Agriculture Reference
In-Depth Information
apples from high Ca (
.
-
.
mg/
g fresh weight) and low Ca (
.
-
◦
C) from
late September to January. Electron micrographs showed that in November
both high- and low- Ca fruits had intact plasma and vacuolar membranes,
mitochondria, plastids and nuclei in the epidermal, hypodermal and many of
the outer cortical cells. In mid-December the high-Ca fruits had intact mem-
branes, mitochondria, plastids and nuclei in most of the epidermal and hypo-
dermal cells and in many outer cortical cells. The low-Ca fruits had much
disorganization of the cytoplasm and separation of the plasma membrane
from the cell wall. By early January the differences were very pronounced,
with relatively little membrane breakdown in the high-Ca fruits but much
membrane breakdown and even complete cytoplasmic disorganization in the
low-Ca fruits. During ripening, solute leakage from tissue discs increases but
this leakage is retarded by elevated Ca concentrations (Sharples and Johnson,
◦
C and
.
mg/
g fresh wt) populations in cold storage (
). Calcium affects physical properties of
plant membranes by regulating their microviscosity or fluidity. Cellular senes-
cence is accompanied by increases in microviscosity and the proportion of
gel-phase lipid of membranes and Ca may diminish these trends (Ferguson,
; Ferguson and Watkins,
).
Calcium, respiration and ethylene production
Calcium status influences respiration, respiration rates of untreated apples be-
ing inversely related to flesh calcium content (Faust and Shear
; Bramlage
et al.
,
), and calcium dips can reduce respiration (Watkins
et al.
,
).
Bramlage
et al.
(
b) considered that the accelerated respiration of Ca-
deficient apples might be linked to effects of calcium on ADP/ATP ratios
in the cell. Calcium ions are essential for the activity of a number of enzymes
includingthemembrane-associated,calcium-dependentATPasesandcalcium
movement across the membrane can directly drive ATP synthesis.
Pyruvate kinase (pyruvate-ATP phosphotransferase, EC
) exerts
considerable control of respiration and the control of its activity is largely a
function of the concentrations of Ca
+
,Mg
+
and K
+
in its environment
(Meli and Bygrave,
.
.
.
). Pyruvate kinase activity
in apple fruits declines throughout the period of fruit growth up to harvest,
is reduced by CaCl
sprays and increased by MgCl
sprays, foreshadowing
subsequent incidence of bitter pit (Witney and Kushad,
; Witney and Kushad,
).
Calcium infiltration of 'Golden Delicious' apples significantly reduces ethy-
lene production (Poovaiah
et al.
,
) and it has been suggested that micro-
somal membranes are the sites of interaction of calcium, and that ethylene
biosynthesis is modulated through its binding with the membrane (Ben-Arie
et al.
,
).
