incubation, two substances were generated from the parent compound. However, only the concentration of one of the substances continually increased whereas the other decreased with increasing concentrations of amiodarone added to the samples. These findings suggest that the inhibition of T3 binding to solubilized nuclear receptors in the presence of amiodarone results from the generation of a specific product from the amiodarone under certain conditions during incubation.     

Does elevated atmospheric CO2 concentration inhibit mitochondrial respiration in green plants?

ATP, adenosine triphosphate
K_m, Michaelis-Menton coefficient
C_a, concentration of CO₂ in the air (μmol mol^–1)
NAD, oxidized nicotin adenine dinucleotide
NADH, reduced nicotin adenine dinucleotide
NADP, oxidized nicotin adenine phosphate dinucleotide
NADPH, reduced nicotine adenine phosphate dinucleotide
R, rate of respiration per unit DW [μmol g
DW^–1], Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase
V_c,_max, maximum in vivo rate of carboxylation at Rubisco (μmol m^–2 s^–1)

There is abundant evidence that a reduction in mitochondrial respiration of plants occurs when atmospheric CO₂ (C_a) is increased. Recent reviews suggest that doubling the present C_a will reduce the respiration rate [per unit dry weight (DW)] by 15 to 18%. The effect has two components: an immediate, reversible effect observed in leaves, stems, and roots of plants as well as soil microbes, and an irreversible effect which occurs as a consequence of growth in elevated C_a and appears to be specific to C₃ species. The direct effect has been correlated with inhibition of certain respiratory enzymes, namely cytochrome-c-oxidase and succinate dehydrogenase, and the indirect or acclimation effect may be related to changes in tissue composition. Although no satisfactory mechanisms to explain these effects have been demonstrated, plausible mechanisms have been proposed and await experimental testing. These are carbamylation of proteins and direct inhibition of enzymes of respiration. A reduction of foliar respiration of 15% by doubling present ambient C_a would represent 3 Gt of carbon per annum in the global carbon budget.     

Does aluminum inhibit pollen germination via extracellular calmodulin?

The effect of aluminum (Al) on pollen germination and its mechanism of action were investigated. Pollen germination and pollen tube elongation were inhibited by Al at pH 4.5. This inhibitory effect was reversed by the addition of purified calmodulin (CaM), whereas neither the calcium binding-protein S-100 nor Al chelator citric acid at the same concentrations had any obvious effect on Al-inhibited pollen germination. The presence of either the membrane-impermeable CaM inhibitor anti-CaM antiserum or Ca2+ chelator EGTA completely suppressed the effect of exogenous CaM. These results indicate the involvement of extracellular calmodulin in the short-term effects of Al on pollen germination and pollen tube elongation.     

Does sphingomyelin inhibit the erythrocyte anion transport system?

The anion transport protein of the human erythrocyte membrane, band 3, was incorporated into unilamellar sphingomyelin vesicles. The vesicles showed a rapid sulfate efflux which could be inhibited by specific inhibitors of the erythrocyte anion transport system. All band 3 molecules contributing to the inhibitor-sensitive flux component were arranged 'right-side-out'. The turnover number of the transport protein for sulfate transport was virtually identical to that in phosphatidylcholine bilayers and around 6 times larger than in human erythrocyte membranes. Thus, in contrast to other claims, sphingomyelin does not inhibit the erythrocyte anion transport system.     

Does hydroxyurea inhibit DNA replication in mouse cells by more than one mechanism?

Cell-free DNA synthesis was performed in a lysed cell system from mouse cell cultures. The in vitro reaction was totally inhibited by N-ethylmaleimide but unaffected by hydroxyurea or fluorodeoxyuridine when these compounds were added to the incubation mixture. However, in a preparation obtained from cells which had been blocked by hydroxyurea before lysis, the rate of DNA synthesis was markedly reduced. This effect could not have been caused by the depletion of the precursor pools as all necessary triphosphates were added to the in vitro incubation mixture. Analysis by alkaline density gradients showed that the ligation of primary synthesis products is retarded in hydroxyurea-pretreated lysed cells and that small fragments accumulate. These results suggest that hydroxyurea interferes with the processing of early replication products, preventing the formation of longer intermediates. Its mechanism is either independent from the well-known inhibition of ribonucleoside diphosphate reductase or it may be the result of an as-yet-unknown function of this enzyme in a later step of replication. This observation could help to explain why cells appear to be blocked by hydroxyurea in the early part of the S phase (rather than at the G1/S border proper) and also why DNA repair synthesis is relatively insensitive to the drug.     

Does feeding area restriction inhibit social learning of toxic weed ingestion in cattle?

Social learning from peers can trigger herd-wide intoxication with white locoweed (Oxytropis sericea), an alkaloid-synthesizing herbaceous legume that grows on rangelands of western North America. We conducted an experiment to test the hypothesis that restriction of the area allocated to animals to feed in would inhibit social facilitation of locoweed ingestion in yearling heifers. Eight heifers that avoided white locoweed (LA) and eight heifers that readily consumed it (LE) were selected from a pool of 40 cross-bred heifers and were randomly assigned to the social facilitation or social interference treatment groups. We conducted 200 10-min feeding trials in three 5-day phases (pre-treatment, treatment, post-treatment) during which animals were presented with a set of bowls arrayed in a test arena, some of which contained ground wheat straw and others contained air-dried ground white locoweed. During the pre-treatment (days 1 to 5) and the post-treatment phases (days 11 to 15) non-social trials were conducted in which the feeding behavior of individual animals was investigated in an 80 m² arena containing 12 feeding bowls. During the treatment phase (days 6 to 10) social learning trials were conducted in which LA + LE pairs from the social interference group were exposed to 12 bowls of food distributed in an 80 m² arena intended to induce social interference, and LA + LE pairs from the social facilitation group were exposed to 36 bowls of food distributed in a 240 m² arena intended to permit social facilitation. During pre-treatment phase, LA heifers consumed detectably less locoweed and wheat straw and exhibited lower preference for locoweed than LE (P ⩽ 0.05) although wheat straw preference of LA and LE was similar. During social learning trials (treatment phase), LA in the social interference group visited similar number of locoweed bowls (mean ± s.e.m.: 0.2 ± 0.12) as they had during non-social learning (0.2 ± 0.20). Conversely, LA heifers in the social facilitation group visited detectably more locoweed bowls during social learning trials (1.6 ± 0.46) compared with the pre-treatment phase (0.2 ± 0.16). Correlation between daily number of locoweed bowls visited by LA and LE during social learning trials was detected in the social facilitation (r = 0.70; P < 0.01), but not in the social interference group (r = 0.15; P = 0.52). During testing trials (post-treatment phase), locoweed and wheat straw intake and preference of LA and LE in both treatment groups was similar. Manipulation of the feeding environment delayed, but did not inhibit social learning of toxic weed ingestion in this study.     

Photosynthetic responses of sun- and shade-grown barley leaves to high light: is the lower PSII connectivity in shade leaves associated with protection against excess of light?

In this study, we have compared photosynthetic performance of barley leaves (Hordeum vulgare L.) grown under sun and shade light regimes during their entire growth period, under field conditions. Analyses were based on measurements of both slow and fast chlorophyll (Chl) a fluorescence kinetics, gas exchange, pigment composition; and of light incident on leaves during their growth. Both the shade and the sun barley leaves had similar Chl a/b and Chl/carotenoid ratios. The fluorescence induction analyses uncovered major functional differences between the sun and the shade leaves: lower connectivity among Photosystem II (PSII), decreased number of electron carriers, and limitations in electron transport between PSII and PSI in the shade leaves; but only low differences in the size of PSII antenna. We discuss the possible protective role of low connectivity between PSII units in shade leaves in keeping the excitation pressure at a lower, physiologically more acceptable level under high light conditions.     

Does artificial light pollution impair problem‐solving success in peafowl?

Behavioral innovations allow animals to adjust their behavior to solve novel problems. While innovative behavior can be important for animals living in new environments, anthropogenic pollution may limit their ability to adapt by impairing cognition or motivation. In particular, exposure to light pollution at night can cause sleep deprivation and may, therefore, hinder innovative behavior. To test this hypothesis, we examined experimentally whether exposure to acute light pollution impacts problem‐solving success in peafowl (Pavo cristatus). After peafowl were exposed to artificial light pollution for one night, they were presented with a problem‐solving task in which they could extract food by piercing the lid of an unfamiliar food bowl. Their problem‐solving success was unrelated to short‐term light pollution exposure. Other factors, including persistence, sex of the bird, and moon illumination, influenced their success in solving the task. The results suggest that short‐term exposure to light pollution does not limit behavioral innovation, but long‐term studies are necessary to further probe this question.     

Fast cyclic electron transport around photosystem I in leaves under far-red light: a proton-uncoupled pathway?

Fast cyclic electron transport (CET) around photosystem I (PS I) was observed in sunflower (Helianthus annuus L.) leaves under intense far-red light (FRL) of up to 200 μmol quanta m⁻² s⁻¹. The electron transport rate (ETR) through PS I was found from the FRL-dark transmittance change at 810 and 950 nm, which was deconvoluted into redox states and pool sizes of P700, plastocyanin (PC) and cytochrome f (Cyt f). PC and P700 were in redox equilibrium with K _e = 35 (ΔE _m = 90 mV). PS II ETR was based on O₂ evolution. CET [(PS I ETR) − (PS II ETR)] increased to 50–70 μmol e⁻ m⁻² s⁻¹ when linear electron transport (LET) under FRL was limited to 5 μmol e⁻ m⁻² s⁻¹ in a gas phase containing 20–40 μmol CO₂ mol⁻¹ and 20 μmol O₂ mol⁻¹. Under these conditions, pulse-saturated fluorescence yield F _m was non-photochemically quenched; however, F _m was similarly quenched when LET was driven by low green or white light, which energetically precluded the possibility for active CET. We suggest that under FRL, CET is rather not coupled to transmembrane proton translocation than the CET-coupled protons are short-circuited via proton channels regulated to open at high ΔpH. A kinetic analysis of CET electron donors and acceptors suggests the CET pathway is that of the reversed Q-cycle: Fd → (FNR) → Cyt c_n → Cyt b_h → Cyt b_l → Rieske FeS → Cyt f → PC → P700 →→ Fd. CET is activated when PQH₂ oxidation is opposed by high ΔpH, and ferredoxin (Fd) is reduced due to low availability of e⁻ acceptors. The physiological significance of CET may be photoprotective, as CET may be regarded as a mechanism of energy dissipation under stress conditions.     

Is peroxidase involved in ethylene biosynthesis?

Wheat (Triticum aestivum L. cv. Jubilar) coleoptile segments convert 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene. This process is totally inhibited by nitrogen atmosphere and severely inhibited by free radical scavengers (sodium benzoate, ferulic acid), inhibitors of reactive -SH groups ( p -chlormercuribenzoate, iodoacetate), CoCl₂ and EDTA. Indole-3-acetic acid, aminoethoxyvinyl glycine, cycloheximide, actinomycin D, pyridoxal phosphate and NADH have no effect on ACC conversion to ethylene. Some in vivo characteristics of this conversion suggest that it could be catalyzed by peroxidase. However, isoperoxidase B1 isolated from wheat seedlings was not able to catalyze in vitro conversion of ACC to ethylene under a wide range of reaction conditions. Therefore, it is concluded that peroxidase is not directly involved in ethylene biosynthesis.     

Does light exposure make plant litter more degradable?

Many field experiments have indicated that litter decomposition in semi-arid areas may be partly or fully controlled by photodegradation. We devised a study to test our hypothesis that light exposure makes plant litter more degradable. Dry, senescent, aboveground plant litter from Miscanthus x giganteus was exposed to light including ultraviolet (UV) radiation for various lengths of time from 0 to 289 days. Weight loss was measured after exposure and appeared to be modest and did not increase with time of exposure. The litter of the longest and shortest exposure time as well as controls were then incubated with soil and moisture for 35 days and CO₂ and N₂O production were measured. The longest exposed litter degraded much faster than any other treatment during incubation with moisture, about twice as fast as the unexposed control. The shortest exposed however, degraded only slightly faster than the unexposed control. This suggests that increasing litter degradability is a more important mechanism for photodegradation than direct light-induced mass loss. N₂O production from decomposition of the exposed litter was high in the beginning, suggesting that nitrogen may be released quickly. The mechanism is probably that light exposure leaves the nitrogen in plant litter easily available to microbial utilisation upon wetting. Such a mechanism might play an important role for nutrient cycling in semi-arid areas.