Quantification of Allelopathic Potential of Sorghum Residues by Novel Indexing of Richards' Function Fitted to Cumulative Cress Seed Germination Curves 1

The inhibitory activity of aqueous extracts of field-grown sorghum ( Sorghum bicolor cv. Bird-a-boo) herbage and roots was quantitatively indexed by three aspects of cumulative cress ( Lepidium sativum cv. Curlycress) seed germination: the germination onset; weighted mean rate; and final germination percentage. Extract potency was greatest for herbage collected four weeks after planting but declined sharply thereafter as the plants matured. About 91% of the inhibitory activity obtained from four-week-old herbage was in a low molecular weight fraction. Differential effects of herbage and root extracts on cress seed germination suggest that the nature and/or proportion of biologically active substances extractable from these plant parts is dissimilar.