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H2O vapor product of photosynthesis (sucrose) H2O vapor H2O vapor H2O mineral ions H2O Fig. 11-1, p. 164 Symplastic and apoplastic flow through roots root hair plasmodesma xylem symplastic flow apoplastic flow cell wall cytoplasm epidermis symplast of endodermis cortex Casparian strip of endodermis stele Fig. 11-7, p. 169 Control of Water Flow • Environmental factors affecting rate of transpiration – Temperature – Relative humidity of bulk air – Wind speed Control of Water Flow • Transpiration – Slow at night – Increases after sun comes up – Peaks middle of day – Decreases to night level over afternoon • Rate of transpiration directly related to intensity of light on leaves LIGHT Events leading to the opening of a stoma: The production of malate and the influx of K+ and Clpowered by the electrical and pH gradients produced by the proton pump increase the concentration of osmotically active solutes in the guard cells. As a result, water flows into the cells by osmosis. starch malic acid malate– plasma membrane ATP H+ ADP + Pi proton pump H+ + K+ CI H+ K+ CI Fig. 11-8a, p. 170 cells connected cellulose microfibrils (radial micellation) reinforced inner wall How radial micellation and reinforcement of guard cell walls force an expanding cell to bow outward. With increased pressure, cell gets longer. Because the outer wall can expand more readily, cell bows outward. Fig. 11-9a, p. 170 Fig. 11-9b, p. 170 MINERAL UPTAKE AND TRANSPORT H2O vapor product of photosynthesis (sucrose) H2O vapor H2O vapor H2O mineral ions H2O Fig. 11-1, p. 164 -P -K Effects of suboptimal concentrations of mineral elements on plant growth -N -S - Mg Fig. 11-10 (a-f), p. 173 Needed in large amounts Needed in small amounts Table 11-1, p. 171 Soil Formation atmospheric gases: CO2 SO2 N2O5 rock acids: H2CO3 H2SO3 HNO3 rain wind and water erode rocks and soil freeze-thaw produces cracks roots: crack rocks through pressure, secrete acid Fig. 11-11, p. 175 Soil Formation • Lichens and small plants start to grow on this “soil solution”: – Rhizoids and roots enlarge fissures in rocks through turgor pressure and emit respiratory CO2, which forms H2CO3, and thus more acid…… • Accelerated soil formation leading to invasion of larger plants species: – Larger roots and more respiratory CO2 , and so on…… ? Needed in large amounts Needed in small amounts Table 11-1, p. 171 Nitrogen Fixation and Symbiosis • Clover root with root nodules that contain the nitrogen fixing bacterium Rhizobium. • Leguminous plants (pea, bean,…) benefit from the nitrogen-fixing association while supplying the bacterial symbiont with photosynthetic products (can be up to 20% of total photosynthesis performed by the plant). Nitrogen • Nitrogen predominantly exists as N2 gas in the atmosphere. Is not directly available to plants. • Nitrogen becomes available after soil bacteria turn it into NH4+ or NO3-. This is called nitrogen fixation. • However, fixed nitrogen is not stably present in soil: - NH4+ (in equilibrium with NH3) is volatile. - NO3- is very water soluble and easily leached from the soil. • Treatment with fertilizers that contain NH4+ or NO3- is very effective in increasing crop yields, since it supplements the soil with an invariably scarce mineral element. Fertilizer use and food production • NH3 in water solution exists as NH4+. • NH3 is made industrially by the Haber-Bosch process: Heat N2(g) + 3H2(g) --------> 2NH3 pressure • H2 is made from light petroleum fractions or natural gas: 700 0C CH4 + H2O(g) --------> CO(g) + 3H2(g) • Energy is needed to make H2 as well as to make NH3from H2 and N2. Mineral uptake H2O vapor product of photosynthesis (sucrose) H2O vapor H2O vapor H2O mineral ions H2O Fig. 11-1, p. 164 Maintenance of Mineral Supply • All plant cells require minerals Especially meristematic regions • Four processes replenish mineral supply – Bulk flow of water in response to transpiration – Diffusion – Active Uptake (requiring ATP) – Growth • As root grows, comes in contact with new soil region and new supply of ions PASSIVE ACTIVE Minerals can passively follow water flow until the endodermis. From there on, active uptake is needed. root hair plasmodesma xylem symplastic flow apoplastic flow cell wall cytoplasm epidermis symplast of endodermis cortex Casparian strip of endodermis stele Fig. 11-7, p. 169 Active Uptake of Minerals Into Root Cells Fig. 11-12, p. 177 root hair After passing the endodermal cell membrane(s), nutrients move into the vascular system to be transported throughout the plant. plasmodesma xylem Symplastic flow Apoplastic flow cell wall cytoplasm epidermis symplast of endodermis cortex Casparian strip of endodermis stele Fig. 11-7, p. 169 Root pressure is generated by an osmotic pump After passing the endodermis, mineral nutrients accumulate in the stele of the root. The endodermal cells provide the differentially permeable membrane needed for osmosis. •Soil saturated with water –Water tends to enter root and stele –Builds up root pressure in xylem –Forces xylem sap up into shoot Fig. 11-13a, p. 178 Guttation: water forced out of hydathodes by root pressure Guttation on a California poppy leaf Fig. 11-13b, p. 178 PHLOEM TRANSPORT Phloem transport H2O vapor product of photosynthesis (sucrose) H2O vapor H2O vapor H2O mineral ions H2O Fig. 11-1, p. 164 Mechanism of Phloem Transport high pressure low pressure sieve tube sucrose sucrose H2O H2O sucrose glucose sucrose H2O glucose H2O H2O source sink sucrose CO2 + H2O parenchyma H2O Fig. 11-14, p. 179 parenchyma Sucrose is actively transported into the sieve tubes at the food source region of the plant (leaves or storage organs) and removed at the sink regions (regions of growth or storage). Water follows by osmosis, increasing the hydrostatic pressure in the sieve tubes at the source region and decreasing the pressure at the sink region. The sievetube contents flow en masse from high(source)- to low(sink)-pressure regions. Phloem Transport – Concentration gradient maintained by • Continual pumping of sucrose at source • Removal of sucrose at the sink – Sink or source behavior of cells is controlled by cell signaling mechanisms (developmental and hormonal controls, see lectures on hormone regulation). – Change in signaling can abruptly switch a cell or tissue from source to sink behavior.
