H. Harmens et al., THE ROLE OF LOW-MOLECULAR-WEIGHT ORGANIC-ACIDS IN THE MECHANISM OF INCREASED ZINC TOLERANCE IN SILENE VULGARIS (MOENCH) GARCKE, New phytologist, 126(4), 1994, pp. 615-621
The possible role of low molecular weight organic acids in the mechani
sm of increased zinc tolerance is investigated in Silene vulgaris (Moe
nch) Garcke. In the leaves, the malate concentration is higher in zinc
-tolerant plants than in zinc-sensitive ones, but no consistent differ
ences are observed in the roots. Therefore, since the mechanism of inc
reased zinc tolerance operates also in the roots themselves, a primary
role for malate in the mechanism of increased zinc tolerance is doubt
ful. Exposure to zinc does not affect the malate concentration in the
plants. The citrate concentration in both the roots and leaves is high
er in tolerant plants than in sensitive ones. A clear-cut effect of zi
nc on the concentration of citrate in the plants is not found. Citrate
does not seem to play an important role in the sequestration of zinc
in root cells, in view of its low concentration in the roots of tolera
nt plants. The possible role of citrate as a carrier for zinc in the c
ytosol is discussed. Oxalate, the most abundant organic acid, might pl
ay an important role in the sequestration of zinc, because of its high
concentration in the plants. However, this cannot explain increased z
inc tolerance, because the oxalate concentration is higher in the sens
itive plants than in the tolerant ones. Neither succinate nor maleate
play a prominent role in the mechanism of increased zinc tolerance. Ma
late and citrate are the only organic acids detected in the xylem flui
d. The malate concentration in the xylem fluid is the same in sensitiv
e and tolerant plants and it is not affected by the concentration of e
xternal zinc. The citrate concentration in the xylem fluid is about th
ree times higher in the tolerant control plants than in the sensitive
ones, but it decreases to the same level as in the sensitive plants af
ter exposure to zinc. Differences in the citrate concentrations in the
xylem fluid of sensitive and tolerant plants are not related to diffe
rences in the allocation of zinc.