BIOL 1902 - Plant Defence

Plant Defence Physical Protection: ○​ What is Physical Protection? ➢​ Refers to the external amour that trees and other woody plants have to protect against predators ○​ Examples: ➢​ Tree: ●​ They have bark to protect them ➢​ Seeds: ●​ Are protected inside their hard coating ●​ Ex. acorns protect the seeds within their tough shells ➢​ Thistle ●​ Have prickly structures on them to keep away attackers, they have spines modified leaves ➢​ Cactus: ●​ Have prickly structures on them to keep away attackers, they have spines modified leaves ➢​ Raspberry / Blackberry bushes ●​ They have prickles, which is epidermal outgrowths (like hairs) ➢​ Hawthorns ●​ They have another type of ‘prickly’ defence, which are big thorns, thorns are modified branches or stems ➢​ Prickly Ash ●​ Also has the defence of thorns ➢​ Honey Locust ●​ Also has the defence of thorns Plant Hair Defence: ○​ What are Irritant Hairs (Trichomes)? ➢​ Trichomes are hairs that, due to their shape and brittleness, break off and cause mechanical damage on skin ➢​ Examples: ●​ Mullein ■​ Have soft, velvety trichomes that provide a physical barrier ●​ Ragweed ■​ The stem is covered in trichomes (resemble the hairs on some caterpillars) ●​ Water Smartweed ■​ Has an inducible defence (when something is not there all the time, but is called upon when needed), it grows trichomes on the leaves when there is no water (no trichomes when in water) ○​ What are Stinging Hairs? ➢​ Specialized hairs that act like hypodermic syringes which inject chemicals through a sharp tip (injector system similar to wasps) ➢​ Example: ●​ Stinging Nettles ■​ This plant contains short hairs (trichomes) and long hairs (stringing hairs) Structures Components: ○​ What are Digestibility Reducers? ➢​ Internal structures in plants that serve as a deterrent and make the plant difficult for predators to digest ○​ Examples: ➢​ Cellulose, Hemicellulose, and Pectin ●​ These are the most popular compounds that make plant tissues hard to digest ➢​ Lignin ●​ Gives leaves stiffness, and nuts and cherry pits their hardness ●​ Keeps water from leaving a plant in dry conditions, and helps keep out insects, fungi, and diseases ➢​ Tannins ●​ Complex chemical substances found in the bark of trees, wood, leaves, buds, stems, fruits, seeds, roots, and plant galls ●​ Tannins are astringent, which means it has a drying effect ➢​ Silica ●​ It is found in Horsetails (Equisetum), it gives them a very tough outer coat, Horsetails are also known as ‘Scouring Rushes’ ●​ Grasses also contain silica ➢​ Calcium ●​ Calcium oxalate crystals serve as a deterrent for both big and small animals because when they get eaten they hurt and burn ●​ Arums contain these crystals, ex. are Skunk Cabbage and Jack-in-the Pulpit Non-Structural Components: ○​ What are Terpenoids? ➢​ A major group of plant secondary metabolites that often taste bitter and serve primarily as repellents ➢​ Terpenoids do not contain Nitrogen ➢​ Examples: ●​ Milkweed ●​ Coniferous Trees ■​ Contains resin, and resin contains terpenoids ●​ Poison Ivy ■​ Contains resin, and resin contains terpenoids ○​ What are Alkaloids? ➢​ A major group of plant secondary metabolites that interfere with digestion by binding to digestive enzymes/proteins and change their structure (some are sugar mimics) ➢​ Alkaloids do contain Nitrogen ➢​ This defence is always present in a plant - a constitutive defence ➢​ Examples: ●​ Asters ●​ Buttercups ●​ Black Cherry ■​ Contains hydrogen cyanide which is a inducible defence ●​ Bracken Fern ■​ Contains hydrogen cyanide which is a inducible defence Hormones: ○​ What are Phytohormones? ➢​ Chemical messengers that travels through the plant initiating biochemical responses ➢​ Some messengers are: jasmonic acid, ethylene, salicylic acid ○​ Types of Hormones: ➢​ Wound Hormones: ●​ A wounded leaf releases these hormones to alert the other leaves to prepare for an attack ➢​ Ecdysone (Moulting Hormone): ●​ This hormone is made from plants so we call them phytoecdysones (instead of just ecdysones), these hormones mimic the animal moulting hormones ●​ Pythomoulting hormone makes an animal grow up too quickly ●​ Examples: ■​ Rock Polybody ■​ Bracken Fern ➢​ Juvenile Hormone: ●​ This hormone keeps the animals in the juvenile stage until the insect is big enough to become and adult ●​ Phytojuvenile hormone keeps an animal in its juvenile state, insects that eat a plant with this hormone do not mature ■​ Balsam Fir contains this ➢​ Reproductive Hormones: ●​ Phytoestrogens, they mess up an animal’s reproductive system ●​ Example: ■​ Red Clover, if it gets eaten the animal will have spontaneous miscarriages or they will fail to give birth ➢​ Phytotoxins ●​ Give off nasty side effects ●​ Example: ■​ St. John’s-wort, the toxins migrate inside the body and go right to the skin and make the body be susceptible to sunburn that can fester and get infected, this is because it makes the skin more sensitive to light ○​ Aposematic Colouring / Smell: ➢​ Plants that contain these chemicals advertise that they are dangerous just like animals do, with aposematic colouring ➢​ Examples: ●​ Mustards ■​ Advertise their defence by a very distinctive smell ●​ Berries ■​ Their colouring, not ripe berries have a greenish colour signalling that they are not ready