Mechanism Of Phloem Transport Research Articles

Teaching the pressure-flow hypothesis of phloem transport in a problem-solving session

Problem solving is an ideal learning strategy, especially for topics that are perceived as difficult to teach. As an example, a format is described for a problem-solving session designed to help students understand the pressure-flow hypothesis of phloem transport in plants. Five key facts and their discussion can lead to the conclusion that a turgor-pressure difference along sieve tubes of the phloem drives a mass flow of sugar solution between source and sink. The given format is suitable for use in the sixth-form classroom when the mechanism of phloem transport is usually first taught to students. Additional information is provided so that school teacher scan lead a class discussion and stimulate student interest in the phloem as a long-distance transport system.

A Mechanism of Phloem Transport Based on Interfacial Flow Controlled by Solute Potential Gradients

A Mechanism of Phloem Transport Based on Interfacial Flow Controlled by Solute Potential Gradients

Solution flow in tubular semipermeable membranes

Solution flow in tubular semipermeable membranes was studied as a model for assimilate transport in sieve tubes. A mass flow of solution was demonstrated both in closed turgid tubes and in open tubes without turgor pressure. These results can be explained in terms of hydrostatic and osmotic pressure differences across the semipermeable membrane without consideration of a decrease in hydrostatic pressure along the direction of solution flow. A theoretical model based on nonequilibrium thermodynamics is developed that is in fairly good quantitative agreement with the experimental results. Münch's original experiment demonstrating solution flow is analyzed and shown not to depend on a gradient of hydrostatic pressure but rather to depend on the same driving forces operative in these experiments. On the basis of these findings a "volume-flow" mechanism of phloem transport is proposed.

The Mechanism of Phloem Transport

The Mechanism of Phloem Transport

Sites of Accumulation in Excised Phloem and Vascular Tissues

Excised pieces of vascular bundle and phloem tissue were allowed to accumulate radioactive phosphate and sulfate, and were then sectioned and autoradiographed so as to detect the sites of accumulation. Special methods were needed to prevent any diffusion of the radioisotope. Some autoradiographs obtained are presented. In excised celery vascular bundles, the most radioactive area and hence the most actively accumulating tissue was the young secondary phloem at the sides of the bundle. In intact plants, the same tissue was the most active in translocating. In excised apple phloem there was some variation in behavior, but again the young secondary phloem was generally the most actively accumulating tissue. Accumulation activities of individual cells in the phloem and vascular tissue were compared. It appeared that all cell types, ray, phloem and xylem parenchyma, cambial cells and sieve tubes, accumulated at least 5 times more actively than did the cortical parenchyma cells. The sieve tubes were among the most actively accumulating cells present, accumulating 20 times more actively than the cortical parenchyma cells. It is concluded that accumulation processes have a primary role to play in the mechanism of phloem transport.