Apoplast characteristics

Agrichemicals which travel mainly in the apoplast characteristically accumulate at the leaf tips and margins of mature leaves, whereas compounds that travel in the phloem accumulate at growing regions (i.e., new leaves, buds, root tips, and storage organs). 

In soil, the chances that any enzyme will retain its activity are very slim indeed, because inactivation can occur by denaturation, microbial degradation, and sorption (61,62), although it is possible that sorption may protect an enzyme from microbial degradation or chemical hydrolysis and retain its activity. The nature of most enzymes, particularly size and charge characteristics, is such that they would have very low mobility in soils, so that if a secreted enzyme is to have any effect, it must operate close to the point of secretion and its substrate must be able to diffuse to the enzyme. Secretory acid phosphatase was found to be produced in response to P-deficiency stress by epidermal cells of the main tap roots of white lupin and in the cell walls and intercellular spaces of lateral roots (63). Such apoplastic phosphatase is safe from soil but can be effective only when presented with soluble organophosphates, which are often present in the soil. solution (64). However, because the phosphatase activity in the rhizo-sphere originates from a number of sources (65), mostly microbial, and is much higher in the rhizosphere than in bulk soil (66), it seems curious that plants would have a need to secrete phosphatase at all. 

There is, however, one means of achieving the conflicting properties required for apoplastic transport without resort to compromise. This is by using precursors which have characteristics favouring uptake and are then converted within the plant to active toxicants which are more readily translocated. The principle is best illustrated by the long-established organophosphorus insecticides of the systox type such as demeton, disulfoton and phorate which are relatively lipophilic... 

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. 

Based on the overall distribution pattern in plants, chemical transport historically has been characterized as being apoplastic or symplastic. Since the mid-1970 s it has been increasingly clear that many compounds are ambimobile (4), in that these chemicals travel in both the apoplast and symplast depending on the physical characteristics of the molecule. In fact, most of the chemicals that were previously characterized as moving only in the apoplast or xylem are now regarded as ambimobile because they penetrate membranes quite readily (4). 

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