37.1. Plants Require Inorganic Nutrients (p. 660)
A. Essential and Beneficial Inorganic Nutrients
1. An essential inorganic nutrient must fulfill the following criteria: have an identifiable nutritional role, no other element can substitute and fulfill the same role, and a deficiency of the element causes the plant to die.
a. These elements are divided into macronutrients and micronutrients by concentration in plant tissue.
b. Essential inorganic nutrients (e.g., carbon, hydrogen, and oxygen) comprise 96% of a plant's dry weight.
1) Carbon dioxide is the source of carbon for a plant.
2) Water is the source of hydrogen.
3) Oxygen can come from either atmospheric oxygen, carbon dioxide, or water.
c. Plants cannot extract nitrogen from air; plants take up ammonium (NH4+) or nitrate (NO3-) from the soil.
2. Beneficial inorganic nutrients are elements that are required for or improve growth of a particular plant: horsetails require silicon as a mineral nutrient, sugar beets show better growth in the presence of sodium, and soybeans use nickel when root nodules are present.
B. How Are Requirements Determined?
1. When a plant is burned, most mineral elements (except for nitrogen) remain in the ash.
2. Hydroponics is the preferred method for determining the mineral requirements for plants. (Fig. 37.1)
a. Hydroponics is the cultivation of plants in water.
b. Nutrient requirements of the plants can be determined by omitting a particular mineral and observing the effects on plant growth.
c. If plant growth suffers, it can be concluded that the omitted mineral is a required nutrient.
C. What Affects Mineral Availability?
1. Location of minerals in soil is critical; if root does not come within few millimeters of ions, no uptake occurs.
2. The downward movement of minerals through the soil is influenced by the quantity and the pH of water.
3. Water can make mineral ions available to roots; often it leaches minerals from soil zone in which roots grow.
4. The lower the pH, the greater the leaching power of water.
5. Soil particles, particularly clay and organic matter, contain negative charges that attract positively charged ions (e.g., calcium [Ca2+], potassium [K+], and magnesium [Mg2+]).
a. This attraction holds these ions and other positively charged nutrients where they are available to plants.
b. In acidic soils, hydrogen ions replace positively charged nutrients; nutrient ions float free and are leached.
6. On the other hand, in very acidic soils, aluminum (Al3+) and iron (Fe3+) ions become available to plants.
a. These minerals are insoluble above a pH of 4, but they become soluble at a more acidic pH.
b. They also displace Ca2+, K+, and Mg2+ on soil particles in the same way as H+ ions do.
c. High levels of aluminum and iron in soils are toxic to plants.
7. Thus, acid deposition on soils harms plants in two ways: it causes the removal of mineral ions that are essential nutrients and it makes available other mineral ions that are toxic at high levels.
D. How Minerals Are Taken In and Distributed
1. Minerals follow the same path as water; they move across the epidermis, through the cortex, and into the xylem to be transported throughout the plant. (Fig. 37.7)
a. Some mineral ions move past the epidermis and through the cortex by way of porous cell walls.
b. Because of the impermeability of the Casparian strip, minerals must eventually enter the cytoplasm of endodermal cells if they are to proceed any farther.
c. Minerals often move directly into cytoplasm of root hair epidermal cells and are transported cell to cell.
d. Mineral nutrient concentration in roots may be 10,000 times more than in the surrounding soil.
e. During their transport throughout the plant, minerals can exit the xylem and enter cells that require them.
2. Mineral ions cross plasma membranes by a chemiosmotic mechanism. (Fig. 37.2) [transp. 198]
a. It has long been known that plants expend energy to actively take up and concentrate mineral ions.
b. Plants absorb minerals in ionic form: nitrate (NO3-), ammonium (NH4+), phosphate (HPO4=), and potassium ions (K+); all have difficulty crossing a charged plasma membrane.
c. Proton pump hydrolyzes ATP to transport H+ ions out of cell; this establishes an electrochemical gradient.
d. The electrochemical gradient causes positive ions to flow into the cells.
e. Negative ions are carried across the plasma membrane, in conjunction with H+ ions, as the H+ ions diffuse down their concentration gradient.
E. How Roots Are Adapted for Uptake
1. Two symbiotic relationships are known to assist roots in acquiring nutrients.
2. Legumes have nodules infected with the bacterium Rhizobium.
a. Plants are unable to make use of atmospheric nitrogen because they do not have the cellular enzymes to break the N t N bond.
b. Rhizobium makes nitrogen compounds available to plants in exchange for carbohydrates.
c. The bacteria live in root nodules---structures on plant roots that contain the nitrogen-fixing bacteria.
d. Rhizobial bacteria reduce atmospheric nitrogen (N2) to ammonium (NH4+) (nitrogen-fixation).
e. Many other plants have a relationship with free-living, nitrogen-fixing microorganisms in the soil.
3. Most plants have mycorrhizae; those lacking mycorrhizae are limited in where they can grow.
a. Mycorrhizae are a mutualistic symbiotic relationship between soil fungi and plant roots.
b. The fungal hyphae may enter the cortex of roots but do not enter plant cells. (Fig. 37.4)
c. Ectomycorrhizae form a mantle that is exterior to the root, and they grow between cell walls.
d. Fungus increases the surface area available for mineral and water uptake and breaks down organic matter.
e. In return the root furnishes the fungus with sugars and amino acids.
4. Orchid seeds are small and contain limited nutrients; they do not germinate until invaded by mycorrhizae.
5. Nonphotosynthetic plants, such as Indian pipe, use their mycorrhizae to extract nutrients from nearby trees.
6. Some plants have poorly developed or no roots since minerals and water are supplied by other mechanisms.
a. The Venus's-flytrap and sundews obtain nitrogen and minerals when leaves capture and digest insects.
b. Epiphytes take their nourishment from the air; their attachment to other plants gives them support.
c. Parasitic plants (e.g., dodders, broomrapes, and pinedrops) send out haustoria (rootlike projections) that grow into the host and tap into the xylem and phloem of the host.
37.2. How Water Moves Through a Plant (p. 664)
A. Transport Tissues
1. Vascular plants have transport tissues as an adaptation to living on land. (Fig. 37.5) [transp. 199]
2. Xylem is the vascular tissue that passively conducts water and mineral solutes upward through the plant, from the roots to the leaves; it contains two types of conduction cells: tracheids and vessel elements.
a. Tracheids are hollow, nonliving cells with tapered overlapping ends; thinner and longer than vessel elements; water crosses end and sidewalls because of pits in secondary cell wall.
b. Vessel elements are hollow, nonliving cells that lack tapered ends; wider and shorter than tracheids; lack transverse end walls; form a continuous pipeline for water and mineral transport.
3. Phloem is the vascular tissue that conducts the organic solutes in plants, particularly from the leaves to the roots; contains sieve-tube cells and companion cells. (Fig. 36.7) [transp. 194]
a. Sieve-tube cells contain cytoplasm but no nucleus; are arranged end to end; have channels in their end walls (thus, the name "sieve-tube") through which plasmodesmata extend from one cell to another.
b. Companion cells are closely connected to sieve-tube cells by numerous plasmodesmata; smaller and more generalized than sieve-tube cells; in addition to cytoplasm, they have a nucleus, which may control and maintain the function of both cells; also involved in transport function of phloem.
4. These systems of transport rely on mechanical properties of water, such as diffusion and hydrogen bonding.
B. Water Potential is Critical
1. Water flows from a region of higher water potential (the potential energy of water) to a region of lower water potential. (Fig. 37.6)
2. Water potential is a measure of the capacity to release or take up water; in cells, water potential includes the following:
a. pressure potential, the effect that pressure has on water potential; water will move from a region of higher pressure to a region of lower pressure; and
b. osmotic potential, the effect that solutes have on water potential; water tends to move by osmosis from an area of lower solute concentration to area of higher solute concentration.
3. Water flows by osmosis into a plant cell that has a greater solute concentration than the surrounding solution.
a. As water enters, pressure increases inside cell; strong plant cell wall allows water pressure to build up.
b. As pressure potential inside cell increases and balances osmotic potential outside cell, water stops entering.
c. Turgor pressure is the pressure potential that increases due to the process of osmosis; critical to plants, since plants depend on it to maintain the turgidity of their bodies.
4. Wilted plant cells have lost water due to reversal in usual water potential differences across membranes.
C. Xylem Transports Water
1. Movement of water and minerals in a plant involves entry into a root, passage in xylem, and through leaves.
2. Water and minerals must enter root cells before they reach the xylem; this occurs by two routes.
(Fig. 37.7)
a. It can simply diffuse between cells; however, a Casparian strip forces water to enter endodermal cells.
b. Water can diffuse into root hairs and diffuse cell-to-cell across the cortex and endodermis.
c. Regardless of the route, water enters root cells because osmotic potential, and therefore the water potential, within root cells is less than that of the soil solution.
3. Water entering root cells creates a positive pressure potential called root pressure.
a. Root pressure occurs primarily at night and tends to push xylem sap upward in the plant.
b. Guttation is the appearance of drops of water along the edge of leaves, as a result of water being forced out of leaf vein endings, and is also a result of root pressure. (Fig. 37.8)
c. Root pressure is not a sufficient mechanism for water to rise to the tops of trees.
D. When Is Water Available?
1. Water availability is function of amount of rain or irrigation, rate water drains away in soil, and evaporation.
2. Speed of drainage can be correlated with permeability of soil; sandy soil is very permeable, clay soil is less permeable.
3. The amount of water remaining in the soil after drainage is referred to as its field capacity.
4. The permanent wilting point is the point at which water is no longer available to the roots; the point at which soil and root water potentials have become the same or have reversed and often occurs in a clay soil.
5. Ideal soil holds water, has air spaces (roots need O2 for cellular respiration), and is easily penetrated by roots.
6. Best agricultural soils are loams that are 10-25% clay with rest being equal mix of sand and silt.
E. To the Leaves
1. Water and dissolved minerals must be transported upward from the roots to the xylem.
2. The cohesion-tension model states that transpiration creates a tension (i.e., a negative pressure potential) that pulls water upward in the xylem.
a. Transport works because water molecules are cohesive with one another and adhesive with xylem walls. (Fig. 37.9) [transp. 200]
b. Transpiration is a plant's loss of water to the atmosphere through evaporation at leaf stomates.
c. Cohesion is tendency of water molecules to cling together, a result of their forming hydrogen bonds.
d. Adhesion is the ability of water (a polar molecule) to interact with molecules comprising the walls of xylem vessels; gives a water column extra strength and prevents it from slipping back down.
e. In daytime, negative water potential created by transpiration extends from leaves to roots; water column must be continuous.
f. Thus, if a water column within xylem is broken (as by cutting a stem), the water column will drop back down the xylem vessel away from the site of breakage, making it more difficult for conduction to occur.
g. At least 90% of the water taken up by roots is lost through the stomates by transpiration.
h. When there is plenty of water, stomates remain open, allowing CO2 to enter leaf and photosynthesis to occur.
i. If plant is under water stress, more water is lost through the leaf than can be brought up and stomates close.
j. Since photosynthesis requires that CO2 enter the leaf, there must be sufficient water so the stomates can remain open and allow CO2 to enter.
F. Opening and Closing of Stomates
1. Each stomate has two guard cells with a pore between them. (Fig. 37.10)
2. Stomates open from turgor pressure when guard cells take up water; when they lose water, turgor pressure decreases and stomates close.
3. Guard cells are attached to each other at their ends; the inner walls are thicker than the outer walls.
4. Radial expansion is prevented by cellulose microfibrils in the walls but the outer walls can expand lengthwise.
5. As they take up water, they buckle out, thereby creating the opening between the cells.
6. Since 1968, it has been known that when stomates open, there is accumulation of K+ ions in the guard cells.
7. A proton pump run by the breakdown of ATP to ADP and P transports H+ outside the cell; this establishes an electrochemical gradient that allows K+ to enter by way of a channel protein. (Fig. 37.10)
8. The blue-light component of sunlight is a signal that can cause stomates to open.
a. There is evidence that flavin pigments absorb blue light.
b. This pigment sets in motion the cytoplasmic response that leads to activation of the proton pump that causes K+ ions to accumulate in the guard cells.
9. There is evidence to suggest that a receptor in the plasma membrane of guard cells brings about the inactivation of the proton pump when CO2 concentration rises, as might happen when photosynthesis ceases.
10. Abscisic acid (ABA), which is produced by cells in wilting leaves, can also cause stomates to close; although photosynthesis cannot occur, water is conserved.
11. If plants are kept in dark, stomates open and close on a 24-hour basis as if they were responding to presence of sunlight in daytime and the absence of sunlight at night; some sort of internal biological clock must keep time.
37.3. How Organic Nutrients Are Transported (p. 670)
A. Phloem Transports Organic Nutrients
1. Marcello Malpighi (1679) suggested that bark is involved in transferring sugars from leaves to roots.
a. He observed the results of removing a strip of bark from a tree, a process called girdling.
b. The bark swells just above the cut and sugar accumulates in the swollen tissue.
c. Today, we know phloem is removed in girdling but the xylem remains; therefore, phloem transports sugars.
2. Radioactive tracer studies using 14C confirm that phloem transports organic nutrients.
a. When 14C-labeled carbon is supplied to mature leaves, radioactively labeled sugar soon moves to roots.
b. Similar studies have confirmed phloem also transports amino acids, hormones, and mineral ions.
3. Hormones are transported from production site to target areas.
4. In the autumn before leaves fall, mineral ions are removed from the leaves and are taken to other locations.
5. Chemical analysis of phloem sap shows it is mainly sucrose; concentration of nutrients is 10-13%.
6. Aphids used in study (Fig. 37.11)
a. It is difficult to take samples of sap from just phloem cells without injuring the phloem.
b. Aphids are small insects that drive their mouth stylets into a sieve-tube cell; samples are easily taken.
c. The aphid body is cut off and the stylet becomes a small needle from which phloem is collected.
B. Phloem Transport Uses Positive Pressure
1. Conducting cells of phloem are sieve-tubes lined end to end.
2. Cytoplasm extends through the sieve plates of adjoining cells to form a continuous tube system.
3. By following 14C-labelled sugar, materials appear to move at the rate from 60-100 up to 300 cm per hour.
4. Pressure-flow model explains transport of sap through sieve tubes by a positive pressure potential.
a. During the growing season, leaves produce sugar. (Fig. 37.12) [transp. 201]
b. Sucrose, a product of photosynthesis, is actively transported into phloem.
c. This is dependent upon an electrochemical gradient established by a H+ pump.
d. Sucrose crosses plasma membrane with H+ ions as they diffuse down concentration gradient.
e. The energy needed for sucrose transport is provided by the companion cells.
f. Water consequently flows into sieve tubes because of their lower osmotic potential.
5. The buildup of water creates a positive pressure potential within the sieve tubes that creates a flow that moves water and sucrose to a sink (e.g., at the roots).
6. A positive pressure potential exists from leaves to roots; at roots, sucrose is transported out and water follows.
7. Consequently, the positive pressure potential gradient causes a flow of water from the leaves to the roots.
8. As water flows with the phloem, it brings sucrose with it.
9. A sink can be at the roots or any other part of the plant that requires nutrients.
10. Because phloem sap flows from source to sink, there is a bidirectional flow within phloem at the same time.