This work examines some of the factors that influence PHS in wheat,
with particular emphasis on the prem ature production of alpha-amylase
(PMAA) in the absence of visible sprouting.
Field trials in 1989 and 1990 studied alpha-amylase activity in relation to
grain development in four winter wheat cultivars. W eather conditions were
generally warmer and drier than average in both years. Differences in drying
rate were induced by covering and wetting treatments. Fenman had a slower
rate of grain drying (40%-23% moisture) than other varieties, but, within
varieties, drying rate appeared not to be related to alpha-amylase levels. This
lack of relationship was possibly due to the fact that all of the drying rates were
relatively fast, and, apart from Fenman, levels of alpha-amylase were low. The
level of dormancy and the lack of visible sprouting indicated that the alphaamylase activity observed in Fenman was due to PMAA. The increase in alphaamylase began when moisture percentage was between 40 and 35 %. Activity
rose steeply, and remained high until harvest. There were differences in alphaamylase activity between the wetting and covering treatments and the control in
1989, but these were not related to effects on grain drying. There was an
increase in the number of grains with high alpha-amylase activity in the covered
and wetted treatm ent and it was suggested that changes in microclimate at an
earlier stage of grain development may have affected sensitivity to GA, leading
to subsequent development of PMAA. This was investigated in 1990. The
onset of sensitivity to GA3 coincided with the increase in alpha-amylase activity
in Fenman, but GA3 sensitivity was also apparent in other cultivars that did not
exhibit PMAA. Fenman showed an earlier onset of GA-sensitivity than the
other varieties, but neither covering nor wetting had a significant effect on GA-sensitivity or PMAA.
The possible role of GA in the regulation of PMAA was investigated in a
trial comparing near-isogenic lines of Maris Fluntsman, differing in GA-sensitivity. Levels of alpha-amylase were reduced in dwarf {Rhtl +2 and Rht3)
and semi-dwarf (Rhtl and Rht2) lines relative to the tall (rht) genotype.
However, this did not appear to be related to variation in GA-sensitivity shown
by developing grains. R htl showed a greater response to GA3 than the other
lines. It was suggested that the genotypes may differ in sensitivity to
environmental effects.
An attempt to identify critical stages of grain development, with respect
to environmental effects on PMAA, was unsuccessful, but did indicate that
constant warm and dry environmental conditions reduce alpha-amylase activity
in Fenman.
Ear culture was used to examine the effect of drying rate on PMAA in
Maris Huntsman. There was no apparent relationship, possibly due to relatively
fast drying rates, but it is suggested that ear culture is a suitable system in which
to study environmental effects.
There is considerable variation in PMAA both within and between ears,
and possible reasons for this were considered. Non-destructive methods of
single grain moisture determination were used to examine the relationship
between alpha-amylase activity and grain moisture content. Response to GA3
appeared to vary between grains in the same ear, but there were too few data to
relate this to incidence of PMAA. There appeared to be an asymmetric
distribution of alpha-amylase within grains.
It is suggested that environmental factors may cause an earlier onset of
GA-sensitivity, and that this may coincide with a moisture content sufficient to
allow alpha-amylase synthesis to begin.
