Aphid retention of maize dwarf mosaic virus (potyvirus): epidemiological implications

In total, 17 589 aphids were assayed for rate of loss of inoculativity and maximum retention times of maize dwarf mosaic (MDMV). The Standard-treatment, involved acquisition access to MDMV-infected tissue followed by confinement of active aphids in Petri dishes. In addition various aphid immobilisation treatments were used to prevent probing on solid surfaces after acquisition access to simulate conditions experienced by wind-borne aphids when aloft. Immobilisation treatments, using nitrogen or argon gases at 25°C, or cold treatments at 6°C after acquisition access greatly increased the efficiency of MDMV transmission by greenbugs, Schizaphis graminum, in an experimental design where insects were individually assayed for transmission over a 7 h period. Further tests in which groups of greenbugs were assayed for MDMV transmission revealed that MDMV strains may be retained for over 21 h, regardless of post-acquisition access treatment. Experiments with other aphid vectors of MDMV (Dactynotus ambrosiae, Macrosiphum euphorbiae, Rhopalosiphum maidis and Myzus persicae) also demonstrated MDMV retention times exceeding 18 h. These results show that the rate at which aphids lose MDMV inoculativity is lower when solid surface probing behaviour is denied, and that MDMV retention times are longer than those previously published. The findings are discussed in relation to the epidemiology of nonpersistent viruses and their dispersal over great distances.     

Effect of Environmental Factors in Aerobiosis and Anaerobiosis on the Growth and Activity of Propionic Acid Bacteria Evaluated by Pressure Measurement

The effects of air, N₂ and CO₂ atmospheres on the growth and metabolism of P. shermanii CIP 10 30 27 under “optimum conditions” (pH 6.5, NaCl 0 %, temperature 30 °C) and “stressful conditions” (pH 5.3, NaCl 2.1 %, temperature 20 °C) simulating cheese ripening conditions were investigated using a pressure measurement technique. Under both conditions, the inhibition of growth was at its maximum when a CO₂ atmosphere with an increase in the lag phase (L_P) from 3 to 80 h and a reduction in the maximum rate of pressure variation (V_max,P) from 2.5 to 0.4 kPa/h was used. The production of metabolites was not affected under “optimum conditions”, but under “stressful conditions”, propionate production increased and higher ratios of propionate to acetate of up to 2.45 were observed. The effects of air and N₂ atmospheres on both growth and metabolite production were also examined.     

Increased shelf life of a bioherbicide through combining modified atmosphere packaging and low temperatures

The effects of air temperatures (4, 14 and 24°C) and modified atmosphere packaging (MAP) (0% CO₂/100% N₂; 20% CO₂/80% N₂ or 40% CO₂/60% N₂) on vigour of a Sclerotinia minor barley formulation during 6 months storage were evaluated. The study was performed using a multilevel factorial experimental design and response surface methodology (RSM) and aimed to determine the optimum combination of the above factors that resulted in retention of S. minor vigour during storage. Temperature and storage duration are the main factors that affect S. minor vigour. CO₂ concentration had no effect on S. minor vigour during storage. However, oxygen displacement from storage containers by CO₂ and N₂ resulted in significant decrease of vigour reduction of S. minor as compared to ambient air control. An acceptable level of S. minor vigour reduction (ALVR) during storage was developed and determined to be ALVR=31.7±14.8% (mean±95% CI). Contour plot analysis indicated that the S. minor barley formulation at 0.4 water activity could be stored for 6, 12 or 26 weeks without exceeding the upper ALVR threshold (ca. 46%) at air temperatures not higher than 20, 17 or 11°C, respectively.     

Impact of modified atmospheres on respiration of last instar larvae of the rice moth,Corcyra cephalonica (Lepidoptera: Pyralidae)

Abstract

Effect of modified atmospheres (MAs) containing CO₂ at 20, 40, 60 and 80% or containing N₂ at 97 and 98% on the mortality of Corcyra cephalonica Stainton (Lepidoptera: Pyralidae) sixth instar larvae was studied to determine the LT values at 30?°C. The respiration rates of untreated and treated larvae with 60% CO₂ and/or 98% N₂ at LT₅₀ were measured using Q-Box RP1LP low range respirometry package. Total protein and triglycerides of treated and untreated larvae were assayed. Complete larval mortality was recorded after 72 and 144?h of treatment with 60% CO₂ and 98% N₂, respectively. Calculated LT₅₀ values were 39.3 at 60% CO₂ and 87.5?h at 98% N₂ MAs. Respiration quotient (RQ) in the light of consumed O₂ and produced CO₂ of untreated larvae was 1.0 while it was 0.85 at 60% CO₂ and 0.72 at 98% N₂. Duration time necessary for produced CO₂ curve to reach the maximum point (2000?ppm) was significantly shorter at untreated larvae (27.64?min) in comparison with that recorded at CO₂ (35.48?min) which also significantly less than that obtained at N₂ (98.54?min). At all treatments, total protein was decreased while triglycerides were increased in comparison with control.     

Phase resetting of the circadian rhythm of carbon dioxide assimilation inBryophyllum leaves in relation to their malate content following brief exposure to high and low temperatures,darkness and 5% carbon dioxide

Leaves ofBryophyllum fedtschenkoi show a persistent circadian rhythm in CO₂ assimilation when kept in continuous illumination and normal air at 15°C. The induction of phase shifts in this rhythm by exposing the leaves for four hours at different times in the circadian cycle to 40° C, 2° C, darkness and 5% CO₂ have been investigated. Exposure to high temperature has no effect on the phase at the apex of the peak but is effective at all other times in the cycle, whereas exposure to low temperature, darkness or 5% CO₂ is without effect between the peaks and induces a phase shift at all other times. The next peak of the rhythm occurs 17 h after a 40° C treatment and 7–10 h after a 2° C, dark or 5% CO₂ treatment regardless of their position in the cycle. When these treatments are given at times in the cycle when they induce maximum phase shifts, they cause no change in the gross malate status of the leaf. The gross malate content of the leaf in continuous light and normal air at 15% shows a heavily damped circadian oscillation which virtually disappears by the time of the third cycle, but the CO₂ assimilation rhythm persists for many days. The generation of the rhythm, and the control of its phase by environmental factors are discussed in terms of mechanisms that involve the synthesis and metabolism of malate in specific localised pools in the cytoplasm of the leaf cells.     

Influence of carbon dioxide gas on German cockroach (Dictyoptera: Blattellidae) knockdown, recovery, movement and feeding

Abstract. Carbon dioxide anaesthesia differentially affects the knockdown and recovery of cockroaches, depending on the strain and the length of time that the colony has been subjected to a CO₂ regime. Adult males from two laboratory and two field-collected strains of German cockroaches, Blattella germanica, are knocked down within 7–45 s after exposure to CO₂. After 5 min of CO₂ exposure, presumptive recovery (i.e. the time for the cockroach to right itself after knockdown) for laboratory strains occurs significantly sooner than for field-collected strains. Control cockroaches, exposed to compressed air rather than CO₂, exit harbourage cups rapidly (≤3.20 min). However, although allowed a recovery period of 5 min, significant movement impairment occurs for all cockroach strains anaesthetized with CO₂. Carbon dioxide exposure significantly reduces consumption of 2.15% hydramethylnon bait and delays mortality even when 24 h is allowed for recovery before bait placement. Cockroaches allowed to recover for 48 h after 5 min of CO₂ exposure consume significantly more bait and die significantly faster than CO₂ exposed groups allowed 24 h of recovery, and mortality is not significantly different from nonanaesthetized bait-fed controls.     

Diurnal Oscillations in Gas Production (O2, CO2, CH4, and N2) in Soil Monoliths

Soil cores (35 cm long, 7 cm diameter) from the Macaulay Land Use Research Institute's Sourhope Research Station in the Scottish Borders were kept and monitored at constant temperature (18± 1°C) for gas production using a 1.6 mm diameter stainless steel probe fitted with a membrane inlet and connected to a quadrupole mass spectrometer. This provided a novel method for on-line, real time monitoring of soil gas dynamics. In closed-system headspace experiments, O₂ and CO₂ (measured at m/z values 32 and 44, respectively) showed anti-phase diurnal fluctuations in low-intensity simulated daylight and under a light-dark (LD, 12:12 h) regime. O₂ increased during periods of illumination and decreased in the dark. The inverse was true for CO₂ production. Ar (m/z = 40) concentration and temperature (°C) remained constant throughout the experiments. The same phase-related oscillations, in CO₂ and O₂ concentrations, were observed at 2 and 5 cm depth in soil cores. The O₂ concentration did not oscillate diurnally at 10 cm depth. In below-ground experiments, CH₄ (m/z = 15) concentration showed diurnal cycles at 2, 5 and 10 cm depth. The CH₄ production had the same diurnal phase cycle as CO₂ but with lower amplitude. Evidence of below-ground diurnal oscillations in N₂ (m/z = 28) concentration was provided at 5 cm depth. The scale of production and consumption of gases associated with soil-atmosphere interactions and below-ground processes, are shown to be a multifaceted output of several variables. These include light, circadian-controlled physiological rhythms of plants and microbes, and the interactions between these organisms.     

Photosynthetic and leaf respiration activity of <Emphasis Type="Italic">Malcolmia littorea</Emphasis> (L.) R. Br. in response to air temperature

Plant traits of Malcolmia littorea growing at the Botanic Garden of Rome and transplanted from the wild population developing along the Latium coast (Italy) were analyzed. The highest photosynthetic rates [P _N, 22.5 ± 0.5 μmol(CO₂) m⁻² s⁻¹], associated to the highest chlorophyll content (Chl, 60 ± 5 SPAD units), and respiration rates [R, 11.1 ± 0.2 μmol(CO₂) m⁻² s⁻¹] were reached in spring, when mean air temperature (T _m) was in the range 17°C to 23°C. P _N, Chl, and R decreased by 86, 38, and 59% in summer when mean maximum air temperature (T _max) was 30.3 ± 2.6°C. Leaf water potential decreased by 34% in summer compared to the spring value, and it was associated to a relative water content (RWC) of 74 ± 4%, and to a water-use efficiency (WUE) of 2.15 ± 0.81 μmol(CO₂) mmol⁻¹(H₂O). Moreover, also low air temperatures determined a significant P _N and R decreases (by 52 and 40% compared to the maximum, respectively). Responsiveness of gross photosynthetic rate (P _g) to R was higher than that to P _N as underlined by the slope of the regression line between the two variables. The results underlined a low tolerance to both high- and low air temperatures of M. littorea. The selected key traits (R, WUE, Chl) by the discriminant analysis might be used to monitor the M. littorea wild population in the long time. The ex situ cultivated plants could be propagated and used to increase the individuals number of the wild population.     

Physiological responses of Opuntia ficus-indica to growth temperature

The influences of various day/night air temperatures on net CO₂ uptake and nocturnal acid accumulation were determined for Opuntia ficus-indica, complementing previous studies on the water relations and responses to photosynthetically active radiation (PAR) for this widely cultivated cactus. As for other Crassulacean acid metabolism (CAM) plants, net nocturnal CO₂ uptake had a relatively low optimal temperature, ranging from 11°C for plants grown at day/night air temperatures of 10°C/0°C to 23°C at 45°C/35°C. Stomatal opening, which occurred essentially only at night and was measured by changes in water vapor conductance, progressively decreased as the measurement temperature was raised. The CO₂ residual conductance, which describes chlorenchyma properties, had a temperature optimum a few degrees higher than the optimum for net CO₂ uptake at all growth temperatures. Nocturnal CO₂ uptake and acid accumulation summed over the whole night were maximal for growth temperatures near 25°C/15°C, CO₂ uptake decreasing more rapidly than acid accumulation as the growth temperature was raised. At day/night air temperatures that led to substantial nocturnal acid accumulation (25°C/15°C.). 90% saturation of acid accumulation required a higher total daily PAR than at non-optimal growth temperatures (10°C/0°C and 35°C/25°C). Also, the optimal temperature of net CO₂ uptake shifted downward when the plants were under drought conditions at all three growth temperatures tested, possibly reflecting an increased fractional importance of respiration at the higher temperatures during drought. Thus, water status, ambient PAR, and growth temperatures must all be considered when predicting the temperature response of gas exchange for O. ficus-indica and presumably for other CAM plants.     

Effect of Modified Atmospheres on the Growth and Extracellular Enzyme Activities of Psychrotrophs in Raw Milk

The conservation of food products within a controlled atmosphere is efficient in packaging. To extend the cold storage of raw milk, the effects of five gas atmospheres enriched with carbon dioxide and nitrogen were investigated. Treated and control milk were stored at 7 °C for 10 days and analyzed for microbial counts, pH, proteolysis and lipolysis. The addition of CO₂, N₂, or their mixture had a significant inhibitory effect on psychrotrophic growth. The generation times of these microorganisms were significantly longer in treated milk, particularly for yeasts where they amounted to 16.63 h. The maximum inhibition was observed when a gas mixture of 50 % CO₂ and 50 % N₂ was used. As a result, psychrotrophic growth was affected to 98 % whereas this inhibition did not exceed 78 % when CO₂ and 41 % N₂ were applied. Milk treatment under the conditions of 50 % CO₂ and 50 % N₂ gave significantly lower counts for all groups of psychrotrophs being more efficient against Enterobacteriaceae with 99.5 % of inhibition. Storage of raw milk under the tested atmospheres had a different effect on extracellular enzyme productions. Significant decreases in protease and lipase activities were observed during the storage at 7 °C. These enzyme activities were not detectable with pure CO₂ and a 50 % CO₂ and 50 % N₂ mixture. N₂ has shown to be the less efficient treatment against lipases (65 %) and proteases (95 %). With regard to growth, the course of the pH and the protease and lipase activities, the tested gas mixture of 50 % CO₂ and 50 % N₂ was more suitable for extending the shelf life of raw milk.     

Effects of n(2) deficiency on transport and partitioning of C and N in a nodulated legume

Nodulated root systems of white lupin (Lupinus albus L. cv Ultra: Rhizobium strain WU425) were exposed to Ar:O₂ (80:20, v/v) or Ar:N₂:O₂ (70:10:20, v/v/v) and C and N partitioning were examined over a 9- or 10-day period in comparison with control plants with nodulated roots retained in air. Accumulation of N ceased in plants exposed to Ar:O₂ or was much reduced in plants exposed to Ar:N₂:O₂, but net C assimilation rates and profiles of C utilization remained similar to those of control N₂-fixing plants. There was, however, a proportional reduction in CO₂ evolution from nodulated roots of the Ar:O₂ treatment. Xylem N levels fell rapidly after application of Ar:O₂. C:N ratios of phloem sap of petioles and of stem base rose during the first day of Ar:O₂ treatment and then fell progressively back to levels close to that of control plants as leaf reserves of N became available for loading of phloem. Stem top phloem sap increased progressively in C:N ratio throughout Ar:O₂ treatment, presumably due to increasing shortage of xylem derived N for xylem to phloem exchange. Reexposure of Ar:O₂-treated nodulated root systems to air prompted a rapid recovery of N₂ fixation and restoration of plant N status. Rates of N₂ fixation in plants whose roots were exposed to a range of N₂ concentrations indicated an apparent K_m of 10% N₂ for the attached intact white lupin nodule.     

Some characteristics of photosynthetic inorganic carbon uptake of a marine macrophytic red alga

Abstract Some characteristics of photosynthetic inorganic carbon uptake by Palmaria palmata, a marine red macroalga, have been measured under physiological conditions in artificial seawater. The apparent affinity of thallus for CO₂ [K_1/2(CO₂)] at pH 8.0 and 15°C was 21.4±3.0mmol m^?3 CO₂ under air, and 25.7±70mmol m^?3 CO₂ under N₂. The corresponding values of V_max were 2.98 ± 0.42 and 3.65±0.87 mmol O₂ evolved g Chr^?1 s^?l. The apparent K_m(CO₂) of isolated ribulose bisphosphate carboxylase was determined at pH 8.0 and 30 °C to be 30.2 mmol m^?3 CO₂, and the corresponding value of V_max was 19.67 μniol CO₂ g protein^?1 s^?1. The CO₂ compensation points of the thallus were measured in artificial seawater at pH 8.0 under air and N₂, using a gas-chromatographic method. The values were relatively low, rising from 10 cm³ m^?3 at 15°C, to 35 cm³ m^?3 at 25°C, but were not affected by the O₂ concentration. The lack of an effect of O₂ on photosynthesis and on compensation point indicates that there is little photorespiratory CO₂ loss in this macroalga. The high affinity of the thallus for CO₂, and the low CO₂ compensation concentrations, are consistent with the occurrence of bicarbonate uptake in this alga.     

Post-harvest treatments for the control of Phytophthora syringae storage rot of apples

Oospores of Phytophthora syringae germinating at 10 and 15°C under artificial light formed one or more sporangia, which yielded zoospores. Furalaxyl and metalaxyl demonstrated eradicant action against infections occurring on zoospore-inoculated apples. Control was obtained at 10°C when treatment was delayed 5–11 days after inoculation. Storage in 5% CO₂+ 3% O₂ and 0% CO₂+ 2% O₂ reduced rotting compared with storage in air.     

Combined effects of elevated CO2 and air temperature on carbon assimilation of Pinus taeda trees

Branches of 22-year-old loblolly pine (Pinus taeda, L.) trees growing in a plantation were exposed to ambient CO₂, ambient + 165 μmol mol^?1 CO₂ or ambient + 330 μmol mol^?1 CO₂ concentrations in combination with ambient or ambient + 2°C air temperatures for 3 years. Field measurements in the third year indicated that net carbon assimilation was enhanced in the elevated CO₂ treatments in all seasons. On the basis of A/C_i, curves, there was no indication of photosynthetic down-regulation. Branch growth and leaf area also increased significantly in the elevated CO₂ treatments. The imposed 2°C increase in air temperature only had slight effects on net assimilation and growth. Compared with the ambient CO₂ treatment, rates of net assimilation were ～1·6 times greater in the ambient + 165 μmol mol^?1 CO₂ treatment and 2·2 times greater in the ambient + 330 μmol mol^?1 CO₂ treatment. These ratios did not change appreciably in measurements made in all four seasons even though mean ambient air temperatures during the measurement periods ranged from 12·6 to 28·2°C. This indicated that the effect of elevated CO₂ concentrations on net assimilation under field conditions was primarily additive. The results also indicated that the effect of elevated CO₂ (+ 165 or + 330 μmol mol^?1) was much greater than the effect of a 2°C increase in air temperature on net assimilation and growth in this species.     

Interaction between light and chilling temperature on the inhibition of photosynthesis in chilling-sensitive plants*

Abstract. Fully expanded leaves of 25°C grown Phaseolus vulgaris and six other species were exposed for 3 h to chilling temperatures at photon flux densities equivalent to full sunlight. In four of the species this treatment resulted in substantial inhibition of the subsequent quantum yield of CO₂ uptake, indicating reduction of the photochemical efficiency of photosynthesis. The extent of inhibition was dependent on the photon flux density during chilling and no inhibition occurred when chilling occurred at a low photon flux density. No inhibition occurred at temperatures above 11.5°C, even in the presence of the equivalent of full sunlight. This interaction between chilling and light to cause inhibition of photosynthesis was promoted by the presence of oxygen at normal air partial pressures and was unaffected by the CO₂ partial pressure present when chilling occurred in air. When chilling occurred at low O₂ partial pressures, CO₂ was effective in reducing the degree of inhibition. Apparently, when leaves of chilling-sensitive plants are exposed to chilling temperatures in air of normal composition then light is instrumental in inducing rapid damage to the photochemical efficiency of photosynthesis.     

Environmental induced variations in leaf dark respiration and net photosynthesis of <Emphasis Type="Italic">Quercus ilex</Emphasis> L.

The relationships between dark respiration rate (R _D) and net photosynthetic rate (P _N) in Quercus ilex L. shrubs growing at the Botanical Garden in Rome were analysed. Correlation analysis of the data sets collected in the year 2006 confirmed the dependence among the considered leaf traits, in particular, R _D was significantly (p<0.05) correlated with P _N (r = 0.40). R _D and P _N increased from March to May [1.40±0.10 and 10.1±1.8 μmol(CO₂) m⁻² s⁻¹ mean values of the period, respectively], when air temperature was in the range 14.8–25.2 °C, underlining the highest metabolic activity in the period of the maximum vegetative activity that favoured biomass accumulation. On the contrary, the highest R _D [1.60±0.02 μmol(CO₂) m⁻² s⁻¹], associated to the lowest P _N rates (44 % of the maximum) and carbon use efficiency (CUE) in July underlined the mobilization of stored material during drought stress by a higher air temperature (32.7 °C).     

Responses of five adult thrips species (Thysanoptera; Thripidae) to high-carbon dioxide atmospheres at different temperatures

The effect of high carbon dioxide atmospheres (60% CO₂) at different temperatures (20, 25, 30 and 34°C) on adult female mortality in Frankliniella occidentalis (Pergande), Frankliniella intonsa (Trybom), Thrips tabaci Lindeman, Thrips palmi Karny, and Thrips parvispinus Karny were tested. Mortality of the five thrips species increased with CO₂ exposure duration at each temperature. Median lethal times (LT₅₀) and times required to achieve 100% mortality due to CO₂ exposure decreased with increasing temperature, for all thrips species. Exposure to 60% CO₂ atmospheres at 30°C is considered to be 100% lethal within 24 h to most pests of fresh agricultural produce. Our findings suggest that CO₂ treatment could be used to propagate thrips-free plants in horticultural nurseries, and as a quarantine tool for controlling insects in/on transported plants.     

Climate drivers alter nitrogen availability in surface peat and decouple N2 fixation from CH4 oxidation in the Sphagnum moss microbiome

Peat mosses (Sphagnum spp.) are keystone species in boreal peatlands, where they dominate net primary productivity and facilitate the accumulation of carbon in thick peat deposits. Sphagnum mosses harbor a diverse assemblage of microbial partners, including N₂-fixing (diazotrophic) and CH₄-oxidizing (methanotrophic) taxa that support ecosystem function by regulating transformations of carbon and nitrogen. Here, we investigate the response of the Sphagnum phytobiome (plant + constituent microbiome + environment) to a gradient of experimental warming (+0°C to +9°C) and elevated CO₂ (+500 ppm) in an ombrotrophic peatland in northern Minnesota (USA). By tracking changes in carbon (CH₄, CO₂) and nitrogen (NH₄-N) cycling from the belowground environment up to Sphagnum and its associated microbiome, we identified a series of cascading impacts to the Sphagnum phytobiome triggered by warming and elevated CO₂. Under ambient CO₂, warming increased plant-available NH₄-N in surface peat, excess N accumulated in Sphagnum tissue, and N₂ fixation activity decreased. Elevated CO₂ offset the effects of warming, disrupting the accumulation of N in peat and Sphagnum tissue. Methane concentrations in porewater increased with warming irrespective of CO₂ treatment, resulting in a ~10× rise in methanotrophic activity within Sphagnum from the +9°C enclosures. Warming's divergent impacts on diazotrophy and methanotrophy caused these processes to become decoupled at warmer temperatures, as evidenced by declining rates of methane-induced N₂ fixation and significant losses of keystone microbial taxa. In addition to changes in the Sphagnum microbiome, we observed ~94% mortality of Sphagnum between the +0°C and +9°C treatments, possibly due to the interactive effects of warming on N-availability and competition from vascular plant species. Collectively, these results highlight the vulnerability of the Sphagnum phytobiome to rising temperatures and atmospheric CO₂ concentrations, with significant implications for carbon and nitrogen cycling in boreal peatlands.     

Measuring and modelling carbon dioxide and water vapour exchange over a temperate broad-leaved forest during the 1995 summer drought

Forests in the south-eastern United States experienced a prolonged dry spell and above-normal temperatures during the 1995 growing season. During this episode, nearly continuous, eddy covariance measurements of carbon dioxide and water vapour fluxes were acquired over a temperate, hardwood forest. These data are used to examine how environmental factors and accumulating soil moisture deficits affected the diurnal pattern and magnitude of canopy-scale carbon dioxide and water vapour fluxes. The field data are also used to test an integrative leaf-to-canopy scaling model (CANOAK), which uses micrometeorological and physiological theory, to calculate mass and energy fluxes. When soil moisture was ample in the spring, peak rates of net ecosystem CO₂ exchange (N_F) occurred around midday and exceeded 20 μmol m^?2 s^?1. Rates of N_K were near optimal when air temperature ranged between 22 and 25°C. The accumulation of soil moisture deficits and a co-occurrence of high temperatures caused peak rates of daytime carbon dioxide uptake to occur earlier in the morning. High air temperatures and soil moisture deficits were also correlated with a dramatic reduction in the magnitude of N_E. On average, the magnitude of N_E decreased from 20 to 7 μmol m^?2 s^?1 as air temperature increased from 24 to 30°C and the soil dried. The CANAOK model yielded accurate estimates of canopy-scale carbon dioxide and water vapour fluxes when the forest had an ample supply of soil moisture. During the drought and heat spell, a cumulative drought index was needed to adjust the proportionality constant of the stomatal conductance model to yield accurate estimates of canopy CO₂ exchange. The adoption of the drought index also enabled the CANOAK model to give improved estimates of evaporation until midday. On the other hand, the scheme failed to yield accurate estimates of evaporation during the afternoon.     

Regulation of the induction of bicarbonate uptake by dissolved CO2 in the marine diatom, Phaeodactylum tricornutum

Physiological properties of photosynthesis were determined in the marine diatom, Phaeodactylum tricornutum UTEX640, during acclimation from 5% CO₂ to air and related to H₂CO₃ dissociation kinetics and equilibria in artificial seawater. The concentration of dissolved inorganic carbon at half maximum rate of photosynthesis (K_0·5[DIC]) value in high CO₂‐grown cells was 1009 mmol m ^{? 3} but was reduced three‐fold by the addition of bovine carbonic anhydrase (CA), whereas in air‐grown cells K_0·5[DIC] was 71 mmol m ^{? 3}, irrespective of the presence of CA. The maximum rate of photosynthesis (P_max) values varied between 300 and 500 μ mol O₂ mg Chl ^{? 1} h ^{? 1} regardless of growth pCO₂. Bicarbonate dehydration kinetics in artificial seawater were re‐examined to evaluate the direct HCO₃ ^? uptake as a substrate for photosynthesis. The uncatalysed CO₂ formation rate in artificial seawater of 31·65°/_oo of salinity at pH 8·2 and 25 °C was found to be 0·6 mmol m ^{? 3} min ^{? 1} at 100 mmol m ^{? 3} DIC, which is 53·5 and 7·3 times slower than the rates of photosynthesis exhibited in air‐ and high CO₂‐grown cells, respectively. These data indicate that even high CO₂‐grown cells of P. tricornutum can take up both CO₂ and HCO₃ ^? as substrates for photosynthesis and HCO₃ ^? use improves dramatically when the cells are grown in air. Detailed time courses were obtained of changes in affinity for DIC during the acclimation of high CO₂‐grown cells to air. The development of high‐affinity photosynthesis started after a 2–5 h lag period, followed by a steady increase over the next 15 h. This acclimation time course is the slowest to be described so far. High CO₂‐grown cells were transferred to controlled DIC conditions, at which the concentrations of each DIC species could be defined, and were allowed to acclimate for more than 36 h. The K_0·5[DIC] values in acclimated cells appeared to be correlated only with [CO_2(aq)] in the medium but not to HCO₃ ^? , CO₃^{2 ?} , total [DIC] or the pH of the medium and indicate that the critical signal regulating the affinity of cells for DIC in the marine diatom, P. tricornutum, is [CO_2(aq)] in the medium.