Metamitron and Shade Effects on Leaf Physiology and Thinning Efficacy of Malus × domestica Borkh

Thinning strategies, namely shade or photosynthetic inhibitors, rely on the reduction of carbon supply to the fruit below the demand, causing fruit abscission. In order to clarify the subject, seven field trials were carried out in Lleida, Girona, and Sint-Truiden (2017 + 2018), using orchards of ‘Golden’ and ‘Gala’ apple trees. At the stage of 9–14-mm fruit diameter, four treatments were implemented: (A) CTR-control, trees under natural environmental conditions; (B) SN-shaded trees, trees above which shading nets reducing 50% of irradiance were installed 24 h after metamitron application date—without application of metamitron—and removed after five days; (C) MET-trees sprayed with 247.5 ppm of metamitron; (D) MET + SN-trees submitted to the combined exposure to metamitron application and shading nets. Low radiation significantly increased metamitron absorption (36–53% in the three locations in 2018) and reduced its degradation. Net photosynthesis and stomatal conductance were strongly reduced in all treatments, with minimum values 2 days after spraying (DAS) and incomplete recovery 10 DAS in MET + SN. All treatments resulted in leaf sucrose and sorbitol decreases, leading to a negative carbon balance. SN and MET + SN promoted the highest thinning efficacy, increasing fruit weight and size, with MET + SN causing over-thinning in some trials. Leaf antioxidant enzymes showed moderate changes in activity increases under MET or MET + SN, accompanied by a rise of glutathione content and a reduction in ascorbate, however without lipid peroxidation. This work shows that environmental conditions, such as cloudy days, must be carefully considered upon metamitron application, since the low irradiance enhances metamitron efficacy and may cause over-thinning.

Characterization of Multiple Herbicide-Resistant Italian Ryegrass (Lolium perennessp.multiflorum) Populations from Winter Wheat Fields in Oregon

Many Italian ryegrass populations in Oregon are resistant to more than one herbicide; therefore, the resistance patterns of these populations must be determined to identify alternative herbicides for management. Two suspected resistant Italian ryegrass populations (R2 and R4) survived flufenacet plus metribuzin applications under typical winter wheat production conditions. Populations R2 and R4 were resistant to clethodim, pinoxaden, quizalofop, mesosulfuron-methyl, flufenacet, but not to acetochlor, dimethenamid-p, metolachlor, pyroxasulfone, imazapyr, sulfometuron, or glyphosate. R4 was resistant to diuron, but R2 was not. The estimated flufenacet doses required for 50% growth reduction (GR50) were 438 g ai ha−1(R2) and 308 g ai ha−1(R4). Both populations were controlled by pyroxasulfone at rates greater than 15 g ai ha−1. An Asp-2078-Gly substitution in the ACCase gene was found in both populations, while an Ile-2041-Asn was found only in the R4 population. A Ser-264-Gly substitution inpsbA gene was found in the R4 population. These mutations previously have been reported to provide resistance to ACCase and photosynthetic inhibitors, respectively. No resistance mutations were identified in the acetolactate synthase (ALS) gene of either population. The addition of the P450 inhibitor, chlorpyrifos, increased the injury resulting from mesosulfuron-methyl on both resistant populations providing indirect evidence that the ALS resistance may be metabolic. Multiple herbicide-resistant Italian ryegrass populations were identified in this study with both target site and nontarget site based mechanisms likely involved. However, several herbicides were identified including pyroxasulfone, a herbicide in the same group as flufenacet, which could be used to control these two populations.

Effects of S-Abscisic Acid and (+)-8′-Acetylene Abscisic Acid on Fruit Set and Stomatal Conductance in Apple

Fruit set of apple can be reduced by cloudy weather, short-term shade treatments, or application of photosynthetic inhibitors when the young fruit are ≈8 to 15 mm in diameter, indicating that fruit are sensitive to a transient carbohydrate stress during this period. We investigated the potential for S-abscisic acid (ABA) and an ABA analog [(+)-8′-acetylene ABA] to chemically thin apple fruit by causing a stomatal limitation of photosynthesis. Stomatal conductance (gS) of ‘Imperial Gala’/M.7 was reduced by 60% 3 h after application of 250 mg·L−1 ABA or 25 mg·L−1 (+)-8′-acetylene ABA. Stomatal conductance began to recover 4 days after application but did not return to control levels until 19 days after treatment. Application of 250 mg·L−1 ABA combined with 100 mg·L−1 6-benzyladenine (6-BA) when mean fruit diameter was ≈10 mm reduced fruit set of ‘Gala’/M.7 but not ‘Pink Lady™’/M.7 or ‘Morganspur Delicious’/MM.111. Fruit set of ‘Pink Lady™’/M.7 was reduced by application of 20 mg·L−1 (+)-8′-acetylene ABA + 100 mg·L−1 6-BA at full bloom or 10 mg·L−1 (+)-8′-acetylene ABA + 100 mg·L−1 6-BA at the 10-mm fruit diameter stage. Fruit set of ‘Morganspur Delicious’/MM.111 was reduced by application of 25 mg·L−1 (+)-8′-acetylene ABA, either alone or in combination with 75 mg·L−1 6-BA, at the 10-mm fruit diameter stage. ABA and (+)-8′-acetylene ABA triggered leaf abscission at rates above 250 mg·L−1 and 25 mg·L−1, respectively. Fruit set and gS data from the present studies indicate the biological activity of (+)-8′-acetylene ABA is 10-fold higher than ABA. These results suggest that ABA and (+)-8′-acetylene ABA reduced fruit set by causing a stomatal limitation in photosynthesis that resulted in a transient carbohydrate stress. Thinning responses to ABA and (+)-8′-acetylene ABA at the concentrations used in these experiments were reduced compared with standard concentrations of currently available chemical thinning agents. However, increasing the concentration of ABA or (+)-8′-acetylene ABA to levels that would achieve comparable thinning are also likely to result in unacceptable leaf abscission.

Amicarbazone, a New Photosystem II Inhibitor

Amicarbazone is a new triazolinone herbicide with a broad spectrum of weed control. The phenotypic responses of sensitive plants exposed to amicarbazone include chlorosis, stunted growth, tissue necrosis, and death. Its efficacy as both a foliar- and root-applied herbicide suggests that absorption and translocation of this compound is very rapid. This new herbicide is a potent inhibitor of photosynthetic electron transport, inducing chlorophyll fluorescence and interrupting oxygen evolution ostensibly via binding to the QB domain of photosystem II (PSII) in a manner similar to the triazines and the triazinones classes of herbicides. As a result, its efficacy is susceptible to the most common form of resistance to PSII inhibitors. Nonetheless, amicarbazone has a good selectivity profile and is a more potent herbicide than atrazine, which enables its use at lower rates than those of traditional photosynthetic inhibitors.