THE PEACH REPLANT PROBLEM IN ONTARIO: II TOXIC SUBSTANCES FROM MICROBIAL DECOMPOSITION PRODUCTS OF PEACH ROOT RESIDUES
Experiments were carried out to evaluate the importance of soil toxins in the peach replant problem and to determine whether the substances produced by the interaction of microorganisms occurring in old peach orchard soils and peach root residues were toxic to living peach roots. A rapid and accurate indication as to whether or not these substances were toxic was obtained by determining their effect on the respiration of excised peach root tips. It was demonstrated that substances which inhibit the respiration of excised peach root tips are produced when peach root residues, and also, chemically pure amygdalin, are acted on by certain microorganisms occurring in old peach orchard soils. Such substances were not produced when the soils were autoclaved before amygdalin was added or when other soils were used in which no breakdown of amygdalin had occurred. Again no such inhibiting substances were produced when other root residues, such as sour cherry, tobacco, or pepper roots, were added to these soils in place of peach root residues or amygdalin. The inhibiting substances were readily obtained by extraction with water and they proved to be highly active physiologic agents; the inhibiting effect on the respiration of peach root tips, which ranged from 40 to 90%, could be detected after tips had been less than half an hour in the toxic leachates. In addition to inhibiting respiration these substances also induced darkening and finally the necrosis of meristematic cells. All of these effects proved to be irreversible after tips had been five hours in the toxic leachates. The root tips were apparently killed after this time. Parallel experiments were carried out also with pure amygdalin, and similar inhibition of respiration and injurious effects on meristematic cells were obtained with microbial degradation products of either amygdalin or peach root residues. Water solutions of amygdalin, emulsin, amygdalin and emulsin combined, and various benzaldehyde dilutions were also tested. Inhibition of respiration and darkening of apical meristems were only obtained, however, in solutions where amygdalin was hydrolyzed by emulsin and in the various concentrations of benzaldehyde in water. They were not obtained in the separate solutions of either the glycoside or the enzyme. It was concluded, therefore, that microbial action on the amygdalin fraction of peach roots is mainly responsible for the toxic factor frequently encountered in old peach orchard soils.