Translocation of sugars in Cucurbita melopepo. VI. The reversible low temperature inhibition of 14C movement and cold acclimation of phloem tissue

Abstract

14C translocation through the node ceases immediately the tissue is chilled (0°) and 14C export from the blade is considerably reduced within the next 5 min, while the assimilation rate remains unchanged. The ratio of 14C distribution among the translocated compounds indicates a uniform deceleration of the entire longitudinal flow through the unchilled length of petiole. Immediately after node chilling an increasing restraint is imposed on the rate of 14C translocation throughout the length of the petiole. This second inhibitory effect of cold treatment is maximally expressed between 30 and 60 min after node chilling and may reflect an accentuated accumulation of translocates within the phloem under these conditions. Translocation through the node resumes immediately the tissue is returned to 25°. Initially the rate is slow but within 90 min of warming the node, when all prechilling restraints on longitudinal movement appear to be removed, the rate of 14C translocation approaches that of control plants. Rapid resumption of transport through the node is matched by a rapid loss of 14C from the blade, again indicating that the entire longitudinal flow moves as a continuum. The longitudinal transport in the phloem and radial movement into the surrounding tissues are both rapidly inhibited at 0 °C. Longitudinal movement will acclimate and within 6 h of continuous cold (1 °C) appreciable transport is resumed through the chilled zone while after 19 to 20 h almost complete recovery of translocation is achieved. There was no evidence for acclimation of the radial movement. If radial movement of 14C reflects cytoplasmic streaming then longitudinal movement likely proceeds by some other mechanism. An evaluation of the extensive cytological observations of phloem tissue produced no clues to explain the mechanism of low temperature inhibition of translocation at the structural level.