38
Effect of pruning and hydrogen cyanam ide on budburst
Australian Journal of Grape and Wine Research 6, 31–39, 2000
ripening to obtain more flexibility during harvesting.
Alternatively, a difference of 7 days in maturity could
make it difficult to optimize the time of machine har-
vesting of a block and in such an instance it would be
sensible to avoid large differences in the time of pruning
of different parts of the block.
between budburst and % fruitful shoots (having at least
one inflorescence) that they reported may have been due
to a combination of two factors. First, the pattern of bud-
burst along canes (i.e. distal buds bursting in advance of
proximal buds) coupled with the well established
(Antcliff and Webster 1955) trend for sultana buds to
have fewer inflorescence primordia toward the base of
canes would contribute to this effect. Secondly, secondary
and tertiary shoots, which burst later than primary
shoots, generally have fewer inflorescences than primary
shoots.
Once budburst commenced, the frequency of buds
bursting per day was sensitive to fluctuations in air tem-
perature, but limited by the number of buds available to
burst (Figures 6 to 10). These patterns may be interpreted
as follows. A wave of dormancy release commences in
the most apical and fertile buds and then proceeds
towards the more proximal and less fertile buds. Once
released from dormancy, each bud will grow in response
to temperature. Rigorous definition of the rules that gov-
ern this behaviour at an intra-vine bud population level
could be valuable in the construction of models designed
to predict phenological events and the productivity of
grapevines in different environments.
The effects of pruning time and reproductive potential
on time of budburst are factors that are not considered in
models that are currently used to predict budburst (e.g.
Pouget 1988, Swanepoel et al. 1990). These temperature-
based models assume that, for a given cultivar, vines will
be able to burst when a critical number of chilling units
have been accumulated, and that budburst will occur
when a minimum number of heat units have been accu-
mulated and a minimum mean daily temperature has
been attained. In our experiment (and others cited
above) the buds on Earlier- and Later-pruned vines of
the same cultivar burst at significantly different times
despite being exposed to an identical set of environmen-
tal conditions. This suggests that the capability of cur-
rently-available models designed to predict the time of
budburst could be improved by the inclusion of a func-
tion or the adjustment of a parameter to take account of
the effects of pruning time and reproductive potential.
In summary, delaying pruning by six weeks delayed
budburst, anthesis, veraison and maturity by approxi-
mately five days. Thus, pruning time may be a source of
variation in the timing of key phenological events in the
vineyard and may be an important factor to consider in
planning vineyard operations. Hydrogen cyanamide did
not advance the times of budburst, anthesis, veraison or
maturity, but caused more shoots to burst, particularly
‘extra’ shoots at the base of spurs and on old wood.
‘Primary’ shoots on clear nodes burst before extra shoots.
The frequency of budburst was sensitive to fluctuations in
air temperature, but limited by the number of buds avail-
able to burst. A primary shoot tended to burst earlier if it
had more bunches. Thus, it may be possible to improve
temperature-based models designed to predict the timing
of phenological events in grapevines by including para-
meters that take account of pruning time and the repro-
ductive potential of classes of buds.
We observed that different types of bud, as defined by
their position on a vine (Table 2) and as indicated by the
different types of shoot that they produced (Table 4),
burst at different times. There was a hierarchy of burst-
ing, with more distal nodes bursting earlier on spurs, and
spur bases bursting earlier than old wood (Table 2). This
pattern is consistent with the familiar phenomenon of
apical dominance, which is a concept that is generally
applied to relationships between buds on a given actively
growing shoot. However, apical dominance could persist
in some form even during dormancy. Perhaps earlier-
pruned vines burst earlier because removal of the distal
buds eliminated a source of inhibition of proximal buds at
an earlier stage, thus increasing their sensitivity to environ-
mental conditions. Alternatively, the effect of pruning
time may be one of pruning itself. Wounding has been
shown to increase respiration from dormant buds
(Shulman et al. 1983) and it may be possible to construct
explanations based on relationships between respiration
and dormancy release. However, the role of respiration in
dormancy alleviation is not well understood (Lang 1989),
with results from various studies being equivocal.
On the spur-pruned Cabernet Sauvignon vines that
we studied, single-inflorescence shoots burst from
primary buds on clear nodes 1 day later, on average, than
double-inflorescence shoots. Buds containing shoots with
no inflorescence primordia burst even later again. The
difference between budburst of ‘fruitful’ primary shoots
(those with at least one inflorescence) and ‘unfruitful’
primary shoots (those with no inflorescence) averaged 3
days. Dunn and Martin (2000, in press) demonstrated
that for Cabernet Sauvignon, double-cluster shoots had
significantly and substantially more flowers (per bunch
and per shoot) than single-cluster shoots. If one intro-
duces the concept that flower number (potential seed
number) and inflorescence number (potential reproduc-
tive sites) indicate a shoot’s ‘reproductive potential’, then
one could speculate that shoots with higher reproduc-
tive potential burst in preference to those with lower
reproductive potential. It is difficult to explain this in
evolutionary terms because the genotypes familiar to viti-
culturists are highly selected, grown unnaturally and not
in their native habitat. However, it is possible that vines
that supported earlier bursting of the shoots most likely to
result in seed dispersal could have gained a selective
advantage. There would also be an advantage for a tree-
climbing plant such as the grapevine in giving preference
to shoots bursting at higher locations.
Antcliff and Webster (1955) have also suggested that
grapevine buds do not burst at random, but that those
with more inflorescences burst earlier than those with
fewer inflorescences. They based this on an analysis of
budburst in cane-pruned sultana over a 3-year period.
However, the highly significant and negative correlation