Production of carpel wall phytoalexins in the developing cotton boll

Abstract

Developing cotton bolls of Deltapine 61, grown under controlled conditions in environmental chambers, were artificially wounded on the carpel surface and treated with an elicitor, composed of cell-free mycelial extracts of Aspergillus flavus, at weekly intervals for eight weeks postanthesis. Two days after treatment, the bolls were harvested from cotton plants and discs containing the treated surfaces were excised and extracted to determine induction of the sesquiterpenoid naphthol phytoalexins, 2,7-dihydroxycadalene and 2-hydroxy-7-methoxy cadalene, their oxidation products lacinilene C and lacinilene C 7-methyl ether, and the coumarin phytoalexin-scopoletin. Maximum concentrations of the cadalenes and lacinilenes were present in bolls treated at seven weeks postanthesis; scopoletin concentrations were highest in bolls treated three weeks postanthesis. To determine the effects of the induced phytoalexins on the growth of A. flavus, a second set of wounded elicitor-treated discs were collected at weekly intervals for eight weeks postanthesis, inoculated with A. flavus and incubated on liquid plant nutrient. Fungal growth displayed a range from 80% of control for treated discs from bolls harvested one week postanthesis to 10% of control for discs from bolls harvested seven to eight weeks postanthesis. The induced phytoalexins on elicitor-treated carpel walls appear to be responsible for the reduction of fungal growth observed.