Osmoregulation and the control of phloem-sap composition inRicinus communisL.

✍ Scribed by J. Andrew C. Smith; John A. Milburn

• Publisher: Springer-Verlag
• Year: 1980
• Tongue: English
• Weight: 662 KB
• Volume: 148
• Category: Article
• ISSN: 0032-0935
• DOI: 10.1007/bf00385438

✦ Synopsis

Phloem-sap composition was studied in plants of Ricinus communis L. grown on a waterculture medium. The sap possessed a relatively high K + : Na + ratio and low levels of Ca 2 + and free H +. Sucrose and K § (together with its associated anions) accounted for 75% of the phloem-sap solute potential (~s). In plants kept in continuous darkness, a decrease in phloem-sap sucrose levels over 24h was accompanied by an increase in K § levels. Measurements of phloem-sap ~s and xylem water potential (~) indicated that this resulted in a partial maintenance of phloem turgor pressure (~p). In darkness there was also a marked decrease in the malate content of the leaf tissue, and it is possible that organic carbon from this source was mobilized for export in the phloem. The results support the concept of the phloem sap as a symplastic phase. We interpret the increase in K § levels in the phloem in darkness as an osmoregulatory response to conditions of restricted solute availability. This response can be explained on the basis of the sucrose-H § co-transport mechanism of phloem loading.