Phloem loading and the control of solute transport in Ricinus communis L

Smith, James Andrew Charles (1978) Phloem loading and the control of solute transport in Ricinus communis L. PhD thesis, University of Glasgow.

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This thesis presents the results of an investigation into the control of phloem loading in Ricinus commtinis L. The metabolic relationship between solute accumulation in the leaves and the longdistance transport pathways was first investigated to assess the significance of phloem-sap composition. The characteristics of phloem-sap exudation from bark incisions were then studied in terms of the osmotic properties of the transport system. Comparison of plant growth on a NO3- -N versus NH4+-N water-culture medium showed that there were marked differences in metabolism. Malate accumulation in the leaves of NO3- -N plants accounted for nearly 70% of the total anion balance, whereas malate levels were negligible in NH4+-N plants. However, the NH4+-N plants possessed much higher levels of organic N in the shoots. Despite these differences in solute accumulation, the organic C and organic N content of the phloem sap in the two groups of plants were very similar. Excess OH- (from the NO3- -N plants) and H+ (from the NH4+-N plants) generated during N assimilation in the roots was excreted to the root medium, but phloem transport did not constitute a quantitatively important mechanism for removal of excess OH- or H+ from the shoots. The differences in metabolism in these plants were considered in relation to control of intracellular pH during growth and solute accumulation. The bulk solute concentration of phloem sap exuding from bark incisions remained relatively constant over prolonged periods in plants maintained under controlled environmental conditions. Exudation rate increased immediately in response to fresh incisions, and these changes were directly related to alterations in solute flux. Calculation of the total capacity of the sieve tubes in the shoot indicated that prolonged exudation was maintained by phloem loading. Fully expanded leaves were the main sources of assimilates for transport. The kinetics of changes in exudation rate indicated that flow occurred through a low-resistance pathway and was influenced by the water relations of the vascular system. Solute flux through the phloem was found to increase when water deficits were imposed on the shoot, which suggested that loading was affected by changes in phloem turgor rather than sap concentration. Under conditions of continuous darkness, a rapid fall in sucrose levels in the phloem was associated with an increase in concentration. This tended partially to maintain phloem turgor, and implied that the mechanism of solute transfer into the tissue was directly involved in osmoregulation. The sites of phloem loading in source leaves were examined by transmission electron microscopy, and the sieve element-companion cell complex was typical of that found in other species. Multiple branched plasmodesmata were associated with wall swellings in this cell junction; they may have served to allow high rates of symplastic transport into the sieve tubes. The osmotic characteristics of sucrose loading were also studied using a leaf-disc system. The results suggested that phloem turgor rather than total water potential affected solute uptake, but the system has a limited bearing on the question of solute loading in the intact plant. The manner in which the solute content of the phloem is controlled supports the concept that the sap constitutes a symplastic phase. In addition, the effects of changes in the osmotic relations of the phloem on solute flux suggested that loading was turgor-pressure dependent. It is proposed that this may represent the mechanism by which phloem transport responds to alterations in the source-sink balance in intact plants.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: J A Milburn
Keywords: Physiology
Date of Award: 1978
Depositing User: Enlighten Team
Unique ID: glathesis:1978-72088
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 24 May 2019 15:11
Last Modified: 24 May 2019 15:11

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