Phytotoxicity penetration

Penetration into and accumulation by fungi are considered to be key factors in the selective toxicity of many fungicides and again polarity appears to be a determining property. Indeed selectivity between fungitoxicity and phytotoxicity... 

All the facts discussed have a distinct bearing on the principles of soil fumigation against nematodes. Plant parasites may be in the roots, loose in the soil, or in the form of eggs enclosed in a mucoid mass. In any case they are probably covered by a film of moisture. The fumigant must (1) be dispersed through the soil, (2) penetrate all barriers, (3) kill, and (4) leave no phytotoxic residue. 

The central issue seems to be whether it is better to keep the deposit near the bark surface or have it penetrate. More spray volume is required to gain good penetration, and this raises the cost. Low volume sprays that do not penetrate significantly would be cheaper and would involve a lesser risk of phytotoxicity, where green trees are sprayed for protection. 

Water-soluble copper compounds are in any case unsuitable as foliage sprays, due to their strong scorching effect. The phytotoxity of water-insoluble copper compounds is weaker, and is to be expected when more soluble copper compounds are formed than needed. Phytotoxicity is caused by the copper(ll) ions penetrating the plant cells. Fortunately, the translocation of copper(II) ions is moderate in plants, so that only local damage occurs and the whole plant is not killed. 

In continously wet, hot weather elemental sulfur may cause injury, particularly to certain sulfur-sensitive plants. Phytotoxicity is manifested by a small or large reduction in photosynthesis and respiration, in the scorching of leaves and, in severe cases, in retarded foliage growth (Hoffman, 1933, 1934, 1936). Turell (1950) attributes phytotoxicity to a decrease in critical temperature and to the absorption of sun rays (lens effect). In lemon cultures, damage due to sulfur which would otherwise occur only at higher temperatures has been observed at lower temperatures. However, the true reason for its phytotoxicity is most probably the fact that sulfur penetrates the plant tissues and, as a hydrogen acceptor, detrimentally influences metabolic processes. 

Ammonium sulfate is a compound with weak herbicidal action. Applied at high rates on the leaves of dicotyledons it has a contact action. Its phytotoxic action can be attributed mainly to the ammonium ions. Ammonia rapidly penetrates to the acid-buffered cell sap, making it alkaline, thus rapidly destroying the cells (Harvey, 1911). Bokomy (1915) assumed that ammonia formed complexes with cell proteins. 

A purely apoplastic movement of monuron is inconsistent with the fact that the site of phytotoxic action is within the chloroplast, and that to reach it the herbicide mtist penetrate the protoplasm and the plasmalemma surrounding the chloroplast (Ashton and Crafts, 1973). 

The herbicides reaching the plant rapidly penetrate the green parts and destroy the tissues by contact action. Absorption, and consequently phytotoxic action, can be increased by nonionic surfactants. Concentrated paraquat solutions are more phytotoxic than dilute ones, and a spray applied in smaller droplets (100/im) is more phytotoxic than one of larger (300 m) drop size (McKinlay et al., 1974). 

The use of plant cell cultures in pesticide metabolism studies has a history of only about 20 years, but pioneers of the technique have laid the groundwork for an increasing number of researchers interested in the advantages the cell culture systems offer. Host have chosen to use suspension cultures, because of the ease with which they can be manipulated, and the increased possibility for standardization of conditions from laboratory to laboratory. As an adjunct to whole plant studies cell cultures provide information as to the changes that structural modifications of a basic molecule may have on phytotoxicity, especially in detecting the inherent toxicity of a molecule that fails to penetrate or translocate in a whole plant. 

Estimation of Phvtotoxicity. Several investigators have used cell cultures to study herbicide phytotoxicity (3. 5. 18-20). Callus and cell suspensions have potential in the estimation of phytotoxicity, especially in detecting the inherent phytotoxicity of a molecule that fails to penetrate into or translocate in a whole plant. Alteration of the molecule to enhance penetrability may provide a usable herbicide. Structure activity relationships have also been examined to determine the comparative potency of different plant growth regulators in cell cultures in comparison to whole plants (21). 

With few exceptions, herbicides are subject to metabolic transformations both in weed and crop species, after they have penetrated the plant tissue and are under way to their target site. As a rule, the herbicide metabolites are more polar than the herbicidal parent compound, and they exhibit reduced phytotoxicity or are completely non-phytotoxic. While often the first step of herbicide metabolism entails a partial or total detoxification of the parent compound, there are other cases where the herbiddaDy active form is generated in the first metabolic reaction (e.g., the hydrolysis of the inactive fenoxaprop-P-ethyl to the herbicidally active free add fenoxaprop-P) followed by detoxification of the molecule in the subsequent metabolic step. 

The effect of surfactants on spray retention efficiency is influenced by droplet size and velocity, leaf angle, and surfactant type and concentration. Examination of spray retention and surface spreading on a range of outdoor-grown crop plants was examined by Anderson et Retention was determined by dynamic surface tension, whereas droplet spreading was related to equilibrium values. They concluded that the basis of selection could be determined by cost, lack of phytotoxicity, and ability to solubilize or aid penetration of the a.i. this would not prejudice the retention properties of the formulation. 

Prepenetration or copenetration of surfactant appears to facilitate transcuticular or transmembrane penetration of the a.i. but to have no direct effect on the efficiency of translocation. In the case of systemic herbicides of low phytotoxicity, increased absorption can result in a proportional increase in the efficiency of translocation to the target sites. The relationships between uptake and translocation are discussed in depth in Chapter 9. 

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