The Nature of the Short-Term Inhibition of Stem Translocation Produced by Abrupt Stimuli

Abstract

The transient inhibitions of phloem translocation which result when stem of Phaseolus vulgaris L. is (a) abruptly cooled by a few degrees Celsius, (b) vibrated vigorously at sonic frequencies, or (c) electrically shocked were examined. Fatigue of the inhibitory response was found to have the following properties: (i) a sequence of widely spaced stimuli often led to a shortened inhibitory period; (ii) for closely spaced stimuli, evidence of fatigue was found for neither temperature followed by vibration nor vibration followed by temperature; (iii) plants grown in strong oscillating wind gusts were fully vibration-sensitive even though two closely spaced vibrations applied to a normal plant led to fatigue; (iv) the vibration-induced inhibition was not sensitive to cytochalasin B. Experiments designed to examine the nature of the inhibition showed that: (i) an inhibitory stimulus essentially halted label import into the plant apex and label motion within the stimulated region but not leaf export, motion basal to the stimulated region, or motion apical to the stimulated region; (ii) the inhibition was affected by neither the callose synthesis blocker 2-deoxy-D-glucose nor the sulfhydryl-specific reagent p-chloromercuribenzenesulfonic acid; Cucumis melo L., whose sap does not gel in vitro, showed a normal inhibitory response. These data are consistent with the hypothesis that the inhibition of phloem translocation by a variety of different stimuli is the result of a local blockage of sieve elements in the region stimulated and that this blockage arises from the occlusion of sieve plate pores.