Tall Fescue Online Monograph

Diazaphenanthrene and Indole Alkaloids

Perloline and its oxidation product perlolidine are diazaphenanthrene alkaloids in tall fescue. Perlolyrine represents the indol group and is present in much smaller amounts than perloline and perlolidine (Fig. 13-1). These alkaloids inhibit in vitro cellulose digestion when present at greater than 10^-4 M concentration, but this effect in vivo has not been documented (Bush et al., 1976). Perloline and perlolidine had similar activity, but perlolyrine was much less. The early investigations were done with paper and thin-layer chromatography. Modern separation techniques have not been applied to these alkaloids because of their apparent lack of association with the incidence of fescue toxicosis. Perlolyrine and perlolidine have been synthesized de novo (Powers and Ponticello, 1968; Jeffreys, 1970). In addition, many simpler alkaloids such as b-carboline, harman, halostachine, histamine, and octopamine are present in tall fescue (Culvenor, 1973; Bush and Jeffreys, 1975; Davis and Camp, 1983). Most, if not all, have some physiological activity.

Perloline occurs in young seedlings as soon as the primary root emerges and before emergence of the shoot during germination (Gentry et al., 1969). In ‘Kentucky 31' tall fescue (KY-31), perloline concentration is greatest in flowering culms, followed in decreasing order by the roots, leaves, inflorescence, and seed. In these studies, vegetative regrowth tissue had perloline concentration of 1500 mg/kg, but the roots contained a higher level, 1900 mg/kg. Forage management, especially N fertilization, alters perloline accumulation. Fertilization levels that normally result in increased herbage production, animal carrying capacity, and animal production usually will result in increased perloline and perlolidine levels. During late summer, the time of year for maximum accumulation of perloline, 224 kg N fertilizer/ha increased the alkaloid level 64% over the unfertilized control, with concentrations in the fertilized fescue of about 1250 mg/kg (Gentry et al., 1969). Phosphorus and K levels had no effect on perloline accumulation. Plants grown in the greenhouse had increased perloline accumulation in the herbage with each increase of fertilizer N. Nutrient solution was added to pots daily in sufficient quantity to replace resident soil moisture. With the highest amount of solution NO₃-N, 50 mM, maximum perloline levels were about 3000 mg/kg (Fig. 13-2) (Bush and Buckner, 1973).

Seasonal Distribution

Perloline occurrence has a seasonal cycle, with greatest concentrations in leaves occurring generally during hot, dry weather, particularly during late summer (Fig. 13-3). Maximum accumulation usually occurred in mid August in Kentucky (Bush et al., 1979). Variability of perloline concentration was great since not only N fertilizer (75 kg N/ha as ammonium nitrate), but rainfall or irrigation events in excess of 50 mm influenced concentration because rate of dry weight accumulation and alkaloid accumulation did not occur together. Perlolidine accumulation patterns were similar to those of perloline.

Genetics

Perloline may accumulate to concentrations of a few thousand milligrams per kilogram, depending on the genotype of the plant and the environmental conditions. Concentrations of more than 10,000 mg/kg have been reported (Buckner et al., 1973). Perlolidine levels are usually about 15% of those of perloline, and perlolyrine levels are even less. Among different fescues, meadow fescue [Lolium pratense (Huds.) Darbysh.] accumulates the most perloline. Broad sense heritability estimates were between 0.57 and 0.80 from among polycross progenies of three groups of genetic materials. Progeny crosses among hybrids of tall fescue and Italian annual ryegrass (Lolium multiflorum Lam.) indicate that perloline is controlled by a few major genes with a high degree of dominance for low perloline (Cornelius et al., 1974).