In this edition: - The European Fruit Magazine

Calcium defi ciency
Calcium defi ciency basically occurs in all varieties
of fruit and principally in those parts of the plant
that show little transpiration, i.e. mainly the fruits.
Apples are particularly susceptible to calcium
defi ciency. On the one hand due to the genetic
make up of the apple, but on the other hand as
apples, because of commercial considerations,
are forced to grow larger than the normal fruit
size for a specifi c variety.
Visible defi ciency symptoms in the leaf are
mainly seen in poorly bearing trees. From the
early summer, pale - later chlorotic - patches appear
at the tip of the leaves, ranging from small
patches to entirely chlorotic leaf tips.
Defi ciency symptoms in fruits can occur with
all types of fruit. Fruits with calcium defi ciency
ripen earlier, the decomposition of chlorophyll in
the skin is encouraged (yellow discoloration) and
the fruits have a low acidity and lack fi rmness.
In addition, fruits with a calcium defi ciency age
quicker (senescence) and the fl esh becomes soft
sooner, mainly after a period of display at room
temperature. The fruits also have a greater susceptibility
to rotting; with apples infected by
gloeosporium, with berries lower resistance to
Botrytis and with stone fruit sensitivity to fruit rot.
On trees with a calcium defi ciency any fruits damaged
by hail, for example, will heal more slowly
than those on trees with an adequate calcium
level. With apples a calcium defi ciency exaggerates
all kinds of physiological disorders such as
fl esh browning, scald, Jonathan-spot,Elisespot,
lenticel blotch and bitter pit.
Table 1. Risk categorisation of Cox’s, Braeburn, Kanzi and Jonagold for physiological
disorders based on the Ca, K and N content in the fl esh
Ca- content in fruits
(in mg/100 g fresh weight)
K/Caratio
N/Caratio
Optimal > 5,5 < 20 < 10
Normal 4,5 - 5,5 20 - 30 10 - 20
Critical < 4,5 30 - 40 > 30
Very critical < 3,5 > 40
16
Figure 3. Occurrence of bitter pit through gradual instability of the middle
lamella
cell with
cell wall
The Auxin pump
Transport over short
distances from cell to
cell is driven by a socalled
‘auxin pump’
mechanism. The auxin
producing tissues in
the plant have a hormonal
sink eff ect and
therefore have priority
in the supply of
nutrients and water
pectin
strands
intact
middle lamella
stage 1 stage 2
starting Cadeficiency
collapse of mutual
cell connections
Bitter pit is caused, in brief, by the following.
With optimal calcium supplies, the protopectin
(strands of pectin plus calcium) create calcium
bridges which impart stability to the middle lamella
(see fi gure 3). With low supplies of calcium,
or displacement of Ca 2+ by Mg 2+ or K + the mutual
links between the cells are destabilised. The
place of calcium is occupied by magnesium. As
magnesium and potassium, contrary to calcium,
lack the ability to bind cells, the cell structures
collapse and turn brown. Magnesium binds to
the sulphates and epsom salt is formed (magnesium
sulphate). This gives the fl esh a bitter fl avour
where the patches are visible (bitter pit).
Risk of disorders
A serious threat of disorders caused by calcium
defi ciency is present in:
• orchards with too low calcium availability in
the soil (low pH, little free calcium);
• trees with rosette leaves damaged by frost or
night frost or rosette leaves with poor nutrient
level (‘Ca-pump’);
• sensitive varieties (for example Kanzi, Braeburn,
Cox’s, Boskoop, Elise and Junami >75 mm);
• apples harvested too early (bitter pit) or too
late (brownish fl esh, gloeosporium);
• fruits too large for the variety concerned;
• newly planted trees with strong growth and
trees with low production (unfavourable calcium
distribution);
• trees with a too high nitrogen, potassium or
magnesium content.
Relevant in the fi rst place are low calcium, potassium,
nitrogen and magnesium contents in
the fl esh. The mutual ratios in particular –especially
the K/Ca and N/Ca ratio –are important
indicators of the potential risk of physiological
disorders in the fruits (see table 1). A few of these
also partially occur while the fruit is still on the
tree (bitter pit, lenticel blotch), others only after
a period of storage (brown fl esh, scald, storage
pit, gloeosporium).
Gloeosporium
in Pinova.