Plant Anatomy and Physiology: Transport and Hormones - PDF

Transport in Plants Lecture outline Transport in plants - vascular tissues in plants Xylem: Transpiration Phloem: Translocation Movement of fluid in xylem vs. phloem Plant hormones Transport in plants What is “Transport”? The movement of things from one place to another. Transport...

Transport in Plants Lecture outline Transport in plants - vascular tissues in plants Xylem: Transpiration Phloem: Translocation Movement of fluid in xylem vs. phloem Plant hormones Transport in plants What is “Transport”? The movement of things from one place to another. Transport in plants? How does a Redwood move water from the soil to the leaves over 30 stories high in the air? How does a carrot transport sugars/amino acids from a leaf to the orange taproot? Vascular System in Vascular Plants Stem The part of the plant that connects the leaves to the roots. Stems provide structural support so they can grow upright and position their leaves towards the sun. Stems are also flexible allowing them to bend in the wind and not snap. Not all stems are similar: Cactus stems are swollen and store water Some stems twist and have grasping tendrils Some stems are covered in thorns Stem visualisation Stem → analogous to chopstick and straws Rubber band Symbolises the dermal tissue → covers outside of the plant stem, and acts as a protective layer. Chopsticks Represent the ground tissue → provides structural support to the stem. Drinking straws Represents the vascular tissue → dicots? Monocots? They transport water, minerals or sugars. Xylem Xylem Cells stacked one on top of the other creating a tube. Xylem Cells stacked one on top of the other creating a tube. Xylem  Transpiration The driving force of movements Water Moved in the xylem. Xylem is a one-way street. Route: soil → roots → stem → leaves → air In xylem In xylem, water molecules move by cohesion and adhesion properties. Transpiration moves water out of the plant body, creates a negative force of water pressure to enhance the flow of water molecules from soil to leaves. How does water enters the root hair xylem? Route 1: Apoplast Water moves through the cell walls and the intracellular spaces of the root cortex. Route 2: Symplast Water moves through the cytoplasm of the root cortex Forces for water Cohesion transport Osmosis (root Adhesion hair) Transpiration (stomata) Phloem Movement of sugars The movement of sugars in a plant is much different than the movement of water. Phloem moves both up and down a plant. This allows the extension of roots, as well as the growth of fruits. 2 terms: Source vs. sink Phloem Phloem tissue is made up of two types of cells that are less rigid and much more lively. Sieve tube elements ○ Separated by perforated plates Companion cells ○ Modulate flows in sieve tube elements ○ Loading food into and out sieve tube elements ○ Provide energy Sugars move from “source” to “sink” Locations that produce or release sugars for the growing Sources plants. Eg. Leaves, certain roots The points of sugar delivery, such as roots, young shoots Sinks and developing seeds. Eg. Apical and lateral meristems, developing leaves, flowers, seed, fruits etc. (active growth) Eg. roots, tubers, bulbs (sugar storage) Translocation Translocation Sugars translocate from source to sink, but how? PRESSURE FLOW MODEL Pressure Flow Model The pressure flow model works like this: A high concentration of sugar at the source creates a low solute potential, which draws water into the phloem from the adjacent xylem. This creates a high pressure potential in the phloem. The high pressure drives movement of phloem sap by “bulk flow” from the source to sink, where the sugars are rapidly removed from the phloem at the sink. Movement of fluid in xylem vs. phloem Some important differences in the mechanisms of fluid movement in the two different vascular tissues: Xylem Phloem Driving force for Transpiration from leaves, Active transport of sugar from fluid movement combined with cohesion and source cells into phloem sieve adhesion of water in the vessel tube elements (energy required). elements and tracheids (passive, no energy required). Cells facilitating Non-living vessel element and Living sieve tube elements fluid movement tracheids. (supported by companion cells) Pressure Negative due to pull from the top Positive due to push from the potential (transpiration, tension) source. Plant Hormones Auxin Also known as indole acetic acid (IAA) It causes: Stem elongation and growth Formation of roots Inhibits leaf loss Promotes cell division Increases ethylene production Stimulates formation of fruits Inhibits the growth of lateral buds in shoots Auxin Francis Darwin noticed that plants bend toward the light. His research team demonstrated that the growing tips of plants sense light, and this ability was not detected in areas behind the shoot apex. Auxin Auxin is produced uniformly by growing shoot tips, but is transported to the shaded side of the shoot. This phenomenon caused the cells on the shaded side to elongate more than the cells on the lighted side. This was achieved by soften the cell wall of the cells on the shaded region, which causes easy expansion of the cell’s cytoplasm. Other effects of auxins Stimulates the formation of fruits - Pollen contains a high concentration of auxin. It serves as a chemical signal that stimulates the formation of fruits. Inhibits the growth of lateral buds in shoots - Production of auxin by the shoot apex inhibits the growth of neighbouring lateral buds (apical dominance). Apical dominance Cytokinin Produced in root apical meristem and by fruits. Stimulates cell division (with auxin) Promotes chloroplast formation Delays leaf aging Promotes formation of lateral buds Inhibits root growth Gibberellin Promotes stem elongation Stimulates enzyme production in germinating seeds Stimulate elongation Breaks seed dormancy Gibberellin Gibberellin increases the size of fruits Gibberellin increases the yield in sugarcane Ethylene Controls shedding of leaves, flowers, fruits. Promotes fruit ripening Produced by apical meristems, leaf nodes, aging flowers, ripening fruits Ethylene is used commercially to ripen green fruits. Carbon dioxide has opposite effect of ethylene. Fruits are often shipped in CO 2, and ethylene applied at the destination.