Physiological effects of secondary plant metabolites

At the cellular level, plant secondary metabolites have five major effects on herbivores (a) alteration of DNA replication, RNA transcription, and protein synthesis (b) alteration of membrane transport processes (c) enzyme inhibition and activation (d) blocking of receptor sites for endogenous chemical transmitters and (e) affecting the conformation of other macromolecules (Robinson, 1979). 

The same diet affects species differently. For instance, mountain hares, Lepus timidus, consume much hirch in their winter diet, while European hares, Lepus europaeus, do not. A high concentration of hirch phenolics in the diet causes massive sodium loss via the urine in European hares, but not in mountain hares (lason and Palo, 1991). Further, among laboratory rodents, hamsters are extremely sensitive to tannins while rats and mice easily adjust within 3 days to doses of condensed tannins that are lethal to hamsters (Mehansho etal, 1987). 

The flavonoid quercetin suppresses growth somewhat in prairie voles, M. ochrogaster, while tannic acid, a hydrolyzable tannin, has an even stronger effect (Lindroth and Batzli, 1984). Quercetin is toxic, but does not bind with protein and, therefore, does not affect protein digestibility. Tannic acid has some effect on protein digestibility but is also toxic. 

Hamsters, unlike mice and rats, are not able to adjust to high levels of tannins in their diet by producing more proline-rich proteins in their saliva. A diet high in tannin has no effect on the salivary glands, and proline-rich proteins are not 

Dry salt grass, Distichlis stricta, has high levels of phenolics after fruiting. Cinnamic acids and their vinyl phenols lower uterus weight, inhibit follicular development, and lead to cessation of reproductive activity in the montane vole Microtus montanus. Specific active compounds are 4-vinylguaiacol and 4-vinylphenol, and alsop-coumaric acid (4-hydroxycinnamic acid) (Berger etal, 1977). 

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