Conditioning period, CO₂ and GR24 influence ethylene biosynthesis and germination of <i>Striga hermonthica</i>

Abstract

Germination of witchweed ( Striga hermonthica [Del.] Benth), an important root parasite on poaceous crops, requires pretreatment ‘conditioning’ in a warm moist environment and a subsequent exposure to a stimulant. The roles of conditioning period, CO 2 and a strigol analogue (GR24) in ethylene biosynthesis and germination of the parasite were investigated. Conditioning increased the seeds’ capacity to oxidize exogenous 1‐aminocyclopropane‐1‐carboxylic acid (ACC). Exogenous CO 2 increased the seeds capacity to oxidize ACC by 3‐ to 9‐fold. A combination of GR24 and ACC increased ethylene production by more than 3‐fold in comparison with the rates obtained using these compounds separately. Aminoethoxyvinylglycine (AVG) completely inhibited ethylene induction by GR24, but not by ACC. A GR24 treatment, made subsequent to conditioning in GR24, did not induce ethylene. However, seeds conditioned in GR24 and then given 1 m M ACC produced 293 nl l −1 ethylene. ACC oxidase (ACCO) activity in crude extracts was increased by conditioning and CO 2 . The enzyme displayed an absolute requirement for ascorbate. Absence of exogenous Fe 2+ reduced enzyme activity only by 14%. GR24 applied during conditioning reduced germination in response to a subsequent GR24 treatment. ACC was, invariably, less effective in inducing S. hermonthica germination than GR24 even at concentrations which induce more ethylene than concurrent GR24 treatments. The results are consistent with a model in which conditioning removes a restriction on the ethylene biosynthetic pathway in S. hermonthica seeds. GR24 modulates the key enzymes in ethylene biosynthesis. The stimulant suppresses ethylene biosynthesis in unconditioned seeds and promotes it in conditioned ones. Germination of S. hermonthica results from the joint action of GR24 and the ethylene it induces.