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PLANT PHYSIOLOGY , Vol 102, Issue 1 13-19, Copyright © 1993 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Direct Demonstration of a Growth-Induced Water Potential Gradient
H. Nonami and J. S. Boyer
College of Marine Studies and College of Agriculture, University of Delaware, Lewes, Delaware 19958
When transpiration is negligible, water potentials in growing tissues are
less than those in mature tissues and have been predicted to form gradients
that move water into the enlarging cells. To determine directly whether the
gradients exist, we measured water potentials along the radius of stems of
intact soybean (Glycine max [L.] Merr.) seedlings growing in vermiculite in
a water-saturated atmosphere. The measurements were made in individual
cells by first determining the turgor with a miniature pressure probe, then
determining the osmotic potential of solution from the same cell, and
finally summing the two potentials. The osmotic potentials were corrected
for sample mixing in the probe. The measurements were checked with a
thermocouple psychrometer that gave average tissue water potentials. In the
elongating region, the water potential was highest near the xylem and
lowest near the epidermis and in the center of the pith. In the basal, more
mature region of the same stems, water potentials were near zero next to
the xylem and throughout the tissue. These basal potentials reflected
mostly the potential of the xylem, which extended into the elongating
tissues. Thus, the high basal potential confirmed the high potential near
the xylem in the elongating tissues. The psychrometer measurements for each
tissue gave average potentials that agreed with the average of the cell
potentials from the pressure probe. We conclude that a radial gradient was
present in the elongating region that formed a water potential field in
three dimensions around the xylem and that confirmed the predictions of
Molz and Boyer (F.J. Molz and J.S. Boyer [1978] Plant Physiol 62: 423-429).
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