2OBJECTIVES The student will be able to… Differentiate between annual, biennial, and perennial plants.
3OBJECTIVES The student will be able to… Describe the processes and steps in seed germination.Enumerate the traits that differentiate plant juvenility from maturity.Describe the steps in the flowering process in plants.Define the stages of dormancy, senescence, and abscission.List four plant hormones and their principal effects.
4In a typical life cycle, plants pass through two states: PLANT LIFE CYCLESIn a typical life cycle, plants pass through two states:Growth and rest.Growth occurs in suitable environmental conditions.Temperature, rainfall, etc.Characterized by flowering, shoot lengthening, and leaf production.Rest, (dormancy) also reflects environment.Cold, drought, or inadequate light.Characterized by slowed/stopped growth, leaf drop, and/or death of above-ground parts of the plant.
5PLANT LIFE CYCLESTo understand importance of growth & dormancy, classify plants according to the length of time they live.Plants fall into one of four botanical groups.Annual, biennial, perennial, and monocarp.
6PLANT LIFE CYCLES Annual An annual plant reproduces itself by seed, grows to maturity, flowers, produces seeds, and dies during one growing season.It experiences dormancy only as a seed.Figure 3-1a Annual plant life cycle. Drawing by Bethany Layport.To most gardeners an annual is a frost-tender flowering or vegetable plant grown in summer.In mild climates there also are winter annuals that are planted in fall to bloom through the winter.
7PLANT LIFE CYCLES Annual Exception must be made for plants that would live more than one year in their warm native climate.These plants, not able to survive winter in cold regions, are not true annuals but function as annuals because of climate.Tomatoes, lantana & impatiens, when grown in cold areas.
8PLANT LIFE CYCLES Biennial A biennial plant completes its life cycle in 2 years.Figure 3-1b Biennial plant life cycle. Drawing by Bethany Layport.A biennial…Is grown from seed.Produces leaves in rosette form for one season.Becomes dormant during winter.Resumes growth in spring.Produces a vertical flower stalk (bolts), and dies.
9PLANT LIFE CYCLES Biennial Biennials are relatively uncommon among cultivated plants.Hollyhocks, parsley, and sweet William.Some vegetable crops are biennials, harvested prior to entering winter dormancy.Cabbages and carrots.
10PLANT LIFE CYCLES Perennial Perennials live for more than two growing seasons.Figure 3-1c Perennial plant life cycle. Drawing by Bethany Layport.In horticulture, the term often refers to flowers such as daylilies & iris, that grow back from the roots each spring.Technically trees, shrubs & bulbs all are perennial.Perennials typically reproduce by growing new plants from the mother plant (vegetatively).In addition to by seed.
11PLANT LIFE CYCLES Monocarp A monocarp can live for many years but will flower only once in its lifetime and die afterward.Figure 3-1d Monocarp plant life cycle. Drawing by Bethany Layport.Bromeliads and century plants (Agave spp.) are two examples of monocarpic plants.These plants bloom after several years of active growth.The tops then die, and new plants are produced by the root system of the old plant.
12STAGES OF PLANT MATURATION Regardless of how long a plant lives, it usually will pass through four stages of maturation.Germination, juvenility, maturity, and senescence.
13STAGES OF PLANT MATURATION Germination Germination starts when a seed absorbs waterEnds when the primary root emerges.Figure 3-2 Germination of a bean seedling.After germination, the seedling goes through establishment.Until it is independent & photosynthesizing.
14STAGES OF PLANT MATURATION Germination A seed must contain…An embryo to develop into a new plant.Figure 3-2 Germination of a bean seedling.A source of energy to supply the embryo during germination and establishment.Carbohydrate, fat, or protein.A covering called the seed coat helps prevent injury & drying.
15STAGES OF PLANT MATURATION Germination In many dicot seeds, carbohydrates for germination are stored in two cotyledons.Figure 3-2 Germination of a bean seedling.These resemble leaves & are sometimes attached to the stem when the seedling emerges from the ground.Frequently called seed leaves, as opposed to true leaves, produced later.
16STAGES OF PLANT MATURATION Germination Cotyledons may serve as storage sites as well as photosynthesizing organs & transfer tissue between the endosperm and the embryo.Figure 3-2 Germination of a bean seedling.
17STAGES OF PLANT MATURATION Germination Cotyledons may serve as storage sites as well as photosynthesizing organs & transfer tissue between the endosperm and the embryo.In other species carbohydrates are mainly stored in an endosperm, a single organ.Figure 3-3a Corn grain showing enlarged endosperm.Figure 3-2 Germination of a bean seedling.
18STAGES OF PLANT MATURATION Germination For a seed to germinate and establish a new plant, correct environmental conditions are necessary.Water must be present.Oxygen must be available.Temperature must be in acceptable range to that species.Uptake of water, the first process, causes the seed to swell & triggers many chemical reactions.Including an increase in the cellular respiration rate.Oxygen is necessary for cellular respiratory process.Without it, cellular respiration will not begin and the seed will not germinate
19STAGES OF PLANT MATURATION Germination Suitable temperature is essential for germination.Temperatures either too high/too low will kill the embryo.Or prevent germination from proceeding.After sufficient swelling and cellular respiration have taken place, the radicle, or primary root, will emerge.It grows down into the soil, beginning the absorption process, and soon afterward the shoot emerges.
20STAGES OF PLANT MATURATION Germination The first shoot of most dicot plants will emerge from the soil in an arch, with the stem of the young plant forming a loop called the hypocotyl.Figure 3-2 Germination of a bean seedling.Once exposed to sunlight, the arched part of the hypocotyl straightens, pulling cotyledons and other delicate tips of the young plant through the soil.
21STAGES OF PLANT MATURATION Juvenility After germination, most plants enter juvenility.May last from several weeks to years.The plant will grow rapidly but not begin reproduction.Pruning, temperature alteration & applications of growth-regulating chemicals can change a plant from juvenile to the mature state, and back again.
22STAGES OF PLANT MATURATION Juvenility Some plants have recognizable traits that signal that the plant is in a juvenile state.Leaf Form - different from that found on the mature plant.Growth Form - variation in form also distinguishes juvenile from mature plants.Juvenile branch form is sometimes found on young fruit trees as whip-like vertical shoots, usually arising from the base of the trunk, they are called suckers, or water sprouts.Thorns - a third common characteristic of juvenility.Leaf Retention - A tendency for juvenile parts of a tree to hold leaves throughout the winter is the fourth characteristic.
23STAGES OF PLANT MATURATION Juvenility Plants may indicate juvenility by other than visible characteristics.In propagation, juvenile parts almost always root faster than mature portions.Figure 3-4 An oak tree retaining its dead leaves in winter, a characteristic of juvenility. Courtesy of Dr. Claud L. Brown, University of Georgia.
24STAGES OF PLANT MATURATION Maturity Maturity is the third stage in plant growth, during which sexual reproduction takes place.Flower Induction - the first indication of maturity.The first of the three phases of flowering.Flower Initiation - the second phase of flowering.Flower Development - length of time from induction to bloom may vary from several weeks to 6 or 8 months.The opening of the flower is the final stage of development, after which the flower is generally receptive to pollination.
25STAGES OF PLANT MATURATION Maturity Induction of flowering is sometimes caused by environmental factors such as temperature or night length.In nature these factors program flowering into a seasonal pattern so that it occurs at the same time each year.People can simulate these conditions to induce flowering at a pre-selected time.
26STAGES OF PLANT MATURATION Maturity Cool temperature is a crucial environmental factor that causes flowering of many plants.Some plants that grow/flower well in northern climates die or grow only vegetatively in warmer locales.They must be vernalized (subjected to cold temperatures) before they can be grown successfully.In addition to certain fruit trees, biennial plants may also require vernalization for flowering.Cold received during the winter following the first season of growth induces flowering to complete the life cycle of the plant.
27STAGES OF PLANT MATURATION Maturity Cabbages, carrots, and beets all are biennial plants that require vernalization.Because they are raised for their roots & leaves, and harvested the first year, vernalization is not important.Vernalization of bulbs such as tulips, hyacinths, and crocuses is important as they are grown for flowers.In warm climates bulbs of these plants are sold prechilled.Vernalized by refrigerated storage.They flower the following spring regardless of climate.They will not bloom more than 1 year unless growing in a climate where they receive yearly natural vernalization.
28STAGES OF PLANT MATURATION Maturity Daily night duration is a second environmental factor that can control flowering in plants.Nights are longest in winter, shortest in summer.Intermediate in spring and fall.The further north or south of the equator, the more extreme the difference between the shortest summer night and longest winter night.Plants that respond to these changes in the night length are called photoperiodic.Common in all but tropical climates near the equator.
29STAGES OF PLANT MATURATION Maturity Two main types of photoperiodic plants have been studied intensively.Commonly called short-day and long-day plants.Actually it is the length of the night that triggers flowering.The confusing designation stems from the first research on photoperiod.It was mistakenly believed length of light rather than darkness caused flowering.
30STAGES OF PLANT MATURATION Maturity Short-day plants flower only when the daily light period is less than their critical photoperiod.For example, 12 hours.Long-day plants will flower only if the light period is more than their critical photoperiod.Or by cutting the dark period into two shorter periods.They can also flower under continuous light.Or during a long dark period interrupted by a short light period.In an otherwise long night, a short period of light has the effect of cutting the dark period into two shorter periods.If each half is less than the critical period, the plant responds as if it were exposed to only short nights & flowering takes place
31STAGES OF PLANT MATURATION Maturity Figure 3-5 The flowering responses of long-day plants to varying dark/light ratios.
32STAGES OF PLANT MATURATION Maturity The way in which night length triggers flowering involves a pigment called phytochrome.Alternates between two forms, each with sensitivity to differing wavelengths of light in the red to far-red range.Light intensity can also control flowering.Plants that are day neutral and not affected by night length frequently respond to intensity.Greater light intensity is required to induce flowering than is sufficient to keep a plant growing vegetatively.Many houseplants bought for their blooms never flower again after being moved from a greenhouse to indoors.The light intensity is simply too low to induce flowering.
33STAGES OF PLANT MATURATION Maturity Like induction, flower initiation is invisible to the naked eye.Changes are taking place in microscopic parts of the plant.During this phase, vegetative meristems at the stem tips or leaf axils change to flower meristems.Capable of developing into blossoms.This change takes place over several days or weeks.
34STAGES OF PLANT MATURATION Maturity Small knobs of cells emerge in a spiral around the center.These are the beginnings of petals, stamens, & other flower parts.The meristem stops lengthening, but development continues.Figure 3-6 Electron microscope photograph of a developing flower meristem of an Easter lily. Photo courtesy of Dr. H. Paul Rasmussen, Logan, Utah.
35STAGES OF PLANT MATURATION Maturity The final stage of development is opening of the flower, after which it is receptive to pollination.Whether it pollinates itself (self-pollination) or is cross-pollinated by another plant will affect the resulting seeds.
36STAGES OF PLANT MATURATION Maturity During pollination, dust-size pollen grains released from the anthers are deposited by wind or insects on the stigma.Figure 3-7 Germinating pollen grains growing toward the ovary. These female parts of a flower are collectively called the pistil.Like seeds, they germinate & grow downward toward the ovary where the eggs are locatedAfter reaching the ovary, the uniting of sperm (contained in the pollen) and eggs (in the ovary) occurs in the process of fertilization.
37STAGES OF PLANT MATURATION Maturity After reaching the ovary, the uniting of sperm (contained in the pollen) and eggs (in the ovary) occurs in the process of fertilization.In some instances pollen produced by a flower may be unable to fertilize eggs produced by that flower.Self-incompatibility is in about 40% of cultivated plants.In some cases the pollen grain simply never germinates, and in others it germinates but grows poorly, never reaching the egg.Thus cross-pollination is quite common.
38STAGES OF PLANT MATURATION Maturity Cross-pollination can be a hindrance as well as an advantage…Sweet corn & field corn can cross-pollinate.Making the sweet corn taste like corn raised for grain.Sweet and hot peppers also will cross-pollinate.Growing a mild pepper with hot-tasting seeds or other changes.Although egg fertilization usually must occur before a fruit will develop, in some cases it is not necessaryAs in when the sperm does not fertilize the egg, but the fruit still develops, resulting in a seedless fruit.Seedless fruits formed in this way are called parthenocarpic.
39STAGES OF PLANT MATURATION Maturity Parthenocarpy can be caused by growth-regulating chemicals or specific environmental conditions.Excessively warm temperatures during pollination can cause seedless tomatoes to form.Parthenocarpic seedless fruits also form by spontaneous death of the seed embryo in early developmental stages.A majority of cultivated fruits need living seeds are necessary for the fruit to develop normally.
40STAGES OF PLANT MATURATION Maturity If only half the eggs in an apple are fertilized, only one side will grow, which produces misshapen fruit.Figure 3-8 A greenhouse-grown cucumber that was not fully pollinated & failed to develop normally. Courtesy of the Cooperative Extension Service of the University of California.Cucumbers can have this problem, with a portion staying small while the remainder develops normally.In cases where the fruit contains only one seed, fruit will drop prematurely if the embryo dies.
41STAGES OF PLANT MATURATION Maturity During fruit development, sugars photosynthesized in the leaves constantly flow into the fruit.Supplying energy for the developing ovary, which will become the fruit.When a fruit reaches the end of its enlargement period, ripening begins.Caused by the production of ethylene gas, a hormone produced by the fruit.It may become soft or change color.Flavor can change from sour to sweet.
42STAGES OF PLANT MATURATION Maturity Softening of fruits is the result of the breaking down of compounds called pectic substances, which strengthen cell walls and cement cells together.In over-ripening, mushiness occurs because too much pectic substance has been lost.Mealiness in apples results from such over-ripening.
43STAGES OF PLANT MATURATION Maturity Color changes result from the breakdown of chlorophyll & accumulation of other pigments.Chlorophyll decreases & other pigments increase intensity.Carotene is a pigment that gives orange color to fruits such as oranges and persimmons.Anthocyanin is the pigment responsible for the red color of ripe strawberries and apples.
44STAGES OF PLANT MATURATION Senescence Senescence is aging of a plant or any of its parts.Part of the natural life cycle of the plant, or as a result of environmental factors.Natural life span often determines when senescence begins.Characterized by dramatic changes in metabolism:Increased respiration.Decreased photosynthesis.Breakdown of larger molecules into smaller ones.Fats, proteins, or carbohydrates.
45STAGES OF PLANT MATURATION Senescence Annual plants begin senescence at flowering, and will die soon after seed formation is complete.Senescence after reproduction also happens in monocarps that live several years before flowering.Senescence of perennial plants is frequently called decline.Asparagus is a perennial that suffers from decline.Productive life of asparagus plants is generally 20 to 25 years.As plants reach this age, yield drops off & the bed is replanted.
46STAGES OF PLANT MATURATION Senescence An entire plant need not die for the term senescence to be applied.Top portions of herbaceous perennials & bulbs senesce annually, but the root system remains alive.A colorful show of leaf senescence occurs during the fall when trees turn color.Coloring is due to the same pigment changes that occur with the ripening of fruit.Shortening of day length in fall & cooler temperature trigger this senescence.
47STAGES OF PLANT MATURATION Senescence The dropping of leaves, flowers, fruits, or other plant parts is called abscission.Involves the manufacture hormones and the formation of a zone of abscission.Figure 3-9 A leaf just prior to abscission.In simple leaves, this zone is formed at the point where the petiole connects to the stem.In compound leaves, the main abscission zone is also at the point where the petiole reaches the stem.Individual leaflets may form abscission layers & drop.
48STAGES OF PLANT MATURATION Senescence Flower abscission is often related to pollination.Pollen contains auxin, which it carries to the pistil, triggering abscission of the flower parts.As petals, stamens, stigma & style are no longer needed, their abscission diverts more carbohydrate to the developing fruit.Fruit abscission can occur at any point in development but is most common after ripening.Dropping of ripe fruit is a type of seed dispersal, with an abscission layer generally forms before a fruit drops.
49STAGES OF PLANT MATURATION Dormancy Dormancy is a stage of plant development in which growth slows or stops, affecting all life stages from seed to maturity.Important in adapting plants to environment & ensuring survival.Dormancy occurs during periods not suitable for plant growth, and is usually seasonally related.Winter dormancy is found in plants in cold-winter areas.Dry-season dormancy is found in areas of distinct wet and dry seasons, such as California.
50STAGES OF PLANT MATURATION Dormancy While photosynthesis can happen at temperatures below freezing, dry winter air can cause plant damage or death.Especially if groundwater is frozen and unavailable.Limited surface area of many evergreen leaves has adapted them to minimize water loss.In other plants, leaf abscission is an indicator of dormancy, signaling the onset of winter.Leaves of these plants cannot withstand subfreezing temperatures, so they abscise in autumn.New leaves survive as buds.
51STAGES OF PLANT MATURATION Dormancy The changeover from dormancy to active growth is called breaking dormancy.Normally results from changing environmental conditions.Seeds usually enter dormancy just prior to fruit abscission, or senescence of the parent plant.Conditions needed to break dormancy & start germination are specifically geared to ensure seedling survival.Many seeds require winter vernalization.
52STAGES OF PLANT MATURATION Dormancy Simulated vernalization is sometimes used to break dormancy in commercial nursery crop production.Called stratification, it involves storing moist seeds at temperatures near freezing for one or more months.Peach & apple tree seeds are stratified before planting.Heat can break dormancy in some seeds.Forest fires destroy existing trees, but also trigger the germination of dormant pine seeds.Only intense forest fire heat will weaken seed coats of some pine seeds that are woody and thick.Most seeds require less drastic measures to break dormancy.
53STAGES OF PLANT MATURATION Dormancy Usually the seed coat erodes enough by weather to permit germination.It can also be weathered artificially by scarification.Cutting, scraping, or otherwise injuring the seed coat enough to allow water absorption & germination.Chemical scarification can weaken the seed coat, such as through the swallowing of seeds.Acid of the stomach eats away at the seed coat, and the seed is deposited along with the feces of the animal.A sort of built-in fertilizer.
54HORMONES & GROWTH-REGULATING CHEMICALS Plant hormones are chemicals, made within the plant, that produce changes in growth.Cause root formation, seedless fruit formation, leaf drop, and stem lengthening.Plant growth regulators are synthesized chemicals like plant hormones, which have the same effects.May be chemically quite similar to hormones.But they do not occur naturally.Nine groups of plant hormones have been identified.It is likely that others will be discovered eventually.
55HORMONES & GROWTH-REGULATING CHEMICALS Scientists researching plant hormones encounter a number of difficulties.Effects of hormones are not the same, species to species.Slight changes in hormone concentrations can alter their effects completely.Two or more hormones are frequently found together.It is difficult to determine which chemical is responsible for a particular effect.
56HORMONES & GROWTH-REGULATING CHEMICALS See the entire list on page 41 your textbook.
57HORMONES & GROWTH-REGULATING CHEMICALS Auxins Auxins, the first hormones discovered have such diverse effects as…Promotion of rooting.Formation of underground tubers and bulbs.Prevention of fruit formation, defoliation.Prevention of abscission of leaves and fruits.Auxins promote juvenility.Auxin made in the terminal bud is responsible for suppressing the sprouting of axillary buds further down the stem of a plant.
58HORMONES & GROWTH-REGULATING CHEMICALS Auxins Synthetic auxins, available in garden centers, have a number of practical home uses.Powders sold to encourage rooting of cuttings are composed of manufactured auxins in a talcum base.2,4-D, sold to control lawn weeds is an auxin which kills many plants at high concentrations.At very low concentrations, it is a growth enhancer.High auxin concentrations can prevent fruit on ornamental trees when it is an undesirable feature.In commercial agriculture, synthetic auxins defoliate plants before harvest, prevent sprouting of potatoes in storage, and prevent premature orchard fruit drop.
59HORMONES & GROWTH-REGULATING CHEMICALS Gibberellins Activity associated with gibberellins:Stem elongationBreaking dormancy of seeds, buds, and tubers.Increases in flower, leaf, and fruit size.Inducing flowering in plants that normally require vernalization or a specific photoperiod.Gibberellins are used in greenhouses to...Form tall tree-form fuchsias and geraniums from cuttings.Increase the size of grapes by elongating flower parts.Substitute for vernalizing azaleas and fruit trees in the South.
60HORMONES & GROWTH-REGULATING CHEMICALS Cytokinins Cytokinins are believed to work in conjunction with light to increase cell division and enlargement.Also been shown to prevent chlorophyll degeneration and break axillary bud dormancy.Their only commercial horticulture use is in tissue culture, where they stimulate callus growth.
61HORMONES & GROWTH-REGULATING CHEMICALS Growth Retardants Synthetic and natural growth retardants are sold under the trade names A-Rest®, B-Nine®, Bonzi®, Sumagic®, and Cycocel® .Used on florist crops such as poinsettia & chrysanthemum.They slow elongation of stems, making sturdier, fuller plants.On fruit crops they improve color, firmness & storage life.
62HORMONES & GROWTH-REGULATING CHEMICALS Growth Retardants Growth retardants can be used on hedges and lawns to slow growth and decrease maintenance.They can also be used to maintain bedding plants in a compact size, giving plants a neater appearance in formal landscapes.Figure 3-10 A shrub treated (left) and untreated (right) with Atrimmec® growth retardant. Courtesy of PBI/Gordon Company, Kansas City, Mo.TREATED UNTREATED
63HORMONES & GROWTH-REGULATING CHEMICALS Growth Retardants Abscisic acid (ABA) is a growth retardant that may induce abscission and does induce dormancy and inhibit seed germination.Its action can be counteracted by growth-inducing chemicals such as auxins, gibberellins, and cytokinins.Appears to be very involved with stress resistance.
64HORMONES & GROWTH-REGULATING CHEMICALS Growth Retardants A number of growth retardants have also been used to control lawn growth & reduce mowing frequency.Maleic hydrazide & mefluidide reduce growth or stop flowering.EPTC and amidochlor reduce growth or flowering.But cannot stop it completely.These chemicals must be used carefully, and the results are not always predictable.Unacceptable color loss, injury and lessened ability of the turf to recover from disease & pest problems can result.
65HORMONES & GROWTH-REGULATING CHEMICALS Growth Retardants Ethylene is a retardant, with use dates to Chinese practices of ripening fruits in incense-filled rooms.Later determined to be ethylene gas given off in the fumes that caused the accelerated ripening.Ethylene is also produced by the ripening fruits themselves, wounded plant parts or by cut flowers.It can be manufactured.Ethylene also ages plant parts such as flowers, causing them to abscise, and induces flowering in a limited number of species.
66HORMONES & GROWTH-REGULATING CHEMICALS Growth Retardants Pineapples and other members of the bromeliad family will bloom when treated with ethylene.In commercial greenhouses, the product used for ethylene generation is Florel® or ethephon.
67HORMONES & GROWTH-REGULATING CHEMICALS Pinching Agents Pinching agent chemicals are used commercially to kill terminal vegetative buds.To promote branching & a more bushy, attractive plant.OffShoot-0® & Atrimmec® are mainly used for this purpose.
68HORMONES & GROWTH-REGULATING CHEMICALS Vitamins Vitamins are sold occasionally as stimulants for plant growth and for use after transplanting.The same vitamins sold for human use.But act more as hormones in plants.Particularly B vitamins.Vitamin effectiveness has not been determined fully.When used on bean seeds, limited experiments have shown improved germination rates.And decrease the time from seed sowing to harvest.Increases in yield have been reported on some species.
69HORMONES & GROWTH-REGULATING CHEMICALS Vitamins Overall, it is very possible that vitamins do improve plant growth in some cases.They should never be used in place of fertilizers or proven-effective chemicals sold for use on plants.