William A. Quick

Influence of drought on graminicide phytotoxicity in wild oat (Avena fatua) grown under different temperature and humidity conditions

Abstract. Controlled environmental experiments were carried out to determine the phytotoxicity of several graminicides on wild oat (Avena futua L.) as influenced by combination of drought and temperature stress or drought and low relative humidity. Compared with unstressed conditions (20/15°C plus adequate soil moisture), imazamethabenz phytotoxicity to wild oat was reduced significantly when plants were exposed to a combination of drought and high temperature (30/20°C) stress. Imazamethabenz phytotoxicity was reduced almost as much by high temperature stress alone as by a combined temperature and drought stress. When herbicides were applied to wild oat plants subjected to drought alone or to drought plus high temperature, the observed reduction in phytotoxicity from greatest to least was: fenoxaprop = diclofop > flamprop > imazamethabenz. Fenoxaprop performance was most inhibited by the combination of drought plus high temperature, although drought alone and to a lesser degree, high temperature alone, inhibited fenoxaprop action. High temperature had an adverse effect on the efficacy of fenoxaprop at lower application rates. Raising fenoxaprop application rates to 400 g ha−1 overcame the inhibition caused by high temperature alone but only partially alleviated the effect of drought combined with high temperature. When plants were grown under a low temperature regimen the imposition of drought stress had little effect on imazamethabenz phytotoxicity but did reduce fenoxaprop phytotoxicity. At 25/15°C drought reduced the phytotoxicity of fenoxaprop and diclofop greatly but had no significant impact on the performance of any of the herbicides examined, regardless of soil moisture regimen.

Influence of water deficit on the phytotoxicity of imazamethabenz and fenoxaprop among five wild oat populations

Abstract Growth chamber studies were carried out on the effect of soil water deficits on the phytotoxicity of imazamethabenz and fenoxaprop in four genetically distinct lines (CS 40, SH 430, AN 51, MON 73) and one natural population of wild oat ( Avena fatua L.). In the absence of drought, SH 430 was relatively more susceptible than the other lines to imazamethabenz and fenoxaprop; MON 73, a short, abundantly tillering line, was somewhat more tolerant to fenoxaprop than the others; and CS 40, the most vigorous of the populations, was slightly more tolerant to both herbicides. Water deficits had limited adverse influence on the phytotoxicity of imazamethabenz in the populations examined. The phytotoxicity of fenoxaprop was greatly reduced by long-term water stress in all populations, but was more variable under short-term drought conditions. The differential responses among the populations to these two herbicides did not appear to be directly related to their intrinsic drought tolerance, characteristic growth habit, or to their level of seed dormancy. Such interaction may still be of adaptive significance to the survival of this troublesome weed.

Influence of temperature and light intensity on absorption, translocation, and phytotoxicity of fenoxaprop-ethyl and imazamethabenz-methyl in Avena fatua

The absorption and translocation of fenoxaprop-ethyl and imazamethabenz-methyl were investigated in wild oat (Avena fatua L.) plants grown under different temperature and light intensity conditions by using 14C tracer techniques. The phytotoxicity of both herbicides, applied as individual droplets, was also determined under similar environments. The absorption of fenoxaprop-ethyl and imazamethabenz-methyl was increased by high temperature (30/20°C) and to a lesser extent by 70% shading; low temperature (10/5°C) had limited effect on the absorption. The basipetal translocation of fenoxaprop-ethyl was not affected by high temperature, and the increase in imazamethabenz-methyl translocation at high temperature was likely the result of the increased absorption. Low temperature decreased total translocation and translocation efficiency in both fenoxaprop-ethyl and imazamethabenz-methyl. Low light intensity tended to reduce the efficiency of basipetal translocation of both herbicides. Fenoxaprop-ethyl phytotoxicity was reduced by high temperature but not by low temperature. Temperature had little effect on imazamethabenz-methyl effectiveness. Under 70% shading, the phytotoxicity of both herbicides was enhanced.

Dormancy Implications of Phosphorus Levels in Developing Caryopses of Wild Oats (Avena fatua L.)

Abstract. The element phosphorus made up 0.5% of the dry weight of dehulled Avena fatua caryopses 7 days after anthesis (DAA), half of it inorganic (Pi). Caryopses detached and pierced 7 DAA germinated in vitro with a rapid drop in Pi levels. By 15–20 DAA caryopsis dry weight had increased three- to fourfold, but phosphorus made up less than 0.04% of the dry weight of this enlarged caryopsis. Caryopses at this stage germinated readily without piercing if incubated in vitro. A further decrease in Pi accompanied by a marked increase in phytate phosphorus began about 15 DAA and continued during later seed maturation. By 20 DAA, when embryos were relatively mature and endosperm cell division had ceased, a decrease in caryopsis water content (as a percentage of dry weight) began, and seed dormancy became apparent. As starch and phytate reserves accumulated, Pi and water levels of the caryopsis diminished. Higher levels of endogenous Pi coincided with the anabolic events of initial seed formation and, to a lesser extent, with anabolic events of seed germination. Decreasing Pi levels coincided with accumulation of nutrient reserves, lowering of water content, and the initiation of dormancy. The data suggest that (1) enzymes associated with the formation and development of the embryo may be activated by the high Pi levels present during initial seed differentiation; (2) embryo quiescence and dormancy are facilitated by the drop of Pi levels which accompanies the accumulation of starch and phytate reserves; and (3) the increase in Pi which accompanies seed afterripening aids in the termination of dormancy and the resumption of germination.

Effect of drought and formulation on wild oat (Avena fatua) control with imazamethabenz and fenoxaprop

Experiments were conducted in the greenhouse to determine the effects of drought, rehydration and herbicide formulation on the phytotoxicity to wild oat of imazamethabenz and fenoxaprop. Under both well-watered and drought conditions, the activity of imazamethabenz with the liquid concentrate formulation tended to be higher than the suspension concentrate formulation, and the activity of the soluble powder formulation was enhanced by the surfactant Agral 90. Long-term drought did not reduce imazamethabenz phytotoxicity to wild oat with any formulations examined, whereas the phytotoxicity of three fenoxaprop formulations, fenoxaprop-ethyl, fenoxaprop-p-ethyl and fenoxaprop-p-ethyl plus safener, was decreased by long-term drought. Fenoxaprop-ethyl activity was reduced by drought stress imposed before spraying, after spraying or, most significantly, both before and after spraying. The adverse effect of drought on fenoxaprop-ethyl activity was still evident even when daily watering was resumed 8 to 48 h before spraying, although plants rehydrated in this way were more susceptible to fenoxaprop-ethyl than plants where rehydration was deferred until after spraying.

Effect of ammonium sulfate on the phytotoxicity, foliar uptake, and translocation of imazamethabenz in wild oat.

Experiments were conducted in greenhouse, growth chamber, and laboratory conditions to determine the effect of ammonium sulfate [(NH4)2SO4] on the phytotoxicity, foliar uptake, and translocation of imazamethabenz on wild oat. Rates of (NH4)2SO4 up to 5% (w/v) applied with a greenhouse sprayer did not affect the phytotoxicity of the herbicide when the mix was applied at the one- to two-leaf stage. However, inclusion of 1 and 2% (NH4)2SO4 increased the phytotoxicity of the herbicide when the mix was sprayed at the two- to three-leaf, or the three- to four-leaf stage. At 10%, (NH4)2SO4 decreased the phytotoxicity of the sublethal dosage of the herbicide. When the herbicide was applied as individual drops to the growth chamber-grown plants, inclusion of (NH4)2SO4 at 1% did not affect phytotoxicity as measured by shoot growth. The presence of (NH4)2SO4 did not affect the amount of imazamethabenz retained by wild oat foliage, but it decreased [14C]imazamethabenz absorption, slightly antagonized acropetal translocation, and increased the basipetal translocation of [14C]imazamethabenz. It was concluded that application methods greatly modify the effect of (NH4)2SO4 on imazamethabenz phytotoxicity. Herbicide absorption and translocation as determined by one method do not necessarily represent the absorption and translocation patterns when different application methods are used. Absorption and translocation were not the factors that were responsible for the observed effect of (NH4)2SO4 on the herbicide phytotoxicity.

Effect of decapitation on absorption, translocation, and phytotoxicity of imazamethabenz in wild oat (Avena fatua L.)

The release of apical dominance by the physical destruction in situ of the apical meristem and associated leaf primordia (decapitation) promoted the growth of tillers in non-herbicide-treated wild oat plants, as indicated by increased tiller lengths and fresh weights. At 96 h after [14C] herbicide treatment following decapitation, the absorption of [14C]imazamethabenz and total translocation of radioactivity were respectively increased by 28% and 49%. By 96 h after [14C]imazamethabenz application, the radioactivity detected in the roots of decapitated plants was 45% higher than that in the roots of nondecapitated plants while the radioactivity in tillers of decapitated plants was 2.6-fold that in tillers of intact plants. Decapitation together with foliar spraying of imazamethabenz at 200 g ha−1 further reduced tiller fresh weight, greatly decreased the total tiller number, and thereafter significantly increased overall phytotoxicity by 32% as measured by total shoot fresh weight. The results of this study support the hypothesis that main shoot apical dominance limits translocation of applied imazamethabenz to lateral shoots, rendering tillers less susceptible to growth inhibition by the herbicide.

Impact of temperature on the phytotoxicity of imazamethabenz and fenoxaprop to wild oat (Avena fatua)

Abstract Controlled environmental experiments were conducted to determine the effect of short- and long-term temperature stresses on the phytotoxicity of imazamethabenz and fenoxaprop to wild oat. Short-term (16 h) temperature stresses, imposed either before or after herbicide spraying, had no influence on imazamethabenz phytotoxicity. However, short-term temperature stresses affected the phytotoxicity of fenoxaprop, with pretreatment high temperature (35°C) reducing the phytotoxicity and post-treatment high temperatures (25–35°C) enhancing the phytotoxicity. Imazamethabenz phytotoxicity was similar at long-term temperatures between 20/15°C and 30/20°C. Constant low temperature (10/5°C) reduced imazamethabenz phytotoxicity, whereas such temperature had no effect if only imposed before or after spraying. Among three constant temperature regimes, fenoxaprop was more effective at 20/15°C, followed by 10/5°C. Constant high temperature (30/20°C) greatly decreased fenoxaprop phytotoxicity. The adverse effect of long-term low temperature on fenoxaprop phytotoxicity was due mainly to the influence of post-treatment low temperature, whereas long-term high temperature imposed either before or after spraying reduced the phytotoxicity of fenoxaprop.

Effects of imazamethabenz on the main shoot growth and tillering of wild oat (Avena fatua L.)

Foliar application of imazamethabenz at sublethal doses of 100 and 200 g a.i./ha to wild oat plants at the two-leaf stage without tillers greatly inhibited the growth of the main shoot but increased tillering. The near cessation of sheath and the main stem elongation indicated that the major effect of imazamethabenz on the main shoot was inhibition of intercalary growth. Low doses of imazameth-abenz treatment resulted in more leaves (including leaf primordia) in the main stem but did not affect mature first and second leaves. Sublethal doses of imazamethabenz only briefly inhibited tiller growth. A later increase in tillering in treated plants resulted from the stimulated resumed growth of tillers and the increased initiation of tiller buds. Such enhanced tillering mainly resulted from the release of apical dominance due to the inhibition or cessation of the main stem growth with imazamethabenz treatment. Both doses of imazamethabenz (100 and 200 g a.i./ha) significantly reduced the biomass of shoots and roots, but increased the ratio of roots/ shoots dry weight.

Tillers Do Not Influence Phytotoxicity of Imazamethabenz in Wild Oat (Avena fatua)

Abstract. The response of wild oat to imazamethabenz varies with the growth stage, but the role of tillers in this regard is unclear. Removal of tillers at the three-leaf stage before spraying with imazamethabenz did not significantly affect the total shoot fresh weight measured 3 weeks later. The leaf area and dry weight of intact plants at the three-leaf stage were 17–21% greater than for plants with coleoptilar and first leaf main shoot tillers (T0 and T1) removed. The greater leaf area may have increased herbicide interception per plant. Similar fresh weight reductions in main shoot, total tillers, and total shoots were found whether imazamethabenz was applied to the plant at the two-leaf without tillers or the three-leaf with two tillers stage. Imazamethabenz applied only to the main shoot reduced total shoot dry weight more than an equivalent amount of imazamethabenz applied only to tiller T1 or applied over the whole shoot. Imazamethabenz had the least inhibitory effect on whole plant growth when applied only to T1. When 14C-herbicide was applied to the first main shoot leaf of plants at the three-leaf stage with two tillers, the 14C translocated 38% to roots, 33% to the main shoot, and nearly 30% to all tillers. When 14C-herbicide was applied to the first leaf of T1 then the 14C translocated 50% to T1, 25% to the main shoot, 20% to roots, and 5% to all other tillers. The translocation pattern and fresh weight values suggested that the presence of early tillers during herbicide application neither increased nor decreased imazamethabenz efficacy in wild oat.