Triazine Resistance in Senecio vulgaris Parental and Nearly Isonuclear Backcrossed Biotypes Is Correlated with Reduced Productivity

Isonuclear triazine-susceptible and triazine-resistant Senecio vulgaris L. biotypes were developed by making reciprocal crosses between susceptible and resistant biotypes to obtain F₁ hybrids and backcrossing the hybrids to the appropriate pollen parent. The electrophoretic isozyme patterns of the enzyme aconitase obtained from leaf extracts of triazine-susceptible parental (S) and backcrossed (S×R_BC6) biotypes, and triazine-resistant parental (R) and backcrossed (R×S_BC6) biotypes verified that the biotypes had the expected nuclear genomes. Atrazine inhibition of chloroplast whole chain electron transport from water to methyl viologen was measured to verify susceptibility or resistance to triazine herbicides. The photosynthetic rate and biomass accumulation of greenhouse grown susceptible and resistant S. vulgaris biotypes were measured 28, 35, 42, 50, 57, and 64 days after planting to determine the effect of altered chloroplast function. S and S×R_BC6 biotypes had CO₂ assimilation rates of 16.2 and 16.6 micromoles CO₂ per square meter per second, respectively, and I₅₀ values (herbicide concentration producing 50% inhibition) of about 0.49 micromolar atrazine. The corresponding values for the R and R×S_BC6 biotypes were 14.7 and 14.6 micromoles CO₂ per square meter per second with I₅₀ values of 65.0 micromolar atrazine. The S biotype was larger and more productive than the R biotype at all harvests. At the harvest 57 days after planting, mean shoot dry weight was 33.2 and 8.7 grams for the S and R biotypes, respectively. The growth effect associated with chloroplast differences was shown in comparisons of the S biotype with the R×S_BC6 biotype and of the S×R_BC6 biotype with the R biotype. The R×S_BC6 biotype had 72% of the shoot dry weight of the S biotype while the R biotype had 55% of the shoot dry weight of the S×R_BC6 biotype. The R×S_BC6 and R biotypes produced about 73 and 62% of the leaf area of the S and S×R_BC6 biotypes, respectively. Relative growth rate was similar in biotypes with the same nuclear genome; however, instantaneous unit leaf rate was higher in the S compared to the R×S_BC6 biotype and in the S×R_BC6 compared to the R biotype. At 57 days after planting, the cumulative leaf area duration (i.e. photosynthetic opportunity) of the R×S_BC6 and R biotypes was 86 and 66% of that of the S and S×R_BC6 biotypes, respectively. Our data indicate that impaired chloroplast function in triazine resistant S. vulgaris biotypes limits growth and productivity at the whole plant level.     

The effects of increased atmospheric carbon dioxide and temperature on carbon partitioning, source-sink relations and respiration

Abstract. Herbaceous C₃ plants grown in elevated CO₂ show increases in carbon assimilation and carbohydrate accumulation (particularly starch) within source leaves. Although changes in the partitioning of biomass between root and shoot occur, the proportion of this extra assimilate made available for sink growth is not known. Root:shoot ratios tend to increase for CO₂-enriched herbaceous plants and decrease for CO₂-enriched trees. Root:shoot ratios for cereals tend to remain constant. In contrast, elevated temperatures decrease carbohydrate accumulation within source and sink regions of a plant and decrease root:shoot ratios. Allometric analysis of at least two species showing changes in root: shoot ratios due to elevated CO₂ show no alteration in the whole-plant partitioning of biomass. Little information is available for interactions between temperature and CO₂. Cold-adapted plants show little response to elevated levels of CO₂, with some species showing a decline in biomass accumulation. In general though, increasing temperature will increase sucrose synthesis, transport and utilization for CO₂-enriched plants and decrease carbohydrate accumulation within the leaf. Literature reports are discussed in relation to the hypothesis that sucrose is a major factor in the control of plant carbon partitioning. A model is presented in support.     

Biomass, reproductive output, and physiological responses of rapid-cycling Brassica (Brassica rapa) to ozone and modified root temperature

Brassica rapa L. (rapid-cycling Brassica), was grown in environmentally controlled chambers to determine the interactive effects of ozone (O₃) and increased root temperature (RT) on biomass, reproductive output, and photosynthesis. Plants were grown with or without an average treatment of 63 ppb O₃. RT treatments were 13°C (LRT) and 18°C (HRT). Air temperatures were 25°C/15°C day/night for all RT treatments.
Ozone affected plant biomass more than did root temperature. Plants in O₃ had significantly smaller total plant d. wt, shoot weight, leaf weight, leaf area and leaf number than plants grown without O₃. LRT plants tended to have slightly smaller total plant d. wt, shoot weight, root weight, leaf weight, leaf area, and leaf number than HRT plants. For all variables, LRT plants grown in O₃ had the smallest biomass, and plants grown in HRT without O₃ had the largest biomass.
Ozone reduced both fruit weight and fruit number; LRT also reduced fruit weight but had no effect on fruit number. Ozone reduced photosynthesis but RT had no effect. Conductance and internal CO₂ were unaffected by O₃ or RT.
These studies indicate that plant growth with LRT might be more reduced in the presence of O₃ than growth in plants with HRT, which might be able to compensate for O₃-caused reductions in photosynthesis to avoid decreased biomass and reproductive output.     

Atrazine resistance entails a limited xanthophyll cycle activity,a lower PSII efficiency and an altered pattern of excess excitation dissipation

Atrazine-resistant (AR) weeds have a modified D1 protein structure, with a Ser₂₆₄→Gly mutation on the D1 protein, near the plastoquinone binding niche. The photosynthetic performance, the light response of the xanthophyll cycle and chlorophyll fluorescence quenching-related parameters were compared in attached leaves of susceptible (S) and AR biotypes of the C₃ dicot Chenopodium album L., Epilobium adenocaulon Hausskn., Erigeron canadensis L., Senecio vulgaris L. and Solanum nigrum L. and the C₄ dicot Amaranthus retroflexus L. grown under natural high-light conditions. No significant difference in CO₂ assimilation rate per leaf area unit was found between the S and AR biotypes of the investigated C₃ plants, whereas the AR biotype of A. retroflexus exhibited a relatively poor photosynthetic performance. The D1 protein mutant plants expressed a reduced activity of light-stimulated zeaxanthin formation. Neither the lower violaxanthin de-epoxidase activity nor the depletion of ascorbate seems to be the cause of the lower in vivo zeaxanthin formation in the AR plants. All the D1 mutant weeds had limited light-induced non-photochemical (NPQ) and photochemical (q_P) quenching capacities, and displayed a higher photosensitivity, as characterized by the ratio (1-q_P)/NPQ and a higher susceptibility to photoinhibition. Analysis of the chlorophyll fluorescence parameters showed that a lower proportion of excitation energy was allocated to PSII photochemistry, while a higher excess of excitation remained in the AR weeds relative to the S plants.     

Movement of paraquat in resistant and susceptible biotypes of Erigeron philadelphicus and E. canadensis

Paraquat (1,1'-dimethyl-4,4'-bipyridinium) resistant biotypes of Erigeron philadelphicus and E. canadensis , from fields where paraquat had been used for weed control, showed more than 100 times higher resistance than the susceptible biotype of both plants. Excised leaves of the susceptible biotypes wilted when supplied with more than 5 μ M paraquatat at the cut ends, but those from the resistant biotypes did not wilt even at 500 μ M. Autoradiographs indicated that (¹⁴CH₃)-paraquat taken up through the cut ends was rapidly distributed through the vascular system in leaves of the susceptible biotype, but was barely translocated in leaves of the resistant biotype. The amount of paraquat taken up during 48 h in the resistant biotype was 0.5% of that in the susceptible biotype in light. This difference in paraquat movement may be correlated with paraquat resistance in Erigeron.     

Disturbance, rainfall and contrasting species responses mediated aboveground biomass response to 11 years of CO2 enrichment in a Florida scrub-oak ecosystem

This study reports the aboveground biomass response of a fire-regenerated Florida scrub-oak ecosystem exposed to elevated CO₂ (1996–2007), from emergence after fire through canopy closure. Eleven years exposure to elevated CO₂ caused a 67% increase in aboveground shoot biomass. Growth stimulation was sustained throughout the experiment; although there was significant variability between years. The absolute stimulation of aboveground biomass generally declined over time, reflecting increasing environmental limitations to long-term growth response. Extensive defoliation caused by hurricanes in September 2004 was followed by a strong increase in shoot density in 2005 that may have resulted from reopening the canopy and relocating nitrogen from leaves to the nutrient-poor soil. Biomass response to elevated CO₂ was driven primarily by stimulation of growth of the dominant species, Quercus myrtifolia , while Quercus geminata , the other co-dominant oak, displayed no significant CO₂ response. Aboveground growth also displayed interannual variation, which was correlated with total annual rainfall. The rainfall × CO₂ interaction was partially masked at the community level by species-specific responses: elevated CO₂ had an ameliorating effect on Q. myrtifolia growth under water stress. The results of this long-term study not only show that atmospheric CO₂ concentration had a consistent stimulating effect on aboveground biomass production, but also showed that available water is the primary driver of interannual variation in shoot growth and that the long-term response to elevated CO₂ may have been caused by other factors such as nutrient limitation and disturbance.     

Maximal biomass production can occur in corn (Zea mays) in the absence of NO3 accumulation in either leaves or roots

In order to investigate effects of limited NO₃ availability in corn ( Zea mays L. cv. Brulouis) 17-day-old plants were grown for a further 25 days on sand in a growth chamber. The plants received frequent irrigation with a complete nutrient solution containing 0.2, 0.6, 1.5 or 3.0 mM NO₃. With 0.2 mM NO; nitrate levels in both roots and leaves diminished rapidly and were almost zero after 10 days treatment. Concurrently, as signs of nitrogen deficiency appeared, shoot growth was restricted, whereas root growth was enhanced. In addition, the concentration of reduced nitrogen and malate in the leaves declined, and in vitro nitrate reductase activity (NRA. EC 1.6.6.1), soluble protein and chlorophyll levels of leaf tissue were depressed and starch concentration was enhanced. With 0.6 mM NO₃ in the nutrient solution, the decrease in NO₃ levels in the tissues and the increase in root development were similar to those observed with 0.2 mM NO₃. However, shoot growth, reduced nitrogen concentration in leaves, and the above-mentioned biochemical characteristics were almost identical to those obtained at 1.5 and 3.0 mM NO₃. This indicates that when supplied with 0.6 mM NO₃, corn plants were able to absorb sufficient NO3 to support maximal biomass production without appreciable NO₃ accumulation in roots or shoot. It is, thus, suggested that the plants responded to low NO₃, availability in medium by enhancing root growth and by maximizing NO₃ reduction relative to NO₃ accumulation.     

Effects of CO2 enrichment on mineral accumulation and nitrogen relations in a submersed macrophyte

1. Six- to eight-week greenhouse experiments with independent control of pH and dissolved CO₂ evaluated the potential for CO₂ enrichment to stimulate the accumulation of Al, Fe, P and N in shoots of Vallisneria americana , particularly at pH 5. These minerals were provided only as they occurred in natural lake sediments.
2. The effect of CO₂ enrichment at pH 5 v pH 7.3 on growth and tissue N concentration was also determined.
3. CO₂ enrichment at pH 5 effected 5.5- and 7-fold increases in total shoot accumulation of Al and Fe, respectively. In a two-way factorial experiment, CO₂ enrichment yielded 6- to 11-fold greater total shoot P accumulation in plants grown on less and more fertile sediments, respectively.
4. In a three-way factorial experiment, CO₂ enrichment stimulated Vallisneria growth, especially at pH 5, and resulted in a 31–58% reduction in tissue [N] for different pH × sediment combinations. These are greater reductions than previously reported. It also increased total shoot N accumulation up to 6-fold, and there were significant interactions with pH and sediment source: the CO₂ enrichment effect on shoot N accumulation was greater at pH 5 than at pH 7.3, and it was greater with the more fertile sediment at pH 5.
5. Water chemistry (pH and/or [CO₂]) and sediment fertility thus both indirectly influenced the accumulation of sediment-derived minerals in macrophyte shoots within the water column.     

Effects of CO2 enrichment on mineral accumulation and nitrogen relations in a submersed macrophyte

1. Six- to eight-week greenhouse experiments with independent control of pH and dissolved CO₂ evaluated the potential for CO₂ enrichment to stimulate the accumulation of Al, Fe, P and N in shoots of Vallisneria americana , particularly at pH 5. These minerals were provided only as they occurred in natural lake sediments.
2. The effect of CO₂ enrichment at pH 5 v pH 7.3 on growth and tissue N concentration was also determined.
3. CO₂ enrichment at pH 5 effected 5.5- and 7-fold increases in total shoot accumulation of Al and Fe, respectively. In a two-way factorial experiment, CO₂ enrichment yielded 6- to 11-fold greater total shoot P accumulation in plants grown on less and more fertile sediments, respectively.
4. In a three-way factorial experiment, CO₂ enrichment stimulated Vallisneria growth, especially at pH 5, and resulted in a 31–58% reduction in tissue [N] for different pH × sediment combinations. These are greater reductions than previously reported. It also increased total shoot N accumulation up to 6-fold, and there were significant interactions with pH and sediment source: the CO₂ enrichment effect on shoot N accumulation was greater at pH 5 than at pH 7.3, and it was greater with the more fertile sediment at pH 5.
5. Water chemistry (pH and/or [CO₂]) and sediment fertility thus both indirectly influenced the accumulation of sediment-derived minerals in macrophyte shoots within the water column.     

Nitrate reductase and molybdenum cofactor in annual ryegrass as affected by salinity and nitrogen source

The influence of salinity on the activity of nitrate reductase (NR, EC 1.6.6.1) and the level of the molybdenum cofactor (MoCo) as affected by the source and concentration of nitrogen was studied in annual ryegrass ( Lolium multiflorum cv. Westerwoldicum). Plants grown in sand were irrigated with nutrient solution with an electrical conductivity of 2 or 11.2 dS m⁻¹, containing nitrogen (0.5 or 4.5 m M ) in the form of NH₄NO₃ or NaNO₃ Salinity-treated (11.2 dS m⁻¹) plants produced less biomass and more organic nitrogen while accumulating more NO⁻₃ than control plants. Increased nitrogen concentration in the irrigation solutions enhanced biomass and organic nitrogen production as well as NO⁻₃ accumulation irrespective of the electrical conductivity. Salinity inhibited shoot growth and increased shoot NR activity of plants receiving 4.5 m M NH₄NO₃ or NaNO₃. Similar effects were observed in roots of plants grown in 4.5 m M NaNO₃. Nitrate added to a complementation medium containing ryegrass MoCo and the NR apoprotein of Neurospora crassa mutant nit-1 stimulated the activity of the reconstituted NR (NADPH-nitrate reductase, EC 1.6.6.3). Increased salinity and nitrogen in the nutrient solutions caused an increase of MoCo content in roots and shoots. Similar results were observed for NR activity in the shoots. The increase of MoCo in response to salinity was more pronounced than that of NR, especially in the roots. We conclude that the pool size of MoCo in ryegrass is not constant, but varies in response to nutritional and environmental factors.     

Elevated atmospheric CO2 influences the interaction between the parasitic angiosperm Orobanche minor and its host Trifolium repens

The influence of the root holoparasitic angiosperm Orobanche minor Sm. on the biomass, photosynthesis, carbohydrate and nitrogen content of Trifolium repens L. was determined for plants grown at two CO₂ concentrations (350 and 550 μmol mol⁻¹). Infected plants accumulated less biomass than their uninfected counterparts, although early in the association there was a transient stimulation of growth. Infection also influenced biomass allocation both between tissues (infected plants had lower root:shoot ratios) and within tissues:infected roots were considerably thicker before the point of parasite attachment and thinner below. Higher concentrations of starch were also found in roots above the point of attachment, particularly for plants grown in elevated CO₂. Elevated CO₂ stimulated the growth of T. repens only during the early stages of development. There was a significant interaction between infection and CO₂ on growth, with infected plants showing a greater response, such that elevated CO₂ partly alleviated the effects of the parasite on host growth. Elevated CO₂ did not affect total O. minor biomass per host, the number of individual parasites supported by each host, or their time of attachment to the host root system. Photosynthesis was stimulated by elevated CO₂ but was unaffected by O. minor . There was no evidence of down-regulation of photosynthesis in T. repens grown at elevated CO₂ in either infected or uninfected plants. The data are discussed with regard to the influence of elevated CO₂ on other parasitic angiosperm-host associations and factors which control plant responses to elevated CO₂.     

Influences of soil volume and an elevated CO2 level on growth and CO2 exchange for the Crassulacean acid metabolism plant Opuntia ficus-indica

Effects of the current (38 Pa) and an elevated (74 Pa) CO₂ partial pressure on root and shoot areas, biomass accumulation and daily net CO₂ exchange were determined for Opuntia ficus-indica (L.) Miller, a highly productive Crassulacean acid metabolism species cultivated worldwide. Plants were grown in environmentally controlled rooms for 18 weeks in pots of three soil volumes (2 600, 6 500 and 26 000 cm³), the smallest of which was intended to restrict root growth. For plants in the medium-sized soil volume, basal cladodes tended to be thicker and areas of main and lateral roots tended to be greater as the CO₂ level was doubled. Daughter cladodes tended to be initiated sooner at the current compared with the elevated CO₂ level but total areas were similar by 10 weeks. At 10 weeks, daily net CO₂ uptake for the three soil volumes averaged 24% higher for plants growing under elevated compared with current CO₂ levels, but at 18 weeks only 3% enhancement in uptake occurred. Dry weight gain was enhanced 24% by elevated CO₂ during the first 10 weeks but only 8% over 18 weeks. Increasing the soil volume 10-fold led to a greater stimulation of daily net CO₂ uptake and biomass production than did doubling the CO₂ level. At 18 weeks, root biomass doubled and shoot biomass nearly doubled as the soil volume was increased 10-fold; the effects of soil volume tended to be greater for elevated CO₂. The amount of cladode nitrogen per unit dry weight decreased as the CO₂ level was raised and increased as soil volume increased, the latter suggesting that the effects of soil volume could be due to nitrogen limitations.     

Mobilization of CS fimbriae-associated plasmids of enterotoxigenic Escherichia coli of serotype O6: K15: H16 or H- into various wild-type hosts

Abstract CS fimbriae-associated plasmids of two enterotoxigenic Escherichia coli strains of serotype O6: K15: H16 or H- (biotypes A and F) with M _r values of 51 × 10⁶ and 72 × 10⁶, respectively, were mobilized into various alternative host bacteria. Expression of CS1 or CS2 fimbriae was obtained when either of the CS fimbriae-associated plasmids was introduced into CS Fim⁻, O6: K15: H16 or H- recipients with rhamnose-negative and rhamnose-positive fermentation phenotypes, respectively, whereas CS3 fimbriae were expressed irrespective of the biotype of the recipient. On transfer into a CS Fim⁻ variant of an enterotoxigenic O8: H9 strain and into two K-12 strains, a CS3-fimbriae-only phenotype was conferred by the presence of either of the plasmids. When a CS Fim⁻ variant of a Rha⁺ CS2-fimbriae-only strain of serotype O6: K15: H16 harboured either of the plasmids, both CS2 and CS3 fimbriae were expressed, indicating that the rare CS2-fimbriae-only wild-type phenotype is probably due to the presence of a defective plasmid in such strains. Mobilization of the 51 MDa CS fimbriae-associated plasmid into five non-enterotoxigenic Rha⁺ porcine isolates of E. coli with O6 serotypes other than O6: K15: H16 or H- yielded CS3-fimbriae-only transconjugants. Thus the correlation between a Rha⁺ fermentation phenotype and expression of CS2 fimbriae does not hold in general for O-group 6 strains.     

Responses to elevated temperature and CO2 in the perennial grass Agrostis curtisii in relation to population origin

1. Evolutionary responses to climate change will depend on the presence of heritable variation within species populations for traits that increase fitness under the changing conditions. Patterns of ecotypic differentiation in relation to latitude in some species suggest that such variation exists in relation to temperature responses. Response to elevated CO₂, whether heritable or not, is not expected to be related to latitudinal or climatic differences within temperate regions.
2. To test these ideas, seeds were collected from 10 populations of the outbreeding perennial grass Agrostis curtisii across its range in Europe from south Wales to Portugal. Plants were grown under ambient and elevated temperature and CO₂ conditions, in a factorial design, in solardomes; two half sibs from each population were planted in separate pots in each of the two replicate domes with each combination of treatments. One half sib was harvested at the end of the first summer, the second at the end of the second summer.
3. Survival was uniformly high and flowering uniformly low across treatments and populations.
4. Responses to temperature and CO₂ treatments varied over time for almost all populations. Treatment effects were not significant on plants harvested in year 1, although there was a trend towards higher shoot biomass under the elevated temperature and CO₂ treatment. In year 2 shoot biomass was significantly higher under the elevated temperature treatment across all populations and there was a strong trend towards decreased biomass under elevated CO₂.
5. There were no significant correlations of plant response to either CO₂ or temperature with climate at origin.
6. These results warn of the dangers of extrapolating evolutionary plant responses to CO₂ from short-term experiments.     

Ozone induced emissions of biogenic VOC from tobacco: relationships between ozone uptake and emission of LOX products

Volatile organic compound (VOC) emissions from tobacco ( Nicotiana tabacum L. var. Bel W3) plants exposed to ozone (O₃) were investigated using proton-transfer-reaction mass-spectrometry (PTR-MS) and gas chromatography mass-spectrometry (GC-MS) to find a quantitative reference for plants' responses to O₃ stress. O₃ exposures to illuminated plants induced post-exposure VOC emission bursts. The lag time for the onset of volatile C₆ emissions produced within the octadecanoid pathway was found to be inversely proportional to O₃ uptake, or more precisely, to the O₃ flux density into the plants. In cases of short O₃ pulses of identical duration the total amount of these emitted C₆ VOC was related to the O₃ flux density into the plants, and not to ozone concentrations or dose–response relationships such as AOT 40 values. Approximately one C₆ product was emitted per five O₃ molecules taken up by the plant. A threshold flux density of O₃ inducing emissions of C₆ products was found to be (1.6 ± 0.7) × 10⁻⁸ mol m⁻² s⁻¹.     

The role of temperature in determining the stimulation of CO2 assimilation at elevated carbon dioxide concentration in soybean seedlings

Soybean ( Glycine max cv. Clark) was grown at both ambient (ca 350 μmol mol⁻¹) and elevated (ca 700 μmol mol⁻¹) CO₂ concentration at 5 growth temperatures (constant day/night temperatures of 20, 25, 30, 35 and 40°C) for 17–22 days after sowing to determine the interaction between temperature and CO₂ concentration on photosynthesis (measured as A, the rate of CO₂ assimilation per unit leaf area) at both the single leaf and whole plant level. Single leaves of soybean demonstrated increasingly greater stimulation of A at elevated CO₂ as temperature increased from 25 to 35°C (i.e. optimal growth rates). At 40°C, primary leaves failed to develop and plants eventually died. In contrast, for both whole plant A and total biomass production, increasing temperature resulted in less stimulation by elevated CO₂ concentration. For whole plants, increased CO₂ stimulated leaf area more as growth temperature increased. Differences between the response of A to elevated CO₂ for single leaves and whole plants may be related to increased self-shading experienced by whole plants at elevated CO₂ as temperature increased. Results from the present study suggest that self-shading could limit the response of CO₂ assimilation rate and the growth response of soybean plants if temperature and CO₂ increase concurrently, and illustrate that light may be an important consideration in predicting the relative stimulation of photosynthesis by elevated CO₂ at the whole plant level.     

The density dependence of plant responses to elevated CO2

1 Stands of the annual Brassica kaber were grown at a range of six densities in both ambient and elevated CO₂ environments, and measurements of shoot growth were made from seedling emergence through to reproduction.
2 Early in stand development (21 days following emergence), CO₂ enhancement (β) for above-ground biomass was highly density-dependent, ranging from 1.41 at the lowest density (20 plants m−2) to 0.59 at the highest density (652 plants m−2).
3 As stands matured and total biomass exceeded a relatively low threshold level (<10.0 g m^{−2; c.}  20% of final yield), the density-dependence of β disappeared. Above this shoot biomass threshold, β-values remained remarkably stable (β = 0.34) across a broad range of stand biomass, independent of a stand's initial density or age.
4 Average stand-level reproductive β-values at a final harvest were very similar to biomass values (β = 0.38) and, as with biomass values at later stages, showed no apparent density-dependence.
5 These results highlight the importance of considering density and the time-course of stand development simultaneously when assessing the potential for CO₂-induced growth enhancements in plants.     

Physiological function of water channels as affected by salinity in roots of paprika pepper

The sap flow (Jv) and the osmotic pressure-dependent hydraulic conductance (L₀) of detached exuding root systems from paprika pepper plants (cv. Albar) were measured. Plants stressed with NaCl (30 m M ) and with six times the macronutrients of the Hoagland nutrient solution (6×HNS) were compared with controls grown in complete Hoagland nutrient solution. Jv of +NaCl and +6×HNS plants decreased markedly, but recovered to values similar to those of controls after removal of the treatments. Hydraulic conductance L₀ was always less in NaCl plants than in controls and 6×HNS. A total increase in the ion concentration of the xylem (except Na⁺ and Cl⁻) was observed with both treatments. In control and 6×HNS plants, HgCl₂ treatment (50 μ M ) caused a sharp decline in L₀ to values similar to those of NaCl-stressed roots, but were restored by treating with 5 m M dithiothreitol (DTT). However, in NaCl roots only a slight effect of Hg²⁺ and DTT was observed. In each treatment, there was no difference in the flux of K⁺ into the xylem after HgCl₂ and DTT application. The results suggest that NaCl decreased L₀ of the roots by reducing either the activity or abundance of Hg-sensitive water channels. The putative reduction in water-channel function of NaCl-treated plants did not seem to be due to the osmotic effect.     

The Effect of Light and Temperature on Competition between Atrazine Susceptible and ResistantBrassica rapa

Biotypes ofBrassica rapasusceptible (S) and resistant (R) toatrazine were grown in competitive replacement series in allpossible combinations of two light levels and three temperatureregimes in controlled growth cabinets. Photosystem II functionwas investigated in all conditions by fluorescence-inductiontechniques. There were no significant differences in the dryweight of the two biotypes when grown in pure stands. In purestands both biotypes produced more biomass under the high lightlevel. Under high light both biotypes yielded more biomass athigh temperature; in low light they did so at medium temperature.Under high light conditions at high and medium temperaturesthe susceptible biotype had a greater photon yield and relativecompetitive ability than the resistant due to the greater vulnerabilityof triazine-resistant biotypes to photoinhibition. However,surprisingly, the resistant biotype was the better competitor,and had a higher photon yield, in the high light/low temperatureregime. In low light no photoinhibition was expected and indeedthere were no significant differences in any fluorescence parametersbetween the resistant and susceptible biotypes. Nevertheless,there were differences in the whole plant performance; the susceptiblebiotype was a better competitor at low and medium temperatures,but the resistant biotype was better at high temperature. Relativelysmall variations in both light and temperature, well withinthe range encountered during British summer time, can have largeeffects on the relative competitiveness of triazine R and Sbiotypes in this species with implications for the spread ofresistance genes through semi-natural communities. In lightof predicted climate changes, interactions between climate andresistance should be studied across a wider range of herbicidetypes and weed species.Copyright 1997 Annals of Botany Company Brassica rapa; chlorophyll fluorescence; competition; light; navew; temperature; triazine resistance     

Effects of drought-rewatering-drought on photosynthesis and growth of maize

Aims Our objective was to investigate the responses of maize photosynthesis and growth to repeated drought.Methods Maize seedlings were exposed to different soil water deficit for three weeks, then rewatering for one week, and again to different water deficit for three weeks, to examine the effects of repeated drought on photosynthesis and growth.Important findings After the first water deficit treatments, under severe drought, plant height, total leaf area of individual plant, shoot and root biomass declined significantly, also transpiration rate (T_r), stomatal conductance (G_s), intercellular CO₂ concentration (C_i), net photosynthetic rate (P_n), maximum net photosynthetic rate (A_max), but light compensation point and dark respiration rate increased significantly. Under medium drought, plant height, leaf area, and shoot biomass decreased significantly, but root biomass did not vary, hence, the ratio of roots to shoots (R/S) increased. Moreover, plants did not show significant differences in photosynthetic parameters. After rewatering, photosynthesis and growth rate of plants previously exposed to water deficit could recover to the levels of well-watered plants, but plant height and leaf area did not recover to the levels of the control. When maize were subjected to recurrent drought, plants pre-exposed to medium drought showed no significant difference in plant height, biomass, and photosynthetic parameters, but a significant decrease in leaf area, compared to plants only exposed to second medium drought. Plants pre-exposed to severe drought had significantly higher T_r, G_s, C_i, P_n, A_max, and, apparent quantum yield but significantly lower plant height, leaf area, and biomass than plants without previous exposure. These results indicated that the first severe drought significantly reduced photosynthetic capacity and maize growth, rewatering could recover photosynthesis and growth rate to the levels of well-watered plants, but could not eliminate the adverse influence of the first drought on growth. The first medium drought could stimulate the growth of maize root system and significantly increased R/S, which can enhance maize drought resistance to subsequent repeated drought, and maintain the total biomass in the control level; the first severe drought could enhance maize drought resistance to subsequent repeated drought in the aspect of photosynthesis, but could not compensate for the adverse effect of early drought on plant growth. Hence, in practice, drought hardening should be limited in the level of medium drought, and avoiding severe drought.