The hidden half of agriculture

Region of water uptake

Path of water transport

The adaptations of roots for absorption of water and minerals 1) Epidermal cells are not covered by cuticle and thus water can easily pass into them.

The adaptations of roots for absorption of water and minerals 2) Numerous root branches and root hairs provide a large surface area for absorption of water and minerals.

The adaptations of roots for absorption of water and minerals 3) The root hairs are long, fine structures.  easily grow between the soil particles

Root hairs

in contact with the soil. Root hairs Soil air Soil water Figure 6.3: Structures unique to roots. Soil particles Epidermis (b) Root hairs on a radish (Raphanus sativus) seedling. Each delicate hair is a unicellular extension of the root epidermis. Root hairs increase the surface area in contact with the soil. Fig. 6-3b, p. 114

Structures Unique to Roots

Herbaceous Eudicot Root

Root Anatomy

(b) A close-up of the stele of the buttercup root. Cortex cells filled with amyloplasts Endodermis cell Pericycle cell Phloem cell Xylem vessel elements Figure 6.4: Structure of an herbaceous eudicot root. Intercellular space (b) A close-up of the stele of the buttercup root. Note the solid core of vascular tissues. Fig. 6-4b, p. 116

Movement upward Endodermis Xylem vessels Casparian strip Epidermis Cortex Symplast: interconnected cytoplasm of living cells Phloem cells Pericycle Figure 6.6: The symplast and apoplast. Water and dissolved minerals that enter the root travel from one cell’s cytoplasm to another through cytoplasmic connections (the symplast) or from cell to cell along the interconnected porous cell walls (the apoplast). On reaching the endodermis, water and minerals can continue to move into the root’s center if they pass through a plasma membrane and enter an endodermal cell. The Casparian strip blocks the passage of water and minerals along the cell walls between adjoining endodermal cells. Plasma membrane Water and dissolved nutrient minerals Plasmodesma Cell wall Apoplast: interconnected cell-wall spaces Root hair Fig. 6-6, p. 119

Symplastic pathway

Radial transport of water

Endodermis and Mineral Uptake

Absorption of water in roots 1 Water moves into the root hairs by osmosis.

Absorption of water in roots 2a Water moves into the neighbouring cortex cells by osmosis.  it moves inwards from cell to cell

Absorption of water in roots 2b Some water moves along the cell wall.

Absorption of water in roots xylem vessel in the stem 3 Water is drawn up the xylem vessel by transpiration pull.

Transpiration Inside the leaves, water is evaporates from all the moist surface of the mesophyll cells into the substomatal spaces which are usually almost saturated with water vapour ( Relative humidity approaches 100% ) If air has R.H. lower than saturation, water vapour diffuses out through the stomata according to the difference in the concentration of water vapour. 100% <100%

Transpiration pull Water lost from the surface of the mesophyll cells is replaced by water in the mesophyll cells. The loss of water lowers the water potential of these mesophyll cells. Water is then drawn from the neignhouring cells by osmosis Water is finally drawn from the xylem vessels, creating water potential gradient. As a result, transpiration pull is set up, causing the water to flow from xylem to the mesophyll cells. Therefore a suction force is created to pull water up the xylem from the root.

Water absorption (briefly)

Transpiration pull Water lost from the surface of the mesophyll cells is replaced by the water in the mesophyll cells The loss of water lowers the water potential of these mesophyll cells, water is drawn from the neighboring cells. The process is repeated and water is finally drawn from the xylem vessels. Water potential gradient along mesophyll cells to xylem vessels transpiration pull

Absorption of Water at Root

Absorption of Water at Root The entry of water increases the water potential of the cytoplasm and vacuole of the root hairs cells, which is higher than the neighboring cells. The water potential of the soil water is usually higher than that of the cytoplasm and cell vacuile of the root hairs cells. Water moves by diffusion along cell wall ( less resistance) 29

Absorption of Water at Root) 1. Through the cytoplasm and vacuole by osmosis 2. Along cell wall and air spaces by diffusion 1 2 Water moves by diffusion along cell wall ( less resistance) 30

Absorption of Water at Root Root hairs are surrounded by soil water Water potential of soil water is higher than that of the cytoplasm of root hairs Water moves from the soil through the selectively permeable cell membrane into the root hairs by osmosis

Absorption of Water at Root Water potential of root hairs is higher than that of the neighbouring cells Water moves into neighbouring cortial cells by osmosis Water moves inwards from cell to cell through the cytoplasm by osmosis or through the fully permeable cell wall by transpiration pull until it reaches the xylem vessels.

Transportation of water and minerals in flowering plants by osmosis

Transportation of water and minerals in flowering plants by osmosis Strong cohesive forces between water molecules and adhesive forces of water to xylem walls. These enable water to move up the vessel without breaking, forming a continuous stream of water.

Transportation of water and minerals in flowering plants by osmosis