Agriculture Reference
In-Depth Information
and fruits (Tukey,
), and has a number of negative effects which are
discussed later. There are also adverse effects of low tissue water potential (high
negative values) on many physiological processes, e.g. cell growth, wall synthe-
sis, protein synthesis and production of new leaf. It follows from equation
,
that plant water potential at any given soil water potential and evaporation
(transpiration) rate will be more negative the higher the plant resistance to
flow. The main source of resistance to water flow in the apple tree is found in
the roots; indeed, root resistance is the main resistance to flow in the complete
soil-plant pathway with soil resistance only becoming significant at low soil
water content (Landsberg and Jones,
.
). This high root resistance can result
in severe mid-day water stress even when soil water is not limiting: such stress
cannot be alleviated by supplying water to the roots but only by shading the
leaves or cooling them by use of over-tree irrigation. In both apple and pear,
unlike some other plant species, root resistance appears to be independent
of flow rate, i.e. transpiration (Jones
et al.
,
), although there is contradic-
tory evidence on this. Dwarfing rootstocks and their graft unions with scions
have higher resistance to flow than more invigorating ones (see Chapter
).
Root resistance to hydraulic flow is increased at low temperature and under
anaerobic conditions.
The sparsity of apple and pear roots suggests that the soil component of the
hydraulic resistance can become significant at higher soil water potentials than
is the case for herbaceous species (Landsberg and Jones,
). It is probable
that both white and woody roots can be effective in water uptake, as they are
in cherry (Atkinson and Wilson,
).
Although the majority of apple and pear roots are usually within the upper
cm of the soil profile, i.e. do not draw water from a greater soil depth than
those of many field crops, the frequent presence of at least some deep roots
enables apple and pear trees to survive droughts for longer than species which
do not have these. Most of the world's high-yielding orchards are, however,
irrigated. In this context the ability of apple and pear roots to become con-
centrated in relatively limited soil volumes maintained at high water potential
by drip, trickle or microjet irrigation is particularly important.
Nutrient uptake
Russell (
) concluded that uptake of nutrients by intact growing plants
is often controlled predominantly by the metabolic demands of the plant as a
whole, and Tromp (
,
) found a linear relationship between uptake of potas-
sium and total plant growth for apple. Faust (
) found calcium uptake by
apple seedlings to be dependent on the presence of healthy young root tips and
either a carbohydrate supply from photosynthesis or a direct supply of sugar.
Atkinson (
) found that the uptake of
P from soil showed a periodicity
