Plant Transport and Nutrition

Plant Transport and Nutrition

Chapter 36: Transport in Plants H 2 O & 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 CO 2 in; O 2 out Stomates o Respiration O 2 in; CO 2 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 H 2 O & 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. 1

Cellular Transport Active transport o Solutes are moved into plant cells via active transport. o Central role of proton pumps. Chemiosmosis 2

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. 3

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 H 2 O potential to low H 2 O potential. o Creates root pressure. 4

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. 5

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 CO 2 CO 2 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 H 2 O potential o Water flows in from xylem vessels Increase in pressure due to increase in H 2 O causes flow. 6

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 (H 2 O) CO 2 Minerals Macronutrients Plants require these nutrients in relatively large amounts o C, O, H, N, P, K, Ca, Mg, S For What & from Where? 1

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 2

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. 3

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 (NO 3 - ) 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 4