Growth Chamber Studies Research Articles

Using an Anion Exchange Membrane to Predict Soil Available N and S Supplies and the Impact of N and S Fertilization on Canola and Wheat Growth

A growth chamber study was conducted to determine the relationships between the supply of soil available nitrogen (N) and sulfur (S) and canola ( Brassica napus) and wheat ( Triticum aestivum L. ‘Biggar’) N and S uptake and yield in three Western Canadian soils. The suitability of one-hour burial with an anion exchange membrane (AEM) was assessed for its utility as a quick test of the available N:S balance in the soil. Canola and wheat were grown on a Luvisolic soil low in available S and on Brown and Black Chernozemic soils low in both available N and S, with different rates and combinations of N and S fertilizers applied. AEM burial was used to assess soil available nitrate and sulfate supply rates after fertilization. Dry matter yield and N and S concentrations in plant tissues were determined after 6 weeks of growth. The soil available N:S ratio determined by AEM burial closely reflected the relative supplies of available N and S as revealed in the N:S ratios of plant tissue dry matter. The highest yields were achieved where the available N:S ratio in soil and plant tissue ranged from 5 to 13. Thus, a one-hour burial of an AEM probe in the field may be a useful tool to quickly test if a balanced N and S supply is present in the soil for optimum crop yield.

Resistance to leaf rust in cultivars of bread wheat and durum wheat grown in Spain

AbstractA set of bread wheat and durum wheat cultivars adapted to Spanish conditions was tested for resistance against leaf rust caused by different pathotypes of Puccinia triticina in field trials and in growth chamber studies. Lower levels of resistance were found in durum wheat than in bread wheat. The most frequent Lr genes found in bread wheat were Lr1, Lr10, Lr13, Lr20, Lr26 and Lr28. In durum wheat, additional resistance genes that differed from the known Lr genes were identified. The level of partial resistance to leaf rust was in general low, although significant levels were identified in some bread wheat and durum wheat cultivars.

Effects of day-night temperature combinations under constant day length on emergence and early growth of sericea lespedeza genotypes

Establishment of sericea lespedeza [Lespedeza cuneata (Dumont de Courset) G. Don.] in southeastern USA is difficult. Seedling emergence may be related to the range of temperatures prevalent during establishment. A growth chamber study was undertaken to measure the effect of temperature on seedling emergence of 56 sericea lespedeza genotypes. Main treatments were: (1) plants grown at three day-night temperature combinations with maximum/minimum temperature difference of 14°C. The temperature combinations were 22/8°C, 27/13°C, and 32/18°C; (2) plants grown at three day-night temperature combinations with maximum/minimum temperature difference of 7°C. This was accomplished by lowering the day temperature and keeping the night temperature the same as above. Emergence was reduced by about 27% with reduction of 7°C in day-night temperature within the range of temperatures used. Plant height, leaf dry weight, stem dry weight and number of branches were very sensitive to temperature combinations. Increases in temperature caused increases in height, leaf dry weight, stem dry weight and number of branches of all genotypes. Further screening of sericea for emergence and growth under low temperature may lead to cultivars with more vigorous seedlings that can be better established early in the season. Key words: Plant growth, temperatures, seedling emergence, Sericea, southern USA

Field Evaluation of Transgenic Wheat Expressing a Modified ADP‐Glucose Pyrophosphorylase Large Subunit

ABSTRACTWheat yield is influenced by the efficacy of seed starch biosynthetic enzymes. ADP‐glucose pyrophosphorylase (AGP) catalyzes a rate‐limiting step in seed starch biosynthesis. We transformed the hard spring wheat (Triticum aestivum L.) cultivar Hi‐Line with a modified maize AGP large subunit sequence (Sh2r6hs) to increase AGP activity. In previously described growth chamber studies, Sh2r6hs conditioned increased AGP activity, seed yield, and plant size. The primary objective of this study was to determine whether a similar yield enhancement could be detected under field conditions. Sh2r6hs transgenics were field tested over four growing years, in three locations, with varying planting density and irrigation. The results indicate that significant yield increases were more likely to occur in space‐planted, irrigated environments than densely planted, rainfed environments, suggesting that limited abiotic resources may subsequently limit Sh2r6hs‐associated yield enhancement. In elite lines, as in the F2–derived trials in which tissue culture derived mutations were reduced by out‐crossing, Sh2r6hs appears to confer a yield advantage only when field conditions are nonlimiting.

Management effects on the incidence of jointed goatgrass inhibitory rhizobacteria

Jointed goatgrass (Aegilops cylindrica Host., JGG), a winter annual grass weed, is a major threat to small grain production in the United States. It infests an estimated 2 million hectares in the US and causes annual losses of $145 million in crop yield and quality. Selective herbicides are available only for certain herbicide-resistant crop varieties and alternative management practices are tillage, burning, or rotation to less profitable crops. We evaluated bacteria that were isolated from winter wheat (Triticum aestivum L., WW), downy brome (Bromus tectorum L., DB), and JGG roots to determine their potential as biological control agents for JGG. The bacteria were sequentially screened in laboratory bioassay, growth chamber and field studies for their effect on JGG and WW root growth. In laboratory studies, 1236 isolates (50%) of the 2450 naturally occurring soil bacteria tested were inhibitory to JGG seedling growth. Only 76 isolates (3.1%) inhibited JGG, but not WW in the in vitro bioassays. In the growth chamber, only 7 (0.3%) of the isolates inhibited JGG growth and not WW. In field studies, 4 of the 7 isolates selected after bioassay and growth chamber tests inhibited JGG more than 30%. Conventional tillage systems maintained higher populations of inhibitory bacteria than no-tillage systems. The number of bacteria inhibitory to JGG was greater in the spring sampling and at the highest level of nitrogen fertilization. Crop rotation had little effect on the incidence of jointed goatgrass inhibitory bacteria. Different JGG accessions from the western US were diverse in their response to inhibitory bacteria. Weed-suppressive bacteria isolated from plant roots and soil have the potential to be used as biological control agents for JGG.

Copper fertilizer practices in Manitoba

Soil testing criteria for copper (Cu) in Manitoba were established in the mid-1980s and were primarily based on growth chamber studies for a range of crop species. A multitude of Cu products and fertilizer placement methods are practiced with insufficient research in support of them. Hence, we attempted to develop agronomic and economic Cu fertilizer management practices for soils of Manitoba through a series of experiments for crop species at two locations with DTPA-extractable Cu levels of 0.12 and 0.25 mg kg-1 soil, respectively. These experiments involved broadcast and incorporation of CuSO4·5H2O (25% Cu), CuEDTA (7% Cu), and a low (<1%) and a high (>60%) water solubility Cu oxysulphate (12.5 and 12% Cu, respectively). Four rates of side-banded liquid CuEDTA (0, 0.28, 0.56 or 1.12 kg Cu ha-1) or seedrow-placed granular products (0, 1.12, 2.24 and 4.48 kg ha-1) as above, were either superimposed or were compared directly with broadcast and incorporation of the same rates. This study confirmed that DTPA extractable Cu levels of less 0.2 mg kg-1 soil are deficient, whereas DTPA-extractable Cu levels of greater than 0.2, but less than 0.4 mg kg-1 are marginal. Broadcast and incorporation of CuSO4·5H2O at rates as low as 2 kg Cu ha-1 or side banding of 0.28 to 0.56 kg Cu ha-1 of liquid CuEDTA provide maximum economic grain yield increases in soils with either deficient or marginal Cu levels. Seedrow applied granular Cu products may provide a maximum agronomic and economic yield increase only in soils with marginal soil Cu levels. Low solubility Cu products do not correct Cu deficiency, whereas CuEDTA, although agronomically equal to CuSO4·5H2O and high solubility oxysulphates, may have a disadvantage due to its high cost. Key words: Deficient, marginal, sulphate, oxysulphate, chelate, seedrow, side band, broadcast

Efficacy of essential oils and biopesticides on Phytophthora infestans suppression in laboratory and growth chamber studies

Control of late blight by alternative products is important for reduction of fungicide inputs and potato production costs. The efficacy of essential oils and biopesticides for inhibition of growth of Phytophthora infestans on media and suppression of late blight on potato plants in growth chambers was examined. Growth of pathogen isolates of diverse genotypes was evaluated on Rye B media amended with essential oils (lavender, thyme, thyme borneal, and oregano) and the biopesticide Serenade (Bacillus subtilis strain QST 713). Over 90% inhibition of pathogen growth was achieved with oregano and Serenade amendments. The protective foliar application of Serenade, an aerated compost tea (ACT), Effective Microorganism mix (EM), and oregano, resulted in disease suppression of 5–40% relative to the untreated control. ACT had no significant suppressive effects (0–15% reduction), EM resulted in mild suppression (15–30% reduction), and oregano and Serenade consistently resulted in moderate disease reduction (20–40%). No oil or biological treatment produced disease control comparable to the chemical control chlorothalonil, which resulted in disease reductions of 80–98%. Both oregano and Serenade resulted in some phytotoxicity at high doses. These results suggest that the natural products and biological amendments tested are not sufficient for effective late blight control by themselves; however, when used in combination with other established disease control practices, these approaches may contribute to improved, integrated, and more sustainable management options for late blight.

Identification, Distribution, and Control of Italian Ryegrass (Lolium multiflorum) Ecotypes with Varying Levels of Sensitivity to Triasulfuron in Texas

Texas wheat producers have observed reduced efficacy and failure to control Italian ryegrass with registered rates of sulfonylurea herbicides that were previously effective. Growth chamber studies were conducted to quantify the sensitivity and distribution of Italian ryegrass ecotypes in Texas to triasulfuron and to determine alternative herbicide management options. Italian ryegrass seed samples were collected from over 40 wheat fields in 13 central and north Texas counties where declining Italian ryegrass control was reported by farmers following sulfonylurea herbicide application(s). Two-leaf Italian ryegrass was screened with an application of 150 g ai/ha triasulfuron, a rate five times the registered herbicide use rate. Sensitivity was determined by the response of an ecotype to that of a known susceptible population. Of the 48 Italian ryegrass ecotypes sampled, nine were comparable to susceptible standard, while the remaining 39 ecotypes were less sensitive to triasulfuron. Four of the least sensitive ecotypes to triasulfuron plus the susceptible standard were selected for a subsequent study. Diclofop, clodinafop, and metribuzin reduced fresh weights by at least 69, 71, and 62% across all ecotypes. No imazamox or triasulfuron treatment reduced fresh weights more than 60%.

Wetland Nightshade (Solanum tampicense) Growth Response to Temperature, and Winter Survival, in Relation to Potential Spread

Greenhouse, growth chamber, and winter survival studies were conducted at Stoneville, MS from 1996 to 2002 to determine growth, time to first flower, and winter survival of wetland nightshade. At 12 wk after emergence, wetland nightshade plants had 58-, 45-, 48-, and 4-cm heights, respectively; 24, 21, 21, and 12 nodes/plant, respectively; 62, 31, 36, and 21 leaves/plant, respectively; and 7.1, 3.9, 5.1, and 0.3 g/plant dry weights, respectively, at temperatures of 26/36, 20/30, 14/24, and 8/18 (±0.5) C at the 14/10 day/night length. Flowering occurred at 79, 85, and 85 days after emergence at 26/36, 20/30, and 14/24 C night/day, respectively at the 14/10 day/night cycle. Wetland nightshade plants did not flower at 8/18 C. Wetland nightshade growth was adequate for flowering and fruit production in additional areas of the southeastern United States with night/day temperatures greater than or equal to 14/24 C. Winter survival was greater than or equal to 33% for established wetland nightshade plants in 5 of 6(1996 to 2002) above water levels and 82% from 20 cm below the water surface. Based on these results, wetland nightshade has the potential to continue to spread in the United States.

Influence of lead acetate on soil microbial biomass and community structure in two different soils with the growth of Chinese cabbage ( Brassica chinensis)

A greenhouse pot experiment was conducted to evaluate the impact of different concentrations of lead acetate on soil microbial biomass and community structure during growth of Chinese cabbage ( Brassica chinensis) in two different soils. The field soils were used for a small pot, short-term 60-day growth chamber study. The soils were amended with different Pb concentrations, ranging from 0 to 900 mg kg −1 soil. The experimental design was a 2 soil × 2 vegetation/non-vegetation × 6 treatments (Pb) × 3 replicate factorial experiment. At 60 days the study was terminated and soils were analyzed for microbial parameters, namely, microbial biomass, basal respiration and PLFAs. The results indicated that the application of Pb at lower concentrations (100 and 300 mg kg −1) as lead acetate resulted in a slight increase in soil microbial biomass, whereas Pb concentrations >500 mg kg −1 caused an immediate gradual significant decline in biomass. However, the degree of impact on soil microbial biomass and basal respiration by Pb was related to management (plant vegetation) or the contents of clay and organic matter in soils. The profiles of 21 phospholipid fatty acids (PLFAs) were used to assess whether observed changes in functional microbial parameters were accompanied by changes in the composition of the microbial communities after Pb application at 0, 300 and 900 mg Pb kg −1 soil. The results of principal component analyses (PCA) indicated that there were significant increases in fungi biomarkers of 18:3 ω6c, 18:1 ω9c and a decrease in cy17:0, which is an indicator of gram-negative bacteria for the high levels of Pb treatments In a word, soil microbial biomass and community structure, therefore, may be sensitive indicators reflecting environmental stress in soil–Pb–plant system. However, further studies will be needed to better understand how these changes in microbial community structure might actually impact soil microbial community function.

Degradation of Polycyclic Aromatic Hydrocarbons in the Rhizosphere of Festuca arundinacea and Associated Microbial Community Changes

ABSTRACT A phytoremediation growth chamber study was conducted to evaluate the contribution of soil microbial diversity to the contaminant degradation. Target contaminant removal from soil was assessed by monitoring concentrations of polycyclic aromatic hydrocarbons (PAHs), along with changes in the bacterial community structure over a time period of 10 months in the presence of tall fescue (Festuca arundinacea). Enhanced degradation of PAHs was observed in rhizosphere soil, with a maximum reduction in pyrene at a rate 36% higher than that noted for the unvegetated control. The dissipation of < 4-ring PAHs, 4-ring PAHs, and > 4-ring PAHs in unvegetated soil was 70%, 54%, and 49% respectively, whereas a higher dissipation rate was observed in tall fescue treated soil of 78%, 68%, and 61% at the end of the study. Microbial enumeration results showed greater total bacterial numbers and PAH-degrading bacteria in rhizosphere soil when compared to unvegetated soil. The results from the terminal restriction fragment length polymorphism (T-RFLP) analysis indicated that there was a shift in the rhizosphere bacterial community during the phytoremediation process.

Influence of Temperature, Relative Humidity, Ascospore Concentration, and Length of Drying of Colonized Dry Bean Flowers on White Mold Development.

Growth chamber studies were conducted using rehydrated dry bean (Phaseolus vulgaris) flowers (RDBF) to assess the influence of temperature (18 and 22°C), relative humidity (RH; 25 and 90%), and ascospore concentrations (102, 103, and 104 ascospores/ml) on white mold incidence in dry bean. Additional studies were carried out to determine the influence of inoculum type (ascospore and mycelium) and to estimate the effect of duration of drying of colonized RDBF on viability of Sclerotinia sclerotiorum and white mold incidence. There was a linear increase in white mold incidence with increase in ascospore concentration but neither temperature nor RH levels significantly affected disease development. In the inoculum type study, both temperature and RH levels significantly affected white mold incidence; however, neither ascospore nor mycelial inocula had a significant effect on white mold incidence. Drying colonized RDBF for up to 96 h did not affect S. sclerotiorum viability; but the amount of white mold incidence depended more on post-inoculation RH and drying duration than on the temperatures tested. Colonized RDBF dried for 96 h took approximately three times longer to achieve 100% white mold incidence compared with colonized RDBF dried for 24 h. These results suggest the potential for greater white mold development with higher ascospore availability and the potential of dry S. sclerotiorum-colonized dry bean flowers as a viable inoculum source.

Characterizing potential bacterial biocontrol agents for suppression of Rhizobium vitis, causal agent of crown gall disease in grapevines

Several bacteria were evaluated in greenhouse studies for their effectiveness at preventing formation of galls induced by Rhizobium vitis, the Gram negative bacterium that causes crown gall disease of grapevines. Three endophytic bacteria isolated from Malus domestica yielded promising results and were selected for further examination: Pseudomonas fluorescens isolate 1100-6, Bacillus subtilis isolate EN63-1, and Bacillus species isolate EN71-1. In planta experiments were initially performed on Nicotiana glauca to facilitate measurement of gall development. The effect of the interval between inoculation of N. glauca with the biocontrol agent and exposure to R. vitis was evaluated. All three bacteria significantly reduced gall size when applied 25 or 86 days before inoculation with R. vitis, and reduced the population of R. vitis relative to control plants . P. fluorescens ‘1100-6’ also reduced the percentage of R. vitis inoculations that yielded galls. P. fluorescens ‘1100-6’ was genetically tagged to allow exploration of its localization in grapevines. In growth chamber studies, P. fluorescens ‘1100-6’ was found to survive in the rhizoplane of grapevines for 6 months. Microscopic examination showed that the genetically modified P. fluorescens ‘1100-6’ predominantly occupied xylem and pith tissues, and occasionally persisted on external surfaces of grapevines.

Bacterial Leaf Spot of Lettuce: Relationship of Temperature to Infection and Potential Host Range of Xanthomonas campestris pv. vitians

Epidemiological aspects, including optimum temperature for infection and host range of Xanthomonas campestris pv. vitians, causal organism of bacterial leaf spot (BLS) of lettuce, were investigated. The optimum temperature for infection was determined to be 22.7°C based on growth chamber studies. Internal populations were monitored over time in lettuce, tomato, pepper, parsley, cilantro, and beet. Each plant species was infiltrated with the bacterium at 105CFU/ml. Highest populations developed in lettuce (108CFU/cm2) followed by pepper with 106CFU/cm2, whereas the other plant species harbored much lower populations (105 to 103CFU/cm2). Infectivity titration endpoints were similar in pepper and lettuce (103 to 104CFU/ml). For other plant species tested, infectivity titration endpoints were 106 to 107 CFU/ml. Electrolyte leakage data and corresponding internal population data support the conclusion that fresh-market tomato is not a host of X. campestris pv. vitians but, instead, interacts in an incompatible response. Electrolyte leakage from cells of tomato plants inoculated with X. campestris pv. vitians or a pepper strain of X. axonopodis pv. vesicatoria peaked at 48 h, suggesting that tomato is not a host for the BLS pathogen. Both electrolyte leakage and population dynamics results point to pepper as a potential host of X. campestris pv. vitians.

Responses of germination and radicle growth of two Populus species to water potential and salinity

The effects of water potential, NaCl and Na2SO4 on germination and radicle growth of two riparian tree species, Populus euphratica Oliv. and P. pruinosa Schrenk (Salicaceae), were tested. Growth chamber studies revealed an optimum temperature range for seed germination of both species between 15–35°C. The final germination percentage of both species decreases with decreasing water potential in all types of solution applied in the experiments. P. pruinosa was less tolerant to low ψW stress than P. euphratica, especially in salt solutions. Germination percentages fell below 20% for P. pruinosa at −0.6 MPa (NaCl) or −0.4 MPa (Na2SO4) and for P. euphratica at −1.2 MPa (NaCl) or −0.6 MPa (Na2SO4). Radicle growth of both species was inhibited by high concentrations of PEG, NaCl and Na2SO4. However, growth was enhanced at −0.13 and −0.29 MPa in PEG or at −0.13 MPa in NaCl solutions compared to distilled water. Radicle growth of P. euphratica was higher than that of P. pruinosa. Germination and radicle growth of both species exhibited ion toxicity. Na2SO4 was more toxic than iso-osmotic solutions of NaCl. Radicle growth proved to be more sensitive than seed germination. Thus, flooding does not only yield the necessary soil moisture for germination but also favors seedling establishment of both species through leaching of salts from the soil surface. The different sensitivity of the species during their early growth stages might, moreover, contribute to the observed differences in their distribution in the Talim Basin (northwest China).

Use of Ion-Exchange Membrane to Assess Nitrogen-Supply Power of Soils

ABSTRACT Soil nitrogen-supply power (NSP), including nitrogen (N) mineralized from organic matter, for crop growth is an important criterion in evaluating soil quality. Ion-exchange membranes in the form of plant root simulator (PRS) probes were used to measure ammonium and nitrate release rates as a measure of NSP in 54 Saskatchewan soils with contrasting pedogenic and management histories. Two incubation systems (aerobic and anaerobic), with ion-exchange membrane probes placed in situ, were conducted to predict NSP. The 54 soil samples were also used in two growth-chamber studies to assess patterns in plant N uptake by canola. Soil type had a profound influence on available N-supply rates, with soils of higher organic-matter content due to climatic conditions, slope position, and past management having higher NSP values. Good relationships (R2 = 0.54 and 0.69) between NSP and canola N uptake were observed. The N-supply rate values used as an index of NSP as predicted by two-week aerobic incubation for Brown, Dark Brown, and Black soils in Saskatchewan were 200–550, 550–1100, and > 1100 μg nitrate-N/10cm2/2 wks, respectively, with different management histories producing significant variation within a region.

Molecular evidence of genetic modification of Sinorhizobium meliloti: enhanced PCB bioremediation

The genome of the nitrogen-fixing soil bacterium Sinorhizobium meliloti does not possess genes for bioremediation of aromatic pollutants. It has the well-known ability to interact specifically with the leguminous alfalfa plant, Medicago sativa. Our previous work has shown enhanced degradation of the nitroaromatic compound 2,4-dinitrotoluene (DNT) when a plasmid containing degradative genes was introduced in it. In this study we report molecular evidence of the transfer of a polychlorinated biphenyl (PCB)-biodegradative plasmid pE43 to S. meliloti strain USDA 1936. Several standard analytical tests and plant growth chamber studies were conducted to test the ability of S. meliloti to degrade 2',3,4-PCB congener. Alfalfa plant alone was able to degrade 30% of PCBs compared with control. No enhanced dechlorination was noted when alfalfa plant was grown with wild-type S. meliloti, and when alfalfa plant was grown with the S. meliloti electrotransformants (genetically modified) dechlorination of PCBs was more than twice that when alfalfa plant was grown with wild-type S. meliloti. When alfalfa plant was grown with uncharacterized mixed culture (containing nodule formers), almost equally significant PCB degradation was observed. The significance of this work is that the naturally occurring nitrogen-fixing soil bacterium S. meliloti (genetically modified) has the ability to enhance fertility of soil in association with the leguminous alfalfa plant while simultaneously enhancing bioremediation of PCB-contaminated soils. Enhanced bioremediation of PCB and robust alfalfa plant growth was also noted when uncharacterized mixed cultures containing alfalfa plant nodule formers were used.

Pathogenicity and real-time PCR detection of Fusarium spp. in wheat and barley roots

The pathogenicity of five Fusarium spp. frequently isolated from wheat and barley roots in southern Idaho was investigated during four growth-chamber experiments and two field studies. A real-time PCR assay for quantifying the presence of F. culmorum from infected root tissue was also developed based on nucleotide sequence for the tri5 gene. Fusarium culmorum, followed by F. acuminatum and F. reticulatum, resulted in the largest root lesions and percent infected root area. However, F. semitectum, followed by F. acuminatum and F. equiseti, had the greatest impact on total root length. The TaqMan-based real-time assay was able to quantify F. culmorum in root tissue from both growth-chamber and field studies down to 61 pg. The assay also detected F. pseudograminearum and F. graminearum but could not distinguish among these three Fusarium spp.

Elevated CO2 reduces disease incidence and severity of a red maple fungal pathogen via changes in host physiology and leaf chemistry

AbstractAtmospheric CO2 concentrations are predicted to double within the next century. Despite this trend, the extent and mechanisms through which elevated CO2 affects plant diseases remain uncertain. In this study, we assessed how elevated CO2 affects a foliar fungal pathogen, Phyllosticta minima, of Acer rubrum growing in the understory at the Duke Forest free‐air CO2 enrichment experiment in Durham, North Carolina. Surveys of A. rubrum saplings in the 6th, 7th, and 8th years of the CO2 exposure revealed that elevated CO2 significantly reduced disease incidence, with 22%, 27%, and 8% fewer saplings and 14%, 4%, and 5% fewer leaves infected per plant in the three consecutive years, respectively. Elevated CO2 also significantly reduced disease severity in infected plants in all years (e.g. mean lesion area reduced 35%, 50%, and 10% in 2002, 2003, and 2004, respectively). To assess the mechanisms underlying these changes, we combined leaf structural, physiological and chemical analyses with growth chamber studies of P. minima growth and host infection. In vitro exponential growth rates of P. minima were enhanced by 17% under elevated CO2, discounting the possibility that disease reductions were because of direct negative effects of elevated CO2 on fungal performance. Scanning electron micrographs (SEM) verified that conidia germ tubes of P. minima infect A. rubrum leaves by entering through the stomata. While stomatal size and density were unchanged, stomatal conductance was reduced by 21–36% under elevated CO2, providing smaller openings for infecting germ tubes. Reduced disease severity under elevated CO2 was likely due to altered leaf chemistry and reduced nutritive quality; elevated CO2 reduced leaf N by 20% and increased the C : N ratio by 20%, total phenolics by 15%, and tannins by 14% (P&lt;0.05 for each factor). The potential dual mechanism we describe here of reduced stomatal opening and altered leaf chemistry that results in reduced disease incidence and severity under elevated CO2 may be prevalent in many plant pathosystems where the pathogen targets the stomata.

Selenium in Australia: Selenium status and biofortification of wheat for better health

Selenium (Se) is an essential micronutrient for humans and animals, but is deficient in at least a billion people worldwide. Wheat ( Triticum aestivum L.) is a major dietary source of Se. The largest survey to date of Se status of Australians found a mean plasma Se concentration of 103 μg/l in 288 Adelaide residents, just above the nutritional adequacy level. In the total sample analysed (six surveys from 1977 to 2002; n=834), plasma Se was higher in males and increased with age. This study showed that many South Australians consume inadequate Se to maximise selenoenzyme expression and cancer protection, and indicated that levels had declined around 20% from the 1970s. No significant genotypic variability for grain Se concentration was observed in modern wheat cultivars, but the diploid wheat Aegilops tauschii L. and rye ( Secale cereale L.) were higher. Grain Se concentrations ranged 5–720 μg/kg and it was apparent that this variation was determined mostly by available soil Se level. Field trials, along with glasshouse and growth chamber studies, were used to investigate agronomic biofortification of wheat. Se applied as sodium selenate at rates of 4–120 g Se/ha increased grain Se concentration progressively up to 133-fold when sprayed on soil at seeding and up to 20-fold when applied as a foliar spray after flowering. A threshold of toxicity of around 325 mg Se/kg in leaves of young wheat plants was observed, a level that would not normally be reached with Se fertilisation. On the other hand sulphur (S) applied at the low rate of 30 kg/ha at seeding reduced grain Se concentration by 16%. Agronomic biofortification could be used by food companies as a cost-effective method to produce high-Se wheat products that contain most Se in the desirable selenomethionine form. Further studies are needed to assess the functionality of high-Se wheat, for example short-term clinical trials that measure changes in genome stability, lipid peroxidation and immunocompetence. Increasing the Se content of wheat is a food systems strategy that could increase the Se intake of whole populations.