Plant Transport and Nutrition

1

Chapter 36: Transport in Plants

• H2O & Minerals

o Transport in xylem

o Transpiration

� Evaporation, adhesion & cohesion

� Negative pressure.

• Sugars

o Transport in phloem.

o Bulk flow

� Calvin cycle in leaves loads sucrose into phloem.

� Positive pressure.

• Gas Exchange

o Photosynthesis

� CO2 in; O2 out

� Stomates

o Respiration

� O2 in; CO2 out

� Roots exchange gases within air spaces in soil.

Why does over-watering kill a plant?

Transport in Plants

• Physical forces drive transport at different scales.

o Cellular

� From environment into plant cells

� Transport of H2O & solutes into root hairs

o Short-distance transport

� From cell to cell

� Loading of sugar from photosynthetic leaves into phloem sieve tubes.

o Long-distance transport

� Transport in xylem & phloem throughout whole plant.

2

Cellular Transport

• Active transport

o Solutes are moved into plant

cells via active transport.

o Central role of proton pumps.

� Chemiosmosis

3

Short Distance (Cell-to-Cell) Transport

• Compartmentalized plant cells

o Cell wall

o Cell membrane

� Cytosol

o Vacuole

• Movement from Cell to Cell

o Move through cytosol

� Plasmodesmata junctions

connect cytosol of neighboring

cells.

� Symplast

o Move through cell wall

� Continuum of cell wall

connecting cell to cell.

� Apoplast

Routes from Cell to Cell

• Moving water & solutes between cells

o Transmembrane Route

� Repeated crossing of plasma membranes.

� Slowest route but offers more control.

o Symplast Route

� Move from cell to cell within cytosol

o Apoplast route

� Move through connected cell wall without crossing cell membrane.

� Fastest route but never enter cell.

Long Distance Transport

• Bulk flow

o Movement of fluid driven by pressure.

� Flow in xylem tracheids & vessels.

• Negative pressure

• Transpiration creates negative pressure pulling xylem sap upwards from

roots.

� Flow in phloem sieve tubes

• Positive pressure

• Loading of sugar from photosynthetic leaf cells generates high positive

pressure pushing phloem sap through tube.

4

Movement of Water in Plants

• Water relation in plant cells is based on water potential.

o Osmosis through aquaporins.

� Transport proteins

o Water flows from high potential to low potential.

Water & Mineral Uptake by Roots

• Mineral uptake by root hairs.

o Dilute solution in soil.

o Active transport pumps.

o This concentrates solutes (~100x) in root cells.

• Water uptake by root hairs

o Flow from high H2O potential to low H2O potential.

o Creates root pressure.

5

Route Water Takes Through Root

• Water uptake by root hairs

o A lot of flow can be through cell wall route

o Apoplasty

Controlling the Route of Water in Root

• Endodermis

o Cell layer surrounding vascular

cylinder of root.

o Lined with impervious Casparian

strip.

o Forces fluid through selective cell

membrane & into Symplast.

• Filtered & forced into xylem

vessels.

Mycorrhizae Increase Absorption

• Symbiotic relationship between fungi &

plant.

o Symbiotic fungi greatly increases surface area for absorption of water & minerals.

o Increases volume of soil reached by plant.

o Increases transport to host plant.

Rise of Water in a Tree by Bulk Flow

• Transpiration pull.

o Adhesion & cohesion.

� H bonding

o Brings water & minerals to shoot.

• Water potential

o High in soil → low in leaves

• Root pressure push

o Due to flow of H2O from soil to root cells.

o Upward push of xylem sap.

6

Control of Transpiration

• Stomate function

o Always a compromise between photosynthesis & transpiration.

� Leaf may transpire more than its weight in water in a day…this loss must be

balanced with plant’s need for CO2 for photosynthesis.

• A corn plant transpires 125 L of water in a growing season.

Regulation of Stomates

• Microfibril Mechanism

o Guard cells attached at tips

o Microfibrils in cell walls

� Elongate causing cells to arch

open = open stomate

� Shorten = close when water is

lost

• Ion Mechanism

o Uptake of K+ ions by guard cells

� Proton pumps

� Water enters by osmosis

� Guard cells become turgid

o Loss of K+ ions by guard cells

� Water leaves by osmosis

� Guard cells become flaccid

• Other Cues

o Light trigger

� Blue-light receptor in plasma

membrane of guard cells

� Triggers ATP-powered

proton pumps causing K+

uptake

• Stomates open

o Depletion of CO2

� CO2 is depleted during photosynthesis (Calvin cycle)

o Circadian rhythm = internal “clock”

� Automatic 24-hour cycle

Transport of Sugars in Phloem

• Loading of sucrose into phloem.

o Flow through symplast via Plasmodesmata.

o Active cotransport of sucrose with H+ protons.

� Proton pumps

Pressure flow in Sieve Tubes (See: Page 753, Figure 36.18)

• Water potential gradient

o “source to sink” flow

� Direction of transport in phloem is variable

o Sucrose flows into phloem sieve tube decreasing

H2O potential

o Water flows in from xylem vessels

� Increase in pressure due to increase in H2O causes flow.

1

Chapter 37: Plant Nutrition

Nutritional Needs

• Autotrophic does not mean autonomous

• Plants Need:

o Sun as an energy source

o Inorganic compounds as raw materials

� Water (H2O)

� CO2

� Minerals

Macronutrients

• Plants require these nutrients in relatively large amounts

o C, O, H, N, P, K, Ca, Mg, S

For What & from Where?

2

Micronutrients

• Plants require in very small amounts.

o Primarily cofactors

Nutrient Deficiencies

• Lack of essential nutrients

o Exhibit specific symptoms

� Dependent on function of nutrient

� Dependent on solubility of nutrient

Magnesium Deficiency

• Symptoms

o Chlorosis = yellowing of leaves

o What is magnesium’s function?

o The chlorosis shows up in older leaves first, because plant moves Mg to newer leaves. Why?

Chlorophyll

3

Water & Mineral Uptake

• Water uptake

o Plants cannot extract all water from soil, only free water

o Osmosis

• Cation uptake

o Cation uptake is aided by H+ secretion by root cells (proton pump)

o Active transport

The Role of Soils

• Plants are dependent on soil quality.

o Texture / structure

� Relative amounts of various sizes of soil particles

o Composition

� Organic & inorganic chemical components

� Fertility

Importance of Organic Matter

• Topsoil

o Most important to plant growth

o Rich in organic matter

� Humus

• Decomposing organic material

o Breakdown of dead organisms, feces, fallen leaves & other organic

refuse by bacteria & fungi

• Improves soil texture

• Reservoir of minerals

o Organisms

� 1 tsp. of topsoil has ~5 billion bacteria living with fungi, algae, protists, insects,

earthworms, nematodes.

• Not taking care of soil health has far-reaching, damaging consequences.

o 1920’s Dust Bowl

� Lack of soil conservation

• Growing wheat & raising cattle

o Land exposed to wind erosion & drought.

4

Fertilizers

• “Organic” fertilizers

o Manure, compost, fishmeal

• “Chemical” fertilizers

o Commercially manufactured

o N-P-K (ex. 15-10-5)

� 15% nitrogen

� 10% phosphorus

� 5% potassium

Nitrogen Uptake

• Nitrates

o Plants can only take up nitrate

(NO3-)

• Nitrogen cycle by bacteria.

o Trace path of nitrogen fixation!

Soybean Root Nodules

• N fixation by Rhizobium bacteria.

o Symbiotic relationship with bean family (legumes).

Increasing Soil Fertility

• Cover crops

o Growing a field of plants just to plow them under

• Usually a legume crop.

• Taking care of soil’s health.

• Puts nitrogen back in soil