Jacek Kwiatkowski

Regulation of phytohormone levels, leaf senescence and transpiration by the strobilurin kresoxim-methyl in wheat (Triticum aestivum)

Summary Using leaf discs and intact wheat plants (Triticum aestivum L.), the physiological effects of the strobilurin-type fungicide kresoxim-methyl were studied in relation to induced phytohormonal changes. Dose-response experiments revealed that kresoxim-methyl shifted the hormonal balance, favouring cytokinins particularly of the dihydrozeatin riboside-type, as opposed to ethylene and its biosynthetic precursor 1-aminocyclopropane-1-carboxylic acid (ACC). This was closely correlated with delayed leaf senescence. Kresoximmethyl was shown to inhibit the induction of ACC synthase in ethylene formation. In addition, kresoximmethyl caused an up to two-fold increase in endogenous levels of abscisic acid, relative to the control. Concomitantly, the stomatal aperture and water consumption of plants were reduced. It is suggested that kresoxim-methyl changes the hormonal constellation in wheat which leads to delayed leaf senescence and water-conserving effects.

Regulation of endogenous abscisic acid levels and transpiration in oilseed rape by plant growth retardants

Summary Plant growth retardants of the norbornanodiazetine (e.g. tetcyclacis) and triazole (e.g. BAS 111. . W, LAB 150 978) type considerably increased endogenous abscisic acid (ABA) levels in cell suspension cultures, detached leaves and hydroponically grown seedlings of oilseed rape ( Brassica napus ). In detached leaves treated via the vascular system, ABA accumulated in proportion to retardant concentration and was closely correlated with a reduction in transpiration. Thus, the growth retardants appear to influence transpiration by regulating ABA metabolism. In suspension cells and shoots of seedlings, maximal ABA content was achieved 4 and 5 days after treatment, respectively. ABA accumulation may be due to a retardant-caused inhibition of the metabolism of ABA, presumably to phaseic acid, as suggested by [ 14 C] ABA feeding experiments with cell cultures. The initial rise in ABA levels was followed by a sharp decline, an effect that might be explained by a stimulated ABA catabolism and/or by an inhibition of its biosynthesis. In contrast, foliar application of BAS 111. . W to plants at the 4th leaf stage did not change either ABA levels in shoots nor water consumption per shoot fresh or dry weight analysed during 10 days after treatment. In leaves of soil treated plants at the 5th leaf stage ABA levels also remained unaffected or dropped below those of controls, especially at higher BAS 111. . W concentration. It is suggested that the growth retardants regulate ABA metabolism in oilseed rape, dependent on the mode of application and intensity, and duration of action.

Induction of abscisic acid is a common effect of auxin herbicides in susceptible plants

Summary For more than forty years synthetic compounds possessing auxin activity have been among the most successful herbicides used in agriculture. However, in spite of extensive research a consensus on the biochemical basis of their phytotoxicity has yet to emerge. Comparing the effects of several representatives of the known chemical classes of auxin herbicides (e.g. dicamba, 2-methyl-4-chlorophenoxyacetic acid, picloram, quinclorac, quinmerac) and auxins (e.g. α-naphthaleneacetic acid, indole-3-acetic acid) in a variety of plant species, we demonstrate, to our knowledge for the first time, that the induction of ABA is a common effect of auxin-type compounds, which appears to be implicated in their inhibition of growth in susceptible plants.

Saflufenacil (Kixor™): Biokinetic Properties and Mechanism of Selectivity of a New Protoporphyrinogen IX Oxidase Inhibiting Herbicide

Abstract Saflufenacil (Kixor™) is a new protoporphyrinogen IX oxidase (PPO) inhibiting herbicide for preplant burndown and selective PRE dicot weed control in multiple crops, including corn. The biokinetic properties and the mechanism of selectivity of saflufenacil in corn, black nightshade, and tall morningglory were investigated. After root treatment of plants at the third-leaf stage, the difference in the phytotoxic selectivity of saflufenacil in corn and the weed species has been quantified as approximately 10-fold. The plant species showed similar selectivity after foliar applications; the plant response to saflufenacil was approximately 100-fold more sensitive compared with a root application. PPO enzyme activity in vitro was inhibited by saflufenacil, a 50% inhibition lay in a concentration range from 0.2 to 2.0 nM, with no clear differences between corn and the weed species. Treatments of light-grown plants and dark-grown seedlings with [14C]saflufenacil revealed that the herbicide is rapidly absorbed by root and shoot tissue. The [14C]saflufenacil was distributed within the plant systemically by acropetal and basipetal movement. Systemic [14C]saflufenacil distribution can be explained by the weak acid character of saflufenacil and its metabolic stability in black nightshade and tall morningglory. Metabolism of [14C]saflufenacil in corn was more rapid than in the weeds. In addition, low translocation of root-absorbed [14C]saflufenacil in the corn shoot was observed. It is concluded that rapid metabolism, combined with a low root translocation, support PRE selectivity of saflufenacil in corn. Nomenclature: Butafenacil; flumioxazin; saflufenacil; black nightshade, Solanum nigrum L., SOLNI; corn, Zea mays L., ZEAMX; tall morningglory, Ipomoea purpurea (L.) Roth. PHBPU; velvetleaf, Abutilon theophrasti Medik., ABUTH.

Accumulation of Abscisic Acid in Cell Suspension Cultures of Oilseed Rape Treated with the Growth Retardant BAS 111 .. W: Effects on Osmotic Potential and Potassium, Water and Sugar Contents

Summary Cell suspension cultures of oilseed rape ( Brassica napus L. cv. Kasan) showed a transient increase in cellular abscisic acid (ABA) content at the beginning of the exponential growth phase, with a maximum after 5 days of subculture. In proportion to the endogenous levels, ABA was excreted into the culture medium. Treatment with 10 -4 mol·L -1 fluridone, an inhibitor of ABA biosynthesis, suppressed the rise in ABA levels almost completely. Thus, ABA might be produced in response to osmotic stress resulting from subculturing the cells in fresh medium with higher sugar and mineral concentrations. This hypothesis was supported by the finding that endogenous ABA accumulation correlated with increasing osmotic stress induced by mannitol (from -0.39 to -0.74MPa) in the medium. Concomitantly, cellular water content and the osmotic potential of the cell sap decreased. Application of the triazole-type growth retardant BAS 111 .. W also increased the amount of endogenous ABA, but without changes in the osmotic potential of the medium. The compound is thought to inhibit ABA catabolism to phaseic acid. In contrast, the cyclohexanetrione-type retardants prohexadione calcium and LAB 198 999 did not alter ABA contents significantly. Dose response curves obtained 5 days after treatment with BAS 111 .. W revealed that ABA accumulation in the cells was accompanied by rising potassium and water levels and osmotic potential of the cell sap. The latter effect could be explained by a restricted uptake of sucrose and/or its hydrolysis products glucose and fructose, in BAS 111 .. W treated suspension cells. Calculated on a cell number and cell dry weight basis, ABA, potassium and water content increased to approximately 210, 160 and 140%, respectively, of the control values after treatment with 10 -5 to 10 -4 mol·L -1 BAS 111.. W. A significant increase was also found in magnesium while calcium was only slightly elevated and sodium levels remained unchanged. The changes in the potassium and water contents following the elevated endogenous ABA levels mediated by BAS 111 .. W were thus opposite to those observed when ABA was increased in response to osmotic stress. The specific effects of ABA on these parameters may thus be elucidated without interference from the effects of simultaneous water stress. It is speculated that retardant-caused ABA accumulation influences endogenous potassium, water and sucrose levels and the osmotic potential of the cell sap.