Impact of Salicylic Acid on the Transport and Distribution of Sugars in Plants - Salicylic Acid: Plant Growth and Development - page 90

(a) (b)

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MES SA10-3 SA10-4 SA10-5 SA10-8 SA-10 MES SA 10--3 SA 10-6 SA10-10

Fig. 3 Effects of different SA concentrations on the DOG-sucrose transport to the root tip. Notes
a at high transport intensity; b at low transport intensity. Exposure duration was 5 h. Y-axis 2-
DOG-sucrose content in 1-cm root tips. X-axis control (Mes buffer) and different SA-

concentrations

Symplastic unloading dominates in strong sinks, such as root tips, developing
fruits, integuments around developing seeds, and in young sink leaves.
In distal differentiated root part, carboxyfluorescein fluorescence was not
observed either at the analysis of the whole root (even in the case of the more
easily viewed arabidopsis root) or on the maize root cut longitudinally through the
middle. However, 14 C-sucrose and deoxysucrose were found in all root parts
(Fig. 1 c, d). 14 C-sucrose, which is easily metabolized, was unloaded in the distal
root parts more intensely than deoxysucrose. A comparison of these data allows a
conclusion that the site of symplast phloem unloading is limited by the interme-
diate zone between mature meristem and the zone of elongation. Apoplastic
unloading evidently occurs along the entire root length.
When seedling root tips were immersed in SA-containing buffer solution,
sucrose distribution in the root changed. The effects of SA on 14 C-sucrose and
deoxysucrose transport into the root tip depended on the exposure duration.
Treatment with 0.1 mM SA for 1 h significantly increased compound accumula-
tion, whereas 5-h exposure in this solution did not exert such an effect (Fig. 2 ).
In the beginning of experiment (in 1 h after sucrose loading on the scutellum)
the rate of sucrose influx was slow [about 1 lg/(g fr wt h)]; with time the influx
was gradually accelerated; and after 5 h it could attain 20 lg/(g fr wt h). It may be
that SA impact on the sucrose accumulation in the root tip depends on the rate of
its influx. This is confirmed by experiments with 5-h exposure to SA of plants with
the slow or high rate of influx (Fig. 3 ). The high SA concentrations (10 -4 M and
especially 10 -3 M) inhibited sucrose incoming from the scutellum independently
of the rate of influx. The lower concentration of SA did not induce any change on