Plants Growth, Structure, Function

Plant Growth  Meristems – actively dividing cells  Plants grow in two ways – primary and secondary growth  Primary growth increases the length of the plant – apical meristems – tips of the roots and shoots  Secondary growth increases the thickness of the plant – lateral meristems

Figure 35.2 Reproductive shoot (flower) Apical bud Node Internode Apical bud Vegetative shoot Leaf Blade Petiole Stem Taproot Lateral (branch) roots Shoot system Root system Axillary bud

Lateral Meristems  Vascular cambium – replaces primary xylem and phloem  Cork cambium – bark  Lenticels – holes for gas exchange

Secondary Growth

Lenticels

Root hair

Roots  A root is an organ with important functions:  Anchoring the plant  Absorbing minerals and water  Storing carbohydrates  In most plants, absorption of water and minerals occurs near the root hairs, where vast numbers of tiny root hairs increase the surface area  Root tip, elongation region, maturation region

Root Body  All roots have epidermis, cortex, and stele  Epidermis – protective covering  Cortex – starch and mineral storage  Stele – vascular tissue  Water and minerals travel through the cortex by apoplast or symplast systems  Apoplast – water travels through porous cell walls  Symplast – through plasmodesmata

Root Growth

Figure 35.14aa Epidermis Cortex Endodermis Vascular cylinder Pericycle Xylem Phloem 100  m (a) Root with xylem and phloem in the center (typical of eudicots) Dermal Ground Vascular Key to labels

Apoplast vs. symplast

Endodermis and Casparian Strip  Regulates passage from cortex into vascular stele  Caparian strip is made of a fatty acid called suberin which blocks water passage between endodermal cells

Figure Sclerenchyma (fiber cells) Phloem Xylem Ground tissue connecting pith to cortex Pith Cortex Vascular bundle Epidermis 1 mm Vascular bundles Epidermis Ground tissue Dermal Ground Vascular Key to labels (a) (b) Cross section of stem with vascular bundles forming a ring (typical of eudicots) Cross section of stem with scattered vascular bundles (typical of monocots)

Leaves  The leaf is the main photosynthetic organ of most vascular plants  Leaves generally consist of a flattened blade and a stalk called the petiole, which joins the leaf to a node of the stem  Leaves can be modified like spines in a cactus or succulent  Leaves can also form traps – Venus flytrap  Pitcher plant creates a slippery slope which traps insects

Figure Key to labels Dermal Ground Vascular Cuticle Bundle- sheath cell Xylem Phloem Sclerenchyma fibers Stoma Upper epidermis Palisade mesophyll Spongy mesophyll Lower epidermis Cuticle Vein Guard cells (a) Cutaway drawing of leaf tissues (b) (c)Cross section of a lilac ( Syringa ) leaf (LM) Surface view of a spiderwort ( Tradescantia ) leaf (LM) Guard cells Stomatal pore Epidermal cell Vein Air spaces Guard cells 50  m 100  m

Vascular Tissues  The vascular tissue system carries out long- distance transport of materials between roots and shoots  The two vascular tissues are xylem and phloem  Xylem conveys water and dissolved minerals upward from roots into the shoots  Phloem transports organic nutrients from where they are made to where they are needed  The two types of water-conducting cells, tracheids and vessel elements, are dead at maturity  Tracheids are found in the xylem of all vascular plants

Figure 35.10d Vessel Tracheids 100  m Tracheids and vessels (colorized SEM) Perforation plate Vessel element Vessel elements, with perforated end walls Pits Tracheids

Sugar conducting cells  Sieve-tube elements are alive at functional maturity, though they lack organelles  Sieve plates are the porous end walls that allow fluid to flow between cells along the sieve tube  Each sieve-tube element has a companion cell whose nucleus and ribosomes serve both cells

Sieve-tube element (left) and companion cell: cross section (TEM) Sieve-tube elements: longitudinal view Sieve plate 3  m Companion cells Sieve-tube elements Plasmodesma Sieve plate Nucleus of companion cell Sieve-tube elements: longitudinal view (LM) 30  m 15  m Sieve plate with pores (LM) Figure 35.10e

Short-Distance Transport of Water Across Plasma Membranes  To survive, plants must balance water uptake and loss  Osmosis determines the net uptake or water loss by a cell and is affected by solute concentration and pressure © 2011 Pearson Education, Inc.

 Water potential is a measurement that combines the effects of solute concentration and pressure  Water potential determines the direction of movement of water  Water flows from regions of higher water potential to regions of lower water potential  Potential refers to water’s capacity to perform work © 2011 Pearson Education, Inc.

How Solutes and Pressure Affect Water Potential  Both pressure and solute concentration affect water potential  This is expressed by the water potential equation: Ψ  Ψ S  Ψ P  The solute potential ( Ψ S ) of a solution is directly proportional to its molarity  Solute potential is also called osmotic potential © 2011 Pearson Education, Inc.

Figure 36.9 Plasmolyzed cell at osmotic equilibrium with its surroundings 0.4 M sucrose solution: Initial flaccid cell: Pure water: Turgid cell at osmotic equilibrium with its surroundings (a) Initial conditions: cellular   environmental  (b) Initial conditions: cellular   environmental  PP  0 PP  0 SS  PP  0 SS  PP  0.7 SS  0.9   0.9 MPa  SS   0.9  0.9 MPa      0.7 MPa SS  0.7  PP 0  0 0 MPa    MPa 

Transport of xylem sap  Water is being “pulled” by transpiration, so must be replenished by soil water.  At night, root cells push mineral ions into the xylem  This lowers the water potential causing water to flow in and force water up the xylem – root pressure  This cause guttation fluid in the morning  Root pressure cannot compete with transpiration at sunrise.

Guttation

Cohesion and Adhesion  The cohesive and adhesive nature of water contribute to transpiration

Transport of Sugars  Translocation is the movement of carbohydrates through the phloem from a source to a sink. The source is leaves, the sink is where the carbohydrate will be used.

Pressure-flow hypothesis 1.Soluble sugars like fructose and sucrose move from palisade mesophyll to sieve tube members by active transport 2.Water then diffuses into the cells 3.Pressure in sieve tube causes water and sugar to flow toward sink 4.Sugars are moved by active transport into neighboring cells 5.Water diffuses back to xylem

Vegetative propagation  Part of a plant can produce more – asexual  Pg. 132 Princeton review book

Tropisms  Phototropism – turning toward light  Gravitropism – shoots growing upward (-), roots growing down (+)  Thigmotropism – growth in response to touch  Auxin’s control plant growth toward the light by thickening on the dark side. They are located at the tip of the plant. They also control cell elongation and fruit development  Other hormones – pg. 133

Phytochromes as Photoreceptors  Phytochromes are pigments that regulate many of a plant’s responses to light throughout its life  These responses include seed germination and shade avoidance  Many seeds remain dormant until light conditions change  Red light (600nm) increased germination, while far-red (730nm) light inhibited germination  The photoreceptor responsible for the opposing effects of red and far-red light is a phytochrome © 2011 Pearson Education, Inc.

Figure RESULTS Red Far-red Dark (control) Dark

Figure Two identical subunits Chromophore Photoreceptor activity Kinase activity

Figure 39.UN01 Red light Far-red light PrPr P fr

Figure Synthesis PrPr P fr Red light Far-red light Slow conversion in darkness (some plants) Responses: seed germination, control of flowering, etc. Enzymatic destruction

The Effect of Light on the Biological Clock Phytochrome conversion marks sunrise and sunset, providing the biological clock with environmental cues Photoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of year Photoperiodism is a physiological response to photoperiod © 2011 Pearson Education, Inc.

Photoperiodism and Control of Flowering Some processes, including flowering in many species, require a certain photoperiod Plants that flower when a light period is shorter than a critical length are called short-day plants Plants that flower when a light period is longer than a certain number of hours are called long- day plants Flowering in day-neutral plants is controlled by plant maturity, not photoperiod © 2011 Pearson Education, Inc.

Figure hours Light Flash of light Darkness Critical dark period Flash of light (b) Long-day (short-night) plant (a)Short day (long-night) plant

24 hours Critical dark period Short-day (long-night) plant Long-day (short-night) plant R R RR R R FR Figure 39.22

Red light can interrupt the nighttime portion of the photoperiod A flash of red light followed by a flash of far-red light does not disrupt night length Action spectra and photoreversibility experiments show that phytochrome is the pigment that receives red light © 2011 Pearson Education, Inc.

Examples of seed dispersal