Studies on phloem exudation from Yucca flaccida Haw. VII. The effect of cooling on exudation

Bleeding inflorescence stalk tips of 10-15 cm length, still attached to the plant, were cooled in melting ice. The result was a considerable prolongation of the exudation period. Apparently the wound-sealing process was strongly retarded. Although the rate of bleeding seemed slightly lower, the dry-matter content of the exudate was 10-20% higher at 0°C than at 20-25 °C. When excised inflorescence tops were cooled as a whole, exudation gradually declined and stopped within 45 minutes. It recommenced within a few minutes when brought to room temperature. It is concluded that both the sugar secretion and sugar uptake by cells surrounding the intact sieve-tubes stops at a low temperature. The secretion process is assumed to be the driving force of the exudation. As the bleeding continues at low temperatures, its driving force will be localized mainly outside the cooled inflorescence.