Root zone

Carsel RF, Mulkey LA, Lorber MN, et al. 1985. The pesticide root zone model (PRZM) A procedure for evaluating pesticide leaching threats to groundwater. Ecological Modeling 30 49-69. 

It is well known that chemical compo.sition of rhizosphere solution can affect plant growth. Particularly, uptake of nutrients may be considerably influenced by the ionic concentration of the rhizosphere solution (40). Despite the difficulty of defining the exact concentration of ions in the rhizosphere surrounding each root (or even root portion), it has been unequivocally demonstrated that plants have evolved mechanisms to cope with the uneven distribution of ions in the root surrounding in order to provide adequate supply of each essential nutrient (41). These mechanisms include expression of transporter genes in specific root zones or cells and synthesis of enzymes involved in the uptake and assimilation of nutrients (40,43). Interestingly, it has been shown that specific isoforms of the H -ATPase are expressed in the plasma membrane of cell roots it has been proposed that the expression of specific isoforms in specific tissues is relevant to nutrient (nitrate) acquisition (44) and salt tolerance (45). 

J. H. Qian, J. W. Doran, and D. T. Walters, Maize plant contributions to root zone available carbon and microbial transformations of nitrogen. Soil Biol. Biochem. 29 1451 (1997). 

This release takes place predominantly in subapical root zones (30). Unlike Felll-citrate, Felll-PS chelates are stable even at high soil pH levels >... 

Aluminium toxicity is a major stress factor in many acidic soils. At soil pH levels below 5.0, intense solubilization of mononuclear A1 species strongly limits root growth by multiple cytotoxic effects mainly on root meristems (240,241). There is increasing evidence that A1 complexation with carboxylates released in apical root zones in response to elevated external Al concentration is a widespread mechanism for Al exclusion in many plant species (Fig. 10). Formation of stable Al complexes occurs with citrate, oxalate, tartarate, and—to a lesser extent— also with malate (86,242,243). The Al carboxylate complexes are less toxic than free ionic Al species (244) and are not taken up by plant roots (240). This explains the well-documented alleviatory effects on root growth in many plant species by carboxylate applications (citric, oxalic, and tartaric acids) to the culture media in presence of toxic Al concentrations (8,244,245) Citrate, malate and oxalate are the carboxylate anions reported so far to be released from Al-stressed plant roots (Fig. 10), and Al resistance of species and cultivars seems to be related to the amount of exuded carboxylates (246,247) but also to the ability to maintain the release of carboxylates over extended periods (248). In contrast to P deficiency-induced carboxylate exudation, which usually increases after several days or weeks of the stress treatment (72,113), exudation of carboxylates in response to Al toxicity is a fast reaction occurring within minutes to several hours... 

Diffusion-mediated release of root exudates is likely to be affected by root zone temperature due to temperature-dependent changes in the speed of diffusion processes and modifications of membrane permeability (259,260). This might explain the stimulation of root exudation in tomato and clover at high temperatures, reported by Rovira (261), and also the increase in exudation of. sugars and amino acids in maize, cucumber, and strawberry exposed to low-temperature treatments (5-10°C), which was mainly attributed to a disturbance in membrane permeability (259,262). A decrease of exudation rates at low temperatures may be predicted for exudation processes that depend on metabolic energy. This assumption is supported by the continuous decrease of phytosiderophore release in Fe-deficient barley by decreasing the temperature from 30 to 5°C (67). 

T. Shepherd and H. V. Davies, Effect of exogenous amino acids, glucose and citric-acid on the patterns of short-term amino acid accumulation and loss of amino acids in the root-zone of sand-cultured forage rape (Bras.sica napiis L.). Plant Soil 158 111 (1994). 

Information is contradictory about the contributions of root-derived C to the C pools available to the root zone and how this readily available C affects the subsequent associated microbial transformations of soil N (107). In a greenhouse experiment that involved growing maize plants and using C natural abundance and isotope "N techniques, 15% of the soil microbial biomass was derived from... 

E. Zagal, S. Bjarna.son. and U. Ols.son, Carbon and nitrogen in the root-zone of barley (Hordeum vulgare L.) supplied with nitrogen fertilizer at two rates. Plant Soil 757 51 (1993). 

In concurrent research involving a longer-term study, the P. fluorescens Pf-5 strain was inoculated into soil used to grow lupine (Lupinus aihus) and barley Hordeum vulgare L.) over a several-week, period (17). It was shown that iron stress was greatest in the zone behind the root tips as compared to older root zones and in the bulk soil. Calibration of the ice-nucleation reporter activity... 

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