DECREASED GROWTH-INDUCED WATER POTENTIAL - PRIMARY CAUSE OF GROWTH-INHIBITION AT LOW WATER POTENTIALS

Cell enlargement depends on a growth-induced difference in water poten tial to move water into the cells. Water deficits decrease this potent ial difference and inhibit growth. To investigate whether the decrease causes the growth inhibition, pressure was applied to the roots of so ybean (Glycine max L. Merr.) seedlings and the growth and potential di fference were monitored in the stems. In water-limited plants, the inh ibited stem growth increased when the roots were pressurized and it re verted to the previous rate when the pressure was released. The pressu re around the roots was perceived as an increased turgor in the stem i n smart cells next to the xylem, but not in outlying cortical cells. T his local effect implied that water transport was impeded by the small cells. The diffusivity for water was much less in the small cells tha n in the outlying cells. The small cells thus were a barrier that caus ed the growth-induced potential difference to be large during rapid gr owth, but to reverse locally during the early part of a water deficit. Such a barrier may be a frequent property of meristems. Because stem growth responded to the pressure-induced recovery of the potential dif ference across this barrier, we conclude that a decrease in the growth -induced potential difference was a primary cause of the inhibition.