Systemic crop protection products are commonly sprayed onto foliage, whereupon the active substances must penetrate into the leaves in order to become biologically active. Penetration of the plant cuticle is the rate-limiting step. The diffusion of organic non-electrolytes within cuticles is a purely physical process that can be described and analysed in the same way as is done for diffusion in synthetic polymer membranes. Solute mobilities in cuticles vary considerably between plant species. For a given species they decrease with increasing solute size, and this size selectivity holds for all of the plant species investigated so far. Wax extraction from leaf cuticles increases the mobility of solutes tremendously, but size selectivity is not affected. Furthermore, diffusion within plant cuticles is extremely temperature dependent. An analogous increase in solute mobility can be achieved by using accelerators, which enhance the fluidity of the polymer matrix and of the waxes. The effects of temperature and plasticizers on the diffusion of non-electrolytes in wax and the cutin matrix have been used to characterize the nature of the lipophilic pathway. The 'free volume' theory can be used to explain the influence of the size and shape of the solute, and its dependence on temperature. The physico-chemical nature of the diffusion pathway has been shown, by thermodynamic analysis, to be identical for a wide range of solute lipophilicities. This approach also explains the mode of action and the intrinsic activity of plasticizers.
Ptant, Cell and Environment (^997) 20,[982][983][984][985][986][987][988][989][990][991][992][993][994],viv In this range, solute mobilities increased up to 1000-fold, which corresponds to temperature coefficients Qio of 3 (IAA in P. armeniaca) to 14 (cholesterol in fi. hetix). For most species, Arrhenius graphs showed good linearity up to 40 °C, and up to 78 °C for some species, while for others activation energies declined with increasing temperature. However, no distinct phase transitions caused by sudden structural changes in the CM were observed. In three species we examined whether heating to 70 °C changed solute mobility irreversibly by comparing Arrhenius graphs for two successive experiments with the same CM. The two graphs were very similar for P, taurocerasus, while mobilities in the second graph were somewhat reduced for C. aurantium and greatly increased (at 25 and 35 °C) for H. helix. This indicates rearrangements of at least some wax constituents when heated to high temperatures. The activation energies of diffusion (/i|,) ranged from 75 to 189 k. F moP' depending on .species and .solute size. Size selectivity and variability between cuticles decreased witb increasing temperature, and this is caused by differences in li|,. An excellent correlation between the pre-exponential factor of the Arrhenius equation and /i,, was observed, which is evidence that organic solutes differing greatly in molecular size (130-349 cm' moP') and cuticle/water partition coefficient (25-10*") use similar diffusion paths in the CM of all 12 plant species tested. Diffusion occurs in regions with identical phy.sicochemical properties and differs only in magnitude.
A major factor which controls sorption and oxidation of Fe(II) at the mineral-water interface is pH, hence buffers are commonly used to control pH in experimental studies. Here, we examined the effects of widely used organic buffers (3-morpholinopropane-1-sulfonic acid (MOPS) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)) on Fe(II) uptake and oxidation by CCl(4) in aqueous suspensions of goethite. Significant sorption of these zwitterionic buffers occurred only at Fe(II)-loaded goethite but not at native goethite. The addition of MOPS and HEPES caused substantial release of Fe(II) from goethite, retarded the oxidation of surface-bound Fe(II) by CCl(4) and changed the reaction pathway as indicated by lower yields of CHCl(3). To explore electrostatic and steric contributions of MOPS and HEPES to the observed phenomena we studied sorption and competitive effects of model sorbates (Ca(2+), sulfonates) which suggest the formation of a complex between surface-bound Fe(II) and MOPS or HEPES. Our study shows for the first time that these frequently used zwitterionic organic buffers may interfere significantly with the surface chemistry and thus with redox reactions of Fe(II) at goethite. Hence, kinetic or mechanistic information obtained in such systems requires careful interpretation.
A laboratory study was undertaken to investigate the leaf systemic properties and the translaminar aphicidal activity of two commercialised neonicotinoid (chloronicotinyl) insecticides. For that purpose [14C]imidacloprid was subjected to uptake and translocation studies in cabbage and cotton after foliar application. Foliar penetration and short-term translocation patterns of imidacloprid were similar in both plant species. Nevertheless imidacloprid penetrated twice as much into cabbage leaves as it did into cotton leaves. It showed a comparable translaminar behaviour and was entirely translocated acropetally, indicating its well-known xylem mobility. The translaminar and acropetal movement of imidacloprid and acetamiprid were quantified by simple laboratory bioassays using the green peach aphid, Myzus persicae (Sulzer), and the cotton aphid, Aphis gossypii (Glover), as typical homopteran pests for cabbage and cotton, respectively. A single dose (7.5 micrograms AI per leaf) applied to the upper leaf surface of cabbage and cotton was tested against aphids feeding on the lower leaf surface both close to and distant from the site of application 1, 5 and 12 days after treatment. The translaminar residual activity of imidacloprid on cabbage leaves was superior to that of acetamiprid, whereas its translaminar efficacy against A gossypii on cotton was inferior to that of acetamiprid. However, oral ingestion bioassays using an artificial double membrane feeding system revealed no significant differences in intrinsic activity between the two neonicotinoids tested.
Solute mobilities in cuticular membranes of six species (Hedera helix, Malus domestica, Populus alba, Pyrus communis, Stephanotis¯oribunda, Strophantus gratus) were measured using plant hormones, growth regulators and other organic model compounds varying in molar volumes from 99 to 349 mL á mol A1 The dependence of mobilities (k*) on molar volume (V x ) was exponential and could be described with equations of the type log k* log k* 0 À b H x . The y-intercepts log k* 0 represent mobilities of a hypothetical solute of zero molar volume. The parameter b¢ is a measure of size selectivity of cuticular membranes and no dierences among the six species were observed. At 25°C the average b¢ was 0.0095 mol á mL A1 . Solute mobility decreased by about a factor of 8.9 when molar volume increased by 100 mL á mol A1 and the mobility of a compound with V x 100 mL á mol A1 was about 700-fold higher than the mobility of a compound with V x 400 mL á mol A1 . Size selectivity decreased with increasing temperatures and for Strophantus b¢-values of 1.6´10 A2 to 8.0´10 -4 mol á mL A1 were obtained for 10 and 30°C, respectively. The-intercepts (log k* 0 ) diered among plant species by 3 orders of magnitude and since size selectivity was the same for all species, solute mobilities for solutes having zero molar volumes were the sole cause for dierences among species in solute mobilities and permeabilities. We argue that these dierences in k* 0 are related to tortuosity of the diusion path. These results were used to derive an equation which predicts rates of cuticular penetration on the basis of k* 0 , the average size selectivity of 9.5´10 A3 mol á mL A1 and the driving forces of penetration.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.