Plant Structure, Function, and Organization (7%)

Stimuli for Stomatal Opening and Closings

*In general open during day and closed at night to prevent water loss when photosynthesis cannot occur* 3 cues: Light, CO2 depletion, internal clock (circadian rhythms) Abscisic acid (ABA) hormone produced in roots and leaves in response to water deficiency, signals guard cells to close stomata (during drought).

Meristems Generate New Cells for Primary/Secondary Growth

1. This year's growth 2. Last year's growth 3. Growth of two years ago 4. Apical bud 5. Bud scale 6. Axillary Buds 7. Leaf Scar 8. Bud Scar 9. Node 10. Internode 11. One year old branch formed from axillary bud near shoot tip 12. Leaf scar 13. Stem 14.Bud Scar 15. Leaf Scar

Transport of Water and Minerals Root Hairs to Xylem

1.) Apoplastic route- uptake of soil solution by hydrophilic walls of root hairs provides access to apoplast. Water and minerals can then diffuse into the cortex along this matrix of walls and extracellular spaces. 2.) Symplastic route- minerals and water that cross the plasma membranes of root hairs can enter symplast 3.) Transmembrane route- as soil solution moves along apoplast route, some water and minerals are transported into protoplasts of cells of the epidermis and cortex and then move inward via symplast. 4.) Endodermis- casparian strip- belt of waxy material that blocks passage of water and dissolved minerals. Only minerals already in symplast or entering that pathway by crossing the plasma membrane of an endodermal cell can detour around casparian strip and pass into vascular cylinder (stele). 5.) Transport in xylem- endodermal cells and also living cells within vascular cylinder discharge water and minerals into walls (apoplast). Xylem vessels then transport water and minerals by bulk flow upward into shoot system.

Secondary Growth (Growth in Thickness)

1.) Primary growth from activity of apical meristem is nearing completion. Vascular cambium formed. 2.) Only secondary growth. Stem thickens, vascular cambium forms secondary xylem to inside and phloem to outside. 3.) Vascular cambium give rise to vascular rays 4.) Tissues external to cambium can't keep pace because cells no longer divide. These tissue (xylem and phloem) and epidermis rupture. Second lateral meristem, the cork cambium, develops from parenchyma cells in cortex. Cork cambium produces cork cells which replace epidermis.

Development of Soybean Root Nodule

1.) Roots emit chemical signals that attract rhizobium bacteria. The bacteria emit signals that stimulate root hairs to elongate and form infection thread via invagination of plasma membrane. 2.) Infection thread containing bacteria penetrates root cortex. 3.) Growth continues in affected regions of cortex and pericycle, these two masses of dividing cells fuse, forming nodule. 4.) Nodule develops vascular tissue that supplies nutrients to nodule and carries nitrogenous compounds into vascular cylinder for distribution 5.) Mature nodule grows to be many times diameter of root. Lignin-rich sclerenchyma cells reduce absorption of oxygen and help maintain anaerobic environment needed for nitrogen fixation.

Secondary Growth Pt. 2

5.) In year two of secondary growth, vascular cambium produces more secondary xylem and phloem. Most of thickening is from secondary xylem. Cork cambium produces more cork. 6.) As stem's diameter increases, outermost tissues exterior to cork cambium rupture and fall off. 7.) Cork cambium re-forms deeper in cortex. When none of cork is left, cambium develops from phloem parenchyma cells. 8.) Each cork cambium and tissues it produces form layer of periderm 9.) Bark consists of all tissue exterior to vascular cambium.

Roots

Absorbs minerals and water, stores carbohydrates and other reserves. Primary root- originates in seed embryo, first root (and first organ) to emerge from a germinating seed. Lateral roots- branch from primary root, enhancing ability of system to absorb water/minerals and further anchoring into soil. Taproot- One, main vertical root (tall, erect plants) usually develops from primary root. In a taproot system role of absorption restricted to tips of lateral roots, facilitates anchorage (prevents toppling allowing plant to grow taller, giving access to more favorable light conditions).

Primary Growth (Growth in Length)

Apical meristem cells in a shoot tip or root tip are undifferentiated. When they divide, some daughter cells remain in the apical meristem, ensuring continuing population of undifferentiated cells. Other daughter cells become partly differentiated as primary meristem cells. After dividing and growing in length, they become fully differentiated cells in mature tissues. Addition of elongated, differentiated cells lengths a stem or root. Root apical meristem is protected by thimble-like root cap.

Types of Meristems

Apical- located at root and shoot tips, provide cells that enable primary growth. Allows roots to extend throughout soil and shoots to increase exposure to light. In herbaceous (non woody) plants, it produces all or most of plant body. Woody plants grow in circumference in the parts of the stem and roots that no longer grow in length. Secondary growth (in thickness)- made possible by LATERAL MERISTEMS: the vascular cambium and cork cambium. Vascular cambium adds secondary xylem and secondary phloem (most thickening is from secondary xylem). Cork cambium replaces epidermis with thicker, tougher periderm. Cells in meristems divide frequently, some new cells remain in meristem and produce more cells, others differentiate and are incorporated into tissues and organs.

Phloem Sap

Aqueous solution that flows through sieve tubes, differs markedly from xylem sap that is transported by tracheids and vessel elements. Most prevalent solute is sugar, typically sucrose. Also contains amino acids, hormones, minerals. Moves from sites of sugar production to sites of storage or sugar use. A sugar source, is a plant organ that is a net producer of sugar (by photosynthesis or starch breakdown). Mature leaf=sugar source. A sugar sink, is an organ that is a net consumer or depository of sugar. Growing roots, buds, stems, and fruits are sugar sinks. Sinks receive from nearest sugar source (upper leaves of branch export to growing shoot tip, lower leaves to roots).

Phyllotaxy

Arrangement of leaves on a stem. Determined by root apical meristem. Xylem- transports water and minerals from roots to shoots Phloem- transports products of photosynthesis from where they are made to where they are stored or need.

Stems

Chief function is to elongate and orient the shoot in a way that maximizes photosynthesis by the leaves. Also elevates reproductive structures for dispersal of pollen and fruit. Green stems can perform a limited amount of photosynthesis. Each stem has an alternating system of nodes, the points at which the leaves are attached, and internodes, the stem segments between nodes. Apical bud- shoot tip (most of growth of a young shoot is concentrated near apical bud) Axillary bud- upper angle formed by each leaf and stem (can form branch, thorn, or flower).

Pulling Xylem Sap: Cohesion-Tension

Cohesion-tension hypothesis- transpiration provides the pull for ascent of xylem sap, and the cohesion of water molecules transmits this pull along the entire length of xylem from shoots to roots. Transpirational pull- Negative pressure potential that causes water to move up through xylem develops at the surface of mesophyll cell walls in the leaf. Cell wall acts as very thin capillary network. Water adheres to cellulose microfibrils and other hydrophilic components of the cell wall. As the water evaporates, the air-water interface retreats farther into the cell wall. High surface tension of water= negative pressure potential. Pulling forces transferred to xylem because water cohesively bound via hydrogen bonds.

Dermal Tissue System

Compose roots, stems and leaves. Tissue system- continuous throughout plant, connecting all organs (each type of tissue forms a system) Dermal tissue system- severs as the outer protective covering of the plant. In nonwoody plants it is a single tissue called the epidermis (layer of tightly packed cells). In leaves/stems the cuticle, a waxy epidermal coating, prevents water loss and disease. In woody plants, protective tissue (periderm) replace the epidermis. In roots, water and minerals absorbed from soil enter through epidermis in root hairs. In shoots, specialized guard cells are involved in gaseous exchange. Trichomes- reduce water loss and reflect excess light in desert species (defend against insects too)

Ectomycorrhizae

Dense sheath or mantle of mycelia over surface of root. Fungal hyphae extend from mantle into soil, greatly increasing surface area for water and mineral absorption.

Arbuscular Mycorrhizae

Embedded within root. Soil hyphae respond to presence of a root by growing towards it, establishing contact, and growing along surface. Hyphae penetrate between epidermal cells and enter root cortex. There they digest small patches of cell walls but don't pierce plasma membrane.

Apoplast and Symplast

Everything external to plasma membrane of living cells (cell walls, ECM..etc) Symplast- entire mass of cytosol of all living cells in a plant, as well as plasmodesmata.

Vascular Tissue System

Facilitate transport of materials through plant, provide mechanical support. Xylem- conducts water and dissolved minerals upward from roots into shoots. Phloem- transports sugars (products of photosynthesis) from where they are made (leaves) to where they are needed or stored- usually roots and sites of growth (new leaves and fruit). Collectively called the stele (greek word for pillar). Arrangement of stele varies depending on species and organ. In angiosperms, root stele is a solid, central vascular cylinder of xylem and phloem. Stele of stems and leaves consists of vascular bundles.

Specialized Stem Functions

Food storage/asexual reproduction. 1.) Rhizomes- (base of iris plant, horizontal shoot grows just below surface. Vertical shoots emerge from axillary buds on the rhizome) 2.) Stolons- Strawberry plant, horizontal shoots that grow along the surface, these "runners" enable a plant to reproduce asexually, as platelets form at nodes along each runner 3.) Tubers- potato plant, enlarged ends of rhizomes or stolons specialized for storing food. Eyes of a potato are clusters of axillary buds that mark the nodes

Ground Meristem

Gives rise to mature ground tissue. Ground tissue of roots, consisting mostly of parenchyma cells, is found in the cortex, the region between the vascular tissue and epidermis. In addition to storing carbohydrates, cells in the cortex transport water and salts from root hairs to center of root. Allows for extracellular diffusion of water, minerals and oxygen from root hairs inward (large spaces between cells). Innermost layer of cortex (endodermis)- a cylinder one cell thick that forms the boundary with vascular cylinder. Selective barrier that regulates passage of substances from soil into vascular cylinder.

Procambium

Gives rise to vascular cylinder, which consists of a solid core of xylem and phloem tissues surrounded by a layer of cells called the pericycle. In most eudicot roots, xylem has a star like appearance cross section, phloem occupies indentations between arms of star. In monocot roots, vascular tissue consists of a core of undifferentiated parenchyma cells surrounded by ring of alternating xylem and phloem tissues.

Stem Growth and Anatomy: Eudicots

Ground tissue consists mostly of parenchyma cells. Collenchyma cells just beneath epidermis strengthen many stems during primary growth. Sclerenchyma cells, especially fiber cells, provide support to parts of stem no longer elongating. Vascular tissue runs length of a stem in vascular bundles. Lateral shoots develop from axillary bud meristems on stems surface and do not disrupt other tissues (unlike lateral roots). Eudicot- vascular bundles arranged in a ring. Xylem in each bundle adjacent to pith, and phloem adjacent to cortex.

Formation of Lateral Root

Lateral root originates in the pericycle, the outermost layer of vascular cylinder of a root, and destructively pushes through the outer tissues before emerging.

Leaf Growth and Anatomy

Leaves develop from leaf primordia, projections shaped like a cows horns that emerge along the sides of the shoot apical meristem. Unlike roots and stems, secondary growth in leaves is minor or nonexistent. Just like roots/stems, three primary meristems give rise to tissues of mature organ. Leaf epidermis covered by waxy cuticle, except where interrupted by stomata. Ground tissue= mesophyll cells, consist of parenchyma cells specialized for photosynthesis.

Leaves

Main photosynthetic organ. Exchange gases with atmosphere, dissipate heat, defense from herbivores/pathogens. Leaf- blade/stalk, the petiole, which joins leaf to stem at node. Grasses and other monocots lack petioles; instead, the base of the leaf forms a sheath that envelopes the stem. Monocots and eudicots differ in the arrangement of veins, the vascular tissue of leaves. Monocots have parallel major veins of equal diameter that run the length of the blade. Eudicots have a branched network of veins arising from major vein that runs down the center of the blade. Compound leaves can withstand wind with less tearing, may concentrate parasites onto one leaflet instead of entire leaf.

Evolutionary Adaptations of Leaves

Modified leaves: Tendrils- by which plants cling to a support. Sometimes modified stems like in grape vines Spines- spines of cacti are actually leaves; photosynthesis carried out by fleshy green stems Storage leaves- bulbs (onion) have a short underground stem and modified leaves that store food Reproductive leaves- (succulents) produce adventitious platelets, which fall off and take root in soil

Ground Tissue System

Neither dermal, nor vascular. Pith- Internal to vascular tissue Cortex- external to vascular tissue. Includes cells specialized for functions such as storage, photosynthesis, support, and short-distance transport.

Roles of Soil Bacteria

Nitrogen cycle --> Nitrogen fixation

Mineral Deficiency

Nitrogen deficiency- yellowing that starts at tip and moves along center of leaf Phosphorus- reddish, purple margins Potassium- drying along tips/margins

Protoderm

Outermost, primary meristem, gives rise to epidermis, a single layer of cuticle-free cells covering the root. Root hairs are most prominent feature of root epidermis. Absorb water/minerals. Root hairs only live a few weeks but make up 70-90% of total root surface area.

Indeterminate Growth

Plant growth not limited to embryonic or juvenile period. Growth occurs throughout life of plant = indeterminate growth (determinant growth=stop growing at certain size). Plants can keep growing because of undifferentiated tissues called meristems containing cells that can divide, leading to new cells that elongate and become differentiated.

Xerophytic Adaptations

Plants adapted to arid environments. Complete short life cycles during brief rainy seasons Stems are fleshy and store water (cacti) Multi-layered epidermal tissue to avoid water loss. CAM- Specialized form of photosynthesis found in succulents. Take in CO2 at night, stomata remain closed during the day.

Primary and Secondary Cell Growth

Primary (growth in length)- made possible by apical meristems at the tips of shoots and roots Secondary (growth in thickness)- made possible by two lateral meristems extending along length of a shoot or root where primary growth has ceased.

Primary Meristems

Protoderm- Dermal Ground meristem- Ground Procambium- Vascular Produce, respectively, three mature tissues of a root or shoot (dermal, ground, vascular)

Nitrogen Fixation

Reduce atmospheric N2 to NH3. All nitrogen fixing organisms are bacteria. N2 + 8e- +8H+ +16ATP --> 2NH3 + H2 + 16ADP + Pi Driven by nitrogenase enzyme. 16ATP for every 2 NH3 molecules, nitrogen-fixing bacteria need rich supply of carbohydrates from decaying material, root secretions, or vascular tissue of roots (rhizobium bacteria/root living bacteria).

Vascular Plant Organs: Roots, Stems, and Leaves

Root system: Roots are never photosynthetic, starve unless photosynthates (sugars and carbs produced during photosynthesis) are imported from shoot system. Shoot system: consists of stems and leaves. Depends on water and minerals absorbed from root system. Vascular plants rely on both systems for survival.

Cation Exchange in Soil

Roots do not absorb mineral cations directly from soil particles, absorb from soil solution. Soil's pH and cation adhesion sites determine capacity to exchange cations

Organization of Primary Tissues in Young Roots

Roots with parenchyma in the center (monocots). Stele of many monocot roots is a vascular cylinder with a. core of parenchyma surrounded by ring of xylem and phloem.

Primary Growth of Shoots

Shoot apical meristem gives rise to three types of primary meristems in the shoot- protoderm, ground meristem, and procambmium. Closer an axillary bud is to an active apical bud, the more inhibited it is (called apical dominance). When gardeners prune shrubs and pinch back house plants, they are reducing the number of apical buds a plant has, thereby allowing branches to elongate and give plant a fuller appearance.

Roots pt 2.

Small vascular plants susceptible to grazing animals that can uproot plant and kill it, these plants are anchored by a fibrous root system (thick mat of slender roots). Includes most monocots, when primary root dies it does not form a taproot. Each root forms its own lateral roots and so on (like grass, also prevents soil erosion). Root hairs- thin, finger-like extensions of root epidermal cells, emerge and increase surface area of root enormously. Mycorrhizal associations- symbiotic interactions between roots and soil fungi, increase plant's ability to absorb minerals.

Roots with Xylem and Phloem in Center

Typical of eudicots. In roots of eudicots and gymnosperms, as well as some monocots, stele is a vascular cylinder appearing in cross section as a lobbed core of xylem with phloem between lobes.

Stem Growth and Anatomy: Monocots

Vascular bundles scattered throughout the ground tissue rather than forming a ring.

Xylem Sap

Water and dissolved minerals in xylem, transported long distances by bulk flow to veins that branch throughout each leaf. Bulk flow MUCH FASTER than diffusion or active transport. Stems and leaves depend on rapid delivery system. Process of transporting xylem sap involves loss of a lot of water (transpiration). If transpired water is not replaced by water transported up from roots, the leaves will wilt and plants will die.

Tracheids

Water conducting cells (vessel elements are also water conducting cells). Tubular, elongated cells that are dead and lignified at functional maturity. Tracheids occur in xylem of all vascular plants.

Root Pressure

Water flows from root cortex generating root pressure, a push of xylem sap. Root pressure sometimes causes more water to enter the leaves than is transpired, resulting in guttation, the exudation of water droplets can be seen in morning on tips or edges of plant leaves. Drives ascent of xylem sap, pushing water only a few meters at most. Positive pressures produced are too weak to overcome gravitational force of the water column in the xylem, particularly tall plants. Root pressure can't keep pace with transpiration after sunrise. For the most part, Xylem sap not pushed from below by root pressure but pulled up.

Zones of Growth

Zone of cell division- Includes stem cells of root apical meristem and immediate products. New root cells produced in this region, including cells of root cap. Zone of elongation- most growth occurs as root cells elongate. Pushes tip farther into soil. Meanwhile, root apical meristem keeps adding cells to younger end of this zone. Three primary meristems (protoderm, ground meristem, and pro cambium) become evident. Zone of differentiation- cells complete differentiation and become distinct cell types.

Mechanisms of Stomatal Opening and Closing

a.) Guard cells of typical angiosperm are illustrated in turgid (open stomata) and flaccid (stomata closed) states. b.) Transport of K+ across plasma membrane and vacuolar membrane causes turgor changes of guard cells. Uptake of anions (malate/chloride) contributes to guard cell swelling.