Lecture Outline - Chapter 10

10.1. Water and Mineral Transport (p. 164)

1. Water mainly enters a plant through root hairs on the root.
2. Water and dissolved minerals move from root tissues into the vascular cylinder.
3. Xylem is the vascular tissue that transports water and minerals from roots to leaves and has two types of conducting cells: (Fig. 10.1)
   • a. Vessel elements lack end walls in the cells and form a hollow pipeline from roots to leaves.
   • b. Tracheids have pitted end walls that also allow for water transport.
4. Root pressure results from root cells pushing xylem sap upwards but this cannot account for water movement to top of tall trees.
5. Atmospheric pressure forces mercury in a hollow tube to height of 76 cm or water height equal to 10.4 meters; since tall trees are 120 meters, other factors are required.
6. The Cohesion-Tension Theory of Water Transport:
   • a. Most likely explanation for transporting water to great heights against gravity.
   • b. Water molecules are polar and adhere to walls of vessel elements.
   • c. Water molecules are cohesive--they cling together.
7. Transpiration:
   • a. Refers to the loss of water from the leaf at the stomata.
   • b. As water evaporates, it creates a pull, or tension, that draws the column of water up the vessel elements from the roots to the leaves.
   • c. Stomates are openings in leaf where evaporated water is lost.
   • d. Therefore, replacement of evaporated water molecules exerts a driving force, a negative pressure or tension, that draws the column of water up the vessel elements from the roots to the leaves.
8. Movement of water up the xylem also transports minerals. (Table 10A)
9. Opening and Closing of Stomates (Fig. 10.2)
   • a. For photosynthesis, stomates must be open to allow CO₂ to enter leaves.
   • b. When stomates are open, water can also leave (transpiration) which can result in water stress.
   • c. When the stomates close to conserve water, photosynthesis stops.
   • d. Each stomate has two guard cells with a pore between them.
   • e. Plant cells will develop turgor pressure when water fills their central vacuole and presses against the cell wall.
   • f. Guard cells are attached to each other at their ends and their inner walls are thicker than their outer walls.
   • g. When they take up water, they buckle out and the stomate opens.
   • h. During photosynthesis, ATP is available from electron transport system in thylakoids.
   • i. The ATP-driven pump transports H⁺ out of cell.
   • j. This electrochemical gradient causes potassium ions to enter the guard cells.
   • k. Water now enters by osmosis and the stomate opens.
   • l. When the pump is not working, potassium ions move into surrounding cells, the guard cells lose water and the stomate closes.
   • m. The blue-light component of sunlight appears to signal stomates to open.
   • n. A receptor in the plasma membrane may inactivate the pump when CO₂ concentration rises, as might happen when photosynthesis ceases.
   • o. Abscisic acid (ABA) from wilting leaves also causes stomates to close.
   • p. Circadian rhythms, 24-hour variations in a biological clock, also open and close stomates daily.

1. Translocation is the movement of organic substances in the phloem.
2. Translocation makes sugars available to parts of a cell that are actively metabolizing and growing.
3. Sieve-tube cells contain cytoplasm but no nucleus; pores in sieve plate in end wall allow cytoplasm (plasmodesmata) to extend into next cell through the seive plate. (Fig. 10.3)
4. A companion cell, a smaller cell with a nucleus, probably controls the sieve-tube cell.
5. Content of phloem sap:
   • a. Mostly sugar.
   • b. Nutrients comprise 10--13% by volume.
   • c. Analysis is by use of aphids whose beaks can tap into just the seive-tube cells.
6. The Pressure-Flow Theory: How Phloem Moves Nutrients
   • a. During growing season, leaves are source of sugar formed by photosynthesis;
   • b. Sugar is actively transported into sieve-tube cells; companion cells provide the necessary energy.
   • c. The pressure is due to water buildup caused by sugar formed by photosynthesis in the leaves; this starts the flow of sap.
   • d. Roots are a sink for sugar (the roots remove sugar) and actively transport it out of the sieve-tube cells.
   • e. As sugar is actively transported by living cell membranes in these tissues, water follows passively by osmosis from leaves (source) to roots (sink).
   • f. This explanation of translocation is the pressure-flow theory; the pressure gradient is lowest in the roots and the greatest in the leaves.
7. In spring before leaves are out and photosynthesizing, flow is reversed due to greater pressure in roots, less in stem where sucrose is being withdrawn and sap moves up.

1. Plants respond to light, day length, gravity, and temperature by changing their growth pattern.
2. Plant growth toward or away from a directional stimulus is called a tropism; for instance, phototropism, gravitropism, and thigmotropism.
3. Growth toward a stimulus is a positive tropism; away from a stimulus is a negative tropism. (Fig. 10.4)
4. Plant hormones are chemical messengers produced by meristematic tissue and transported to other tissues, others move directly between other tissues, and still others are used where they are made.
5. Plant growth regulators include:
   • a. Growth promoters: (Table 10.1)
     • i. Auxins such as indoleacetic acid cause cell elongation and prevent fruit from dropping; pruning a terminal bud eliminates auxins and allows growth of lateral buds; weak solutions promote root development; auxins move to lower surfaces relative to gravity, causing roots to curve downward, stems upward.
     • ii. Gibberellins include gibberellic acid that causes stem elongation and seed germination.
     • iii. Cytokinins such as zeatin cause cell division.
   • b. Growth inhibitors:
     • i. Abscisic acid causes stomatal closure, maintains dormancy.
     • ii. Ethylene promotes fruit ripening, abscission and fruit drop; inhibits growth.
6. Phototropism
   • a. Classic experiment involves coleoptiles, sheath of oat seedlings. (Fig. 10.5)
   • b. Coleoptile tips are cut and set on agar blocks that soak up its secretions.
   • c. Such an agar block placed on edge of coleoptile causes bending away from block, even in absence of light.
   • d. Bending was caused by increase growth from plant hormone "auxin" which comes from Greek word aux, "to grow."
   • e. In variable sunlight, yellow pigment related to riboflavin is photoreceptor to blue light; auxin migrates from bright to shady side; cells elongate on shady side and curve stem toward light.
   • f. Mode of action of auxin is to bind to receptors and activate ATP-driven proton (H⁺) pump. H⁺ ions cause cell wall to become acidic, breaking hydrogen bonds; cell wall is weakened; solutes and water enter the cell and turgor pressure stretches the cell wall producing elongation. (Fig. 10.6)
7. Photoperiodism
   • a. Photoperiodism is the plant response based on the proportion of light to darkness in a 24-hour cycle. Photoperiodism is particularly obvious in temperate zone plants.
   • b. Plants respond to increasing day length in spring by initiating growth and cease growth in decreasing fall day length; violets and tulips are triggered to flower in spring, asters and goldenrods flower in fall.
   • c. Short-day plants initiate flowering when photoperiod is shorter than critical length; for example, cocklebur is a short day plant that will not flower if a long night is interrupted by a flash of light.
   • d. Long-day plants flower when photoperiod is longer than minimum value; for example: if clover on a long night is interrupted by a flash of light, it will flower, but interrupting the day with darkness has no effect. This shows that flowering is controlled by length of continuous darkness, not length of day.
   • e. Phytochrome:
     • i. Is a blue-green plant pigment whose chemical structure changes when it is exposed to light and darkness.
     • ii. Is probably part of a biological clock that controls flowering.

1. Sexual reproduction is defined as reproduction requiring gametes (sperm and egg) and they are located in the flower.
2. Anatomy of Flower Structure (Fig. 10.8)
   • a. Sepals are usually green structures that form a whorl about the petals.
   • b. Petals are colored on many flowers.
   • c. Pistil is vaselike structure composed of:
     • i. Stigma, an enlarged sticky knob to which pollen grains attach;
     • ii. Style, a slender stalk;
     • iii. Ovary or enlarged base that contains ovules (becomes the seed after fertilization).
   • d. Stamens are composed of the:
     • i. Anther, a saclike container that will form pollen grains.
     • ii. Filament, a slender stalk that attaches to the anther.
3. The Flowering Plant Life Cycle
   • a. Life cycle is called an alternation of generation because it alternates between two generations, the sporophyte and gametophyte.
   • b. Sporophyte generation is diploid (2n) that produces haploid (n) spores via meiosis.
   • c. Flowers produce two types of spores:
     • i. Microspores are produced in the anthers of stamens; they mature into a pollen grain, which is a sperm-containing microgametophyte, also called a male gametophyte.
     • ii. Megaspores are produced within ovules, they become an egg-containing embryo sac, which is the megagametophyte or female gametophyte.
   • d. Following fertilization, the zygote develops into an embryo in a seed; when the seed germinates, the new sporophyte plant begins to grow.
   • e. In an ovule, a megaspore parent cell undergoes meiosis, three megaspores disintegrate and one divides mitotically to form a megagametophyte or embryo sac consisting of eight haploid nuclei embedded in mass of cytoplasm; the cytoplasm differentiates into cells, one of which is an egg and one is the endosperm cell with two nuclei (polar nuclei). (Fig. 10.9)
   • f. The anther has pollen sacs with numerous microspore parent cells; each undergoes meiosis to produce four haploid microspores. Microspores usually separate into pollen grains. Each pollen grain (microgametophyte) contains two nuclei, the generative nucleus and the tube nucleus.
   • g. Pollination
     • i. Occurs when pollen is windblown or carried by insects, birds, or bats to the stigma of the same type of plant.
     • ii. Pollen grain germinates and becomes a microgametophyte with a long pollen tube.
     • iii. The pollen tube grows within the style until it reaches ovule.
     • iv. The pollen tube discharges the sperm; the generative nucleus has divided to form two sperm.
     • v. One sperm combines with egg of ovule to form diploid zygote.
     • vi. The other sperm combines with two polar nuclei to form a 3n (triploid) endosperm, which is used as food for the developing plant embryo.
     • vii. The ovule becomes the seed.
     • viii. Ovary becomes the fruit.
     • ix. Note that flowering plants have double fertilization.
   • h. Fruits
     • i. Seeds are enclosed within a fruit that develops from the ovary or other accessory parts.
     • ii. A fruit is a mature ovary that contains seeds; for example, tomatoes, peas, beans, and cucumbers.
     • iii. Fruits protect seeds and may provide extra nourishment and aid in seed dispersal.
     • iv. The ovary wall or pericarp assumes many forms:
       - the almond husk
       - flesh of peaches and plums
       - apple (develops from both ovary and receptacle)
       - tomato (reflects compound ovary with several seed-filled cavities)
       - bean and pea fruits (dry fruits that split along seams)
       - rice, corn, wheat, and barley (kernel fruits)

       v. Aggregate fruits form from separate ovaries; for example blackberry.

       vi. Strawberry is aggregate fruit with flesh from receptacle.

       vii. Pineapple is fused multiple fruit from multiple flowers.
     i. How Seeds Disperse and Germinate
     • i. Animals help disperse seeds
       - via attachment to fur or clothing by hooks spines, or burs;
       - feeding on fruits and passing seeds when defecating;
       - when squirrels gather and bury seeds.

       ii. Coconuts and other buoyant seeds are carried by water currents.

       iii. Wind carries seeds with woolly hairs, plumes, wings, and parachutes.

       iv. Touch-me-not plant shoots out seeds.
   • j. Germination of Seeds
     • i. Dormancy is time during which no seed growth or germination occurs.
     • ii. Some desert seeds must have moisture.
     • iii. Some temperature zone seeds must be exposed to cold for a time.
     • iv. Germination is often inhibited by chemicals in fleshy fruits.
     • v. Mechanical action, bacterial action, water and fire can be for germination, allowing water uptake and causing the seed coat to burst.
   • k. Germination of Dicots and Monocots
     • i. Monocots have one cotyledon that absorbs food from endosperm and dicots have two cotyledons that replace the endosperm and act as source of food for the embryo.
     • ii. The epicotyl is the portion of embryo above the attachment of the cotyledon; contains the apical meristem of the shoot and sometimes bears young leaves termed the plumule. (Fig. 10.11)
     • iii. The hypocotyl is portion of embryo below attachment of cotyledons; becomes a portion of stem; lower end may form the radicle (embryonic root). (Fig. 10.11)
     • iv. Endosperm is food storage tissue in monocots.
     • v. If seeds are planted too close to the surface, they fail to germinate. Seedlings grown in dark etiolate (stems increase in length, leaves remain small) but when exposed to sunlight, they begin to grow normally.

1. Plants contain nondifferentiated meristem tissue allowing asexual reproduction by vegetative propagation.
   • a. Strawberries develop from runners, above-ground horizontal stems, and new plants grow from nodes of stolons.
   • b. Violets grow from rhizomes, horizontal underground stems.
   • c. White potato plants grow from the "eyes" of a potato, buds on tubers.
2. Plants Propagate in Tissue Culture
   • a. Tissue culture is growth of tissues in artificial liquid culture medium.
   • b. Allows micropropagation, commercial production of millions of identical seedlings.
   • c. Meristem culture uses proportions of auxin and cytokinin to develop continuous production of many new shoots from a single callus.
   • d. Cell suspension culture dissociates plant cells into nutrient suspensions.
3. Genetic Engineering of Plants
   • a. Protoplasts are plant cells that lack a cell wall.
   • b. Some protoplasts can take up foreign DNA; others must be injected with DNA using various techniques.
   • c. This allows the introduction of genes for insect resistance, herbicides, etc., and production of higher protein with less water and fertilizer.