Agriculture Reference
In-Depth Information
becausethemechanicalsupportscanbeusedintree-trainingtogivethedesired
branch angles and also to support heavy crops without branch breakage. The
alternative to such supportwould generallyinvolvepruning to stiffen branches,
which inevitably delays cropping. An appreciable improvement in anchorage
per se
can be achieved by budding high on the rootstock stem in the nursery
and then planting to a greater depth in the orchard. This results in a second
root system arising from the newly buried stem (Rogers and Parry,
; Parry,
), i.e. in root systems distributed over a greater depth.
Almost all of the seedling
Pyrus
rootstocks for pear give good anchorage
whereas trees on clonal quince rootstocks require support (Lombard and
Westwood,
).
Water uptake
Only a very small part of the water uptake by plants is needed to meet their
direct metabolic requirement. Most of it is to replenish evaporative water loss
through the leaves. This loss is a consequence of the need to have open stomata
in the leaves and an internal leaf structure permitting ready gas exchange
(including water loss), if carbon dioxide is to be taken up for photosynthesis.
Van den Honert (
) considered water fluxthrough a plant to be governed
by an Ohm's-type law, which can be written
=
soil
−
leaf
r
p
F
(
.
)
where
F
=
transpirational flux,
soil
is the water potential at the soil-root
interface,
leaf
is the leaf water potential and
r
p
is the resistance to liquid flow
in the plant.
If the soil is saturated with water,
soil
is
.
leaf
is negative when transpira-
−
−
tion is taking place but seldom falls below
.
MPa (
bar) unless there
are very severe soil water deficits (Jones
et al.
,
).
The water potential of plant cells is in a state of dynamic equilibrium with
the transpirational water fluxin the ylem. The water potential of an indi-
vidual plant cell (ignoring matric potential which is usually small) depends
on the osmotic potential of its sap (
s
) and its (hydrostatic) turgor pressure
potential (
p
):
cell
=
s
+
p
(
.
)
s
is negative because the presence of solutes lowers water potential. If the
water potential of the transpiration stream is lowered, i.e. becomes more neg-
ative, as when greater tensions develop in the xylem resulting from an increase
in transpiration rate, water tends to move from the living cells into the xylem.
This is very obvious at mid-day in apple, with actual shrinkage of tree trunks
