Exudate collection

Exudate collection in trap solutions usually requires subsequent concentration steps (vacuum evaporation, lyophilization) due to the low concentration of exudate compounds. Depending on the composition of the trap solution, the reduction of sample volume can lead to high salt concentrations, which may interfere with subsequent analysis or may even cause irreversible precipitation of certain exudate compounds (e.g., Ca-citrate, Ca-oxalate, proteins). Therefore, if possible, removal of interfering salts by use of ion exchange resins prior to sample concentration is recommended. Alternatively, solid-phase extraction techniques may be employed for enrichment of exudate compounds from the diluted trap solution (11,22). High-molecular-weight compounds may be concentrated by precipitation with organic solvents [methanol, ethanol, acetone 80% (v/v) for polysaccharides and proteins] or acidification [trichloroacetic acid 10% (w/v), per-... 

The application of various antibiotics such as rifampicin/tetracycline (63), cefatoxime/trimethoprim (64), or bacteriostatic compounds such as Micropur (Roth, Karlsruhe, Germany) (65) used for root pretreatment or added to collection media is another strategy to prevent biodegradation during root exudate collection. However, depending on dosage and plant species, also phytotoxic effects of antibiotics have been reported (Table 3). Antibiotics in the soil environment... 

Various techniques for collection of root exudates are associated with the risk of root injury by rupture of root hairs and epidermal cells or rapid change of the environmental conditions (e.g., temperature, pH, oxygen availability) during transfer of root systems into trap solutions, application of absorbtion materials onto the root surface, and preparation of root systems for exudate collection. The possible impact of those stress treatments may be assessed by measuring parameters of plant growth in plants either. subjected or not subjected to the collection procedure (6) and by comparing exudation patterns after exposure of roots to the handling procedures with different intensity. 

Inhibitors applied during the 2-h period of exudate collection into distilled water, starting at the beginning of the light period. In each row, significant differences are indicated by different characters. 

Table 1 Effect of Iron Stress on Chemical Composition of Root Exudates Collected from Barley Plants, as Determined by Combined NMR and GC-MS Analysis (Fan et al., 1997)...

Variance in root size of 5-day old plants was not taken into account in either the calculation of pmol/root or pmold exuded/root. The weight of filtered and lyophilized exudate of 375 plants was used to extrapolate to the figure of 3245 individuals used for exudate collection in the analysis of exudation/root. 

It is a fire hazard because, when the exudate collects on the floor of a storage magazine, it might accidently catch fire, especially if it contains red particles of the expl iron salt... 

Post-WWH investigations conducted at Picarinny Arsenal showed that a dark reddish-brown exudate collected ftom 105mm shells loaded with TNT consisted principally of a mixture of a-TNT with some /3- and yTNT and, sometimes, traces of 2,4-DNT and other products of nitration of impurities in toluene, it is also probable that alcohol present in "NRC" sealing compound and/or other extraneous material might favor exudation. 

Nevertheless, in view of the gravity of the illegal drug problem, it may be worth re-appraising the idea from the platform of modem science. Take the poppy plant, Papaver somniferum, as an example. The plant produces opium, which is the air-dried milky exudate collected from incised unripe poppy-seed capsules. Opium contains a mixture of about 20 alkaloids which amount to ca 25% by weight of the opium. Of these 20 alkaloids the main ones are ... 

Fig. 7. The pH values and organic carbon contents of the root exudates collected from the aqueous cultures of maize, wheat, pea and soybean. Means and standard deviations are shown.

Fig. 3. DPASV polarograms obtained for (a) the control (exudate-free) and (b) the exudate solutions obtained from Fe-starved ryegrass, after additions of four known amounts of Cu and Zn. A bacteriostatic (Micropur) was added to the solutions prior to exudate collection (same for the control solution).

Fig. 2. Total cytokinin activity of butanol-soluble fraction of leaf (O) and root ( ) exudates collected at various times after the start of the 22-h inductive LD. Exudation started at the indicated times and lasted 16 h. Exudates showed no activity in the aqueous fraction...

All the natural SLs have weak to moderate UV absorption with a maximum at around 240-250 nm [1] and thus can be detected by UV or PDA (photodiode array) detector only when their concentration (>10 M) and purity are high enough. In the case of plant root exudates collected and thus concentrated by absorption on activated charcoal, it is sometimes possible to detect SLs without any purification by HPLC equipped with a UV detector. Of course, various compounds have rather strong absorption in this UV region, and concentrations of SLs in plant root exudates are generally quite low. Consequently, it is quite difficult to analyze SLs in crude samples with HPLC-UV detector system. 

Of the ancient, water-soluble commercial gums that are dried, gummy exudations collected by hand from the trunks and branches of various trees and shrubs, only gum arable (Chap. 4) is yet in significant use. 

Gum tragacanth is a plant exudate collected from Astragalus species shrubs grown in the Middle East (Iran, Syria, Turkey). 

Crude g. is an exudation collected from the roots of Ferula galbaniflua (Boiss.) and F. rubricaulus (Umbelliferae/Apiaceae), which are wild-growing in Iran, Syria, Turkey and Lebanon. Initially, a milky latex-like substance, it dries on contact with air into a gum resin, which is then either steam-distilled to produce g. oil or extracted with hydrocarbon solvents to yield g. resinoid. 

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