1.10.2 Water Absorption Function (Main Root)
Water absorption occurs via two mechanisms:
• A passive process via mass flow, and
• An active process through aquaporins that is energy-dependent.
Mass flow depends on solar energy. Solar radiation on leaves induces water
evaporation through stomata in leaves that connect with xylem. Water is absorbed
mainly from the main root, is transported through the xylem, and finally
evapotranspires from the stomata (Fig. 1.34). As water evapotranspiration occurs
through porous structures (stomata), the liquid water inside plants transforms into a
gas, removing heat from the leaves and cooling the leaf surface.
As mass flow depends on solar energy, this high-energy process causes a high
water potential of approximately À10 Pa in the main root. If the xylem of the main
root is not composed of lignin, the cell in the main root will collapse due to the
extremely high pressure.
Nutrients are mainly absorbed from lateral roots. The cell walls of lateral cell
roots are composed of cellulose and hemicellulose, not lignin. Then, as lateral root
cells are relatively soft, if they connect directly with xylem, the cell will collapse.
The process of water absorption, which is related to nutrient absorption, depends on
the chemical energy supplied by respiration, which is why the oxygen supply is a
key limiting factor.
Main Root
(Lignin
Shive)
Lateral Root
Leaf
Stem/Trunk
Land Surface
Water
(Passive:
Mass Flow)
Nutrients &
Water (Active:
Energy-dependent
(Aquaporin)
Oxygen
Little Oxygen
& Little CH 2 0
CO 2
H 2 O
(Mass Flow)
CH 2 O
CH 2 0
High Water
Potential
-10 Pa
Xylem
Phloem
Fig. 1.34 Root function of lateral roots and main roots
50
M. Osaki et al.
Water absorption occurs via two mechanisms:
• A passive process via mass flow, and
• An active process through aquaporins that is energy-dependent.
Mass flow depends on solar energy. Solar radiation on leaves induces water
evaporation through stomata in leaves that connect with xylem. Water is absorbed
mainly from the main root, is transported through the xylem, and finally
evapotranspires from the stomata (Fig. 1.34). As water evapotranspiration occurs
through porous structures (stomata), the liquid water inside plants transforms into a
gas, removing heat from the leaves and cooling the leaf surface.
As mass flow depends on solar energy, this high-energy process causes a high
water potential of approximately À10 Pa in the main root. If the xylem of the main
root is not composed of lignin, the cell in the main root will collapse due to the
extremely high pressure.
Nutrients are mainly absorbed from lateral roots. The cell walls of lateral cell
roots are composed of cellulose and hemicellulose, not lignin. Then, as lateral root
cells are relatively soft, if they connect directly with xylem, the cell will collapse.
The process of water absorption, which is related to nutrient absorption, depends on
the chemical energy supplied by respiration, which is why the oxygen supply is a
key limiting factor.
Main Root
(Lignin
Shive)
Lateral Root
Leaf
Stem/Trunk
Land Surface
Water
(Passive:
Mass Flow)
Nutrients &
Water (Active:
Energy-dependent
(Aquaporin)
Oxygen
Little Oxygen
& Little CH 2 0
CO 2
H 2 O
(Mass Flow)
CH 2 O
CH 2 0
High Water
Potential
-10 Pa
Xylem
Phloem
Fig. 1.34 Root function of lateral roots and main roots
50
M. Osaki et al.