Adequacy of cellular phytoalexin concentrations in hypersensitively responding cotton leaves

Abstract

Average cellular phytoalexin concentrations at infection sites were determined in Xanthomonas campestrispv. malvacearuminfected cotton leaves to test the hypothesis that sesquiterpenoid phytoalexins play a major role in heritable resistance of cotton to bacterial blight. Bacteriostasis was achieved in leaves of resistant cotton line WbM(0.0) by 5–7 days after inoculation. Between days 3 and 7, phytoalexin amounts per gram of tissue were above basal levels but were far below their peak values. Nevertheless, when the phytoalexin quantities present on day 7 were divided by the volume of water contained in the hypersensitively responding cells defining the scattered infection centres, the concentrations computed were ones previously found to be highly inhibitory in in vitrobioassays. In necrotic cells of susceptible line WbM(4.0), the values were <3% of those in the resistant line. In a comparison of highly resistant lines OK1.2 and Im216, concentrations of phytoalexins far exceeding inhibitory levels in vitrowere calculated for hypersensitively necrotic cells in both lines on days 3 and 4 post-inoculation. Analysis of the clustering of necrotic cells indicated that in OK1.2, only 25% of the infection sites had mounted a reaction by day 3 and nearly all had responded by day 4, while in Im216, 50% or more had responded hypersensitively by day 3 and all sites had shown a reaction by day 4. Bacterial inhibition occurred on the day following appearance of HR-cell clusters at all sites. The phytoalexins present in the HR cells had access to the bacteria, as indicated by loss of plasmalemma integrity of the yellow-green fluorescent, hypersensitively responding cells and by diffusion of the phytoalexins into the bathing medium.