Horticulture Exam 2

Photosynthetically active radiation (PAR)

Light spectrum plants can see

Sun loving plants

Evolved to grow in areas of high light —> might survive in shade but will likely display light seeking behavior

Etiolation

Classic light seeking behavior —> leggy plants that look tall and stretched (internodal stretching) and pale

Heliotropism

Light seeking behavior where plants stretch and turn towards light

Phytochrome

Photoreceptor that detects changes in R:Fr

R:Fr

Red light : Far red light

Pfr

Active form of phytochrome —> inhibits internodal stretching, causes leaf and chloroplast development

Pr

Inactive form of phytochrome

Shade loving plants

Evolved in areas with more continuous shade —> thin and broad leaves to maximize photosynthesis, less sensitive to R:Fr

Abscisic acid (ABA)

Plant hormone responsible for water conservation —> when root tips detect soil dryness they produce more ABA, signals stomata to close

Temporary wilt (aka “Sun wilt”)

Caused by a plant losing water by rapid transpiration on a hot, dry day (not soil dryness) —> will recover overnight when stomata close

Permanent wilt

Caused by low levels of water in the soil

Drought

Prolonged period of low rainfall leading to a shortage of water —> may occur randomly, seasonally, or semi-permanently depending on location

Drought avoidance

Mechanism that some plants have evolved to withstand drought conditions by avoiding them

4 major types of drought avoidance

1) Germination inhibition —> seeds are coated with water soluble germination inhibitor hormone, water must wash away coating

2) Geophytes —> go dormant and lose all above ground foliage after blooming, avoid losing water via transpiration during drought

3) Succulents —> store water in stems and leaves, shallow roots

4) Deep roots —> allow plants to harvest deep ground water reservoirs

Drought tolerance

Mechanism that some plants have evolved to withstand drought conditions —> can endure more water stress in their cells than other plants

6 drought tolerance adaptations

1) Sunken stomata

2) Reflective leaves

3) Small leaves

4) Leaf hairs (trichomes)

5) Ability to shed leaves during times of drought

6) C4 and CAM photosynthesis

C4 and CAM photosynthesis

Adapted by drought tolerant plants that cannot rely on their stomata being open during the day because of the risks of transpirational water loss —> use other means of harvesting CO₂ for photosynthesis

PEP carboxylase

Important enzyme that catalyzes carbon fixation in C4 and CAM photosynthesis

C4 photosynthesis

Open and close stomata throughout the day depending on rate of water loss (common in monocot crops) —> CO₂ captured by PEP carboxylase in mesophyll and fixed into 4 carbon chain

CAM (Crassulacean Acid Metabolism) photosynthesis

Stomata closed during the day and open at night (common in succulents), think waxy cuticle helps withstand high temperatures (no transpiration to cool during day) —> CO₂ collected at night, fixed by PEP carboxylase into malic acid and accumulated in vacuoles, malic acid is broken down to create CO₂ during the day

Halophytes

Plants that have adapted to tolerate salt water —> some have specialized glands on their roots filter salt out of water, others store excess salt in vacuoles or secrete it from specialized glands on leaves

Aerenchyma

Specialized tissue made up of long interconnected gas-filled spaces that span from shoot to root —> plants with roots in aqueous/low oxygen soil harvest atmospheric gas via stomata and shuttle oxygen to their roots through aerenchyma

Aquatic plants

Plants that have evolved several strategies for growing in aqueous environments —> most have stomata on top surface of leaves (not undersides), some have gas filled sacs that allow them to float on water’s surface, some have long petioles that stretch so leaves float atop water, some have no stomata (submerged, harvest gas using specialized glands)

Pest

any organism that is undesirable in a gardener’s environment —> subjective

Pesticide

Catch-all term for chemical used to eradicate a pest

Herbicide

Chemical treatment to kill weeds

Insecticide

Chemical treatment to kill insects

Acaricide

Chemical treatment to kill spiders and spider mites

Fungicide

Chemical treatment to kill fungal pathogens

Ovicide

Chemical treatment to kill the eggs of insects and mites

Pathogen

Fungal, bacterial, or viral disease

Pest insect

Cause plant damage by feeding on the sap, leaves, stems, roots, flowers, or pollen

4 invasive characteristics of pest insects

1) Rapid reproduction

2) Complex lifecycle makes it hard to eradicate at all life stages at once

3) Most are small, fast, experts at hiding

4) Some create nests in the cracks and crevices where they cannot be seen until populations explode

Mites

• Suck sap from foliage with stippling mouthpart —> create stippling marks

• Pale and mottled leaves

• Severe infestation —> webbing at shoot tips

• Thrive in hot, dry environments —> hate light and getting wet

• Beneficial = predatory mites

Whitefly

• Feed on plant sap

• Notorious for spreading damaging plant viruses

• Live and lay eggs on underside of leaves

• Beneficial = parasitic wasps (larvae hatch and eat whitefly eggs)

Scale

• Adults are non-motile

• Hard shell —> can extend into plant tissue

• Adults lay eggs underneath their shell —> make small opening for juvenile crawling scale

Thrips

• Eat plant tissue, sap, and pollen with their rasping mouthpart

• White patches and streaks on leaves

  • Undersides usually contain black specks (thrip waste)

• Pupate in soil —> makes infestation hard to eradicate

• Adults are fast but weak fliers

Aphids

• Suck sap —> produce sticky “honeydew” waste that attracts other pests and pathogens

• Soft-bodied and slow

• Give live birth and born pregnant —> populations boom overnight

• Easily washed away

• Often concentrated around new growth

Mealybugs

• Soft-bodied and slow

• Squeeze into tiny cracks and crevices

  • Build cottony protective nests

Nematodes

• Microscopic roundworms

• Live in soil or within plant

• Have a small mouthpart that allows them to suck sap from plant cells

Vector

Organism that spreads disease

Beneficial insect

Helps reduce pest insect populations by eating or parasitizing them

Minute pirate bug (beneficial)

• Equal opportunity omnivore —> eats many types of soft-bodied insects and pollen

• Can eat 45 thrips in a day

• Loves ornamental pepper flowers

Green lacewing larvae (beneficial)

• Big, fast, very hungry

• Will eat any soft-bodied organism (even each other)

• Can eat 60 aphids in a day

Beneficial nematodes

Can eat pests living in the soil —> mixed with water and drenched into potting media

Disease triangle

3 points must be present for a pest or pathogen outbreak to occur…

1) Plant host

2) Pest or pathogen

3) Ideal environmental conditions for host and pathogen to survive

Fungal pathogen

Grow between or through plant cells secreting enzymes, toxins, or hormones that cause disease symptoms —> harvest nutrients released as plant cells die —> reproduce sexually or asexually via spores that are created in the fruiting body of fungus

Rust (fungal pathogen)

• Appears on leaves

• Characteristic rusty color

• Looks raised, grainy texture

• Control

  • Reduce leaf wetness

  • Biofungisides

  • Prune affected foliage

Mildew

• Characteristic silver/gray blotches look raised and fuzzy

• May contain black dots above or below leaf (fungal fruiting body)

• Spreads quickly via splashing water

• Sooty mildew —> indicative of sap-sucking insects

• Control

  • Reduce leaf wetness

  • Prune affected foliage

  • Biofungisides

Hyphae

Tiny tube-like structures that fungal pathogens use to invade plant cells/intercellular space

Fruiting body

Spore producing structure created by fungus —> used to diagnose specific fungal pathogens (rest of body can be difficult to ID)

Viral pathogen

Cannot move, must be introduced to a plant via a vector —> extract nutrients from living cells without killing them, use plant cells to reproduce

Mosaic virus

• Blotchy, mottled, stunted foliage

• Spread via vector

• Control

  • Sterilize plants, tools, and soil before greenhouse introduction

  • Reduce pest populations to control vectors

  • Destroy heavily affected plants

Bacterial pathogen

Grow in or between plant cells in slimy single-cell masses, secreting enzymes and toxins to kill plant tissue and harvest nutrients the cells release as they die —> favor warm, wet conditions, often spread by water (or via a vector)

Bacterial leaf spot

• Foliar infection with a characteristic yellow halo

• Spreads via splashing water

• Control

  • Reduce leaf wetness

  • Space for increased air flow

  • Prune affected foliage

Bacterial fruit blotch

Water soaked spots —> lesions expand rapidly

Bacterial, stem, and crown rot

• Soil borne infection affecting the vascular tissues

• Water-soaked lesions giev rise to open cankers

• Crown rot will collapse small or rosette plants

• Wilt diseases —> entire plant wilts and dies despite adequate water

Weeds

Pest plants that have evolved to quickly overtake and out-compete surrounding plants

7 weed strategies

1) Produce massive quantities of seeds

2) Rapid germination

3) Reproduction via runners

4) Tolerance to temperature and precipitation extremes

5) Vining and choking behaviors

6) Thorns or other irritants that make them hard to remove

7) Extensive root systems

Cultural control methods

“Best practices” of growing, inhibit and prevent pest/pathogen infestation

8 cultural control strategies

1) Plant selection for pest/pathogen resistance

2) Soil management

3) Crop rotation

4) Spacing for air flow

5) Pruning

6) Manipulation of environmental controls

7) Hygiene

9) Quarantine

Physical control methods

Physical removal of a pest or physical barriers to prevent pest infestation

Examples

• Picking caterpillars off by hand

• Installing weed cloth

• Applying horticultural oils and soaps

Horticultural soaps and oils

Clog insect pores and cause them to suffocate —> plants vary in tolerance to their application

Biological control methods

Using natural predators to reduce pest populations (beneficial insects)

Selective pesticide

Targets a specific pest

Broad spectrum pesticide

Kills most or all pests (including beneficial organisms)

Contact pesticide

Kills pest on contact —> either immediately or over a series of days

Systemic insecticide

Chemical is taken up by the plant and distributed via the phloem to all plant parts —> renders plant toxic to herbivores

Targeted spraying

Spraying of specific plants that are being affected by a pest or pathogen

Broadcast spraying

Spraying of entire crop, regardless of pest or pathogen presence on individual organisms

Pesticide resistant organism

Strain of pest/pathogen that is not affected by pesticide —> result of consistent pesticide over use

Integrated pest management (IPM)

Series of strategies for growing plants without relying on chemical pesticides (in response to pesticide overuse in 1980s)

1) Set action threshold

2) Monitor and identify pests (physical control)

3) Prevention (cultural control)

4) Control —> evaluate proper control methods for both effectiveness and risk (pesticides are last resort)

Re-entry interval (REI)

Amount of time after spraying a pesticide that it is safe for a person not wearing PPE to enter the greenhouse

Mode of action (MOA)

How a pesticide kills a pest, on a physiological level —> rotating chemicals with different MOAs when spraying is key to avoid pesticide resistance

USDA APHIS (Animal and Plant Health Inspection Service)

Prevent the movement of the most harmful plant pests and pathogens —> inspection stations at major airports and seaports to oversee all plant product import/export

Size

In horticulture refers to height and width of plant

Form

Refers to lateral growth and overall shape

Gibberellins

Group of plant hormones responsible for stem elongation and branching

Auxins

Group of plant hormones responsible for rooting and cell elongation

Cytokinins

Group of plant hormones responsible for cell division

Phototropins

Type of photoreceptor that moves auxins out of tissue that is in direct sun

Dwarf variety

Plants bred to be miniatures of wild type of same species —> lower concentration of gibberellins

Apical dominance

Phenomenon whereby main/central stem of plant grows more strongly than side/lateral branches —> controlled by auxin (produced in terminal bud and flows down to roots), inhibits development of lateral buds as it moves downward

Plant growth regulators (PGRs)

Chemicals applied as foliar spray that alter form of plant —> less labor intense than pruning and will not affect bloom time

Growth retardant PGR

Chemicals that inhibit action of gibberellins to create shorter, less leggy plants (effects wear off with time)

Growth enhancement PGR

Synthetic gibberellins applied to increase stem length and encourage branching

Root:shoot

Ratio of root biomass to shoot biomass —> plant maintains fairly constant root:shoot ratio, pruning one without the other will not reduce plant size

Notching

Cutting off a small wedge of tissue directly above lateral bud to encourage its development —> reduces flow of auxins from terminal bud

Nicking

Cutitng off a small wedge of tissue directly below a lateral bud to discourage its development —> damaging vascular tissue reduces flow of water and nutrients

Dwarf rootstock

Many dwarfs are created by grafting a desirable scion onto a dwarf rootstock —> dwarf rootstock won’t grow very large, limits growth of aboveground biomass

Cordon

Technique common in fruit trees, reduced to a single stem and grown on an angle —> eliminating branches makes the trees easy to spray and allows fruit to get light from all sides (encourages larger fruit)

Espalier

Ornamental technique in which several horizontal branches on each side of the trunk are allowed to develop (sometimes in a pattern) along a wall/fence

Coppice

Tree is cu down to a foot or less above ground level —> encourages fast growth of many new canes emerging from coppiced trunk

Pollard

All upper branches of tree are cut off —> keeps flowers on ornamental trees at eye level (Crepe myrtles)

Topiary

Form of garden art where bushes are pruned to a decorative shape

Bonsai

Centuries old art form where trees are intentionally dwarfed and pruned in such a way as to resemble a large, old tree