Mechanisms of cross and multiple herbicide resistance in Alopecurus myosuroides and Lolium rigidum

L’Alopecurus myosuroides et le Lolium rigidum ont développé des résistances croisées et multiples à des herbicides ayant différents modes d'action et provenant de diverses classes chimiques. Un biotype d'Amyosuroides, Peldon AI, possède une capacité élevée de dégradation métabolique des herbicides de types urée substituée et aryloxypheno-xypropionate (APP), lui conférant une résistance croisée non reliée à la cible d'action. Un biotype australien de L. rigidum, SLR 31, possède de multiples mécanismes de résistance, comprenant à la fois des mécanismes de résistance croisée non reliés à la cible et d'autres reliés à la cible. La majorité des individus de la population SLR 31 a une capacité élevée de dégradation métabolique du chlorsulfuron et du diclofop-méthyl, en plus d'un mécanisme associé à une altération de la membrane cellulaire, lequel est corrélé avec la résistance à plusieurs herbicides de types APP et cyclohexanedione (CHD). De plus, une faible proportion des individus de cette population possède une cible d'action modifiée conférant une grande résistance à tous les APP et CHD. Bien que la biologie d'A. myosuroides et de L. rigidum présente beaucoup de points communs, ces deux espèces ne sont pas uniques. Nous prédisons que les résistances aux herbicides de type croisée non reliée à la cible et les résistances multiples vont se développer chez d'autres espèces. Les implications potentielles de ces types de résistance justifient l'adoption de mesures préventives.Alopecurus myosuroides and Lolium rigidum have developed resistance to herbicides with several modes of action in many herbicide classes. A. myosuroides biotype Peldon A1 from England exhibits non-target site cross resistance to substituted urea and aryloxyphenoxypropionate herbicides (APP) due to enhanced metabolism. L. rigidum biotype SLR 31 from Australia has multiple resistance mechanisms, including both non-target site cross resistance and target site cross resistance. The majority of the SLR 31 population has enhanced metabolism of chlorsulfuron and diclofop-methyl and a mechanism correlated with altered plasma membrane response, which correlates with resistance to some APP and cyclohexanedione (CHD) herbicides. A small proportion of the population also has target site cross resistance to APP and CHD herbicides. While A myosuroides and L. rigidum share common biological elements, they are not unique. Non-target site cross resistance and multiple herbicide resistance is predicted to develop in other weed species. The repercussions of cross and multiple resistance warrant proactive measures to prevent or delay onset

Resistance to ACCase-inhibiting herbicides in sprangletop (Leptochloa chinensis)

This study reports evolved resistance to fenoxaprop-P in a population of sprangletop from a rice field in Thailand (BLC1). After eight applications of fenoxaprop-P, the herbicide appeared no longer effective. To confirm herbicide resistance in the BLC1 population, three experiments were conducted. First, glasshouse experiments revealed that the BLC1 population survived 600 g ai ha-1 of fenoxaprop-P without visual injury. Second, the BLC1 population was treated with fenoxaprop-P and other acetyl coenzyme A carboxylase (ACCase)-inhibiting herbicides (quizalofop-P, cyhalofopbutyl, and profoxydim) under field conditions; BLC1 exhibited resistance to all of these herbicides. Third, seeds of susceptible SLC1 and resistant BLC1 were germinated on 0.6% (v/v) agar across a range of herbicide concentrations. The resistant BLC1 population exhibited 61-, 44-, 9- and 8-fold resistance to fenoxaprop-P, cyhalofop, quizalofop-P, and profoxydim, respectively, compared with a susceptible SLC1 population. At the enzyme level, ACCase from the resistant BLC1 exhibited 30, 24, 11, 4, and 5 times resistance to fenoxaprop, cyhalofop-butyl, haloxyfop, clethodim, and cycloxydim, respectively. The spectrum of resistance at the whole plant level correlated well with resistance at the ACCase level. Hence, the mechanism of resistance to ACCase-inhibiting herbicides in this biotype of sprangletop is a herbicide-resistant ACCase. The specific mutation(s) of the ACCase gene that endows resistance in this population remains to be investigated

Changes in photosynthetic capacity, carboxylation efficiency, and CO 2 compensation point associated with midday stomatal closure and midday depression of net CO 2 exchange of leaves of Quercus suber

The carbon-dioxide response of photosynthesis of leaves of Quercus suber , a sclerophyllous species of the European Mediterranean region, was studied as a function of time of day at the end of the summer dry season in the natural habitat. To examine the response experimentally, a “standard” time course for temperature and humidity, which resembled natural conditions, was imposed on the leaves, and the CO 2 pressure external to the leaves on subsequent days was varied. The particular temperature and humidity conditions chosen were those which elicited a strong stomatal closure at midday and the simultaneous depression of net CO 2 uptake. Midday depression of CO 2 uptake is the result of i) a decrease in CO 2 -saturated photosynthetic capacity after light saturation is reached in the early morning, ii) a decrease in the initial slope of the CO 2 response curve (carboxylation efficiency), and iii) a substantial increase in the CO 2 compensation point caused by an increase in leaf temperature and a decrease in humidity. As a consequence of the changes in photosynthesis, the internal leaf CO 2 pressure remained essentially constant despite stomatal closure. The effects on capacity, slope, and compensation point were reversed by lowering the temperature and increasing the humidity in the afternoon. Constant internal CO 2 may aid in minimizing photoinhibition during stomatal closure at midday. The results are discussed in terms of possible temperature, humidity, and hormonal effects on photosynthesis.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47468/1/425_2004_Article_BF00397440.pd

Effects of temperature at constant air dew point on leaf carboxylation efficiency and CO 2 compensation point of different leaf types

The effect of temperature on photosynthesis at constant water-vapor pressure in the air was investigated using two sclerophyll species, Arbutus unedo and Quercus suber , and one mesophytic species, Spinacia oleracea . Photosynthesis and transpiration were measured over a range of temperatures, 20–39° C. The external concentration of CO 2 was varied from 340 μbar to near CO 2 compensation. The initial slope (carboxylation efficiency, CE) of the photosynthetic response to intercellular CO 2 concentration, the CO 2 compensation point (Γ), and the extrapolated rate of CO 2 released into CO 2 -free air ( R i ) were calculated. At an external CO 2 concentration of 320–340 μbar CO 2 , photosynthesis decreased with temperature in all species. The effect of temperature on Γ was similar in all species. While CE in S. oleracea changed little with temperature, CE decreased by 50% in Q. suber as temperature increased from 25 to 34° C. Arbutus unedo also exhibited a decrease in CE at higher temperatures but not as marked as Q. suber . The absolut value of R i increased with temperature in S. oleracea , while changing little or decreasing in the sclerophylls. Variations in Γ and R i of the sclerophyll species are not consistent with greater increase of respiration with temperature in the light in these species compared with S. oleracea .Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47470/1/425_2004_Article_BF00397389.pd