Abstract:
Hydrogen cyanamide (CY) has been used to break rest
in temperate crops, but levels used to overcome rest are
sometimes toxic to plants. The rest breaking effect of CY
is thought to involve peroxide metabolism. Inhibition of
catalase by CY has been proposed as a mechanism for
overcoming dormancy in plants.
The objectives of this study were to determine
effects of CY on: (1) the viability of bromegrass cells;
(2) the reduction of SH groups from SH-containing
compounds, and the concentration of SH groups in
bromegrass cells; and (3) the relationship between TTC
reduction and the amount of SH groups; and (4) the mode
of inhibition of cyanamide on both bovine liver catalase
from Sigma and on catalase extract from bromegrass cells.
The 2,3,5-triphenyltetrazolium chloride (TTC) method
was used to determine cell viability. The TTC test
indicates that the effect of cyanamide on cell viability
is both concentration and time dependent. Relatively low
concentrations of CY (1 and 5 mM) and short exposure
times increased the reduction of TTC above the level of
the controls, but reduction decreased with increasing
exposure or higher concentrations.
Sulfhydryl (SH) groups are known to function as an
antioxidant in cells and, therefore, have been proposed
to protect cells against physical and chemical stresses.
Hydrogen cyanamide reduced the titratable SH groups of
SH-containing compounds in in vitro assays and the effect
on SH groups depended on the ratio of CY concentration
and the concentration of SH-containing compounds: the
rate of reduction of titratable SH groups was fastest in
dithiothreitol (DTT) followed by glutathione (GSH) and
lastly by cysteine.
Hydrogen cyanamide also stimulates the production of
nonprotein SH compounds in bromegrass cells in vivo. The
effect of CY on levels of SH compounds is concentration
and time dependent. The concentration of SH compounds
increased following relatively short exposure time (4
hours) to low concentrations of CY and then decreased
with additional time. The level of TTC reduction was
positively correlated with the concentration of SH groups
in bromegrass cells.
Hydrogen cyanamide inhibited catalase activity and
catalase activity was restored to near the level of the
untreated control after the removal of CY by dialysis
suggesting that hydrogen cyanamide is a reversible
inhibitor of catalase. Enzyme kinetic studies indicate
that the inhibition of catalase by cyanamide is of the
mixed-type inhibition.
