Effects of Helminthosporium carbonum Toxin on Absorption of Solutes by Corn Roots

Susceptible corn roots exposed to the host-selective toxin of Helminthosporium carbonum took up and retained more NO₃⁻, Na⁺, Cl⁻, 3-o-methylglucose, and leucine than did control roots. Stimulatory effects on uptake were more pronounced with freshly cut roots than with roots that were washed and aged. Solutes were accumulated against a concentration gradient, and toxin-treated tissues developed a steeper gradient than did control tissues. Toxin affected both the low and high affinity uptake systems for Na⁺ and Cl⁻. Toxin did not affect uptake of Na₂⁻, K⁺, Ca²⁺, phosphate ion (H₂PO₄⁻ and HPO₄⁻), SO₄⁻, and glutamic acid. No toxin-induced leakage of any solute tested was detected within 5 to 6 hr after initial exposure to toxin. The data suggest that toxin from H. carbonum does not cause the general plasma membrane derangement caused by other host-selective toxins. Instead, H. carbonum toxin may cause specific changes in characteristics of the plasmalemma, which result in increased uptake of certain solutes.