Water Balance in Plants. An organism usually exists in a state of gradients s gradient keeps sugar...

Water Balance in Plants

An organism usually exists in a state of gradients

• s gradient keeps sugar flowing down the phloem to supply the needs of the various cells in the root.

• p gradient : without these pressure differences, bulk flow would cease, the shoot would wilt and the root would starve.

• gradient from the soil water to the xylem provides the potential differences needed to bring water and minerals in from the soil to be lifted up the xylem.

• gravitational water(1) : the water that drains from the soil by gravity. Move to lower soil levels.

• When all gravitational water has drained, the soil is saturated and to be at field capacity.

• Most of water is held by surface tension in small pores in the soil : capillary water(2), which is easily absorbed by plant

• When the tension is equal with the ability to extrach it, the remaining water in soil reach permanent wilting point. This is the lower limit of soil moisture that can support plant growth

• Hygroscopic water is all remaining unbound water which is held tightly by small soil particles. Plants cannot extract hygroscopic water from the soil.

SOIL WATERThe type of water in soil affects the ability of plant to obtain water :1 – bound water, locked in particle – unavailable to plants2 – unbound water, avalailable to plants:

The Path of Water into Root

How water and mineral enter the root

The path water and mineral into the root

• Root (hairs) epidermal cells take up water and nutrients• Active uptake of mineral into root cells s cells

• Water osmosis enter between cells (apoplast) or through cells and cells membrane (symplast)

• Endodermis : one layer cells with casparian strip in cell wall (apoplast changes to symplast) to regulate the quantity and type of minerals and ions reach the xylem

• Further transport through symplast has to pass through selective membrane : control of which minerals/ions reach root xylem

• Active uptake of minerals is facilitated by transpiration. Low transpiration will be supported by root pressure

Casparian strip: a waxy layer in the endodermis cells to regulate the quantity and type of minerals and ions reach the xylem

Transmembrane pathway

Symplastic route In order for water and minerals

to reach the stele (xylem) the highly regulated (cytoplasmic) must be taken.

The symplastic route involves special openings between adjacent cell walls called plasmodesmata.

Apoplastic route in the nonliving parts of the

root — that is, in the spaces between the cells and in the cells walls themselves.

This water has not crossed a plasma membrane.

Therefore, to enter the stele, apoplastic water must enter the symplasm of the endodermal cells. From here it can pass by plasmodesmata into the cells of the stele.

Water and mineral normally can travel through the porous cell walls of the root cortex.

The apoplastic route is blocked by the casparian strip.

Casparian strip a waxy layer in the endodermis

cells to regulate the quantity and

type of minerals and ions reach the xylem

Pericycle, Stele :xylem, phloem

The water potentials of apoplast and symplast for a given tissue are the same but the two components of water potential are different

In the symplast : - hydrostatic pressure is

positive (turgor pressure), and

- the osmotic pressure is the larger of the two components because of the dissolved solutes in the cytoplasm.

• In the apoplast : - Hydrostatic pressure is

negative. The apoplast is under tension.

- osmotic pressure is small because there are fewer dissolved solutes in the apoplastic water.

Cells involve in pathway of water

Xylem vessels

• no cell contents (dead)• form continuous tubes• lignin fibres strengthen the cell walls• so do not collapse when pressure

inside falls

Water Movement in Xylem :

1. Transpiration : the pulling of water up through the xylem of a plant utilizing the energy of evaporation and the tensile strength of water.

2. Cohesion is the attractive force between molecules of the same substance. Water's cohesive force within xylem give it a tensile strength.

• A combination of adhesion, cohesion, and surface tension allow water to climb the walls of small diameter tubes like xylem. This is called capillary action.

3. Adhesion is the attractive force between water molecules and other substances. Because both water and cellulose are polar molecules there is a strong attraction for water within the hollow capillaries of the xylem.

4. Tension. This pulling force is created by the surface tension which develops in the leaf's air spaces.

5. Root pressure. It occurs when no transpiration

Capillarity Water rises up narrow tubes due to the adhesive forces between the water molecules and the wall of the tube

Xylem vessels are very narrow

Water rises higher in narrower tubes

Transpiration pathway through a plant

the xylem the petiole the veins of the leaf the finest veins the cells of the spongy and palisade layers.

Here some of the water may be used in metabolism, but most is lost in transpiration.

The purposes of transpiration :

• supplies water for photosynthesis • transports minerals from the soil to all parts of

the plant • cools leaf surfaces some 10 to 15 degrees by

evaporative cooling • maintains the plant's shape and structure by

keeping cells turgid

Transpiration depends on two major factors:

1. Difference in water vapor concentration2. Diffusional resistance (r)a) Leaf stomatal resistance (rs)b) Boundary layer resistance (rb)

Cl CaTranspiration flow =

Rs Ra

Transpiration from the Leaf

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Guard cell and water transport

• The physical structure of guard cells : A stoma is a physical gap between two special epidermal cells called guard cells.

• If the plant water deprivation it will wilt. To compensate the guard cells become flaccid and the stoma is closed.

• When the pair of guard cells are turgid -- full of water -- they bow in such a way as to increase the gap -- stoma -- between them. WHY ????

Guard Cell Physiology

Radially oriented cellulose microfibril• The structure of guard cells explains why they bow apart when

turgid. – The two guard cells are fused at their ends. – The inner cell walls which form the stoma are

thicker than the outer walls. – Cellulose microfibrils are oriented radially rather

than longitudinally.

Microfibrils are made of cellulose and are oriented transversely to the long axis of the cell resulting in cell expansion in the direction of its long axis because the cellulose reinforcement offers the least resistance at right angles to its orientation.

The sequence of events which result in stomatal opening

• The immediate cause is an increase in turgor pressure - water enters the central vacuole by osmosis

• Turgor pressure increases because of a negative water potential due to an influx of potassium ions (K+). The cell becomes hypertonic to its environment

• The reversible uptake of K+ ions takes place because of the membrane potential created when H+ are actively pumped out of the cell - consuming ATP. The cell's interior becomes negative compared to the surroundings.

The stoma is closed at night when the large central vacuole is isotonic, even hypotonic to surrounding fluids. K+ ions are outside of the cell, and H+ ions by and large remain attached to the weak organic acids within the cell.

Blue light is absorbed by a membrane protein which somehow causes an increase in the activity of proton pumps which use ATP to transport H+ out of the cell.

With H+ on the outside K+ readily diffuse into the cell to compensate for the negative electrical potential. The hypertonic conditions within the cell attract water molecules and the stoma opens as turgor pressure increases.

Guard cell transport

• GC membrane-transport processes during stomatal opening (left) and stomatal closing (right).

• Transporters in a conducting state are green; in a non conducting state, red; conducting state not specified, blue.

• Note, as indicated, that one icon may represent several transporters having different properties. The traffic-control icon by a transporter indicates inhibition (red, upper) or activation (green, lower); effectors act directly and indirectly. The roles of some transporters are not assigned with certainty.

stomatal opening

• It is initiated by H+ extrusion (1), which hyperpolarizes the PM and acidifies the apoplast.

• These effects increase the driving force for K+ uptake and activate the voltage-regulated K+-in channel (2).

• Cl- (3) and suc (4) may also be taken up. • H+ pumping into the vacuole (5, 6) provides for K+ antiport

(7) and Cl- uptake (8). • GC solute increase leads to H2O uptake and therefore an

increase in GC volume and aperture size.

Stomatal closing

• It is initiated by activation of the A- channel (9), which depolarizes the PM.

• This effect increases the driving force for K+ efflux and activates the voltage-regulated K+-out channel (10).

• Several channels (11) provide for solute release from the vacuole.

• Internal and external ABA receptors (12) cause the release of Ca2+ through cellular messengers (e.g., IP3); ABA also causes Ca2+ influx (13).

• ABA also induces stomatal closure by Ca2+-independent means. Compartments are not to scale.

An organism usually exists in a state of gradients so that flows can occur, both

between and around its cells.

Microfibrils

• In guard cells, microfibrils are made of cellulose and are oriented transversely to the long axis of the cell resulting in cell expansion in the direction of its long axis because the cellulose reinforcement offers the least resistance at right angles to its orientation.

• Keunikan dari sel penjaga adalah serat halus sellulosa (cellulose microfibril) pada dinding selnya tersusun melingkari sel penjaga,

• Pola susunan ini dikenal sebagai miselasi Radial (Radial Micellation). Karena serat sellulosa ini relatif tidak elastis, maka jika sel penjaga menyerap air mengakibatkan sel ini tidak dapat membesar diameternya melainkan memanjang.

• Akibat melekatnya sel penjaga satu sama lain pada kedua ujungnya memanjang akibat menyerap air maka keduanya akan melengkung ke arah luar. Kejadian ini yang menyebabkan celah stomata membuka.

The sequence of events which result in stomatal opening

• The immediate cause is an increase in turgor pressure - water enters the central vacuole by osmosis

• Turgor pressure increases because of a negative water potential due to an influx of potassium ions (K+). The cell becomes hypertonic to its environment

• The reversible uptake of K+ ions takes place because of the membrane potential created when H+ are actively pumped out of the cell - consuming ATP. The cell's interior becomes negative compared to the surroundings.