Plant vascular system

Pesticides may also be divided into two main types contact or nons-ystemic pesticides and systemic pesticides. Contact or surface coating pesticides do not appreciably penetrate plant tissue and are consequently not transported, or translocated, within the plant vascular system. The earlier pesticides were of this type their disadvantages were that they are susceptible to the effects of the weather and new plant growth was not protected. 

In contrast, most of recently developed pesticides are systemicaUy active and therefore they penetrate the plant cuticle and move through the plant vascular system. Examples of systemic fungicides are benomyl and hexacona-zole. These systemic agents can not only protect a plant from attack but also inhibit or cure established infections. They are not affected by weathering and also confer immunity to all new plant growth. 

West, G. B., Brown, J. H. and Enquist, B. J. (1999b). A general model for the structure and allometry of plant vascular system. Nature, 400,664-7. 

Pesticides are chemicals or biological substances used to kill or control pests. They fall into three major classes insecticides, fungicides, and herbicides. Others classes are rodenticides, nematicides, molluscicides, and acaricides. These chemicals can be natural compounds or synthetic ones. Pesticides may also be divided into contact and systemic pesticides. Pesticides that were earlier produced were of the contact type. They do not appreciably penetrate plant tissue, so they are susceptible to the effects of the weather and moreover new plant growth is not protected. Vice versa, most of the recently developed pesticides are systemically active. They move through the plant vascular system and thus they not only can protect a plant from attack but also inhibit or cure established infections. Systemic pesticides are not affected by weathering and also confer immunity to all new plant growth. In 1993, 57% of all the pesticides used in the United States were herbicides, while 23% and 12% were insecticides and fungicides, respectively [1]. 

Mosses and liverworts (Bryophyta) are more complex than algae. Some of the larger species have structures that superficially appear similar to roots, stems and leaves, but they lack the internal conducting systems present in the vascular plants (Tracheophyta). Internal transport systems (vascular systems) make possible the large sizes of terrestrial plants where the soil is the source of some requisites (water, mineral nutrients) and the air is the source of others (CO2, sunlight). The different groups of vascular plants are characterized primarily by their methods of reproduction. Vascular plants are the source of all wood. 

These results corroborated that alfalfa absorbed the Au(0) from the medium and translocated it through the vascular system. The EXAFS results (Figure 5, Table 3) showed that the Au atoms in plant samples had longer distances that the Au atoms in the tetrachloroaurate, but had equivalent distance to the Au atoms in the gold foil, which confirmed that the Au in plant samples was Au(0). 

Signs Lesions appear on the leaves of rice plants and vary in size. They are usually diamond shaped and have a gray or white center with a brown or reddish-brown border. Crop loss of 50-90% has been reported. Lesions also appear on the rice head but are brown or black in color. Rice grains do not develop properly. In severe neck infections, the stem will break and the head will drop off. The fungus can infect the roots and also invade the plant s vascular system blocking the transport of nutrients and water from the roots. ... 

Xanthomonas campestris causes leaf blight in cabbage, and, generally, Xanthomonas species are plant pathogens. In this invasion of the plant, the viscous, extracellular polysaccharides may bind strongly to the cell-wall polysaccharides of the vascular system, and... 

Similiarly variable is the systemic activity of these compounds, that is the translocation in the vascular system of the plant. In this respect, cymoxanil with its very localized distribution, and fosetyl with its fast and strong translocation both acropetally and basipetally, are the extremes (Table IV). From this point of view, fosetyl is the most remarkable structure it is the only commercial pesticide showing effective acropetal and basipetal translocation at normal use rates. 

Metalaxyl and most of its active analogues are chiral molecules. Chirality is caused by the asymmetric carbon atom in the alkyl side chain of the alanine moiety. The two optically pure enantiomers S (+) and R (-) differ widely in their biological activity both in vitro and in vivo. In all experiments, the R (-) enantiomer was more active than its antipode S (+) (22, 24, 30). The main characteristics of metalaxyl have been discussed in detail by several authors (J, 21, 28, 29, 32> 38). Of particular value is the rapid uptake of metalaxyl by the plant tissue, especially under the wet conditions that favor foliar Oomycete diseases. Acylalanines are easily translocated in the vascular system of the plant after foliar, stem or root treatment (35, 47). The predominant route of transport is the transpiration stream, thus apoplastic (12, 35). Symplastic transport occurs but is much less evident (35, 47). In potatoes treated by foliar sprays of metalaxyl concentrations (0.02-0.04 ppm), Bruin et al. (SO were able to demonstrate protection of harvested tubers from late blight. 

Roots modify their environment quite extensively in many ways. The most important of these are pH change, exudation and microbiological activity in the rhizosphere. Root exudates contain compounds such as hydroxycarboxylic acids and amino acids and these are capable of complexing trace metals. Bowling (1976), Farago (1986) and Streit and Stumm (1993) have discussed the theories of mineral uptake by plant roots the first suggests that there are four links in the uptake chain movement of ions or complexes in the soil to the roots uptake into the root transport across the root to the vascular system and movement to the shoot. 

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