Agriculture Reference
In-Depth Information
acids transported to the leaves are synthesized into proteins, primarily into
the photosynthetic enzyme ribulose
-bisphosphate (RUBP) carboxylase. In
mid-July this can amount to more than
,
% of total protein in apple leaves
(Kang and Titus,
), while more than
% of total amino acid and protein
N is in proteins.
Export from senescing leaves
Much leaf nitrogen is exported from the leaves before their abscission. When
abscissionisinducedbylowtemperatures,
%oftheinitialleafNislostbefore
leaf shed (Shim
et al.
,
). Millard and Thomson (
) found a similar per-
centage loss from unfertilized 'MM.
' rootstock leaves between
September
and
November, with a smaller relative loss from leaves of plants well supplied
with N. Earlier studies showed the amount of N lost during senescence to be
from
). Leaf N begins to decline from
the onset of senescence, the time varying with availability of nutrients, crop
load and climatic conditions, especially temperature. The loss of leaf N results
from loss of leaf protein. Up to
%to
% (Kang and Titus,
% of the soluble protein lost was identi-
fied as being ribulose
-bisphosphate carboxylase-oxygenase (RuBPC/O),
there being preferential loss of this compared with other soluble leaf proteins
(Millard and Thomson,
,
). The measured decline in protein represents the
difference between synthesis and degradation over the period of senescence. It
is generally accepted, though without direct evidence, that amides are the ma-
jor forms of nitrogenous compounds transported from senescing leaves. This
leaf N migrates back into spurs and branches but is eventually translocated
to the older wood and root system (Murneek and Logan,
, cited by Titus
and Kang,
).
Storage over winter
After the cessation of shoot extension in summer there is a gradual increase of
nitrogen in wood and bark of all parts of the apple tree including roots (Mason
and Whitfield,
). The autumn application of N in late October and early
November increases the N content mainly in the roots during the late autumn
and winter (Tromp,
). The seasonal course of N reserves in apple roots
(Tromp,
. The main storage in the roots is in a
soluble form with arginine as the main component of the soluble N fraction.
The amides, especially asparagine, also appear to be important sources of N
reserves, especially in the roots (Hill-Cottingham and Cooper,
) is shown in Figure
.
; Cooper
et al.
,
).
Both bark and wood of stems also store nitrogen over the winter, with bark
storage probably being the most important with respect to remobilization
(Mason and Whitfield,
). Much storage is in the form of proteins.
