AGRC 111 W1 Handout 2024 PDF - Plant Sciences Laboratory 1

Agriculture Science 111.3 PLANT SCIENCES LABORATORY 1 2024 Crop Emergence & Development Learning Objectives  Describe differences that exist between monocotyledonous and dicotyledonous plants.  Distinguish...

Agriculture Science 111.3 PLANT SCIENCES LABORATORY 1 2024 Crop Emergence & Development Learning Objectives  Describe differences that exist between monocotyledonous and dicotyledonous plants.  Distinguish between a monocotyledon embryo and a dicotyledon embryo.  List and describe two germination types in dicotyledonous plants.  Identify two main types of internode elongation in monocotyledonous plants. Key Terms and Concepts aleurone layer hull bran hypocotyl caryopsis hypogeal germination cereal grain mesocotyl coleoptilar node micropyle coleoptile monocotyledon coleorhiza pericarp cotyledon plumule dicotyledon Poaceae embryo quiescence embryonic axis radicle endosperm scutellum epicotyl secondary adventitious root system epigeal germination shoot apical meristem (SAM) fruit seminal root system germ sub-crown internode grain testa hilum Introduction to Monocotyledons and Dicotyledons The plants of Saskatchewan’s agriculture system fall into two major scientific classifications, the monocotyledons (monocots) and dicotyledons (dicots). Because these two classes represent a major division among our most important agricultural crops, some of the primary differences between monocots and dicots should be noted and understood. The number of cotyledons (an embryonic structure that is most often swollen with nutritive substances to help feed geminating seedlings within the seeds) is a fundamental and reliable difference between monocot plants and dicot plants. The names of the two groups are based upon cotyledon number present in the seed. Monocots always have one cotyledon (mono/cotyledon) in the seed, named the scutellum which is specialized for absorption of the endosperm. Dicots have two cotyledons (di/cotyledon). 1  Agriculture Science 111.3 PLANT SCIENCES LABORATORY 1 2024 Taxonomic Hierarchy Wheat Pea Kingdom Plants Plants Subkingdom Vascular Plants Vascular Plants Superdivision Seed Plants Seed Plants Division Flowering Plants Flowering Plants Class Monocotyledons Dicotyledons Subclass Commelinidae Rosidae Order Cyperales Fabales Family Grass Family (Poaceae) Pea Family (Fabaceae) Genus Triticum Pisum Species Triticum aestivum Pisum sativum The primary difference between monocot plants and dicot plants is summarized in the following table. Monocot Plants Dicot Plants One cotyledon (scutellum) per seed Two cotyledons per seed Parallel leaf venation Net or branched leaf venation Flower parts tending to be in units of Floral parts tending to be in units of five or three. sometimes four *No vascular cambium present *Vascular cambium present *Vascular cambium is the specialized layer which allows dicotyledonous plants to increase in girth. Monocots (Cereals) Cereal crops will be mainly used as examples of monocot plants. A cereal grain is defined as any member of the grass family that is grown for the harvested product of its mature seed. The grass family’s formal scientific name is Poaceae. The name Poaceae is derived from the genus name Poa, a well-known group of grasses, to which the standard “aceae” suffix was added to indicate the family level of classification. Any time a botanical term ends in “aceae” it is specifying a family of plants. In order to pronounce the word Poaceae think of the highest ranked playing card in a deck of cards — the ace — and pretend you are spelling it aloud. To do so you would say: “A” – “C” – “E.” Plant family names are pronounced much the same way, so the grass family, Poaceae is pronounced “Po–A–C–E.” 2  Agriculture Science 111.3 PLANT SCIENCES LABORATORY 1 2024 The Monocot (Cereal) Embryo Saskatchewan’s typical cropping year begins at spring seeding time (some crops are seeded in the autumn) with the sowing of hundreds of billions of “seeds”. Within each seed is an embryo that, given proper conditions, should develop into a new and productive plant. The embryo of a cereal grain is composed of three distinct parts. These are the plumule, the radicle and a cotyledon known as the scutellum. The Radicle and the Grass Root System The radicle is the primary root in embryonic form. The radicle in grass species is protected by a unique sheath- like structure called the coleorhiza. Its role is similar to that of the coleoptile. The word radicle comes from the Latin “radicula” meaning “small root” and is a fitting name because this tiny root is the first structure to emerge from almost any germinating seed. The seedling root system in grass (cereal, monocot) species is formed from the radicle and the lateral seminal roots erupting from a node located in the scutellum. The seedling root system is known as the seminal root system. The seminal root system may stay intact during the duration of the plant’s life cycle but the main sustaining root system for the plant is developed in an area above the seminal root system. The main root sustaining system in grass species is known as the secondary adventitious root system. This root system is developed from the coleoptilar node which is found at the base of the coleoptile and other nodes above the area where the seminal root system is located. The Plumule The plumule is the embryonic shoot which develops into above ground plant structures. It generally consists of two pre-formed leaves and the shoot apical meristem (SAM). The SAM is a small growth point between the two pre-formed leaves. The leaves remain in embryonic form until expanding at germination. The plumule is so named because the tiny leaves inside the seed were originally thought to resemble small feathers, thus the name comes from Latin (plume/ule = feather/small). Grass plumules are protected by a unique structure — the coleoptile. Grass plumules are enclosed within a specialized sheath called a coleoptile which protects the delicate leaves of the plumule as they push up through abrasive soil. In addition to its protective role, the coleoptile is also highly sensitive to light, permitting it to guide the expanding plumule toward the soil surface. The coleoptile is phototropic – it grows towards light. Once the tubular, elongated coleoptile is above ground, the leaves of the plumule emerge through a slit at the top. The leaves then begin to use sunlight as their energy source in the manufacturing of food through photosynthesis. At this point a seedling is no longer reliant upon stored food, and has become an independent plant capable of manufacturing its own food. Monocots: Together the plumule (embryonic shoot) and radicle (embryonic root) form what is called the embryonic axis, a central column that is embedded within the single cotyledon/scutellum of the grass seed. The Cotyledon (Scutellum) Some of the food that sustained the germinating grass seedling prior to its reaching the sunlight and becoming self-sustaining will have been stored in the cotyledon/scutellum, a structure that serves as a food storage role. The Endosperm Endosperm is a food storage tissue outside the embryo but within the seed. The name “endo/sperm” actually means “inside/seed.” Endosperm is highly charged with starch granules intended to supply the germinating seedling with necessary carbohydrate energy to power its journey to the soil surface during germination. 3  Agriculture Science 111.3 PLANT SCIENCES LABORATORY 1 2024 The Outer Bits Every cereal grain is a small dry fruit.  Botanically, a fruit is the structure that develops from the ripened ovary wall of any flower.  An individual cereal grain is a botanical fruit. Although cereal grains may appear to be little more than “seeds” cereal grains and all other grass “seeds” are actually simple dry fruits.  Cereal grains meet the botanical criteria for being fruits because the outermost wall of a grain is derived from the dried ovary wall, called the pericarp (peri/carp = around/carpel).  The ripened ovary wall/pericarp encloses a seed.  The protective outer covering of a seed is called the testa.  The outermost layer of a cereal grain is therefore the pericarp fused tightly to the testa.  Just below the seed coat is the aleurone layer, which is a layer that consists of high protein cells surrounding the starch storage cells of the endosperm.  The function of the protein cells in the aleurone layer is to secrete hydrolytic enzymes for digesting food reserves in the endosperm. Together the aleurone layer, pericarp and testa make up the bran of the cereal grain (caryopsis), which is most often removed during milling. The germ of a cereal grain (caryopsis) is composed of those parts directly involved in “germ”ination, so it consists of the plumule, radicle and cotyledon (scutellum). Thus, the germ is composed of the embryo. Dicots (Oilseeds and Pulses) Oilseed crops and pulse crops will be mainly used as examples of dicot plants Oilseed crops grown in Saskatchewan include Argentine and Polish canola, flax for both industrial and edible oil markets, yellow mustard, brown mustard, Oriental mustard, sunflower and safflower. The predominant pulse crops are field pea and lentil. Acreages of pinto bean, chickpea and other types of dry bean are presently on the rise. Oilseeds and pulses are part of a larger classification of plants called dicotyledons (di/cotyledon = two/cotyledons) because their seeds always contain two cotyledons. They are usually called dicots for short. The Dicot Embryo As in monocots, the same three basic parts make up the dicot embryo. These are the radicle (embryonic root), which develops into the primary root system, the plumule (embryonic shoot), which develops into the above ground shoot system, and always two cotyledons for food storage. Dicots: The embryonic axis formed by the plumule (embryonic shoot) and radicle (embryonic root) is imbedded neatly between the two cotyledons, which form a matched pair. The cotyledons are easily separated once the testa (seed coat) is removed. Unlike monocots, dicot seeds typically lack a distinct endosperm because early in the seeds’ development the endosperm tissue is absorbed into the cotyledons, leaving the cotyledons entirely responsible for food storage. The testa surrounds the entire embryo. There is often a visible scar left on the testa where the seed was attached inside the fruit. This scar (equivalent in human terms to a belly button!) is called the hilum. A very small hole 4  Agriculture Science 111.3 PLANT SCIENCES LABORATORY 1 2024 called the micropyle may also be visible in the testa. This hole permits entry of moisture into the seed at the beginning of germination. Germination and Emergence Germination is a PROCESS involving some measure of seedling development. Cereal Crop Emergence  The radicle is the first organ to emerge, followed by the coleoptile enclosing and helping guide the plumule to the soil surface.  Depending on seeding depth, underground stem internodes elongate. In some grasses, the coleoptile encloses this underground structure called the sub-crown internode.  In other grasses, the coleoptile is pushed upward by an underground stem called the mesocotyl.  In either case, secondary adventitious roots develop from the coleoptilar node. o The coleoptilar node is the node at the very base of the coleoptile. o Secondary adventitious roots are those that cannot trace their origin back to the radicle. o Secondary adventitious roots form a large component of the root system of a grass. Emergence of Oilseeds and Pulses Two forms of seed germination occur, which is based on the fate of the cotyledons; hypogeal germination and epigeal germination. In hypogeal germination, the cotyledons remain below the soil surface; in epigeal germination, the cotyledons are raised above the soil surface. Dicot seed emergence is either epigeal or hypogeal; hypogeal germination is characteristic of all cereal crops. The different germination type in oilseeds and pulses is based upon the location of the cotyledons in relation to the soil surface when germination is complete. Hypogeal Germination  In this type of emergence the cotyledons remain below the soil’s surface supplying their stored energy and providing anchorage to the emerging seedling.  The epicotyl (epi/cotyl = above/cotyledon) is the unit of stem directly above the cotyledons.  The epicotyl emerges from between the cotyledons, bent over to draw the delicate plumule through the soil behind it as a means of protecting this structure from direct abrasion with soil.  Once above ground the epicotyl straightens to elevate the plumule upward toward the light. The term “hypogeal” is derived from Latin roots and it literally means (hypo/geal = below/earth) because the cotyledons remain below the ground.  Only the leaves of the plumule rise above the soil surface, so only true leaves are seen above ground.  The cotyledons remain below ground. Epigeal Germination  The word “epigeal” means upon or above earth (epi/geal = above/earth) because the cotyledons are drawn up and out of the soil.  This is accomplished by the elongation and arching of the hypocotyl (hypo/cotyl = below/cotyledon), which is the stem unit directly below the cotyledons.  Again the delicate plumule is protected from soil abrasion, but in this case, it is by remaining enclosed between the two cotyledons until they have broken the soil surface. o For a short time the above ground cotyledons will take on a photosynthetic role, acting as leaves but soon after the true leaves expand — those of the plumule — the cotyledons wither and fall 5  Agriculture Science 111.3 PLANT SCIENCES LABORATORY 1 2024 away. The seedling is then an independent plant responsible for manufacturing its own food supply. 6