solid horizontal line in Fig. 3A. Water potential of the elongating
tissue measured with the psychrometer indicated the
average cell water potential, and it is clear from Fig.3A that
cell water potential measurements were accurately performed.
The number of conducting xylem vessels in the elongating
region of soybean stems was 41___2 and that in the basal
region of soybean stems was 106___10 (Nonami et al. 1997).
The average xylem vessel diameter in the elongating region
was 10.0_+3.0/~m, and that in the basal region was 20.7_+0.8
/zm (Nonami et al. 1997). Because cells surrounding the
xylem vessels were small (Fig. 2A) and had low diffusivity for
water (Fig. 2B), the steepest water potential gradient was
present at cells adjacent to the xylem vessels as seen in the
corrected water potential profile shown as dotted curves in
Fig. 3A.
Water in the elongating stem tissue must move from the
xylem to the surrounding cells, and thus, the water potential
field in three dimensional geometrical space (i.e., x, y, zcoordinates)
can be expressed in the four dimensional
space. Figure 4A shows that the water potential field associated
with cell elongation. The xylem region has the