Agriculture Reference
In-Depth Information
Water shoot growth
Water shoots arise from trace buds buried in older stem tissues after heavy
pruning. They grow very strongly, with all their assimilates directed to the
rebuilding of new shoots, limbs and trunks (Mika,
). They never supply
assimilates to fruits even if these are nearby. This appears to illustrate the
general principle that fruit trees react to severe pruning in such a way as to
restore the functional equilibrium between shoots and roots.
Secondary thickening
In the stems of woody plants the cambium begins to differentiate from the
procambium in a given region just before that region ceases to elongate (Eames
and MacDaniels,
). A continuous cambial cylinder forms and produces
annual increments of secondary vascular tissues for the life of that part of the
tree. Evert (
) found this production of new secondary vascular tissues in
pear to start in April in Montana, USA, reaching a peak in June and early
July. Knight (
) found new secondary xylem in apple stems in England by
April. Gross measurements of trunk girth as an indicator of cambial activity
show this to occur from June to September, or from May to August, in different
studies (Pratt,
). Girth often continues to increase after extension growth
has ceased.
The increments in trunk girth each year as a result of secondary thickening
reflect factors influencing growth in general. Nutrient deficiencies and wa-
ter stress reduce trunk growth, as do dormant-season pruning, fruiting, root
pruning and the use of dwarfing rootstocks.
Water stress, however, reduces trunk growth less than it reduces new stem
growth (Maggs,
).
The negative effect of heavy fruiting on trunk growth is greater in the year of
fruiting than in the following year. This has been demonstrated experimentally
by Blasco (
) but more than it reduces fruit enlargement (Iancu,
), as shown in Table
.
and in regression studies by Barlow
(
b).
Secondary thickening in relation to trace buds
and to branches
Secondary xylem produced by cambium at the nodes surrounds and buries
leaf and branch traces.
The base of a side branch becomes buried in growth rings of secondary
tissues of the trunk. This produces a mechanically strong crotch, in contrast to
the weak crotch between approximately equal leading branches, as shown in
Figure
.
. Secondary xylem in general has high mechanical strength. Apple
