Reproduction in Plants

Alternation of Generation Plant life cycles undergo alternation of generation of two multicellular stages This involves alternating between diploid (2N) and haploid (1N) generation The sporophyte is diploid, and in all angiosperms, the roots, stems, leaves and flowers are part of the sporophyte generation

Flower Structure Flowers Are composed of four floral organs: sepals, petals, stamens, and carpels, which are modified leaves

Sepals - green leaflike parts that protect developing flower

Stamen is the male reproductive organ The anther located at the tip of the stamen, is where meiosis occurs to produce pollen grains Pollen grains contain sperm Carpel (pistil) is the female reproductive organ The ovary located at the base of the carpel contains ovules

Flowers that lack one or more of the four parts are incomplete flowers Flowers that contain both stamens and carpels are said to be perfect, those with only stamens or carpels are imperfect

Staminate flowers have only stamens (imperfect) Carpellate flowers contain only carpels (imperfect) Flowers that are imperfect, and contain both male (staminate) and female (carpellate) flowers on the same plant are monoecious

Many variations in floral structure Have evolved during the 140 million years of angiosperm history

Pollination Pollination is the transfer of pollen from an anther to the stigma of a carpel Self-pollination occurs if the pollen is from the same plant Pollination can occur by wind, water, insects, birds, bats, and other mammals

Pollination enables gametes to come together within a flower In angiosperms, the dominant sporophyte Produces microspores that develop within flowers into male gametophytes (pollen grains)

Pollen Develops from microspores within the sporangia of anthers

Embryo sacs Develop from megaspores within ovules

Double Fertilization The ovule contains the embryo sac which is the female gametophyte Pollination must precede fertilization,most angiosperms depend on animals for fertilization The tube cell gives rise to the pollen tube, and the generative cell divides to form two sperm One sperm fertilizes the egg to become the zygote, and the other fertilizes the large diploid cell to become the triploid (3N) endosperm which nourishes the embryo

Gametophyte Development and Pollination In angiosperms If pollination is successful, a pollen grain produces a structure called a pollen tube, which grows down into the ovary and discharges sperm near the embryo sac

Growth of the pollen tube and double fertilization

After double fertilization From Ovule to Seed After double fertilization The ovary develops into a fruit enclosing the seed(s)

Endosperm Development Usually precedes embryo development In most monocots and some dicots The endosperm stores nutrients that can be used by the seedling after germination In other dicots

Structure of the Mature Seed The embryo and its food supply

In a common garden bean, a dicot

The seeds of other dicots, such as castor beans Have similar structures, but thin cotyledons

The embryo of a monocot Has a single cotyledon, a coleoptile, and a coleorhiza

From Ovary to Fruit A fruit Develops from the ovary Protects the enclosed seeds

Fruit Types and Seed Dispersal Simple Fruits Simple fruits are derived from single or several united carpels Legumes are fruits that split along two sides when mature Dehiscent - Split open

Simple Fruits Fleshy Dry Drupe Berry Pome Follicle Legume Capsule Achene Nut Grain

Simple Fruits Dispersal Many seeds are dispersed by wind Woolly hairs, plumes, wings Fleshy fruits - Attract animals and provide them with food Peaches, cherries, tomatoes Accessory fruit - Bulk of fruit is not from ovary, but from receptacle

Compound fruits develop from several individual ovaries Aggregate Fruits Ovaries are from a single flower Blackberry Multiple Fruits

Fruits are classified into several types Depending on their developmental origin

Seed Germination As a seed matures It dehydrates and enters a phase referred to as dormancy

Environmental requirements for seed germination Availability of oxygen for metabolic needs Adequate temperature for enzyme activity Adequate moisture for hydration of cells Light (in some cases)

Length of time seeds retain their viability is quite variable Seed Germination Length of time seeds retain their viability is quite variable Some seeds do not germinate until they have been through a dormant period Temperate zones - Cold Weather Deserts - Rain

Seed Germination In some seeds, the seed coat (testa) must be disrupted or scarified before water uptake Often must pass through digestive tract of animals

Seed Dormancy: Adaptation for Tough Times Increases the chances that germination will occur at a time and place most advantageous to the seedling The breaking of seed dormancy

Seed metabolism during germination Uptake of water Gibberellin released after water uptake Amylase results in the hydrolysis of starch (stored in endosperm or cotyledons) into maltose Maltose broken down into glucose which can be used in cellular respiration or to build cellulose for cell walls

From Seed to Seedling Germination of seeds depends on the physical process called imbibition, the uptake of water due to low water potential of the dry seed The hydrated seed expands, ruptures the seed coat, and triggers metabolic changes that make the embryo start to grow

The radicle In many dicots Is the first organ to emerge from the germinating seed In many dicots A hook forms in the hypocotyl, and growth pushes the hook above ground

Monocots The coleoptile Use a different method for breaking ground when they germinate The coleoptile

Phytochromes as Photoreceptors

A phytochrome is the photoreceptor responsible for the opposing effects of red and far-red light

The pigment phytochrome responds to the presence or absence of light by changing forms between Pred (Pr) and Pfar red (Pfr) Pr is changed by exposure to red light to become Pfr Pfr is changed back to Pr by exposure to far-red light, conversion in darkness, or enzymatic degradation The leaves produce a hormone that induces flowering in response to levels of Pfr

Phytochromes exist in two photoreversible states With conversion of Pr to Pfr triggering many developmental responses

Biological Clocks and Circadian Rhythms Many plant processes Oscillate during the day

Many legumes lower their leaves in the evening and raise them in the morning

Many plants open their flowers during the day, and close them at night

Cyclical responses to environmental stimuli are called circadian rhythms and are approximately 24 hours long Circadian rhythms occur with or without external stimuli such as sunrise and sunset The light-dark cycle of day and night provide cues that fine tune biological clocks and keep them precisely synchronized to a period of exactly 24 hours This biological clock is located in a cluster of nerve cells located in the hypothalamus of the brain in humans and mammals

Researchers have identified genes that control biological clocks in rodents, fruit flies, bacteria, and some plants These genes encode a transcription factor that builds up over time At high concentrations, the same gene is turned off When concentrations fall, transcription begins again

The Effect of Light on the Biological Clock Phytochrome conversion marks sunrise and sunset

Photoperiodism and Responses to Seasons Photoperiod, the relative lengths of night and day Is the environmental stimulus plants use most often to detect the time of year Photoperiodism

Photoperiodism and Control of Flowering Some developmental processes, including flowering in many species

Short day plants flower when day length is below a critical value, or more correctly, short day plants flower when the length of night exceeds the minimum critical night length Short day plants are really long night plants Long day plants bloom when day length exceeds the critical value, or when the length of night does not exceed the maximum critical night length Long day plants are short night plants

Critical Night Length In the 1940s, researchers discovered that flowering and other responses to photoperiod Are actually controlled by night length, not day length

Phytochrome is the pigment that receives red light, which can interrupt the nighttime portion of the photoperiod

Short bursts of red or white light can cause the critical night length (length of continuous darkness) to be interrupted A flash of far-red light reverses this effect and does not interrupt length of continuous darkness

A Flowering Hormone? The flowering signal, not yet chemically identified