Ralph C. Kirkwood

Recent developments in our understanding of the plant cuticle as a barrier to the foliar uptake of pesticides

The plant cuticle is a highly complex membrane which forms the outer surface of the aerial portion of plants. The nature of the plant cuticle is reviewed with particular regard to its action as a potential barrier to the penetration of pesticide molecules; the role of the cuticular waxes is highlighted. The physicochemical properties of the cuticle influence the behaviour of spray droplets and, in turn, may affect the rate and efficiency of cuticle penetration. The permeation of active ingredients is influenced by their solubility characteristics as indicated by octanol/water (log K ow ) and cuticle/water (K cw ) partition coefficients. Penetration of hydrophilic compounds (low log K ow ) may be enhanced by hydration of the cuticle, while transcuticular transport of non-polar solutes (high log K ow ) is increased by factors which reduce wax viscosity. The use of in-vitro models involving isolated cuticle membranes, isolated cuticle waxes, or isolated leaves has helped to focus on the activities of the cuticle in the absence of other physiological factors. Using these systems, the role of the waxes as a transport-limiting barrier has been identified and the factors influencing sorption, permeance and desorption examined. The action of surfactants, in vitro and in vivo, has been briefly addressed in regard to their role in facilitating cuticle penetration; other steps involving surfactant/solute/cuticle are complex, and synergy appears to depend on a number of factors including test species, concentration of active ingredient, surfactant type and concentration. Adjuvants may greatly influence the surface properties of the droplet, predispose the cuticle to solute transport, and enhance pesticide activity. The nature of these complex inter-relationships is discussed.

Adsorption, Desorption and Mobility of Four Commonly Used Pesticides in Malaysian Agricultural Soils

Working with Malaysian agricultural soils, high Freundlich adsorption distribution coefficients (K ads(f) ) were observed for paraquat (28.7 and 1419) and glyphosate (83.8 and 417) and lower values for 2,4-D (0.57 and 5.26) and lindane (2.65 and 14.1) in a sandy loam and a muck soil, respectively. Desorption of 2,4-D and lindane from the muck soil occurred. The adsorption of the pesticides was not affected by temperature (20°C/30°C), pH or addition of the pesticides as a mixture. Leaching of 2,4-D and lindane was evident under a high water influx (200 mm). Comparable results in the leaching of 2,4-D were observed between laboratory studies and a VARLEACH model prediction.

Biology and control of Phalaris minor Retz. (littleseed canarygrass) in wheat

Abstract Phalaris minor Retz. (littleseed canarygrass) is an important winter season weed of several crops across many continents. P. minor is a prolific and competitive weed especially in wheat crops. This review considers its distribution, biology and agro-ecology. Special importance is attached to considering the value and limitations of cultural and chemical control methods together with a crop management blueprint. While the use of selective herbicides is critical to maintain economic returns of wheat production, there are issues associated with their continuous use. Undoubtedly the development of herbicide-resistant biotypes of P. minor is an epidemic in India of economic, cultural and scientific importance. Consideration is given to the nature of the resistance problem and management approaches designated to minimise the impact of resistance and perhaps avoid further spread of the epidemic. Future research strategies are discussed to address the nature of this important grass weed problem. These include the importance of agronomic and physiological research to understand the basis of weed behaviour.

Absorption, localisation, translocation and activity of glyphosate in barnyardgrass (Echinochloa crus-galli (L) Beauv) : influence of herbicide and surfactant concentration

The influence of sub-lethal and lethal doses of glyphosate (5 µg and 10 µg per plant) applied to the fourth leaf of barnyardgrass (Echinochloa crus-galli) was examined over a treatment period of up to 14 days. Assessments were undertaken on plant growth, chlorophyll fluorescence, absorption and translocation of [14C]glyphosate. Electronic autoradiography and image analysis were used to examine the distribution of [14C]glyphosate over the duration of the study. Major sinks affecting glyphosate distribution included the emerging fifth leaf, the roots and ‘shoot’ (meristem area). At 10 µg per plant, chlorophyll fluorescence declined over the treatment period; in the source and sink leaves effects were particularly evident at 5 DAT. Absorption and translocation of [14C]glyphosate (5 and 10 µg per plant) was rapid during 1–2 DAT, remaining relatively constant thereafter. Approximately 70% of the application was absorbed and, of this, 70% was translocated. The concentration of glyphosate increased in the sinks (the emerging fifth leaf, the roots and shoot (meristem) area) to a maximum at 3 DAT, thereafter declining. This decline was coincident with a decrease (2–3 DAT) in the level of photosynthesis (fluorescence) in the source and sink leaves of plants treated with 10 µg glyphosate. Incorporation of the surfactant MON 0818 at 0.5, 1.0 or 2.0 ml litre−1 enhanced herbicidal activity, absorption, translocation and sink accumulation of [14C]glyphosate (5 µg per plant), with absorption and translocation greatest at 0.5 ml litre−1 at 5 DAT. Herbicidal activity at 12–14 DAT, however, was greatest at the 1.0 ml litre−1 concentration. © 2000 Society of Chemical Industry

Effect of ABT on the activity and rate of degradation of isoproturon in susceptible and resistant biotypes of Phalaris minor and in wheat

The effect of the monooxygenase inhibitor, 1-aminobenzotriazole (ABT) on isoproturon phytotoxicity and metabolism was studied in resistant (R) and susceptible (S) biotypes of Phalaris minor and in wheat (Triticum aestivum). Addition of ABT (2·5, 5 and 10 mg litre-1) to isoproturon (0·25, 0·5, 1, 2 and 4 mg litre-1) in the nutrient solution significantly enhanced the phytotoxicity of isoproturon against the R biotype. Isoproturon at 0·25 mg litre-1 reduced the dry weight (DW) of the S biotype by 77%, whereas the R biotype required 4·0 mg litre-1 for similar reduction. Addition of 10 mg litre-1 of ABT to the 0·25 mg litre-1 isoproturon caused 71 and 82% reduction in DW of R and S biotypes, respectively. Wheat was more sensitive to the mixture of isoproturon and ABT than the R biotype of P. minor. Reduced concentrations of ABT in the mixture from 10 to 2·5 mg litre-1 increased the DW of the R biotype more than that of the S biotype. The R biotype metabolised [14C]isoproturon at a faster rate than the S biotype. ABT (5 mg litre-1) inhibited the degradation of [14C]isoproturon in both biotypes of P. minor and in wheat. In the presence of ABT, about half of the applied [14C]isoproturon remained as parent herbicide in all the three species after two days. The metabolites were similar in the R and S biotypes and wheat as determined by co-chromatography with reference standards and mass spectroscopy (MS). ABT inhibited the appearance of the hydroxy and monomethyl metabolites and their conjugates in all the test plants. These results suggest that the activity of the enzymes responsible for the degradation of isoproturon is greater in the R than in the S biotype of P. minor, resulting in its rapid detoxification. Incorporation of the monooxygenase inhibitor ABT into the nutrient solution greatly inhibited the degradation of [14C]isoproturon in the R biotype and increased its phytotoxicity. Both hydroxylation and N-dealkylation reactions were found to be sensitive to ABT; inhibition of hydroxylation was greater than that of demethylation. Since ABT could not completely suppress isoproturon degradation, it is possible that more than one monooxygenase is involved.

Pathways and Mechanisms of Uptake of Foliage-Applied Herbicides with Particular Reference to the Role of Surfactants

The activity of a foliage-applied herbicide depends upon its effect on the target enzyme(s), and, in turn, this must be influenced by the efficiency of target site delivery. It is well known that delivery is influenced by a series of interrelated factors including the efficiency of cuticle retention and penetration, absorption into the leaf tissues, and possibly translocation from the absorption site. Immobilization or metabolism of the active ingredient (a.i.) at any stage may reduce the concentration reaching the target site. Interspecies differences in activity (selectivity) of a herbicidal compound may be due to variation in the rate or efficiency of any of these processes. In this chapter, the mechanism and factors influencing cuticle wetting, retention, and penetration prior to absorption into the leaf tissues will be considered, with particular reference to the role of surfactants. The processes and factors determining the transport of the a.i. away from the absorption site are reviewed in Chapter 9.

Effect of phenoxyalkanoic acid herbicides on the fermentative production of l-lysine

SummaryThe effect of the herbicides MCPA, MCPB, mecoprop, dichlorprop, 2,4-D, 2,4-DB, and 2,4,5-T on l-lysine fermentation was investigated using a lysine-producing mutant of Corynebacterium glutamicum. Stimulation of l-lysine production by 6% to 36% was observed in shaken flask experiments when the test herbicides were added at a concentration of 5 · 10-4 M to growing cultures after 24 h of cultivation. The most effective stimulators were MCPA, mecoprop and dichlorprop.Detailed studies of the effect of MCPA (5 · 10-6 M to 5 · 10-3 M) showed that the degree of stimulation depended on medium composition and aeration. In the synthetic medium, maximum production of 50 g · l-1 lys · HCl occurred at 5 · 10-4 M MCPA and an oxygen transfer rate (OTR) of 1.97 g O2 · l-1 · h-1, while 61.7 g · l-1 of lys · HCL was formed at 5 · 10-3 M MCPA and an OTR of 3.75 g O2 · l-1 · h-1. In the amino-nitrogen rich medium, maximum production of 42 g · l-1 lys · HCl was observed at 5 · 10-6 M MCPA and an oxygen transfer rate of 1.5 g O2 · l-1 · h-1. Results from batch l-lysine fermentation in a fermenter showed similar stimulatory effects, with an optimal concentration of MCPA for l-lysine production of 5 · 10-5 M. Without herbicide addition, the test strain produced 16.25 g · l-1 of product and with addition of 5 · 10-5 M MCPA, the same strain produced 52.1 g · l-1 lys · HCl after 72 h of fermentation.