A comparative histochemical study of plant polyphenols in southern African grasses

Abstract

Recent anatomical studies have shown that tannin‐like substances (TLS) occur in the epidermal cells of a number of southern African tropical grasses, and the presence of condensed tannins in grasses has been confirmed by chemical analyses. A number of species from four of the five subfamilies of the Poaceae were compared for their responses to a range of histochemical tests which differ in their specificity for phenolic compounds. These included: ferrous sulphate, acidified vanillin, diazotized sulphanilic acid, Fast Blue‐BB, dimethoxybenzaldehyde and nitrous acid, Safranin and Fast Green. In addition, the radial diffusion test for protein precipitation was used. Comparative histochemical tests indicated that most taxa known to contain TLS showed comparable responses to the tests used here, with variations in intensity and hue of the coloured products formed. These qualitative differences suggest the presence of a number of different compounds including oligomeric procyanidins, oligomeric prodelphinidins, monomeric and/or dimeric flavan‐3‐ols and flavan‐3,4‐diols. The presence flavan‐4‐ols has been confirmed in the andropogonoid grasses by previous workers. Histochemical tests are adequate to identify the presence of condensed tannins and their precursors in plant tissue. However, they do not provide a means to identify those compounds which precipitate protein and function as digestibility‐reducing compounds in plant‐herbivore interactions.

Onlangse anatomiese studies het getoon dat tannienagtige stowwe (TLS) in die epidermis selle van ‘n aantal tropiese grasse van suidelike Afrika voorkom, en die teenwoordigheid van gekondenseerde tanniene in grasse is deur chemiese analises bevestig. ‘n Aantal spesies van vier van die vyf subfamilies van die Poaceae is vir hul reaksies op ‘n reeks van histochemiese toetse, wat verskil in hulle spesifisiteit vir fenoliese verbindings, vergelyk. Dit sluit in ferrosulfaat, aangesuurde vanillien, gediasotiseerde sulfanielsuur, Vas Blou‐BB, dimetoksibensaldehied en salpetrigsuur, en die Safranien en Vas Groen prosedure. Die radiale diffusie toets vir proteïen presipitasie is bykomend gebruik. Vergely‐kende histochemiese toetse het getoon dat meeste taksa wat bekend is dat hulle TLS bevat, vergelykbare reaksie toon op die toetse hier gebruik, met variasies in intensiteit en skakering van die gevormde gekleurde produkte. Hierdie kwalitatiewe verskille dui op die teenwoordigheid van ‘n aantal verskillende verbindings, insluitend oligomeriese prosianidiene, oligomeriese prodelfinidiene, monomeriese en/of dimeriese flavan‐3‐ole en flavan‐3,4‐diole. Die teenwoordigheid van flavan‐4‐ole is deur vorige werkers in die andropogonoïede grasse bevestig. Histochemiese toetse is voldoende om die teenwoordigheid van gekondenseerde tanniene en hulle voorgangers in plantweefsel te identifi‐seer. Hulle verskaf egter nie ‘n metode om daardie verbindings wat proteïene presipiteer en as verteerbaarheidsredu‐serende verbindings in plant‐herbivoor interaksies funksioneer, te identifiseer nie.

Additional index words:

• Condensed tannins
• flavan‐4‐ols
• proanthocyanidins