= Root exudate
A <root exudate> is material released by a <root (botany)> into its immediate surroundings, including the <rhizosphere>. An exudate sample is different from a root-tissue extract: collecting released material asks what reaches the environment, whereas disrupting tissue also exposes intracellular contents.
One released compound can have a striking effect on another organism. In 1966, investigators isolated the <germination> stimulant <strigol> from <cotton> root exudates and demonstrated activity on a root-parasitic <Striga>. Chemical isolation linked a constituent of the released material to a measured developmental response.<Germination of Witchweed (Striga lutea Lour.): Isolation and Properties of a Potent Stimulant by C. E. Cook and others>[Abstract: crystalline stimulant isolated from cotton root exudates and germination activity]{ref}
For an original quantitative example, suppose a collection contains 30 nanomoles of a compound after 3 hours in contact with 2 grams of roots. With zero initial amount, no background contribution and no losses, the mean net appearance rate is
$$
\frac{30\,\mathrm{nmol}}{(3\,\mathrm h)(2\,\mathrm g)}=5\,\mathrm{nmol\,g^{-1}\,h^{-1}}.
$$
The assumptions matter: degradation, uptake, damaged-cell leakage or material from associated <microorganisms> could change the interpretation. A root-free collection blank and an explicit mass/time normalization help identify what was actually measured.
A <germination assay> on the same collection asks an additional question about biological activity. Amount released and potency per unit amount are separate properties; neither measurement alone specifies the other.
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