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Topic 9: Plant Science Heirarchy • Plants are still composed of cells, tissues, organs and organ systems • 3 major plant organs – Roots (root system) – Stems (shoot system) – Leaves (shoot system) Roots • Roots have a variety of jobs, including: • Anchoring to the soil, • absorbing minerals and water, • often storing carbohydrates • Taproots often hold carbohydrates for use by the plant during flowering Roots • Types – Taproot: penetrate deep into the soil. This is a “main root” with lateral roots growing off from the main – Adventitious roots: tend to have a fibrous root system with no one main root. These tend to be seen in shallow soils – Also a variety of modified roots: prop roots, storage roots, aerial roots, buttress roots, pneumatophores Stems • Stem jobs include: growth, food storage and asexual reproduction – Some of these jobs are represented in modified stems such as Rhizomes, Bulbs, Stolons, and Tubers • A stem is made up of nodes (point at which leaves are attached) and internodes (space in between) Stems • The angle between each leaf and a stem is a region known as the axillary bud. The axillary bud can give rise to a branch. – Most axillary buds are dormant on young plants because elongation is concentrated at the shoot tip (aka apical bud or terminal bud) Leaves • Main photosynthetic organ in most vascular plants • The petiole joins the leaf to the stem at the node • There can be several different arrangements of leaves: simple, compound, doubly compound – Veination will also vary (monocot / eudicot) • Modified leaves often act in defense or reproduction, or may protect against desiccation – Tendrils, spines, storage leaves, reproductive leaves, bracts Plant Tissues • Dermal Tissue – outer covering – Ex: epidermis, peridermis (woody plants) or cuticle • Vascular Tissue – carries out long distance transport of materials – Ex: xylem, phloem, stele • Ground Tissue – part of neither the dermal or vascular tissue – Ex: Pith (internal to vascular), cortex (external to vascular) Plant Cells • Parenchyma – “typical” plant cell with large vacuole • Collenchyma – offer support without limiting growth – often found in young parts of plant • Sclerenchyma – Offer support, but cannot elongate – Occur in parts of the plant that are not growing • Tracheids and vessel elements – Water conducting of xylem – Dead at functional maturity, leaving behind the supportive cell wall • Sieve Cells – Sugar Conducting cells of phloem – Alive, but lacking many cell parts (such as nucleus, ribosomes, etc.) allowing for easier passage of materials Meristems • Plants have indeterminate growth, meaning they grow throughout their lives • This is possible because of meristems (embryonic tissues) – Apical (primary growth) – located at the tips of roots and shoots and in axillary buds – Lateral (secondary growth) – growth of vascular cambium and cork cambium Primary Growth (Roots) – Root cap protects the apical meristem and secretes a polysaccharide slime to lubricate the soil – 3 zones above the root tip • Zone of cell division – New root cells produced • Zone of elongation – Cells increase in size • Zone of differentiation – Cells become distinct cell types Primary Growth (Shoots) • Leaf primordia give rise to leaves along the side of the apical meristem. Axillary buds then develop from the remains of the apical meristems. • Stems – arrangement of vascular tissue dependent on monocot (scattered) or dicot (ring) • Leaves – stomata and guard cells interrupt the epiderm – Mesophyll is ground tissue of the leaf • Dicots have palisade and spongy mesophyll Monocots vs. Dicots Resource Acquisition and Transport: Overview • Leaves: – CO2 is taken and O2 leaves through stomata – Sugars are produced – Transpiration causes xylem sap to be pulled upward • Roots: – Gases are exchanged through soil – Water and minerals are absorbed • Stem: – Xylem transports water and minerals from roots to shoots – Phloem moves sugar from sites of production to storage (can move up or down within plant) Shoot Architecture • Leaf size is dependent on environmental conditions – Ex: Largest leaves are found in tropical rain forests; smallest are found in cold or dry areas • Phyllotaxy (arrangement of leaves) is related to sun exposure for the leaf – May be to increase sun exposure or to allow shade in certain environments • Leaf orientation may be horizontal or vertical for similar reasons Root Architecture • Plants with taproots tend to be taller because of the strength of the root system in anchoring. • Approximately 80% of extant land plants are involved in mutualistic mycorrhizae relationships (roots and fungi) – Better able to absorb water and minerals (esp. phosphate) Transport: A Review • Solutes can be moved from cell to cell via several methods both actively and passively Osmosis and Water Potential • In animal cells, water moves from an area of lower solute concentration to an area of higher solute concentration • In plant cells, have to also take into account the physical pressure of the cell wall pushing back against protoplast – This is water potential (both solute concentration and physical pressure) – Free water (not bound to surfaces or solutes) moves from regions of higher water potential to regions of lower water potential Water Potential • Ψ=ψs+ψp – ψS= solute potential, always negative unless pure water (0) • Those water molecules that are bound to solute have less potential to move and do work – Adding solute always lowers water potential – ψP= pressure potential, can be positive or negative Pathways of Transport • Water generally moves using the apoplastic, symplastic or transmembrane routes – Apoplastic: water and solutes move along the continuum of cell walls and extracellular space – Symplastic: water and solutes move along the continuum of cytosol within the cell tissue – Transmembrane route: water and solutes move out of one cell, across the cell wall and into the neighboring cell Ascent of Xylem Sap • Xylem sap is pulled upward via transpiration • Cohesion aids in this by keeping water molecules sticking together • Adhesion aids in this by water sticking to the xylem walls, offsetting gravity Stomata • Stomata are the major source of water loss in plants – Mechanisms that control when they open and close can help to control this water loss – Solute concentration and water movement control the guard cells which open and close the stomata • Open because of light, CO2 depletion and an internal clock in the guard cells • May close because of environmental conditions (drought) or hormonal reactions Phloem • Sucrose/Sugars will go from their source (where they are made) to a sink (where they are stored) until they are needed • Movement is caused by positive pressure (sugar concentration is higher at sources than at sinks) Transpiration • Levels of transpiration will vary depending on environmental conditions – This includes light, humidity, wind & temperature • Example related to extremes is xerophytes – These are plants adapted to live in dry environments • Tend to have specially adapted leaves (waxy, spines, smaller or fewer stomata) • Many use a different metabolic pathway for photosynthesis (C4 or CAM) Angiosperm Life Cycle •Angiosperms have structures unique to their life cycles, including flowers and fruit •Flowers allow for fertilization to occur which then give rise to fruit that includes seeds •Seeds then give rise to the next generation Development of Gametophytes •Males: –Pollen grains develop within the microsporangia of anthers via meiosis –Becomes mature when it generative nucleus divides, forming 2 sperm (generally when it lands on the stigma) •Females: –Embryo sac develops within an ovule, enclosed by the ovary at the base of the carpel –Megasporocyte divides via meiosis, generally only 1 survives as the megaspore –Megaspore undergoes 3 mitotic divisions to form embryo sac Flower Pollination •Flowers can be pollinated in a variety of ways including –Abiotic: Wind, Water –Biotic: Bees, Moths and butterflies, Flies, Birds, Bats Seed Development •As seeds develop, they will include certain parts including endosperm, cotyledons and the embryo. The exact structure varies between monocots and dicots •Late in development, the seed dehydrates and enters dormancy •The seed will exit dormancy when conditions are right for development Germination and Development •To germinate, the seed must imbibe water, expanding and breaking the seed coat. This also triggers metabolic changes that resume growth. •Other needs for germination include oxygen, particular temperatures, and the presence of gibberellin, a plant hormone that stimulates mitosis. •The first organ to emerge is the radicle, followed by the shoot breaking through the soil surface. Fruit Development •As seeds develop from ovules, fruit develops from the ovary •Fruits and seeds are then dispersed in a variety of ways including wind, water and animals Plant Hormones Auxin • Auxins are related to a variety of plant activities including: – Cell elongation – Lateral and adventitious root formation – Increased cambrial activity related to secondary growth – Fruit growth – Can also be used as herbicides via hormonal overdoes Cytokinins • Cytokinins are related to – Control of cell division and differentiation – Control of apical dominance – Anti-aging effects Gibberellins • Gibberellins are associated with: – Stem elongation – Fruit growth – Germination Brassinosteroids • These are also related to growth • Were originally thought to be an auxin • Chemically similar to cholesterol and sex hormones Abscisic Acid • ABA is responsible for slowing growth • Also responsible for seed dormancy. Often ABA must be removed to allow for germination to occur Ethylene • Ethylene plays a role in a variety of important events – Triple response to mechanical stress – Senescence – Leaf Abscision – Fruit Ripening Light and Plants • Plants can detect the direction, intensity and wavelength of light • Reception of light allows plants to measure the passage of days and seasons • Red and blue light are important in photomorphogenesis; thus, there are two photoreceptors – Blue-light photoreceptors • Phototropism, light-induced opening of stomata, light-induced slowing of hypocotyl elongation – Phytochromes • Seed germination, shade avoidance Light and Plants • Plant processes vary throughout the day in response to variation in light, temperature and relative humidity – In perfectly maintained conditions, some processes still occur on a cycle of 21-27 hour frequency, such as opening and closing of stomata and “sleeping” in legumes – These are circadian rhythms Light and Plants • Short Day vs. Long Day Plant: – Short: flowers when the dark period is long enough. If the dark period is interrupted, the plant will not flower. – Long: flowers when the light period is long enough. If the dark period is interrupted, the plant will flower even if the light period is not long enough. • If red light and far red light are used to effect phototropism, far red light will offset the affects of red light