3.2. Na+ concentration
The Na+ concentrations of the shoot, root, and xylem sap
in ungrafted and self-grafted cucumber were similar (Table 2,
Figs. 1 and 2). A similar result was also obtained for ungrafted
and self-grafted pumpkin. This result implies that the grafting process
itself did not significantly affect the Na+ concentrations of the
plant tissues and xylem sap in either species. Ungrafted and selfgrafted
cucumber showed consistently higher Na+ concentrations
in the shoot, but lower Na+ concentrations in the root compared
with ungrafted and self-grafted pumpkin (Fig. 1). When the plants
were treated with 91 mM NaCl for 10 or 30 days, the Na+ concentrations
in the cucumber grafted onto pumpkin shoots decreased
by 69% or 71% compared with those of self-grafted cucumber,
whereas compared with self-grafted pumpkin, the Na+ concentrations
in pumpkin grafted onto cucumber shoot increased by 203%
or 501%, respectively (Fig. 1). In addition, the Na+ concentration
in pumpkin grafted onto cucumber shoot was higher than that
of the self-grafted cucumber plants, but this discrepancy did not
decrease the salt tolerance of pumpkin grafted onto cucumber to
a value lower than those of self-grafted cucumber plants (Table 1
and Fig. 1). This result suggests that the pumpkin is still more tolerant
to salt than cucumber, regardless of the rootstock used. The
Na+ concentrations in the root were similar in ungrafted cucumber,
self-grafted cucumber, and pumpkin grafted onto cucumber.
A similar result was obtained in ungrafted and self-grafted pumpkin.
However, the Na+ concentration in the cucumber grafted onto pumpkin root was significantly lower than those in ungrafted and
self-grafted pumpkin (Fig. 1).
Quantitative analysis indicates the average AF value of Na+
in the plants with cucumber roots was 114.6% under 91 mM
NaCl, whereas this value was 49.5% in plants with pumpkin roots
(Table 3). These results indicate that the pumpkin root excludes
50.5% ofthe potential amount of Na+ available for absorption under
91 mM NaCl, whereas nearly no Na+ exclusion in the cucumber
root was observed. The average RF value for Na+ in the plants with
pumpkin roots was 15.9% under 91 mM of NaCl (Table 3), indicating
that from the total amount of Na+ absorbed by the plants
with pumpkin rootstocks, the retention of Na+ in the pumpkin
root decreased the amount of Na+ in the shoot by 15.9% compared
with the amount when Na+ was uniformly distributed in the whole
plant. In contrast, the RF value for Na+ in the plants with cucumber
roots was −4.9% (Table 3), suggesting that, in practice, no Na+ was
retained by the cucumber root.
When the plants were grown under 91 mM NaCl for 30 days,
the volumes of xylem sap collected from above and below the graft
union were similar for all plants (Fig. 2A). This result suggests that
the graft union is not a barrier for water transport. However, the
graft union is a barrier for Na+ transport when pumpkin is used
as rootstock. The average Na+ concentration in the xylem sap of
the plants with pumpkin rootstocks decreased from 6.5 mM in the
root (rootstock) to 1.9 mM in the shoot (scion), decreased by 71%.
In contrast, when cucumber was used as rootstock, the graft union
was not a barrier for Na+ transport (Fig. 2B).