Absorption & Transportation of plant

                                                                                                                                                      

I.             Water potential

                A.            Water potential (Ψ) = pressure potential (Ψp) + osmotic potential (Ψπ) + matrix potential (Ψm)

                                1.             Water potential - a measure used to predict the direction of water flow (from high to low)

                                                a)            Pressure potential - turgor pressure (associated with cell growth & expansion)

                                                b)            Osmotic potential - can be used to regulate turgor - regulates plant water movement

                                                c)             Matrix potential - force with which water is held to plants by adsorption and capillarity

II.            Water and mineral absorption by roots

                A.            Absorption of water by roots

                                1.             Driven by transpiration - negative pressure in xylem draws water in through free space - water must move through living cells of endodermis

                                2.             Root pressure (driving force when transpiration is low - high humidity)

                                                a)            Solutes (sugars) built up in roots cause an osmotic drive of water from surrounding media solution to inner root (i.e., water moves from higher potential to lower potential)

                                                b)            Xylem transport driven by positive pressure - source for guttation

III.          Uptake of mineral nutrients

                A.            Passive uptake of minerals (mineral ions move freely into free space of cortex)

                                1.             Movement of ions by the "sweeping effect"

                                                a)            Ions can cross the endodermal cell membranes passively by being "swept" into the stele with water

                B.            Active uptake of minerals

                                1.             Energy-requiring transport of ions into cells of the cortex (mostly mineral in low abundance in soil solution - nitrate, potassium, sulfate, phosphate, etc.)

                                2.             Movement into xylem is blocked by special barrier (Casparian strip of an endodermis) - promotes active transport

                                                a)            Endodermis (with casparian strip) requires that molecules pass through a plasma membrane to enter (or leave) the vascular cylinder

IV.          Xylem & phloem transport

                A.            Mechanism of xylem transport (cohesion-adhesion-tension hypothesis)

                                1.             Tracheids and vessels usually dead, empty cells

                                2.             Transport by bulk flow - driven by transpiration

                                                a)            Transpiration causes "suction" and negative pressure on water in xylem

                                3.             Important characteristics of water

                                                a)            Cohesion - attraction of water molecules to each other

                                                b)            Adhesion - attraction of water to other molecules (like cell walls)

                                                c)             Tension - ability of water to withstand negative pressure

                B.            Mechanism of phloem transport (pressure-flow hypothesis)

                                1.             Source is high pressure;  sink is low pressure

                                2.             "Source-sink" directionality (photosynthesis is source; meristem is sink)

                                                a)            Sugar (photosynthate) is actively transported into sieve tube at a source

                                                b)            Water moves into sieve tube by osmosis

                                                c)             Water uptake pushes sieve tube sap (photosynthate) towards sink

                                                d)            Sap (photosynthate) is unloaded at sink;

                                                e)             Water returns to xylem

V.            Mineral nutrition

                A.            CHOPKNS CaFe Mg B Mn CuZn Cl Mo

                                1.             CHO - carbohydrates

                                2.             P - ATP                 K - enzymes & stomates                   N - proteins

                                                S - amino acids    Ca - membranes                                 Fe - ETS & photosynthesis

                                                Mg - chloro.         B - CHO breakdown                          Mn, Cu, Zn - enzymes

                                                Cl - OEC               Mo - enzymes