Tenuazonic acid (TeA) is a natural phytotoxin produced by Alternaria alternata, the causal agent of brown leaf spot disease of Eupatorium adenophorum. Results from chlorophyll fluorescence revealed TeA can block electron flow from QA to QB at photosystem II acceptor side. Based on studies with D1-mutants of Chlamydomonas reinhardtii, the No. 256 amino acid plays a key role in TeA binding to the QB-niche. The results of competitive replacement with [14C]atrazine combined with JIP-test and D1-mutant showed that TeA should be considered as a new type of photosystem II inhibitor because it has a different binding behavior within QB-niche from other known photosystem II inhibitors. Bioassay of TeA and its analogues indicated 3-acyl-5-alkyltetramic and even tetramic acid compounds may represent a new structural framework for photosynthetic inhibitors.

Curvularia eragrostidis, a causal agent of head blight on the weed (Digitaria sanguinalis), did not cause disease on the turfgrass Festuca arundinacea. Different extracellular esterase isoenzymes were detected in saprophytic and parasitic phases during the fungal germination. The epicuticular waxes of D. sanguinalis were more efficient to induce the secretion of esterases from the fungus than that of F. arundinacea, but were more rapidly degraded by the fungal enzymes. Component analysis indicated that the epicuticular waxes from D. sanguinalis were mostly composed of alcohols, with 54.3% being 9,12-Octadecadien-1-ol. The main component of F arundinacea waxes was alkyl compounds, with 49.8% being olefin, 9-Tricosence. More long-chained esters were found in D. sanguinalis waxes, which were easier to be digested than those in F. arundinacea waxes by extreacellular esterases of the fungus. Epicuticular waxes play a role in varying pathogenicity of C. eragrostidis on D. sanguinalis and F arundinacea.

AAC-toxin, a putative nonhost-selective phytotoxin, was obtained from Alternaria alternata causing a brown leaf spot disease of Crofton weed (Eupatorium adenopborum). The effect of AAC-toxin on the electron transfer reaction of chloroplasts showed that the activity of photosystem II, but not photosystem I, was completely inhibited by the toxin. AAC-toxin affected the following chlorophyll fluorescence parameters: coefficient of photochemical quenching (qp), the half-time value of fluorescence rise, and the O-J-I-P fluorescence reduction kinetics curve, but not the ratio values of Fv/Fm (the quanttml yield of photosystem II) and the half-time value of fluorescence quenching. It was concluded that the toxin inhibited electron transfer from QA to QE (primary and secondary quidine acceptors of photosystem II) in photosystem II by competing with QE for the binding site in D1 protein on the thylakoid membrane.