The role of potassium as an osmoticum in barley leaf cells.
1994
A recent X-ray analytical technique for the measurement of inorganic solutes
in dried vacuolar sap has been improved and tested and was used to measure the
content of barley leaf epidermal vacuoles. This method was compared with a
technique which measured inorganic solutes in vacuoles of frozen leaf sections. The
methods gave comparable results. They were used in conjunction with a range of
other microanalytical methods to investigate the role of potassium as an osmoticum
in individual leaf epidermal cells of barley grown in either low (0.2 mM) or control
(4.0 mM) levels of potassium.
In the low potassium plants, both turgor and osmotic pressure were at
first relatively low, but eventually rose to levels similar to the control plants. In both
treatments, vacuolar potassium and its counterion accounted for all the osmotic
pressure in young leaves. However, during leaf development, the potassium was
replaced to
.
varying extents by calcium. During this time gradients of ion
concentrations developed between adjacent cells, depending on their proximity to
vascular tissue.
In older leaves, the cellular concentrations of calcium and potassium were
negatively correlated. The nature of this relationship was affected by potassium
nutrition. In control plants, the ratio of calcium to potassium concentration was 2: 3
(0.67). This value is consistent with the maintenance of osmotic pressure which was
observed. In the low potassium plants, the Ca: K ratio was 0.76, which explained the
increase in osmotic pressure over time in these plants. It is proposed that the epidermis behaves as a storage tissue for potassium
which is retranslocated to younger tissue as the leaf ages. The rate of export depends
on the potassium nutrition of the plant. These events are also consistent with the
hypothesis that turgor and osmotic pressure are regulated to compensate for the arrival
of soluble calcium in the leaf.
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