Mechanism of Cyanide Inhibition of Phloem Translocation

Abstract

Petiolar application of potassium cyanide inhibited '4C-assimilate translocation without affecting source leaf photosynthesis or phloem loading of sucrose in Phaseolus vulgaris. The inhibition of transport was correlated with disruption of the structural integrity of the sieve tubes (sieve pore blockage) rather than impairment of a metabolic process in the translocation path driving translocation. Although the osmotically generated pressure flow hypothesis is generally accepted as the mechanism of phloem transport, several workers postulate that translocation is driven by meta-bolic processes residing in the sieve tubes of the translocation path (16). The most commonly cited evidence for these activated mechanisms is the inhibition of translocation caused by either localized path chilling or by the application of metabolic inhibi-tors to the translocation path. Both treatments have been inter-preted as being specific for metabolic processes. Recently, it was shown that the conflicting reports in the literature of path chill-ing and translocation appear to result from the varying degrees of chilling sensitivity of the various species and ecotypes studied (5, 9). In addition, the inhibition of translocation by path chilling in the chilling-sensitive bean was correlated with the loss of structural integrity of the sieve tubes rather than impairment of a metabolic process driving transport (9). Like the chilling effects on translocation, the literature on the effects of locally applied chemical inhibitors in relation to the energy requirement of the translocation path is marked with controversy. These studies range from no effect of local anaero-biosis or cyanide treatment (15), to transient effects (14), to marked inhibition (10-13, 17). It is difficult to interpret the