Leaf disks

The regulations require three samples from the treated plot (one from each subplot) and a single sample from the control plot at each sampling interval. For foliage the preferred technique is to collect leaf punch samples. Leaf punch samplers are available in 5-, 2.5- and 1.25-cm punch areas. Common practice requires a sample of 40-5-cm leaf disks to provide a 400-cm sample using both the top and bottom of the leaf disk to calculate sample surface area. 

If a smaller leaf punch is used, an increased number of leaf disks must be generated for each sample. Only sample when leaf surfaces are dry from application or dew. Leaf disks are placed in glass jars for further analysis. 

In the oxamyl tomato study, the DFR samples were obtained using a 5-cm Birkestrand sampler (10-cm disk size using the upper and lower surfaces). The sample consisted of 40 leaf disks or 400 cm from each subplot. The samples were collected impartially or in a nondirected approach from the middle two rows. The plot was four rows wide and the tractor came into contact with the first and fourth rows as the application was made. The middle two rows should be undisturbed by this movement and should therefore provide a more representative sample. 

Six control sample jars of 0.01% Aerosol OT detergent in distilled water solution (200 mL each) were prepared at each site on sampling days 1,14 and 28 by dislodging leaf disks taken from the untreated control area. Triplicate samples were fortified at each of two concentrations. Fortification levels were 50 and 400 qg of oxamyl per sample. 

To show that the method is consistent, it is best to conduct the method validation over two or three sets with different fortification levels carried out in each set. Fortify the leaf disk sample, dislodge, then add the extraction solvent and perform the extraction procedure. 

Leaf water potential and osmotic potential were measured using a Wescor Dewpoint Microvoltmeter (Model HR-33) coupled with C-51 and C-52 sample chambers. Two plants from each group were sampled each day by taking two 7-mm diameter leaf disks from each plant, one for water potential and one for osmotic potential. Plants from which leaf disks were obtained were discarded. The water potential of a leaf disk was read following a 2-hr equilibration period in a sample... 

Seasonal variations in the metabolic fate of adenine nucleotides prelabelled with [8—1-4C] adenine were examined in leaf disks prepared at 1-month intervals, over the course of 1 year, from the shoots of tea plants (Camellia sinensis L. cv. Yabukita) which were growing under natural field conditions by Fujimori et al.33 Incorporation of radioactivity into nucleic acids and catabolites of purine nucleotides was found throughout the experimental period, but incorporation into theobromine and caffeine was found only in the young leaves harvested from April to June. Methy-lation of xanthosine, 7-methylxanthine, and theobromine was catalyzed by gel-filtered leaf extracts from young shoots (April to June), but the reactions could not be detected in extracts from leaves in which no synthesis of caffeine was observed in vivo. By contrast, the activity of 5-phosphoribosyl-1-pyrophosphate synthetase was still found in leaves harvested in July and August. 

Adedipe et a// found that Bel W, tobacco leaves were more sensitive to ozone when attached, rather than detached or used as leaf disks. Effects were seen as visual injury and change in chlorophyll content. 

Table V, Cotton Leaf Disk "Choice" Bioassay of Azadirachtin with...

The ozone treatment had no significant effect on the vitro NR activity, indicating that it did not inactivate the NR protein (Table III), Leaf extracts that would couple the oxidation of fructose-1, 6-diphosphate to nitrate reduction were prepared from leaves exposed to either 0 or 980 yg/m ozone ( ), Ozone depressed the in vitro coupled NR activity 58% (Table III), indicating that the observed ozone depression of nitrate reduction in the vivo leaf disk assay resulted from a depression in the rate of NADH formation by GPD,... 

Small leaf disks (-1 cm2) were cut out of the central part of the Q. kelloggii leaves using a hole punch. Edges were cut off of the N. solandri leaves (complete leaves were small enough... 

The safe levels established for parathion + paraoxon, azinphosmethyl + azinphosmethyl oxon and methidathion + methi-dation oxon on foliage have absorbance values determined by the rapid field method (4 ) equal to those given in Table III. Absorbance values greater than those listed in Table III signal an unsafe working condition. Field testing can also be conducted by standard gas chromatographic analysis of the leaf disk samples by state-approved laboratories. 

A 2.54-cm diameter leaf punch sampler is normally used to collect leaf samples from citrus, while a 1.8-cm diameter punch sampler is more suitable for peaches with narrow leaves. A forty-1.8-cm diameter leaf disk sample represents a total surface area of 200 cm. An additional 40 leaf disks, therefore, must be collected to give a sample representing 400 cin as required by the method. 

The influence of Ag+ and CO on ethylene action was observed in senescing tobacco leaf disks in which loss of chlorophyll was taken as an index of senescence (Fig. 7). 

Figure 7. Influence of Ag (10 ppm), C02 (10% ), and AVG (O.lmM) on chlorophyll retention in aging tobacco leaf disks senesing in the dark for six days...

Carbon dioxide and Ag+ ions clearly suppressed senescence, as determined by chlorophyll loss. Aminoethoxy vinylglycine (AVG), the inhibitor of ethylene biosynthesis, also significantly suppressed senescence, as determined by preservation of chlorophyll in the leaf disks aging in the dark.Combinations of C0-, Ag ions and AVG were especially effective on preserving chlorophyll, presumably by suppressing both ethylene biosynthesis and action at the two receptor sites. After 6 days aging at 25° in the dark, the controls contained only 7% of the chlorophyll present at the start, whereas 84% of the chlorophyll was retained by the leaf disks treated with a combination of CO-, Ag and AVG. 

We used a leaf disk bioassay adapted from Rowland et al.35 to test the insecticidal activity of lichen secondary products on B. tabaci. Two lichen secondary compounds, (-)-usnic acid and vulpinic acid, showed significant results when compared to the controls. Vulpinic acid had an average mortality of 18%, and (-)-usnic acid had an average mortality of 14%. From the dose response of (-)-usnic acid, LD50 was not reached at 1000pM, but a positive correlation was established with increasing concentration and whitefly population response (data not shown). 

Other bioassays have been developed to evaluate weed pathogen effects using leaf disks. Culture extracts of three fungal pathogens (Fusarium oxysporum, Cylindrocarpon destructans and Colletotrichum dematium) were bioassayed using... 

The leaf disk method is a conventional type of bioassay in which disks of constant area are cut from leaves of a certain plant and are coated with the extract or pure compound (in solution). These are then presented to insects in a petri dish. After a certain amount of time, the percentage of the leaf that has been consumed is estimated, visually17 or photographically with or without software.18 A leaf that is coated with the solvent only is used as the control. This bioassay can be used with small variations, for example, instead of leaves, an artificial diet containing sucrose, flour, or even calcium alginate19 can be mixed with the compound. 

Treatment of potato leaf disks with 10 -clerodanes isolated from Teucrium and with 10 of their synthetic derivatives resulted in a significant antifeedant activity against L. decemlineata, although in choice and no-choice bioassay concentrations of 1000 ppm were used. For the most active compounds, effective concentration to inhibit 50% of the feeding (EC50) ranged from 53 to 394 ppm.75 Of the 12 most active antifeedants 128-139,... 

The Chinese mangrove, Xylocarpus granatum (Meliaceae), contains a large number of 8,9,30-phragmalin orthoesters, called xyloccensins.12 These were isolated from either the seeds or the stem bark of the Chinese mangrove. In total 25 xyloccensins have been isolated, xyloccensins A—V, Y, Zl, and Z2. Tested with a conventional leaf disk method against the third-instar larvae of M. separata... 

Figure 9. Cotton leaf disk "choice" bioassay.

In fact, abyssinol A (II), B (III) and C (IV), isolated monitoring by the leaf disk assay> did not show any Insect growth inhibitory activity, as is shown in Table III. 

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