Foliar sprays against potato common scab: compounds related to 3,5-dichlorophenoxyacetic acid

In glasshouse tests against the soil-borne disease potato common scab (Streptomyces scabies), foliar sprays (0 · 9 mm) of several compounds inhibited symptom development as effectively as 3,5-D (3,5-dichlorophenoxyacetic acid). These compounds were of two main types: (1) compounds very similar in structure to 3,5-D, viz. 3,5-disubstituted phenoxyacetic acids in which the substituents were dibromo-, di(trifluoromethyl)- or any combination of bromo-, chloro- and iodo- except diiodo-; (2) compounds which could be metabolized to 3,5-D within the plant, viz. 4-(3,5-dichlorophenoxy)butanoic acid, 3,5-dichlorophenoxy-acetamide, 3,5-dichlorophenoxy-N,N-diethylacetamide and 2-(3,5-dichlorophenoxy)ethylamine. No compound of either type was significantly more effective than 3,5-D, and those which were as effective shared its effects on tuber yield, number and shape. About 30 other compounds related to 3,5-D were less effective or ineffective.     

Physico-chemical factors determining the phloem translocation of benzoic acids

A series of ¹⁴C-labelled benzoic acids, chosen to permit assessment of the role of pK_a and lipophilicity in determining movement in plants of these herbicide analogues, was synthesised and their phloem translocation investigated. Following application of substituted benzoic acids to castor bean, Ricinus communis L., by injection into the petioles, the compounds of intermediate lipophilicity (2-fluoro-, 4-chloro- and 3,4-dichlorobenzoic acids) gave highest concentrations in phloem exudates; 4-methyl-2,3,5,6-tetrafluoro- and pentafluorobenzoic acids were less well translocated, perhaps because their pK_a values are much less than those of the other benzoic acids studied. The polar 4-ureidobenzoic acid and the lipophilic 3-(4-methylphenoxy)benzoic acid were much less efficiently translocated in phloem. These results are similar to those previously obtained for phenoxyacetic acids, and provide further support for the role of ionisation in the accumulation and retention of chemicals in phloem sieve tubes.     

Translocation of surface litter carbon into soil by Collembola

Soil invertebrates are important in nutrient cycling in soils, but the degree to which mesofauna such as Collembola are responsible for the direct movement of carbon (C) from the litter layer into soil has not yet been ascertained. We used naturally occurring stable C isotopic differences between a C₄ soil and alder leaves (C₃) to examine the effect of the collembolan Folsomia candida on C translocation into soil in laboratory microcosms. Collembolan numbers greatly increased in the presence of alder, but despite large collembolan populations there were no changes in decomposition rate (measured as litter mass loss, cumulative respired CO₂ and alder C:N ratios). Small changes in the δ¹³C values of bulk soil organic matter were detected, but could not be assigned to collembolan activity. However, mean δ¹³C values of soil microbial phospholipid fatty acids (PLFAs) were significantly lower in the presence of alder and Collembola together, demonstrating that collembolan activities resulted in greater availability of litter-derived C to the soil microbial community. Additionally, the presence of Collembola resulted in the translocation of alder-derived compounds (chlorophyll and its breakdown product pheophytin) into soil, demonstrating that Collembola modify soil organic matter at the molecular level. These results are consistent with deposition of collembolan faeces in underlying soil and demonstrate that despite their small size, Collembola contribute directly to C transport in the litter-soil environment.     

Physicochemical factors affecting the uptake by roots and translocation to shoots of amine bases in barley

The uptake by barley roots from nutrient solution and subsequent transport to shoots of two series of amine bases were measured over 6 to 72 h. The compounds were chosen to span systematically ranges of lipophilicity (assessed using 1-octanol/water partition coefficients, K_ow) and pKa that would include commercial pesticide amines. In a series of six substituted phenethyl amines, strong bases with pKa∽9·5, all the compounds were strongly taken up by roots from solutions of pH 8·0; uptake declined substantially as the pH was lowered to 5·0, especially for the compounds of intermediate lipophilicity (log K_ow 2 to 3). This uptake could be ascribed to three processes: (i) accumulation of the cation inside the root cells due to the negative charge on the plasmalemma, as given by the Nernst equation and important only for the polar compounds which have low permeation rates through membranes; (ii) accumulation into the vacuole by ion-trapping, which was the dominant process at high pH for all compounds and at all pH values for the compounds of intermediate lipophilicity; (iii) partitioning on to the root solids, substantial only for the most lipophilic compounds. Translocation to shoots was proportional to uptake by roots, this ratio being independent of external pH for each compound and being optimal for the compounds of intermediate lipophilicity. Such proportionality was also observed in a series of three weaker bases of intermediate lipophilicity, in which compounds of pKa 7·4 to 8·0 were also well taken up and translocated whereas the very weak base 4-ethylaniline (pKa 5·03) was much less so. Tests with quaternised pyridines confirmed that organic cations move only slowly through membranes. The observed behaviour of the amines could be modelled reasonably well assuming that transport within the plant was dominated by movement across membranes of the non-ionised species, and this appeared to be true even for the most lipophilic phenethylamine (log K_ow 4·67) studied, though its long-distance movement would be as the protonated species. © 1998 Society of Chemical Industry     

Anion exclusion in microbial and soil polysaccharides

Microorganisms in soil, especially those associated with plant roots, are surrounded by envelopes of polysaccharides. These originate from both microbes and roots and are a characteristic feature of the rhizosphere. We have shown these materials to selectively restrict the diffusion of anions by the measurement of diffusion potentials. Using xanthan gum as a model microbial polysaccharide, increasing polymer concentration or polysaccharide layer thickness or the removal of acetyl and pyruvyl groups have been shown to increase the degree of anion exclusion. The anion-exclusive behaviour of xanthan has been validated independently by direct measurements of diffused ion concentrations. Data is presented showing this phenomenon to operate in KCl, KNO₃ and KH₂PO₄ systems. In all cases, the anion exclusion appears to be partial, restricting the diffusion of anions in the presence of a layer of 3% xanthan by 50-80%. By measurement of diffusion potentials, scleroglucan and polysaccharides produced by two soil bacteria, Azotobacter chroococcum and Enterobacter cloacae, were also shown to behave anion-exclusively. Ca-polygalacturonate, which has been used as a model root surface polymer, showed little ion-exclusive behaviour compared to polymers extracted from bulk soil and the rhizosphere and root surface of pea, which all showed high levels of anion exclusion. By chemical characterisation of all polymers under study, it was possible to link the presence of uronic acids within the gel to anion-exclusive behaviour. The results suggest that anion exclusion is a common property of microbial and soil polysaccharides. The ability of these materials to restrict the diffusion and thus the availability of nutrient anions at the microorganism or root cell surface may be of significance to the survival and growth of polysaccharide-producing organisms in soil.