No abstract

The background of bio-potentials in relation to transport is briefly reviewed and the chief difficulties lying in the way of an electrical theory of transport are pointed out. A re-examination of electro-osmosis and streaming potentials in xylem and phloem vessels suggests that the bio-electrical forces are sufficiently large in magnitude to cause transport and that they certainly operate in a direction to assist it. The productions of the potentials through metabolic activity and membrane diffusion is discussed.

A 3-min pulse of 11CO2 was fed to leaves of various plant species to allow us to compare the patterns of movement of 11C translocate over 90 min. Three groups of profiles were found. (1) In Helianthus, Nymphoides, Tropaeolum, Ipomoea, and Fraxinus, a mass flow passed successive detectors, rising steadily over a 90-min period. (2) In Zea and Triticum, the mass flow remained at a peak for 5–10 min and fell steadily thereafter. (3) In Picea and Pinus, no obvious mass-flow pattern was detected in 90 min but rather a series of waves or packets. In every case, the activity opposite certain positions accumulated faster than others, suggesting that points of local buildup of translocate occur along a stem or petiole.By using high activity 11CO2 counting times of 5 s or 1 s, and the five-point mean technique of analysis of data, we have been able to detect aberrations in the tracer profile opposite each detector. These aberrations are due in part to the movement of tracer at different speeds in parallel veins, in part to the superposition of reverse flow of tracer, and in part to apparent discontinuities of flow or small waves of tracer. We conclude that some pulsatory sucrose loading mechanism is possible in the leaves, but a nonsteady-state translocatory mechanism is also possible. We have developed techniques for detecting the points of time of the onset of mass flow and the method of following small peaks of tracer activity past successive detectors. Waves of translocate moved at 0.2 cm min−1 in pine and spruce and 0.5–10 cm min−1 in ash and the angiosperms. Some synchrony of flow causes activity reinforcement or interference to occur in transient fashion.

General expressions for electrokinetic phenomena of relevance in biology are derived using the methods of irreversible thermodynamics and Onsager coefficients, not only for a Helmholtz-Smoluchowski model but also for a factional model and the model of Schmid. These last two models would seem to be more appropriate for biological cell membranes.Some applications of these expressions to plant physiology include the following: the pressure contribution of electro-osmosis to the turgor of Nitella or Chara cells is found to be negligible; the power used by an electro-osmotic pump can never be less than that used by a pressure mechanism; electro-osmosis may account for the present discrepancies between calculations of membrane conductance using tracer ions fluxes and those using applied potential differences; the streaming potentials developed by pressures across biological membranes would be too small to detect, but in large pores such as xylem or phloem vessels or in cell walls small pressures would result in easily measured potentials.

Continuous records of bioelectric stem potentials have been made throughout a season for three kinds of tree: Ulmus americana L., Pinus resinosa Ait., and Acer saccharum Marsh. Daily measurements of potential and resistance for a 15-month period have also been made on a maple and some direct measurements of current are also given. A yearly rhythm was clearly visible, also daily cycles, and during the growing season cycles of 5-minute frequency. The yearly and daily rhythms have been interpreted in terms of electro-osmosis or streaming potential and seem to be correlated with the transport within the tree. The 5-minute oscillations have been related to cambial growth.