Agriculture Reference
In-Depth Information
iron in soils is very low, particularly in calcareous soils or where drainage
problems exist. Iron deficiency leads to chlorosis and loss of photosynthetic
efficiency (Sun
et al.
,
). It cannot reliably be diagnosed from the Fe content
of the leaves (Korcak,
) but can be proved by response to foliar-supplied Fe
and diagnosed and predicted using a test based on peroxidaze enzyme, an iron
haemoprotein. Pears are much more subject to iron chlorosis than apples.
Pyrus
communis
rootstocks are more tolerant than
P. calleryana
,
P. ussuriensis
or quince
(Korcak,
), although some quince rootstocks are said to be tolerant of high
pH soils (Webster,
). The pear cultivars 'Bartlett', 'Nelis', and 'Comice'
are more subject to iron chlorosis than 'Hardy' and 'Clairgeau'.
Iron can exist at either the ferric (Fe
+
) or the ferrous (Fe
+
) level. The ox-
idized ferric form gives highly insoluble ferric hydrate precipitates especially
above pH
. The activity of Fe
+
decreases
-fold for each unit increase in
pH. Well-aerated acid soils have much more soluble Fe than calcareous soils. In
calcareous soils the bicarbonate ion, rather than CaCO
, content is the most
important factor associated with lime-induced chlorosis and the bicarbonate
ion level is increased by high soil water content. Iron deficiency chlorosis prob-
lems can arise when roots initially growing in surface soil penetrate underlying
calcareous zones.
Before plants can utilize Fe
+
this must be solubilized and then reduced to
Fe
+
. Solubilization is thought to be by malate as an organic acid supplied by
the roots into the rhizosphere (Korcak,
). Reduction to
Fe
+
is thought to be exocellular. The uptake requires energy and needs both
O
and photosynthate. It also responds to plant Fe status: roots under iron
stress take up
; Sun
et al.
,
-
times more Fe than unstressed roots. In barley, uptake is
by the root zone
cm from the tip. With blueberry, it has been shown that
fertilizer supply of NH
-N gives a more acid rhizosphere than NO
-N and
this is thought to be relevant to Fe uptake (Korcak,
-
).
Iron is transported upwards in the xylem in a complexed form as a ferric-
citrate chelate. Once in the leaf iron moves rapidly across the plasmalemma
into the symplast via nicotianamine, a divalent cation chelator. It is found in
a complexed form in the phloem at higher concentration than in the xylem
(Robson and Pitman,
).
Temporary control of iron deficiency can be achieved by trunk injection
with ferrous sulphate or ferric citrate. FeSO
applied to the roots is also useful
and can be provided as a mixture of manure and ferrous sulphate inserted in
holes around the tree. Chelated (EDTA) iron is effective on acid soils, with
FeEDDHA being more effective on calcareous soils because of its greater
stability at high pH. Chelated Fe can be used in much smaller quantities
and gives higher foliar leaf levels than unchelated iron. Although there are a
number of problems in its use due to fixation and adsorption (Korcak,
),
soilapplicationofFeEDDHAchelateisconsideredtobethebestwaytocorrect
