Increased Heat Sensitivity of the Photosynthetic Apparatus in Triazine-Resistant Biotypes from Different Plant Species

Leaf fragments from five species of triazine-resistant and -susceptibleplants were exposed for brief periods to temperatures above38?C. In resistant leaves, a greater increase in temperature-inducedfluorescence emission was observed than in susceptible leaves,indicative of greater heat sensitivity of the PS II electrontransfer linked to triazine resistance. The kinetics of fluorescenceinduction under low or strong light excitation revealed twodistinct effects of heat exposure; A DCMU-type inhibition whichwas reversed upon cooling and a decrease in the variable fluorescence,due mainly to an increase in the constant fluorescence (O level)in resistant leaves, which was only partially reversible. Agreater deterioration of electron transfer activity in chloroplastsisolated from resistant plants also was detected, under exposuresto a high pH, temperature of 25–35?C and to ferricyanide.This indicates that there is greater instability of the oxygenevolving system in triazine-resistant plants. (Received July 26, 1984; Accepted January 8, 1985)     

Comparison of Triazine-Resistant and -Susceptible Biotypes of Senecio vulgaris and Their F(1) Hybrids

The relationship of triazine resistance to decreased plant productivity was investigated in Senecio vulgaris L. F₁ reciprocal hybrids were developed from pure-breeding susceptible (S) and resistant (R) lines. The four biotypes (S, S × R, R, R × S) were compared in terms of atrazine response, electron transport, carbon fixation, and biomass production. Atrazine response, carbon fixation rate, and PSII and whole-chain electron transport rates of hybrids were nearly identical to those of their respective maternal parents. Significant differences occurred between the two susceptible (S, S × R) and two resistant (R, R × S) biotypes in atrazine response (I₅₀), carbon fixation rate, and PSII and whole-chain electron transport rates; PSI rates were identical in all four biotypes. Coupled and uncoupled, whole-chain electron transport rates of thylakoids of the two susceptible biotypes were approximately 50% greater than those of the two resistant biotypes at photon flux densities greater than 215 micromoles per square meter per second. Carbon exchange rates of the two susceptible biotypes were 23% greater than those of the two resistant biotypes. Hybrid biotypes (S × R, R × S) were not identical to their maternal parents in biomass production. The S, S × R, and R × S plants all achieved greater biomass than R plants. These results suggest that while the resistance mutation influences thylakoid performance, reduced productivity of triazine-resistant plants cannot be ascribed solely to decreases in electron transport or carbon assimilation rates brought about by the altered binding protein. Since the F₁ hybrids differed from their maternal parents only in nuclear genes, it appears that the detrimental effects of the triazine resistance mutation on plant growth may be attenuated by interactions of the plastid and nuclear genomes.     

Similar Photosynthetic Performance in Low Light-Grown Isonuclear Triazine-Resistant and -Susceptible Brassica napus L

Triazine-resistant plants grown under moderate to high photon flux density (PFD) conditions exhibit decreased photon yield, decreased light-saturated O₂ evolution and slower growth than triazine-susceptible plants. In this study we tested the hypothesis that the comparable growth previously observed in resistant and susceptible Brassica napus L. lines grown under low PFD was accompanied by comparable photon yield and light-saturated O₂ evolution. We measured photon yield, O₂ flash yield, fluorescence decay kinetics, fluorescence transient kinetics, and quenching components, F_v/F_m and light saturated O₂ evolution in leaf disks of low PFD-grown triazine-resistant and susceptible B. napus isogenic lines. Results indicated that slow electron transfer from the primary to secondary quinone electron acceptors of photosystem II was still present in the resistant line but photon yield and light-saturated O₂ evolution were similar in the two B. napus lines. We conclude that the alteration in the D1 protein that confers resistance does not necessarily cause decreased photosynthetic performance. Decreased photon yield in resistant plants grown at high PFD is not a direct consequence of the alteration in D1, but represents secondary damage.     

Characterization of Chloroplasts Isolated from Triazine-Susceptible and Triazine-Resistant Biotypes of Brassica campestris L

Chloroplasts isolated from triazine-susceptible and triazine-resistant biotypes of Brassica campestris L. were analyzed for lipid composition, ultrastructure, and relative quantum requirements of photosynthesis. In general, phospholipids, but not glycolipids in chloroplasts from the triazine-resistant biotype had a higher linolenic acid concentration and lower levels of oleic and linoleic fatty acids, than chloroplasts from triazine-susceptible plants. Chloroplasts from the triazine-resistant biotype had a 1.6-fold higher concentration of t-Δ3-hexadecenoic acid with a concomitantly lower palmitic acid concentration in phosphatidylglycerol. Phosphatidylglycerol previously has been hypothesized to be a boundary lipid for photosystem II. Chloroplasts from the triazine-resistant biotype had a lower chlorophyll a/b ratio and exhibited increased grana stacking. Light-saturation curves revealed that the relative quantum requirement for whole chain electron transport at limiting light intensities was lower for the susceptible biotype than for the triazine-resistant biotype. Although the level of the chlorophyll a/b light-harvesting complex associated with photosystem II was greater in resistant biotypes, the increased levels of the light-harvesting complex did not increase the photosynthetic efficiency enough to overcome the rate limitation that is inherited concomitantly with the modification of the Striazine binding site.     

Contribution of photosynthetic rate to growth and reproduction in Amaranthus hybridus

While it is known that genetic variation for photosynthetic and growth traits exists in natural populations, the functional significance of this variation remains unclear, particularly for photosynthetic traits. To test the hypothesis that photosynthetic rate has direct effects on reproduction as well as contributing indirectly to reproduction through effects on growth, we compared wild-type Amaranthus hybridus families to those with a single gene mutation that confers a lower photosynthetic rate. Wild-type and photosynthetic-mutant families were grown in competitive and non-competitive environments and we compared size, biomass allocation, architecture, and reproduction at three developmental stages. To assess the contributions of individual growth traits to reproduction, we calculated covariances between standardized traits and relative fitness (selection differentials), and compared selection between the two biotypes. Finally, we used path analysis to calculate the indirect effects of photosynthetic rate on fitness through growth. The size, allocation, and architecture of photosynthetic mutants did not differ from those of the wild type in either the competitive or non-competitive environment, with the exception that they were taller by the last developmental stage. However, the reproductive biomass of the photosynthetic mutants was significantly reduced compared to the wild type. In the competitive environment, the wild type achieved greater fitness because, while similar in size to the mutants, at any given size it produced more reproductive biomass. This suggests that photosynthetic rate affected the linkage between plant size and reproduction and is evidence of an indirect contribution to fitness. In the non-competitive environment, there were fewer differences in selection differentials between the two plant genotypes, suggesting fewer indirect effects. Path analysis showed that variation in photosynthetic biotype had indirect effects on reproductive biomass, via growth traits, and that there were no direct effects. Photosynthetic rate appears to have fitness consequences primarily through multiple contributions to growth throughout development. Received: 27 March 1998 / Accepted: 28 August 1998     

Acetolactate synthase mutation conferring imidazolinone-specific herbicide resistance in Amaranthus hybridus

Acetolactate synthase (ALS) catalyzes the first common step in the biosynthesis of branched-chain amino acids in plants and is the target of several herbicides. ALS inhibitors have enjoyed popularity as herbicides due to numerous attributes, although their current adequacy in weed control programs is hampered by herbicide resistance. Most cases of ALS-inhibitor resistance have resulted from selection of an altered target site. The study herein reports on an alanine by threonine amino acid substitution at position 122 of ALS as the basis for imidazolinone-specific resistance in an A. hybridus population from Illinois. In vitro inhibition of enzymatic activity (I(50)) required 1000-fold greater concentration of imazethapyr in the resistant population compared with a susceptible control. This mutation represents the second ALS alteration associated with herbicide resistance in a natural A. hybridus population.     

Growth and Photosynthetic Characteristics of Two Biotypes of the Weed Black-grass (Alopecurus myosuroides Huds.) Resistant and Susceptible to the Herbicide Chlorotoluron

Response of two biotypes of black-grass (Alopecurus myosuroidesHuds.) to the herbicide, chlorotoluron, was characterized inglasshouse and laboratory studies. ED₅₀values, defined as theamount (kg active ingredient ha^-1) of chlorotoluron requiredto reduce fresh mass by 50% under standard conditions, weredetermined for a resistant biotype (39.3 kg a.i. ha^-1) collectedfrom Peldon, Essex, UK and a susceptible biotype (0.93 kg a.i.ha^-1) obtained commercially, giving a resistance factor of 42.The resistance factor was calculated as the ratio of ED₅₀valuesand describes the increase in amount of herbicide needed toreduce fresh mass by 50% in the resistant, compared to the susceptible,biotype. Resistance was further characterized by measurementsof whole plant growth and photosynthesis. Relative growth rate,number of tillers, leaf area and mean fresh mass were the samein untreated plants of both biotypes, and rates of photosynthesisat both high and low photon flux were similar, with no differencein apparent quantum yield. Photosynthesis by whole plants wasstudied over a 24 h period following chlorotoluron treatment.Resistant plants showed no reduction in photosynthesis overthis period, whereas photosynthesis by susceptible plants ceased10 h after treatment and did not recover. Alopecurus myosuroides ; black-grass; herbicide resistance; chlorotoluron     

Molecular Mechanisms Endowing Cross-resistance to ALS-Inhibiting Herbicides in Amaranthus hybridus from Argentina

Amaranthus hybridus L. is one of the most problematic weeds in summer crops in Argentina. However, 20 years after the detection of the first case of resistance to ALS-inhibiting herbicides in this country, no extensive reports of the molecular mechanisms endowing resistance were published. In this work, we sequenced the acetolactate synthase gene of resistant plants belonging to five different populations of A. hybridus from Santa Fe and Cordoba provinces. We found that every population presented at least one of the previously documented substitutions W574L and D376E in ALS amino acid sequence. These results explain the cross-resistance to ALS-inhibiting herbicides and should alert about the usage of herbicides with a different site of action after an ineffective control of this species. This is the first report of these target-site mechanisms endowing resistance to ALS-inhibiting herbicides in A. hybridus populations from Argentina.

     

Reduction of Nitrate and Nitrite in Lambsquarters (Chenopodium album) Biotypes Resistant and Susceptible to Atrazine Toxicity

The nitrite-reducing activity of the normal susceptible biotype of lambsquarters (Chenopodium album L.) was strongly inhibited by atrazine in the assay medium, both in the case of the in vivo assays of leaf discs in light, and in vitro photoreduction assays of crude extracts. In vitro assays of crude extracts with methylviologen or ferredoxin supplying the reducing potential were not inhibited by atrazine. In the resistant biotype, inhibition of nitrite reduction did not occur with any of the above assays. Thus, it appears that atrazine does not inhibit nitrite reductase itself, but rather the availability of photosynthetically supplied electrons for the reduction. Atrazine had no effect when added to the media for either in vivo or in vitro assays of nitrate reduction by either the susceptible or resistant biotype.     

微波法和超声波法提取绿穗苋多糖响应面优化研究

绿穗苋是一种药食兼用作物,其中多糖成份具有很高的药食价值。本研究以绿穗苋地上部分为材料,以微波和超声波两种方法对绿穗苋多糖进行提取,并通过响应面分析法对提取进行优化,确定最佳工艺,通过水提醇沉的方法得到绿穗苋粗多糖。综合比较两种提取工艺,提取效果最佳工艺为微波提取法。其条件为:提取时间41.42 min,提取功率211.65 W,料液比1:33.338 (g/mL),实际得率为13.25%。该研究结果促进绿穗苋资源的有效利用,为绿穗苋多糖的提取工艺及产品的开发利用提供理论依据。     

Isoproturon Resistance in Phalaris minor: Study of Physiobiochemical Parameters in Isoproturon Susceptible and Resistant Biotypes and Their Hybrids

An experiment was conducted to study physio-biochemical parameters to delineate the molecular basis of resistance against isoproturon in Phalaris minor. The reciprocal crosses of isoproturon resistant and susceptible biotypes of P. minor were made during the first year of an experiment and during the next year, these F₁ hybrids with their respective parents were evaluated for physio-biochemical and growth parameters after spraying the weed @ 0.0, 0.5, 1.0 and 1.5 kg ha^?1 with isoproturon at 22^nd and 30^th DAS. The F₁s showed heterosis and the order of resistance against isoproturon in the four biotypes was R×S > R > S×R > S. This indicated that the resistance against isoproturon in P. minor is predominantly governed by cytoplasmic inheritance, but the nuclear inheritance also has some contribution.     

Changes in size and composition of pigweed (Amaranthus hybridus L.) calcium oxalate crystals under CO2 starvation conditions

The functional role(s) of plant calcium oxalate (CaOx) crystals are still poorly understood. Recently, it was shown that crystals function as dynamic carbon pools whose decomposition could provide CO₂ to photosynthesis when stomata are closed (e.g. under drought conditions) and CO₂ starvation conditions may be created within the mesophyll. This biochemical process, named as ‘alarm photosynthesis’, can become crucial for plant survival under adverse conditions. Here, we study crystal decomposition under controlled CO₂ starvation conditions (either in the shoot or in the root) to obtain a better insight into the process of crystal formation and function. Hydroponically grown pigweed plants were kept in CO₂‐free air and/or CO₂‐free nutrient medium for 9 days. Crystal volume was monitored daily, and carbon stable isotope composition (δ¹³C) and Fourier transformation Raman spectra were obtained at the end of the experiment. A considerable reduction in the leaf crystal volume was observed in shoot‐CO₂‐starved plants at the end of the experiment. The smallest crystals were isolated from the plants in which carbon was excluded from both the shoot and the root and contained potassium nitrate. Crystal δ¹³C of CO₂‐starved plants was altered in a predicted way. Specifically, it depended on the average calculated isotope fractionation of all carbon fixation processes considered to be contributing in each experimental treatment. The results of the present study confirmed the correlation between CO₂ starvation conditions and the CaOx crystal decomposition. Inorganic carbon fixed in the root may represent a major carbon source for CaOx formation.     

Regulation of Photosynthesis in Triazine-Resistant and -Susceptible Brassica napus

The response of photosynthetic carbon assimilation and chlorophyll fluorescence quenching to changes in intercellular CO₂ partial pressure (C_i), O₂ partial pressure, and leaf temperature (15-35°C) in triazine-resistant and -susceptible biotypes of Brassica napus were examined to determine the effects of the changes in the resistant biotype on the overall process of photosynthesis in intact leaves. Three categories of photosynthetic regulation were observed. The first category of photosynthetic response, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco)-limited photosynthesis, was observed at 15, 25, and 35°C leaf temperatures with low C_i. When the carbon assimilation rate was Rubisco-limited, there was little difference between the resistant and susceptible biotypes, and Rubisco activity parameters were similar between the two biotypes. A second category, called feedback-limited photosynthesis, was evident at 15 and 25°C above 300 microbars C_i. The third category, photosynthetic electron transport-limited photosynthesis, was evident at 25 and 35°C at moderate to high CO₂. At low temperature, when the response curves of carbon assimilation to C_i indicated little or no electron transport limitation, the carbon assimilation rate was similar in the resistant and susceptible biotypes. With increasing temperature, more electron transport-limited carbon assimilation was observed, and a greater difference between resistant and susceptible biotypes was observed. These observations reveal the increasing importance of photosynthetic electron transport in controlling the overall rate of photosynthesis in the resistant biotype as temperature increases. Photochemical quenching of chlorophyll fluorescence (q_P) in the resistant biotype never exceeded 60%, and triazine resistance effects were more evident when the susceptible biotype had greater than 60% q_P, but not when it had less than 60% q_P.     

Comparison of Atrazine-Resistant and -Susceptible Biotypes of Senecio vulgaris L.: Effects of High and Low Temperatures on the in vivo Photosynthetic Electron Transfer in Intact Leaves

The effects of temperature on the yield of in vivo modulatedchlorophyll fluorescence were measured in intact leaves of atrazineresistantand -susceptible biotypes of the weed Senecio vulgaris L. At25 ?C, the photochemical quenching (q_Q of steady-state chlorophyllfluorescence was reduced by around 30% in the atrazine-resistantmutant as compared to the susceptible wild type, indicatinga higher reduction state of the primary electron acceptor Q_Aof photosystem II in the former biotype. Moderately elevatedtemperatures (above 30 ?C) further increased the steady-stateconcentration of reduced in the mutant. Analysis of the temperature dependence of both the photochemicalquenching q_Q and the initial fluorescence level F_o clearly indicateda drastically enhanced heat-sensitivity of the photochemicalapparatus in the atrazine-resistant Senecio leaves. The heat-inducedchanges in F₀ and q_Q were closely correlated, suggesting thatthe phenomenon responsible for the rise in F₀ was also involvedin the inhibition of the photosynthetic electron flux. Low temperaturesalso affected reoxidation but, in contrast to heating, no apparent differences were observed in the behaviourof the two biotypes exposed to chilling stress. Key words: Atrazine-resistance, chlorophyll fluorescence, heat stress, Senecio vulgaris     

Photosynthetic characteristics of Amaranthus tricolor,a C4 tropical leafy vegetable

The gas exchange characteristics are reported for Amaranthus tricolor, a C₄ vegetable amaranth of southeastern Asia. Maximum photosynthetic capacity was 48.3±1.0μmol CO₂ m^?2s^?1 and the temperature optimum was 35°C. The calculated intercellular CO₂ concentration at this leaf temperature and an incident photon flux (400–700 mm) of 2 mmol m^?2s^?1 averaged 208±14 μl l^?1, abnormally high for a C₄ species. The photosynthetic rate, intercellular CO₂ concentration, and leaf conductance all decreased with an increase in water vapor pressure deficit. However, the decrease in leaf conductance which resulted in a decrease in intercellular CO₂ concentration accounted for only one fourth of the observed decrease in photosynthetic rate as water vapor pressure deficit was increased. Subsequent measurements indicated that the depence of net photosynthesis on intercellular CO₂ concetration changed with water vapor pressure deficit.     

Photosynthetic characteristics of Amaranthus tricolor,a C4 tropical leafy vegetable

The gas exchange characteristics are reported for Amaranthus tricolor, a C₄ vegetable amaranth of southeastern Asia. Maximum photosynthetic capacity was 48.3±1.0 μmol CO₂ m^-2 s^-1 and the temperature optimum was 35°C. The calculated intercellular CO₂ concentration at this leaf temperature and an incident photon flux (400–700 mm) of 2 mmol m^-2 s^-1 averaged 208±14 μl l^-1, abnormally high for a C₄ species. The photosynthetic rate, intercellular CO₂ concentration, and leaf conductance all decreased with an increase in water vapor pressure deficit. However, the decrease in leaf conductance which resulted in a decrease in intercellular CO₂ concentration accounted for only one fourth of the observed decrease in photosynthetic rate as water vapor pressure deficit was increased. Subsequent measurements indicated that the dependence of net photosynthesis on intercellular CO₂ concentration changed with water vapor pressure deficit.     

Chloroplast Doublets and Differential Staining of the Cell Walls in Photosynthetic Tissues of Amaranthus dubius Mart

Ultrastructural studies of the leaves of Amaranthus dubius revealpresence of chloroplast doublets in mature leaf tissues. Theyalso highlight the difference in the structural organisationof the bundle sheath and the mesophyll cell walls, a featurewhich may have functional significance. Amaranthus dubius Mart., Calaloo, mesophyll, bundle sheath, chloroplast doublets, plastid fusion, plastid division, differential staining of the cell walls