Root membranes

Salt damage is due to the product of xylem concentration and transpiration rate over the life of the leaf. There are several pathways for salt entry into the root. In spite of many reports in the literature of correlative associations between the lipid analyses of relatively crude root membrane preparations and salt resistance (see Kuiper, 1985), we have not been able... 

Phytostabilization on the root membranes. Proteins and enzymes directly associated with the root cell walls can bind and stabilize the contaminant on the exterior surfaces of the root membranes. This prevents the contaminant from entering the plant. 

Phytostabilization in the root cells. Proteins and enzymes present on the root cell walls can facilitate the transport of contaminants across the root membranes. Upon uptake, these contaminants can be sequestered into the vacuole of the root cells, preventing further translocation to the shoots. 

Varanini, Z., and Pinton, R. (2007). Root membrane activities relevant to nutrient acquisition at the plant-soil interface. In The Rhizosphere Biochemistry and Organic Substances at the Soil-Plant Interface, 2nd edition, Pinton, R., Varanini, Z., and Nannipieri, P., eds., CRC Press, Boca Raton, FL, pp. 151-172. 

Clearly, in the presence of the high level of silicon, aluminum was prevented from binding at gill epithelial surfaces and systemic absorption. This exclusion occurred at the interface between creature and the external environment, and a fundamental question is, Is this a general effect, not only at the fish gill, but also at plant root membranes and in the gastrointestinal tract of mammals and humans ... 

Chloride toxicity appears to be similar to Na toxicity. Excessive accumulations in tissues near plant tips, the end of the plant transpiration stream, lead to necrosis, death of leaf tips and margins, and eventual death of the plant. Some plants are able to screen out such ions through their root membranes. In addition, different rootstocks may possess varying abilities to exclude sodium or chloride from above-ground parts (Table 11.5). Some grape rootstocks exhibit up to 30-fold differences in their abilities to exclude chloride ions. Selection of a rootstock that screens out ions may prevent toxic accumulations in plant tops. 

The lack of involvement of photosynthetic inhibitions in lipid inhibitions by pyridazinones is also supported by results in non-photosynthetic tissue (11, 12). The phospholipids predominate in root membranes. The data in Table V show that BASF 13 338 does not influence the distribution of lipid between the various classes of phospholipids, but BASF 13 338 specifically decreases the linolenic acid content of phosphatidylcholine and phosphatidylethanolamine. 

These organic promoters and inhibitors must be at sufficient concentrations and be present for sufficient length of time to modify plant function and growth of receiving plants either directly (e.g., impact on root membranes and/or cell process) or indirectly (e.g., impact on nodule or mycorrhizae formation,... 

Pro 1 Phenolic acids do not need to be taken up by roots and translocated to the shoots (i.e., leaves) to be effective. All that is required is root membrane contact. The primary effect of phenolic acids on sensitive seedlings appears to be associated... 

Liljenberg, C. and Kates, M., 1985, Changes in lipid composition of oat root membranes as a function of water-deficit stress. 

EFFECT OF SALINITY ON THE STEROL CONTENT OF SOYBEAN ROOT MEMBRANES... 

Plasma root membranes play an important role in the ability of plants to cope with salinity. Indeed PM exercises a high degree of control on ion fluxes via their H+ pumps and transport proteins. More precisely, the chemical structure and physical state of the lipid environment (in particular sterol) is important for membrane protein function. Therefore we have chosen to study the sterol composition of root PM to gain further insight into the mechanism of Na" " exclusion. 

Unlike organic contaminants, nutrient and metal ions are generally actively transported into roots since the rate of passive transport of charged compounds across hydrophobic root membranes is limited. Plants take up most nonnutrient metals incidentally while acquiring nutrients for growth (Pulford and Watson, 2003 Saison et al., 2004). We do not address uptake of metals and nutrients in this chapter. The reader is referred to the above references and to phytoremediation reviews by Salt et al. (1995), Raskin and Ensley (2000), and Weis and Weis (2004). 

Once the chemical passes through the root membrane, and depending on its properties, it can be transported to other parts of the plant by way of the flow of sap in the xylem and phloem channels. Xylem channels conduct the unidirectional flow of water and nutrients from roots to the photosynthetic sections of the plant while phloem is the bidirectional flow that distributes sugars and other photosynthetic products throughout the plant. However, within the xylem, lateral movement to adjacent cells occurs and may provide a pathway for contaminants to partition into the phloem (Hendrix, 1995 Marschner, 1995). Xylem transport rates are on the order of 10 cm/min while phloem transport rates are much slower, 1 cm/min (Lang, 1990). Xylem transport rates are directly related to transpiration rates while the rate of phloem transport is governed by differences in solute concentrations between sites of solute synthesis (source) and consumption (sink) (Marschner, 1995). 

Partitioning of nonionized organic chemicals into the lipophilic root membranes and the root sap has been described as a root concentration factor (RCF), the ratio between the chemical concentration in the roots and that in the exposure media (water or soil) contacting the roots (Briggs et al., 1982,1983 Topp et al 1986). 

Ionized compounds also enter the roots, but do not readily cross the root membranes and enter the xylem. For organic acids and bases, the pH of the system and the pA a of the compound can be used to determine the fraction of the neutral form of the chemical most available for uptake (Briggs et al., 1987 Trapp, 2000, 2004). These ionic substances undergo a variety of additional processes such as dissociation, ion trapping, and electrical interactions with soils and plant cells (Trapp, 2004). 

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