Shoot turnover times

Of great importance, for these studies, was knowledge of both pool size and turnover. This knowledge permitted calculation of the specific activity of the applied isotopically labeled compound at any desired time. Thus, radioactivity, from a labeled compound applied to the endosperm and appearing in the shoot could be translated in amounts of compound moved from seed to shoot. 

In soils, lead concentrates in organic-rich surface horizons. In one instance, only 17.0 mg of soluble Pb/kg was found in soils days after the addition of 2784.0 mg of lead (as lead nitrate)/kg. The estimated residence time of lead in soils is about 20 years complete turnover in topsoil is expected every few decades. In forest litter, however, the mean residence time of lead is lengthy estimates range from 220 years to >500 years. Lead may leach from loamy soils of clay target shooting sites, where soils contain about 50,000.0 mg Pb/kg (about 40% Pb as particulate lead shot in the most contaminated areas) leachates at 100 mm depth contained up to 3.4 mg Pb/L vs. little or no lead in leachates collected... 

More recently we have measured the turnover rate of lAA in tomato shoots after different incubation times, as part of a study of lAA biosynthesis in that tissue. lAA was supplied to the surface of the youngest leaf, 1 cm or longer, of excised shoots from twenty, 4-week-old tomato plants, as a 10 /xl drop (2 1 1, ethanol propan-2-ol H2O) containing 3711 Bq pH]IAA (1.11 GBq//xmol). Uptake was allowed for 2 h, surface radioactivity was then removed by several washes with 50% aqueous propan-2-ol followed with water, and the plants equilibrated for 1 h. After 0,4,10 or 20 h incubation in continuous light, lAA was extracted, purified, derivatised to form the pentafluorobenzyl ester [10] and the specific activity measured by liquid scintillation counting and GC-ecd. The purity and identity of selected samples was confirmed by combined GC-MS. 

Table 1. Change in specific activity of pH]IAA and t,/., for lAA turnover with increasing incubation time after application of pH]IAA to the youngest leaf of 4-week-old tomato shoots...

In three experiments with different incubation times the half-times for lAA turnover varied between 3.1 h to 5.6 h but there was no consistent increase or decrease with incubation time. However, the ti/2 values from shoots supplied with [ H]IAA via the transpiration stream were significantly smaller (an average of 4.2 h) than those from plants supplied via the youngest leaf (10.3 h). Table 2. The... 

If dilution of specific activity alone is used to measure turnover, care should be taken to use small amounts of labelled compound, if possible less than 1% of endogenous. The compound should be applied so that it will be transported by the same routes as endogenous. Finally, sufficient time should be allowed for equilibration before starting the experiment. Using these guidelines we have measured the turnover of lAA in tomato shoots. Confirmation of the order of magnitude of the turnover rate was obtained from incorporation results. Using either the half-life of lAA turnover (10.3 h) or the rate constant (0.0673 h ) the rate of lAA metabolism was calculated from Eq. (3) and the lAA concentration of the shoots (9.3 ng/g fresh wt). A value of 3.6 pmol/g fresh wt/h was obtained. This can be compared in future experiments with rates of lAA synthesis from different potential precursors. 

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