Transpiration stream

Assume that the entering material is rapidly mixed so that the composition is always uniform in the radial direction. The transpiration rate per unit length of tube is = q(z) with units of m /s. Component A has concentration Utrans = o-transi/) in the transpired stream. The component balance, Equation (3.4), now becomes... 

Upon formation of a metal chelate or complex, the next rate-limiting step in delivering iron to the cell is the diffusion of iron complexes through the. soil in response to diffusion gradients. In the vicinity of plant roots, metal chelates and complexes may also move by bulk flow in the transpiration stream as water moves from the soil into the plant. However, depending on their charge characteristics and hydrophobicity, metal chelators and complexes can become adsorbed to clay and organic matter, which may then decrease their mobility and bioavail-... 

Water is absorbed by the plant through the roots and transpired through the leaves, mainly during daylight. The transpiration stream provides soil nutrients in solution and keeps the grass plants turgid. This turgidity ensures optimal photosynthesis by the leaves. 

Estimating the Transpiration Stream Concentration Factor (TSCF) and Root Concentration Factor (RCF) for Some Typical Contaminants (8)... 

TSCF Transpiration stream concentration factor (dimensionless) u Air velocity (m/s)... 

Atrazine enters plants primarily by way of the roots and secondarily by way of the foliage, passively translocated in the xylem with the transpiration stream, and accumulates in the apical meristems and leaves (Hull 1967 Forney 1980 Reed 1982 Wolf and Jackson 1982). The main phytotoxic effect is the inhibition of photosynthesis by blocking the electron transport during Hill reaction of photosystem II. This blockage leads to inhibitory effects on the synthesis of carbohydrate, a reduction in the carbon pool, and a buildup of carbon dioxide within the leaf, which subsequently causes closure of the stomates, thus inhibiting transpiration (Stevenson et al. 1982 Jachetta et al. 1986 Shabana 1987). 

Canny MJ. What becomes of the transpiration stream New Phytol 1990 114 341-368. 

Transport of NH4+ to the roots in Kirk and Solivas experiment was mainly by diffusion. The additional transport resulting from mass flow of soil solution in the transpiration stream would have increased the influx across the roots by about QQaVa/0.5bD% where Va is the water flux (Tinker and Nye, 2000, pp. 146-148), or about 4% in Kirk and Solivas experiment. A sensitivity analysis showed that rates of diffusion will generally not limit uptake in well-puddled soils, but they may greatly limit uptake in puddled soils that have been drained and re-flooded and in unpuddled flooded soils. 

It has long been recognized that boron is required by higher plants [61, 62], and recent research indicates the involvement of boron in three main aspects of plant physiology cell wall structure, membrane function, and reproduction. In vascular plants, boron in solution moves in the transpiration stream from the roots and accumulates in the stems and leaves. Once in the leaves, the translocation of boron is limited and requires a phloem transport mechanism. The nature of this mechanism was only recently elucidated with the isolation of a number of borate polyol compounds from various plants [63-65]. These include sorbitol-borate ester complexes isolated from the floral nectar of peaches and mannitol-borate ester complexes from the phloem sap of celery. The implication is that the movement of boron in plants depends on borate-polyol ester formation with the particular sugar polyol compounds used as transport molecules in specific plants. 

It became clear, however, that in the whole plant the major site of cytokinin synthesis was in the roots from whence it was transported to the rest of the plant via the xylem transpiration stream. Perhaps kinins were a kind of rhizocaline ... 

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. 

The incorporation of label from mevalonate into ABA, a sesquiterpenoid, has been demonstrated in different parts of plants ( . . 41). This indicates that ABA can be synthesized throughout the plant. In addition to the direct incorporation of three isoprene units, derived from mevalonate, into ABA, an indirect biosynthetic pathway via carotenoids has been proposed. This idea stems from the finding that xanthophylls, in particular violaxanthin, can either photochemically or enzymatically be converted to the neutral inhibitor xanthoxin (42) (Figure 3). When labeled xanthoxin was fed in the transpiration stream to bean or tomato shoots, ca. 10% was converted to ABA over an 8-hr period (43). However, the importance of the biosynthetic route to ABA via xanthophylls and xanthoxin in normal metabolism remains to be established, and most of the evidence favors the direct synthesis route via a precursor (see 2). 

Fluorides are readily translocated to the tip and margin of leaves in the transpiration stream. If atmospheric levels of HF are low enough, the intercostal injury will not develop, and the fluoride concentration will increase at the periphery of the leaf. Acute fluoride intoxication at the margin of dicotyledonous plants, according to Solberg et al. (18), is first characterized by a collapse of the spongy mesophyll and lower epidermis, followed by distortion and disruption of the chloroplasts of the palisade cells, and finally, distortion and collapse of the upper epidermis. The injured area soon turns brown during hot, dry weather, but this symptom may be delayed if the weather is cool and damp. 

A root-derived suppressor negatively regulating the formation of roots has been hypothesized.497 This proposed mechanism, termed root apical dominance, is analogous to the means for apical dominance in shoots. The cytokinin, tZR, in root xylem sap is hypothesized to play the role of the main suppressor, and tZR transported from roots to shoots via the transpiration stream negatively regulates the formation of adventitious roots.365... 

One characteristic activity of ABA is its effect on stomata, which protects plants from water stress. At 10 7moll 1, ABA given through a transpiration stream from cut ends of shoots causes stomatal closure.702 The activity is more effective in epidermal strips floated on a buffer solution than in shoots. At 10-10 mol 1 1, ABA closes stomata in epidermal strips at pH 5.5.703 The activity at pH 6.8 is 103 times lower than that at pH 5.5, suggesting that the active form of ABA is not a dissociated acid but an undissociated acid. The simulation study... 

E. What is the capacitance and the time constant for water release from the trunk, if stored water equivalent to 0.8% of the trunk volume enters the transpiration stream Assume that the resistance involved is the same as... 

During photosynthesis, chloroplasts convert CO2, water and Pj to triose phosphates that are exported to the cytosol. Phosphate is therefore a substrate of this process and the continued operation of the RPP cycle is dependent on the utilization of triose phosphate for the synthesis of starch (in the chloroplast) and sucrose (in the cytosol). These synthetic processes release Pj, preventing the level of free Pj in the cell from falling to a concentration where photosynthesis may be limited by its availability. Such a limitation of photosynthesis is observed during O2-insensitive CO2 assimilation [56] and is suggested by the increase in CO2 fixation detected on feeding P via the transpiration stream to a cut leaf [57]. It has long been known that isolated chloroplasts require a continuous supply of P in order to sustain photosynthesis. 

To understand how 36 is metabolized, it was fed to bean shoots through the cut ends via a transpiration stream before extraction with methanol and purified after methylation to afford the methyl esters of metabolites 37-39. These were individually isolated, but slowly converted to an equilibrium mixture in a 37/38/39 ratio of 7 6 1 in methanol at 25°C. These findings suggest that the 8 -hydroxylated metabolite 37 is thermodynamically and kinetically more stable than 8 -HOABA. We examined quantitatively the equilibrium ratios and the isomerization rates of 8 -HOABA/PA and 37/38 in aqueous solution buffered at various pHs and temperatures, and discussed the thermodynamic and kinetic characteristics of the isomerization process on the basis of a computer-aided molecular orbital analysis of model compounds in addition to experimental results [90]. 

In monocots, Fe and Zn are taken up as phytosiderophore chelates by YSL transporters in the epidermis. Fe can also be taken up by OsIRTI. Metals move through the symplastic space to the vasculature. The citrate effluxer FRDLl is important for loading of citrate into the xylem and subsequent Fe transport to the shoot through the transpiration stream. YSL transporters also may play a role in unloading the xylem into the shoot and the phloem. Fe is unloaded from the phloem by OsYSL2 and OsIRTI into shoot and seed tissue. 

Root uptake has been proven to be an important pathway for contaminants with intermediate octanol-water partitioning coefficients (Aiow)- Variable uptake of an organic compound by different plants has been observed. Plant species such as Daucus carota (carrot) and Pastinaca sativa (parsnip) with swollen storage roots did not translocate chemicals as well as expected from barley experiments. While the lipid content was considered a factor, plant structure, root types, and other properties may all play a role. The effect of the chemical itself was best illustrated by the increasing root concentration factor (RCF) and the bell-shaped transpiration stream concentration factors (TSCF) relative to logAiow- The physiochemical properties of compounds, including the Ko, solubility, and... 

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