Agriculture Reference
In-Depth Information
Preston,
). Potassium plays an important role in cell expansion and other
phenomena, e.g. stomatal behaviour, which depend on cell osmotic potential.
It is also the most abundant cation in the cytoplasm with important roles in
enzyme activation, pH stabilization, protein synthesis, etc.
Calcium nutrition
This is dealt with in Chapter
.
Magnesium nutrition
Apple trees have a greater demand for magnesium than many other fruit trees
and readily show Mg deficiency symptoms. These can arise because of Mg
deficiency in the soil or because uptake is depressed by competing cations. Of
theseK
+
isusuallythemostimportantbutH
+
(lowpH),NH
+
,Ca
+
andMn
+
also compete. In aerobic soils of neutral pH the Mg content of the soil solution
is usually quite high (Marschner,
) and mass flow to the root surface
should give an adequate supply (Robson and Pitman,
). It is said not to
be transported in the symplast (Luttge,
), but unlike Ca it reaches much
higher concentrations in the phloem than in the xylem (Robson and Pitman,
) and may be re-transported within the plant. During April and May
when leaf growth is rapid, the amount of Mg in leaves increases sharply with
corresponding withdrawals of the element from the bark and wood of shoots,
branchesandrootstock(MasonandWhitfield,
).Thebiggestwithdrawalis
fromwood.DevelopingfruitsarethenabletowithdrawMgfromneighbouring
leaves (Shear and Faust,
% of the Mg
absorbed after foliar application is exported to permanent woody tissues and
roots. Oland (
). According to Forshey (
),
), however, found that there is no change in leaf Mg content
during senescence and Wittwer and Teubner (
) classed Mg as being an
immobile element. The complexity of results reported may reflect different
modes of transport in relation to different processes. A high proportion of
Mg in plants, often over
%, is diffusible and is associated with inorganic
anions and organic acid anions such as malate and citrate. About
%
of Mg is in the chloroplast. Less than half is bound to chlorophyll; the rest
serves as an activator of ribulose-bisphosphate carboxylase. It is also important
for protein synthesis and in the transfer of high energy phosphate in ATP
metabolism (Faust,
-
). Magnesium is supplied to fruit trees as kieserite or
as magnesian limestone to correct initial soil inadequacy. Where there are
leaf symptoms three or more foliar sprays at two-weekly intervals with
%
magnesium sulphate (Epsom salts) beginning at petal fall can alleviate these,
