Agriculture Reference
In-Depth Information
deficiencies, an extreme case of the latter being the 'little-leaf' disease induced
by zinc deficiency. Spur leaves are smaller than extension shoot leaves. On ex-
tension shoots the leaves are larger the more vigorous the shoot (Barlow,
).
Sources of carbon and nitrogen for leaf growth
Early in spring carbohydrates move upwards in the tree. Both Hansen (
b)
) found the new leaves to contain
C that had been supplied
as
CO
to the trees in the previous autumn. Hansen (
and Quinlan (
) found that a third
to a half of all building materials used in spurs up to the time of flower colour
development, and in shoots during the development of the first
leaves,
originated from reserves. These reserves also provide the energy for early
leaf growth. Young, growing leaves use the majority of the photosynthates
that they produce. Hansen (
or
% of the
C supplied
to young leaves was incorporated into the structural material of the leaf
(methanol-insoluble substances) whereas fully developed leaves incorporated
only
a) found that
-
) found that growing leaves consumed most of the
CO
supplied to them but usually exported some to younger leaves and the
shoot tip. They also received some
C when
CO
was supplied to mature
leaves in the apical part of the same shoot. Jankiewicz
et al.
(
-
%. Quinlan (
) also found
that young leaves retained most of their assimilates and received assimilated
C from mature leaves on the same shoot; moreover, the youngest leaves on
the leading shoot receive
C when
CO
is supplied to lateral shoots.
At the beginning of leaf development the supplies of nitrogen, phosphorus
and potassium used in the formation of new tissue are derived mainly from
tree reserves, principally the bark of branches and stem (Mason and Whitfield,
). Subsequently N is translocated upwards from the roots in the form of
amino acids in the xylem (Titus and Kang,
), together with other nutrient
elements.
Leaf senescence and shed
Premature leaf shed can be induced by pest and disease attack and nutrient
deficiencies. Leaf retention is favoured by the presence of fruits.
The general process of leaf senescence and abscission in autumn is primarily
controlled by temperature. Jonkers (
) showed that apple and pear trees
grown under controlled temperature conditions as well as outdoors had accel-
erated leaf drop when kept at
◦
Cor
◦
C but greatly retarded drop when
◦
C,
◦
Cor
◦
C. He also found that at
◦
C all apple
maintained at
◦
C,
◦
Cor
leaves had yellowed by end-November whereas in trees kept at
◦
C this did not occur until late January. Lakso and Lenz (
) showed that
