Osmoregulation, solute distribution, and growth in soybean seedlings having low water potentials

Soybean (Glycine max (L.) Merr.) seedlings osmoregulate when the supply of water is limited around the roots. The osmoregulation involves solute accumulation (osmotic adjustment) by the elongating region of the hypocotyls. We investigated the relationship between growth, solute accumulation, and the partitioning of solutes during osmoregulation. Darkgrown seedlings were transplanted to vermiculite containing 1/8 (0.13 x) the water of the controls. Within 12-15 h, the osmotic potential of the elongating region had decreased to-12 bar, but it was-7 bar in the controls. This osmoregulation involved a true solute accumulation by the hypocotyls, since cell volume and turgor were virtually the same regardless of the water regime. The hypocotyls having low water potentials elongated slowly but, when deprived of their cotyledons, did not elongate or accumulate solute. This result indicated a cotyledonary origin for the solutes and a dependence of slow growth on osmotic adjustment. The translocation of nonrespired dry matter from the cotyledons to the seedling axis was unaffected by the availability of water, but partitioning was altered. In the first 12 h, dry matter accumulated in the elongating region of the 0.13 x hypocotyls, and osmotic adjustment occurred. The solutes involved were mostly free amino acids, glucose, fructose, and sucrose, and these accounted for most of the increased dry weight. After osmotic adjustment was complete, dry matter ceased to accumulate in the hypocotyls and bypassed them to accumulate in the roots, which grew faster than the control roots. The proliferation of the roots resulted in an increased root/shoot ratio, a common response of plants to dry conditions.Osmotic adjustment occurred in the elongating region of the hypocotyls because solute utilization for growth decreased while solute uptake continued. Adjustment was completed when solute uptake subsequently decreased, and uptake then balanced utilization. The control of osmotic adjustment was therefore the rate of solute utilization and, secondarily, the rate of solute uptake. Elongation was inhibited by unknown factors(s) despite the turgor and substrates associated with osmotic adjustment. The remaining slow elongation depended on osmotic adjustment and represented some optimum between the necessary inhibition for solute accumulation and the necessary growth for seedling establishment.