1.8.2.3 K Metabolism (Fig. 1.20)
Potassium is a univalent cation (K
+
). The function of K
+ is still unknown because K
+
does not form chemical compounds; however, its functions are hypothesized as
follows (Marschner 1995):
• Stomatal movement by osmoregulation: One of the functions of K
+ functions is to
regulate turgor changes in the guard cell for stomatal movement. When stomata
are stimulated by solar energy, the K
+ channel is stimulated, followed by a K
+
influx from the cytosol into the stomata. Then, stomata open, and CO 2 and H 2 O
diffuse from and into the atmosphere, respectively. When H 2 O evaporates due to
solar energy through porous structures such as stomata, the H 2 O changes into a
gas, removing the vaporization energy (latent heat), which causes the leaf to cool
down. K
+ is very important for cooling forests.
• Enzyme activation: A large number of enzymes are either completely dependent
on or stimulated by K
+
, inducing conformational changes in the enzyme protein.
All macromolecules such as enzyme proteins are highly hydrated and stabilized
by firmly bound water molecules forming an electrical double layer. K
+
-induced
conformational changes in enzymes increase the rate of catalytic reactions. Thus,
as the K
+ intensity or concentration in cells is important for enzyme activity, all
metabolic activity in all tissues of plants depends on the K
+ concentration.
• Phloem transport activation: Potassium has important functions in both the
loading of sucrose and in the rate of mass flow-driven solute transport in sieve
tubes. In particular, as photosynthate transport to roots or underground organs
decreases due to K
+ deficiency, the K
+ concentration in leaves is related to
maintaining root activity (Yamada et al. 2002).
• Cation-anion balance: In charge compensation, K
+ is the dominant cation for
counterbalancing immobile anions in the cytoplasm and chloroplasts as well as
mobile anions in the vacuoles, xylem and phloem. The role of K
+ in cation-anion
balance is almost reflected in the nitrate metabolism, in which K is often the
dominant counterion for NO 3
À in long-distance transport in the xylem. As a
consequence of NO 3
À reduction in leaves, the remaining K
+ requires the stoichiometric synthesis of organic acids such as malic acid for charge balancing and
pH homeostasis; part of this newly formed potassium malate may be
retranslocated to the roots for the subsequent utilization of K
+ as a counterion
for NO 3
À within the root cells and for xylem transport (Fig. 1.18). In nodulated
legumes, this K
+ recirculation may serve a similar function for the xylem transportation of amino acids.
1.8.3 Micronutrient-Related Carbon Metabolism
Micronutrients are key elements in the electron transport system as follows:
Photosynthesis: (Mg), Fe, Mn, Cu, Zn
34
M. Osaki et al.
Potassium is a univalent cation (K
+
). The function of K
+ is still unknown because K
+
does not form chemical compounds; however, its functions are hypothesized as
follows (Marschner 1995):
• Stomatal movement by osmoregulation: One of the functions of K
+ functions is to
regulate turgor changes in the guard cell for stomatal movement. When stomata
are stimulated by solar energy, the K
+ channel is stimulated, followed by a K
+
influx from the cytosol into the stomata. Then, stomata open, and CO 2 and H 2 O
diffuse from and into the atmosphere, respectively. When H 2 O evaporates due to
solar energy through porous structures such as stomata, the H 2 O changes into a
gas, removing the vaporization energy (latent heat), which causes the leaf to cool
down. K
+ is very important for cooling forests.
• Enzyme activation: A large number of enzymes are either completely dependent
on or stimulated by K
+
, inducing conformational changes in the enzyme protein.
All macromolecules such as enzyme proteins are highly hydrated and stabilized
by firmly bound water molecules forming an electrical double layer. K
+
-induced
conformational changes in enzymes increase the rate of catalytic reactions. Thus,
as the K
+ intensity or concentration in cells is important for enzyme activity, all
metabolic activity in all tissues of plants depends on the K
+ concentration.
• Phloem transport activation: Potassium has important functions in both the
loading of sucrose and in the rate of mass flow-driven solute transport in sieve
tubes. In particular, as photosynthate transport to roots or underground organs
decreases due to K
+ deficiency, the K
+ concentration in leaves is related to
maintaining root activity (Yamada et al. 2002).
• Cation-anion balance: In charge compensation, K
+ is the dominant cation for
counterbalancing immobile anions in the cytoplasm and chloroplasts as well as
mobile anions in the vacuoles, xylem and phloem. The role of K
+ in cation-anion
balance is almost reflected in the nitrate metabolism, in which K is often the
dominant counterion for NO 3
À in long-distance transport in the xylem. As a
consequence of NO 3
À reduction in leaves, the remaining K
+ requires the stoichiometric synthesis of organic acids such as malic acid for charge balancing and
pH homeostasis; part of this newly formed potassium malate may be
retranslocated to the roots for the subsequent utilization of K
+ as a counterion
for NO 3
À within the root cells and for xylem transport (Fig. 1.18). In nodulated
legumes, this K
+ recirculation may serve a similar function for the xylem transportation of amino acids.
1.8.3 Micronutrient-Related Carbon Metabolism
Micronutrients are key elements in the electron transport system as follows:
Photosynthesis: (Mg), Fe, Mn, Cu, Zn
34
M. Osaki et al.