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Estimation of Osmotic Gradients in Soybean Sieve Tubes by Quantitative Autoradiography

Qualified Support for the MÜnch Hypothesis ¹

^a Department of Botany, University of Georgia, Athens, Georgia 30602

An attempt was made to evaluate Münch's hypothesis of osmotically generated pressure flow in soybean (Glycine max L.) sieve tubes from velocity measurements and calculations of pressure potentials and sieve tube resistances. Pressure potential was estimated from values for water potentials and osmotic potential. Leaf water potential measurements were made by isopiestic thermocouple psychrometry, while the water potential of the nutrient solution was made with a vapor pressure osmometer. Osmotic potential was measured by first bringing the sucrose pools in the entire plant to the same specific radioactivity by steady-state-labeling of the shoot with constant specific radioactivity ¹⁴CO₂ for 5 to 8 hours. Sucrose concentrations in sieve tubes were calculated from the disintegration rate per unit volume in sieve elements as measured by absolute quantitative microautoradiography of freeze-substituted, Eponembedded source (leaf) and sink (root) tissues.

Conductivity of the sieve tubes was calculated from measurements of their dimensions in the petiole, stem, and root. The total pressure drop required for pressure flow at the observed velocities was calculated from the conductivity, velocity, and path length.

In all experiments, the calculated sucrose concentration in source sieve tubes was greater than that in sink sieve tubes, with an average ratio (source to sink) of 1.79:1. However, the absolute sucrose concentrations (average values of 46.4 mg cm^-3 in the source and 23.9 mg cm^-3 in the sink) would have been insufficient to maintain positive turgor in the sieve elements, and the expected pressure differences would not have accounted for movement at the observed velocities. However, the low values for sucrose concentrations almost certainly were due to loss of sucrose during tissue preparation but, for technical reasons, such loss could not be accurately quantified.

Assuming a sucrose concentration sufficient to maintain zero turgor in the root sieve tubes, a xylem water potential gradient (_w [sink] - _w [source]) of 2 bars between source and sink, and the measured ratio of sucrose concentrations in source and sink (1.79:1), the average turgor gradient between source and sink (_p [sink] - _p [source]) would have been about -1.6 to -3.5 bars, which compares favorably with the -1.07 to -2.41 bars average gradient that would have been required to drive translocation at the observed velocities.

³ To whom reprint requests should be addressed.

¹ Supported by National Science Foundation Grant GB 33905.