The objectives of this study were (i) to evaluate the relative
involvement of fructans from leaf sheaths and from
elongating leaf bases to regrowth after defoliation of Lolium perenne L.
by following fructan exohydrolase (FEH),
sucrose-sucrose fructosyl transferase (SST) and invertase (INV) activities and
(ii) to examine whether gibberellins
could be implicated in regulation of FEH activity. In stubble, 36% of fructans
are located in leaf bases and 64%
in leaf sheaths. During the first phase of regrowth, the depletion of
carbohydrates was mainly due to the decline
of fructans, which represented 76% and 50% of the carbohydrates mobilized
from leaf sheaths and elongating leaf
bases respectively. Despite a decrease in protein content, FEH activity
increased 2·5-fold in leaf sheaths and six-fold in elongating leaf bases,
so that the fructan-hydrolysing activity was four times higher in growing leaves than
in leaf sheaths, 2 d after defoliation. INV activity also increased,
whereas the pattern of SST activity was inversely
related to the variations of both hydrolysing activities. SST activity, which
is highest in growing leaves, declined
approximately threefold in leaf sheaths and elongating leaf bases at the
beginning of regrowth.
The increase in activity of FEH in stubble was strongly inhibited by an
inhibitor of the biosynthesis of
gibberellin, uniconazole. FEH activity was decreased to c. 67%, 45%
and 33% of the level in nontreated plants
24, 48 and 72 h following defoliation, respectively. The inhibition could
be overcome by a subsequent treatment
with gibberellic acids (GAs). For the first time, data are provided to
support the view that GAs might play a role
in the regulation of FEH activity, and the implication of this result is discussed.