Agriculture Reference
In-Depth Information
reduction in weight per unit length of shoot, rather than in the length of the
individual shoots ( Jackson and Palmer,
a).
Effects of water stress
Tromp (
a) found that reducing water stress by growing apple trees at
%Relative Humidity (RH), as contrasted with
% RH, increased total
shoot growth by more than
%. Part of this effect was a result of increase
in individual shoot length but most of it was a result of the emergence of
more lateral shoots. Maggs (
), working with pot-grown trees disbudded so
as to give only one shoot per plant, found that growth of this shoot under a
high water supply regime, expressed as dry weight increment, was more than
twice as great as under a low water supply regime. The shoots grown under
conditions of ample water supply were also about
% longer (taller) than
those with limited water supply. These results are compatible with results from
field trials that showed the main effect of irrigation to a minimal soil water
deficit (
mm) in England, as compared with no irrigation, was to increase
the number of shoots, but that in a dry season mean shoot length was increased
by
% also (Goode and Ingram,
).
Effects of nitrogen and other nutrients
The nitrogen reserves available at the beginning of the season determine the
length of time for which the shoots will grow without further supplies. Such
suppliesdelaytheformationofterminalbudsandprolongtheseasonofgrowth
(Hill-Cottingham,
).
Deficiencies of other mineral nutrients, or toxic levels of these, can also
reduce shoot growth as part of their general effects of inducing stunting, often
associated with poor leaf development, as in the little-leaf syndrome induced
by zinc deficiency, or with leaf shed.
Effects of plant hormones
Shoot extension is generally considered to be controlled by both growth in-
hibitors and growth promoters and the rate of extension to reflect the balance
between these.
Auxins stimulate cell enlargement, cell division and stem growth (Davies,
). They also have a major effect on root system development and, through
this, on the growth of shoots. There is, however, also evidence that the api-
cal dominance effect of shoot-tip-derived auxin reduces the growth of lateral
shoots as well as influencing lateral bud dormancy.
Abscisic acid (ABA), which is generally regarded as a growth inhibitor,
stopped the elongation of apple shoots and induced terminal bud formation
