Allelopathy in a Grass-Legume Association: A Case Study With Hemarthria altissima and Desmodium intortum

Abstract

In Hawaii, the legume Desmodium intortum (Mill.) Urb. could not be established in a pasture of the tetraploid Hemarthria altissima (Poir.) Stapf and Hubb cv. bigalta limpograss, but intortum was easily established in a sward of the less vigorous greenalta limpograss. The effects of root residues and root exudates of intortum and the limpograsses on the growth, nodulation and nitrogen fixation of intortum were studied in experiments designed to avoid competition between root systems of the grass and legume for nutrients, water, and space and between aerial plant parts for light. Finally, allelochemicals in the root exudates of bigalta and greenalta limpograsses were collected, isolated and partially characterized. The growth of intortum in fertilized soil which contained root residue of bigalta limpograss was significantly less than the growth of intortum in the presence of greenalta limpograss. The residue treatments had little or no effect on the mineral nutrient contents of intortum tops. The effects of root exudates of the limpograsses were studied in vermiculite cultures watered with Hoagland's or a dilute nutrient solution. Intortum was grown with each of the limpograsses in divided pots where the root systems were separated and in pots where the root systems were intermingled. The growth of intortum seedlings and cuttings was inhibited as much as 75% in divided pots and 88% in undivided pots by exudates from bigalta limpograss. Exudates from bigalta limpograss were much more inhibitory than those of greenalta. It is concluded from the data that the inhibition of intortum growth by bigalta limpograss was allelopathic. The inhibition of intortum seedling growth by exudates from established intortum cuttings showed that intortum was autotoxic. Nodule fresh weight and acetylene reduction per pot of intortum grown with bigalta, greenalta or intortum were significantly lower than the control in pots with and without dividers which were irrigated with Hoagland's nutrient solution. However, the specific nodule activity of intortum was not reduced by root exudates from the two grasses. Specific nodule activity of intortum grown with intortum was significantly less than the control. A new method was developed for the extraction of hydrophobic allelochemicals from root homogenates and exudates using the resin Amberlite XAD-4. Allelochemicals in aqueous root homogenates of the limpograsses were extracted with equal efficiency by methanol or XAD-4 resin as shown by a lettuce seed germination bioassay. A unique continuous trapping system for the collection of root exudates was developed by connecting a column containing XAD-4 resin to the container used for growing limpograsses in sand culture. Chemicals trapped by the resin were eluted by organic solvents and separated into neutral, acidic, and basic fractions. The neutral fraction from bigalta limpograss inhibited lettuce seed germination more than that of greenalta. Allelochemicals were isolated only from the neutral and acidic fractions by paper chromatography. Most zones on the chromatograph containing inhibitors showed a positive color reaction when sprayed with diazotized p-nitroaniline followed by 10% sodium carbonate indicating that the allelochemicals were mainly phenolic compounds. Root residues of bigalta and greenalta limpograsses and intortum added to soil increased the populations of some fungi and bacteria and the soil levels of the enzymes amylase, cellulase, invertase and dehydrogenase. However, the lowest levels of fungi were found in the pots containing residues from bigalta limpograss. The levels of the four enzymes were highest in soil containing intortum root residues and lowest in soil containing bigalta residues and in the control soil.