A factor toxic to seedlings of the same species associated with living roots of the non-gregarious subtropical rain forest tree Grevillea robusta.
Monocultures of Grevillea robusta, a non-gregarious species, grow poorly in South Queensland where the tree occurs naturally. In G. robusta plantations, G. robusta does not regenerate (though other species do) but it regenerates freely in plantations of the gregarious Araucaria cunninghamii and along the edges of rain forest. In plantations of G. robusta the tips of the leaves of seedlings of this species become blackened and the seedlings die. The same phenomena were reproduced under experimental conditions and blackening symptoms and death were shown to follow contact of seedlings' roots with actively growing roots of other plants of G. robusta. Similar symptoms and death were produced in single seedlings in pots watered with nutrient solution plus leachates from older G. robusta plants in sand. Control seedlings receiving nutrient solution only remained healthy.
It is concluded that G. robusta fails to regenerate in G. robusta plantations because of some water-transferable factor associated with the rhizosphere of this species, in which antagonistic microflora may be involved. F.s.-D.B.