Sensory Systems in Plants

http://plantsinmotion.bio.indiana.edu/plantmotion/movements/tropism/tropisms.html

Outline

• Photomorphogenesis

• Phototropisms

• Gravitropisms

• Thigmotropism and Thigmonasty

• Turgor Movement

• Dormancy

• Surviving Temperature Extremes

• Plant Hormones

Introduction

• At every stage in the life of a plant, sensitivity to the environment and coordination of responses are evident.

• One part of a plant can send signals to other parts.

• Plants can sense gravity and the direction of light.

• A plant’s morphology and physiology are constantly tuned to its variable surroundings by complex interactions between environmental stimuli and internal signals.

• At the organismal level, plants and animals respond to environmental stimuli by very different means

• Animals, being mobile, respond mainly by behavioral mechanisms, moving toward positive stimuli and away from negative stimuli.

• Rooted in one location for life, a plant generally responds to environmental cues by adjusting its pattern of growth and development.

• Plants of the same species vary in body form much more than do animals of the same species.

• At the cellular level, plants and all other eukaryotes are surprisingly similar in their signaling mechanisms.

• All organisms, including plants, have the ability to receive specific environmental and internal signals and respond to them in ways that enhance survival and reproductive success.

• Like animals, plants have cellular receptors that they use to detect important changes in their environment.

• These changes may be an increase in the concentration of a growth hormone, an injury from a caterpillar munching on leaves, or a decrease in day length as winter approaches.

• In order for an internal or external stimulus to elicit a physiological response, certain cells in the organism must possess an appropriate receptor, a molecule that is sensitive to and affected by the specific stimulus.

• Upon receiving a stimulus, a receptor initiates a specific series of biochemical steps, a signal transduction pathway.

• Plants are sensitive to a wide range of internal and external stimuli, and each of these initiates a specific signal transduction pathway.

• This couples reception of the stimulus to the response of the organism.

Plants Respond to Light

• Photomorphogenesis

• nondirectional, light-triggered development

• red light changes the shape of phytochrome and can trigger photomorphogenesis

How Phytochrome Works

Phototropisms

• Phototropic responses involve bending of growing stems toward light sources.

• Individual leaves may also display phototrophic responses.

• auxin most likely involved

Plants Respond to Gravity

• Gravitropism is the response of a plant to the earth’s gravitational field.

• present at germination

• auxins play primary role

• Four steps

• gravity perceived by cell

• signal formed that perceives gravity

• signal transduced intra- and intercellularly

• differential cell elongation

Gravitropism

• Increased auxin concentration on the lower side in stems causes those cells to grow more than cells on the upper side.

• stem bends up against the force of gravity

• negative gravitropism

• Upper side of roots oriented horizontally grow more rapidly than the lower side

• roots ultimately grow downward

• positive gravitropism

Plants Respond to Touch

• Thigmotropism is directional growth response to contact with an object.

• tendrils

Other Tropisms

• Electrotropism - electricity

• Chemotropism - chemicals

• Traumatropism - wounding

• Thermotropism - temperature

• Aerotropism - oxygen

• Skototropism - dark

• Geomagnetotropism - magnetic fields

Turgor Movement

• Turgor is pressure within a living cell resulting from water diffusion.

• After exposure to a stimulus, changes in leaf orientation are mostly associated with rapid turgor pressure changes in pulvini.

• multicellular swellings located at base of each leaf or leaflet

• turgor movements are reversible

Circadian Clocks

• Circadian clocks are endogenous timekeepers that keep plant responses synchronized with the environment.

• circadian rhythm characteristics

• must continue to run in absence of external inputs

• must be about 24 hours in duration

• can be reset or entrained

• can compensate for temperature differences

Dormancy

• The ability to cease growth and enter a dormant stage provides a survival advantage.

• environmental signals initiate and terminate dormant phases

• freezing

• drought

• dormancy may last extremely long period of time

Surviving Temperature Extremes

• Chilling and freezing acclimation rely on high levels of unsaturated fatty acids, supercooling, and synthesis of antifreeze proteins.

• heat shock proteins stabilize proteins at high temperatures to avoid folding errors

Plant Hormones

• Many internal signaling pathways involve plant hormones.

• substances produced in small quantities by an organism, and then transported elsewhere for use

• have capacity to stimulate and/or inhibit physiological processes

• seven major plant hormones:

• auxins, cytokinins, gibberellins, brassinosteroids, oligosaccharins, ethylene and abscisic acid.

Auxin

• Auxin increases the plasticity of plant cell walls and is involved in stem elongation.

Acid Growth Hypothesis

Auxin

• Synthetic auxins

• widely used in agriculture and horticulture

• prevent leaf abscission

• prevent fruit drop

• promote flowering and fruiting

• control weeds

Cytokinins

• Cytokinins, in combination with auxin, stimulate cell division and differentiation.

• most produced in root apical meristems and transported throughout plant

• inhibit formation of lateral roots

• auxins promote their formation

• have been used against plants by pathogens

Gibberellins

• Gibberellins are named after the fungus Gibberella fujikuroi which causes rice plants to grow abnormally tall.

• synthesized in apical portions of stems and roots

• important effects on stem elongation

• in some cases, hasten seed germination

Brassinosteroids and Oligosaccharins

• Brassinosteroids

• have a broad spectrum of physiological effects

• can be triggered by environment

• Oligosaccharins

• believed to signal defense responses

• regulate plant growth and development

Ethylene

• Ethylene is produced when auxin is transported down from the apical meristem of the stem.

• plays major role in fruit development

• Ethylene production increases rapidly when the plant is exposed to toxic chemicals, temperature extremes, drought, pathogens and other stresses.

Abscisic Acid

• Abscisic acid is produced chiefly in mature green leaves and in fruits.

• suppresses bud growth and promotes leaf senescence

• also plays important role in controlling stomatal opening and closing

Summary

• Photomorphogenesis

• Phototropisms

• Gravitropisms

• Thigmotropism and Thigmonasty

• Turgor Movement

• Dormancy

• Surviving Temperature Extremes

• Plant Hormones

Plant Movements

• Movements From Internal Stimuli

• Nutations - Slight spiraling

• Nodding - Side-to-side oscillations

• Twining - Very defined spiraling

• Contraction - Contractile roots

• Nastic - Non-directional

• Epinasty - Permanent downward bending

Plant Movements

• Movements From External Stimuli

• Tropisms can be divided into three phases:

• Initial Perception

• Transduction

• Asymmetric Growth

Plant Movements

• Phototropism

• Positive - Growth towards a light source.

• Negative - Growth away from a light source.

• Different light intensities bring about different phototrophic responses.

• Gravitropism

• Growth responses to the stimulus of gravity.

• Primary plant roots are positively gravitropic, while shoots forming the main axis are negatively gravitropic.

Plant Movements

• Other Tropisms

• Thigmotropism - Physical Contact.

• Chemotropism - Chemicals

• Thermotropism - Temperature

• Traumotropism - Wounding

• Electrotropism - Electricity

• Skototropism - Dark

• Aerotropism - Oxygen

Plant Movements

• Turgor Movements

• Turgor movements result from changes in internal water pressures and are often initiated by contact with objects outside of the plant.

• Turgor contact movements are not confined to leaves.

• Many flowers exhibit movements of stamens and other parts facilitating pollination.

Plant Movements

• Circadian Rhythms

• Members of the Legume Family exhibit movements in which leaves or petals fold in regular daily cycles.

• Fold in the evening and unfold in the morning.

• Controlled by a biological “clock” on approximately 24 hours cycles.

• Appear to be controlled internally.

Plant Movements

• Solar Tracking

• Leaves often twist on their petioles and, in response to illumination, become perpendicularly oriented to a light source.

• Heliotropisms - Growth is not involved.

• Should be phototorsion.

• Water Conservation

• Many grasses have special thin-walled cells that lose their turgor and roll up or fold during periods of insufficient water.

Plant Movements

• Taxes

• Taxic Movement refers to movement involving either the entire plant, or their reproductive cells.

• In response to a stimulus, the cell or organisms, propelled by a flagella or cilia, moves toward or away from the stimulus.

• Chemotaxic - Chemicals

• Phototaxic - Light

• Aerotaxic - Oxygen Concentrations

Photoperiodism

• Photoperiodism refers to the fact that day length is directly related to the onset of flowering in many plants.

• Short-Day Plants will not flower unless the day length is shorter than a critical period.

• Long-Day Plants will not flower unless periods of light are longer than a critical period.

Photoperiodism

• Intermediate-Day Plants will not flower if the days are too short, or too long.

• Day-Neutral Plants - Will flower under any day-length, provided they have received the minimum amount of light necessary for normal growth.

Phytochromes and Cryptochromes

• Phytochromes - Pale blue proteinaceous pigments associated with light absorption.

• Two stable forms:

• P_r - Absorbs red light.

• P_fr - Absorbs far-red light.

• When either form absorbs light, it is converted to the other form.

• Cryptochromes - Blue, light-sensitive pigments that play a role in circadian rhythms and help control reactions to light.

Temperature and Growth

• Each plant species has an optimum temperature for growth which may vary with a plant’s growth stage, and a minimum temperature, below which growth will not occur.

• Lower night temperatures often result in higher sugar content in plants and may also produce greater root growth.

• Growth of many field crops is roughly proportional to prevailing temperatures.

Dormancy and Quiescence

• Dormancy - Period of growth inactivity in seeds, buds, bulbs, and other plant organs even when environmental requirements are met.

• Quiescence - Sate in which a seed cannot germinate unless environmental conditions normally required for growth are present.

What causes plants to grow toward light?

• Selection of an experimental system

• should be easy to manipulate

• should be simple

• should show the response under study

• For phototropism studies the oat coleoptile was chosen

Coleoptiles as a model system

Phototropism experiments with coleoptiles

Plant Movements

From Internal Stimuli

• Nutations: spiraling movements

• Twining: cells elongate to different extents causing the twining of tendrils

Plant Movements

From External Stimuli

• Tropisms: growth movement in place in response to an external stimulus

• Perception of stimulus by plant

• Transduction – uneven distribution of hormones

• Asymmetric growth – uneven distribution of hormones

Plant Movements

• Positive tropism: toward stimulus

• Negative tropism: away from stimulus

• Phototropism: light stimulus; bending tropism toward light

• Gravitropism: gravity is stimulus; growth toward or away from gravity

Photoperiodism

Photoperiodism

• Def: day length (night length) determines when plants flower during the growing season

• Short-day plants (long night): those that flower when the day length is shorter than a critical length

• Spring and fall flowering

Photoperiodism

• Long Day Plants (Short Night): plants that flower when the day length is longer than a critical length

• Summer flowering plants

• Intermediate-day length plants: will not flower if day length is too short or too long.

Photoperiodism

• Day-Neutral Plants: flower under any day length provided minimum amount of light necessary

Photochromes and Photoperiodism

• Active Forms

• Red form, P_r, absorbs red light (660nm)

• Far-red form, P_fr, absorbs far-red light (730nm)

• P_r à red light à P_fr

• P_fr à far-red light à P_r

Photochromes and Photoperiodism

Photochromes and Photoperiodism

• In Nature

• Normal light causes

• P_r à red light à P_fr instantaneously

• P_fr à P_r in dark, slowly

• P_fr (or red light) stimulates seed germination

• P_r (or far-red light) inhibits seed germination

Photochromes and Photoperiodism

Flowering Response

Flowering and Phytochrome

• P_fr: inhibits short-day flowering; promotes long-day flowering

• P_fr produced during long days

• Short nights not long enough to convert P_fr to P_r

• As nights get long enough (short days) to convert P_fr to P_r then flowering occurs

Photochromes and Photoperiodism

Flowering Response

• P_r: promotes short-day flowering; inhibits long-day flowering

• P_fr produced during long days

• As days get shorter, nights get longer, P_fr converted to P_r flowering occurs

• Short nights not long enough to convert P_fr to P_r

Photochromes and Photoperiodism

Movement in Plants

§ Turgor movements (changes in turgor pressure in selected cells)

§ Growth movements (elongation of selected cells in response to stimulus)

¦phototropism

¦geotropism

¦thigmotropism

Turgor movement
Mimosa pudica L. (sensitive plant)

Pulvinus of Mimosa pudica

Tropic responses

Directional movements in response to a directional stimulus

Growth movement

Phototropism

Statoliths

Thigmotropism

1. Signal-transduction pathways link internal and environmental signals to cellular responses.

• Plant growth patterns vary dramatically in the presence versus the absence of light.

• For example, a potato (a modified underground stem) can sprout shots from its “eyes” (axillary buds).

• These shoots are ghostly pale,
have long and thin stems,
unexpanded leaves, and
reduced roots.

• These morphological adaptations, seen also in seedlings germinated in the dark, make sense for plants sprouting underground.

• The shoot is supported by the surrounding soil and does not need a thick stem.

• Expanded leaves would hinder soil penetration and be damaged as the shoot pushes upward.

• Because little water is lost in transpiration, an extensive root system is not required.

• The production of chlorophyll is unnecessary in the absence of light.

• A plant growing in the dark allocates as much energy as possible to the elongation of stems to break ground.

• Once a shoot reaches the sunlight, its morphology and biochemistry undergo profound changes, collectively called greening.

• The elongation rate of the stems slow.

• The leaves expand and the roots start to elongate.

• The entire shoot begins

• to produce chlorophyll.

• The greening response is an example of how a plant receives a signal - in this case, light - and how this reception is transduced into a response (greening).

• Studies of mutants
have provided

• valuable
insights into the

• roles
played by various
molecules in the three
stages of cell-signal
processing: reception,
transduction, and
response.

• Signals, whether internal or external, are first detected by receptors, proteins that change shape in response to a specific stimulus.

• The receptor for greening in plants is called a phytochrome, which consists of a light-absorbing pigment attached to a specific protein.

• Unlike many receptors, which are in the plasma membrane, this phytochrome is in the cytoplasm.

• Unlike many receptors, which are in the plasma membrane, this phytochrome is in the cytoplasm.

• The importance of this phytochrome was confirmed through investigations of a tomato mutant, called aurea, which greens less when exposed to light.

• Injection of additional phytochrome into aurea leaf cells produced a normal greening response.

• Receptors such as phytochrome are sensitive to very weak environmental and chemical signals.

• For example, just a few seconds of moonlight slow stem elongation in dark-grown oak seedlings.

• These weak signals are amplified by second messengers - small, internally produced chemicals that transfer and amplify the signal from the receptor to proteins that cause the specific response.

• In the greening response, each activated phytochrome may give rise to hundreds of molecules of a second messenger, each of which may lead to the activation of hundreds of molecules of a specific enzyme.

• Ultimately, a signal-transduction pathway leads to the regulation of one or more cellular activities.

• In most cases, these responses to stimulation involve the increased activity of certain enzymes.

• This occurs through two mechanisms: stimulating transcription of mRNA for the enzyme or by activating existing enzyme molecules (post-translational modification).

• During the greening response, a variety of proteins are either synthesized or activated.

• These include enzymes that function in photosynthesis directly or that supply the chemical precursors for chlorophyll production.

• Others affect the levels of plant hormones that regulate growth.

• For example, the levels of two hormones that enhance stem elongation will decrease following phytochrome activation - hence, the reduction in stem elongation that accompanies greening.

1. Research on how plants grow toward light led to the discovery of plant hormones

• The concept of chemical messengers in plants emerged from a series of classic experiments on how stems respond to light.

• Plants grow toward light, and if you rotate a plant, it will reorient its growth until its leaves again face the light.

• Any growth response that results in curvatures of whole plant organs toward or away from stimuli is called a tropism.

• The growth of a shoot toward light is called positive phototropism.

• Much of what is known about phototropism has been learned from studies of grass seedlings, particularly oats.

• The shoot of a grass seedling is enclosed in a sheath called the coleoptile, which grows straight upward if kept in the dark or if it is illuminated uniformly from all sides.

• If it is illuminated from one side, it will curve toward the light as a result of differential growth of cells on opposite sides of the coleoptile.

• The cells on the darker side elongate faster than the cells on the brighter side.

• In the late 19th century, Charles Darwin and his son observed that a grass seedling bent toward light only if the tip of the coleoptile was present.

• This response stopped if the tip were removed or covered with an opaque cap (but not a transparent cap).

• While the tip was responsible for sensing light, the actual growth response occurred some distance below the tip, leading the Darwins to postulate that some signal was transmitted from the tip downward.

• Later, Peter Boysen-Jensen demonstrated that the signal was a mobile chemical substance.

• He separated the tip from the remainder of the coleoptile by a block of gelatin, preventing cellular contact, but allowing chemicals to pass.

• These seedlings were phototropic.

• However, if the tip were segregated from the lower coleoptile by an impermeable barrier, no phototropic response occurred.

• In 1926, F.W. Went extracted the chemical messenger for phototropism, naming it auxin.

• Modifying the Boysen-Jensen
experiment, he placed excised
tips on agar blocks, collecting
the hormone.

If an agar block with this
substance were centered on a
coleoptile without a tip, the
plant grew straight upward.

• If the block were placed on one
side, the plant began to bend
away from the agar block.

• The classical hypothesis for what causes grass coleoptiles to grow toward light, based on the previous research, is that an asymmetrical distribution of auxin moving down from the coleoptile tip causes cells on the dark side to elongate faster than cells on the brighter side.

• However, studies of phototropism by organs other than grass coleoptiles provide less support for this idea.

• There is, however, an asymmetrical distribution of certain substances that may act as growth inhibitors, with these substances more concentrated on the lighted side of a stem.

2. Plant hormones help coordinate growth, development, and responses to environmental stimuli

• In general, plant hormones control plant growth and development by affecting the division, elongation, and differentiation of cells.

• Some hormones also mediate shorter-term physiological responses of plants to environmental stimuli.

• Each hormone has multiple effects, depending on its site of action, its concentration, and the developmental stage of the plant.

Introduction

• Because of their immobility, plants must adjust to a wide range of environmental circumstances through developmental and physiological mechanisms.

• While light is one important environmental cue, other environmental stimuli also influence plant development and physiology.

1. Plants respond to environmental stimuli through a combination of developmental and physiological mechanisms

• Both the roots and shoots of plants respond to gravity, or gravitropism, although in diametrically different ways.

• Roots demonstrate positive gravitropism and shoots exhibit negative gravitropism.

• Gravitropism ensures that the root grows in the soil and that the shoot reaches sunlight regardless of how a seed happens to be oriented when it lands.

• Auxin plays a major role in gravitropic responses.

• Plants may tell up from down by the settling of statoliths, specialized plastids containing dense starch grains, to the lower portions of cells.

• In one hypothesis, the aggregation of statoliths at low points in cells of the root cap triggers the redistribution of calcium, which in turn causes lateral transport of auxin within the root.

• The high concentrations of auxin on the lower side of the zone of elongation inhibits cell elongation, slowing growth on that side and curving the root downward.

• Plants can change form in response to mechanical perturbations.

• Such thigmomorphogenesis may be seen when comparing a short, stocky tree growing on a windy mountain ridge with a taller, slenderer member of the same species growing in a more sheltered location.

• Because plants are very sensitive to mechanical stress, researcher have found that even measuring the length of a leaf with a ruler alters its subsequent growth.

• Rubbing the stems of young plants a few times results in plants that are shorter than controls.

• Mechanical stimulation activates a signal-transduction pathway that increase cytoplasmic calcium, which mediates the activity of specific genes, including some which encode for proteins that affect cell wall properties.

• Some plant species have become, over the course of their evolution, “touch specialists.”

• For example, most vines and other climbing plants have tendrils that grow straight until they touch something.

• Contact stimulates a coiling response, thigmotropism, caused by differential growth of cells on opposite sides of the tendril.

• This allows a vine to take advantage of whatever mechanical support it comes across as it climbs upward toward a forest canopy.

• Some touch specialists undergo rapid leaf movements in response to mechanical stimulation.

• For example, when the compound leaf of a Mimosa plant is touched, it collapses and leaflets fold together.

• This occurs when pulvini, motor organs at the joints of leaves, become flaccid from a loss of potassium and subsequent loss of water by osmosis.

• It takes about ten minutes for the cells to regain their turgor and restore the “unstimulated” form of the leaf.

• One remarkable feature of rapid leaf movement is that signals are transmitted from leaflet to leaflet via action potentials.

• Traveling at about a centimeter per second through the leaf, these electrical impulses resemble nervous-system messages in animals, although the action potentials of plants are thousands of times slower.

• Action potentials, which have been discovered in many species of algae and plants, may be widely used as a form of internal communication.

• In the carnivorous Venus flytrap, stimulation of sensory hairs in the trap results in an action potential that travels to the cells that close the trap.

• Occasionally, factors in the environment change severely enough to have an adverse effect on a plant’s survival, growth, and reproduction.

• These environmental stresses can devastate crop yields.

• In natural ecosystems, plants that cannot tolerate an environmental stress will either succumb or be outcompeted by other plants, and they will become locally extinct.

• Thus, environmental stresses, both biotic and abiotic, are important in determining the geographic range of plants.

• On a bright, warm, dry day, a plant may be stressed by a water deficit because it is losing water by transpiration faster than water can be restored by uptake from the soil.

• Prolonged drought can stress or even kill crops and the plants of natural ecosystems.

• But plants have control systems that enable them to cope with less extreme water deficits.

• Much of the plant’s response to a water deficit helps the plant conserve water by reducing transpiration.

• As the deficit in a leaf rises, the guard cell lose turgor and the stomata close.

• A water deficit also stimulates increased synthesis and release of abscisic acid in a leaf, which also signals guard cells to close stomata.

• Because cell expansion is a turgor-dependent process, a water deficit will inhibit the growth of young leaves.

• As many plants wilt, their leaves roll into a shape that reduces transpiration by exposing less leaf surface to dry air and wind.

• These responses also reduce photosynthesis.

• Root growth also responds to water deficit.

• During a drought, the soil usually dries from the surface down.

• This inhibits the growth of shallow roots, partly because cells cannot maintain the turgor required for elongation.

• Deeper roots surrounded by soil that is still moist continue to grow.

• Plants in flooded soils can suffocate because the soil lacks the air spaces that provide oxygen for cellular respiration in the roots.

• Some plants are adapted to very wet habitats.

• Mangroves, inhabitants of coastal marshes, produce aerial roots that provide access to oxygen.

• Less specialized plants in waterlogged soils may produce ethylene in the roots causing some cortical cells to undergo apoptosis, which creates air tubes that function as “snorkels”.

• An excess of sodium chloride or other salts in the soil threaten plants for two reasons.

• First, by lowering the water potential of the soil, plants can lose water to the environment rather than absorb it.

• Second, sodium and certain other ions are toxic to plants when their concentrations are relatively high.

• The selectively permeable membranes of root cells impede the uptake of most harmful ions, but this aggravates the problem of acquiring water.

• Some plants produce compatible solutes, organic compounds that keep the water potential of the cell more negative than that of the soil, without admitting toxic quantities of salt.

• Still, most plants cannot survive salt stress for long.

• The exceptions are halophytes, salt-tolerant plants with adaptations such as salt glands that pump salts out across the leaf epidermis.

• Excessive heat can harm and eventually kill a plant by denaturing its enzymes and damaging its metabolism.

• Transpiration helps dissipate excess heat through evaporative cooling, but at the cost of possibly causing a water deficit in many plants.

• Closing stomata to preserve water sacrifices evaporative cooling.

• Most plants have a backup response that enables them to survive heat stress.

• Above a certain temperature - about 40^ºC for most plants in temperature regions - plant cells begin to synthesize relatively large quantities of heat-shock proteins.

• Heat-shock proteins may embrace enzymes and other proteins and help prevent denaturation.

• One problem that plants face when the temperature of the environment falls is a change in the fluidity of cell membranes.

• When the temperature becomes too cool, lipids are locked into crystalline structures and membranes lose their fluidity, solute transport and the functions of other membrane proteins are adversely affected.

• One solution is to alter lipid composition in the membranes, increasing the proportion of unsaturated fatty acids, which have shapes that keep membranes fluid at lower temperatures.

• This response requires several hours to days, which is one reason rapid chilling is generally more stressful than gradual seasonal cooling.

• Freezing is a more severe version of cold stress.

• At subfreezing temperatures, ice forms in the cells walls and intercellular spaces of most plants.

• Solutes in the cytosol depress its freezing point.

• This lowers the extracellular water potential, causing water to leave the cytoplasm and, therefore, dehydration.

• The resulting increase in the concentration of salt ions in the cytoplasm is also harmful and can lead to cell death.

• Plants native to regions where winters are cold have special adaptations that enable them to cope with freezing stress.

• This may involve an overall resistance to dehydration.

• In other cases, the cells of many frost-tolerant species increase their cytoplasmic levels of specific solutes, such as sugars, which are better tolerated at high concentrations, and which help reduce water loss from the cell during extracellular freezing.

Cold Protection of Ornamental Plants1

Dewayne L. Ingram and Thomas H. Yeager2

Winter temperatures in Florida are frequently low enough to cause cold injury to tropical, subtropical, and occasionally temperate plants not adapted to Florida climatic conditions.

Tropical plants and summer annuals do not adapt or harden to withstand temperatures below freezing and many are injured by temperatures below 50°F (10°C).

Subtropical plants can harden or acclimate (become accustomed to a new climate) to withstand freezing temperatures and properly conditioned temperate plants can withstand temperatures substantially below freezing.

Freezing conditions occur annually in north and central Florida, while below freezing temperatures are rare for south Florida.

Freeze probabilities for various locations in Florida are published in IFAS Bulletin 777, Freeze Probabilities in Florida.

Freezes can be characterized as radiational or advective.

Radiational freezes or frosts occur on calm, clear nights when heat radiates from the surfaces of objects into the environment.

These surfaces can become colder than the air above them due to this rapid loss of heat or long wave radiation.

When the air is moist, a radiant freeze results in deposits of ice or frost on surfaces.

Dry radiational freezes leave no ice deposits but can cause freeze damage.

Plant damage from a radiational freeze can be minimized by reducing radiant heat loss from plant and soil surfaces.

Advective freezes occur when cold air masses move from northern regions causing a sudden drop in temperature.

Windy conditions are normal during advective freezes.

Although radiant heat loss occurs during an advective freeze, the conditions are quite different from a radiational freeze.

Plant protection during advective freezes is more difficult.

The ability of plants to withstand freezing temperatures is affected by temperature fluctuations and day lengths prior to a freeze

A gradual decrease in temperature over a period of time increases the ability of plants or plant parts to withstand cold temperatures.

A sudden decrease in temperature in late fall or early winter usually results in more damage than the same low temperature in January of February.

Short durations of warmer temperatures in midwinter can deacclimate some plants resulting in bud break or flowering.

Deacclimated plants are more prone to freeze injury.

Preconditioning of tropical plants to withstand chilling temperatures has not been well documented.

Cold injury can occur to the entire plant or to plant parts such as fruits, flowers, buds, leaves, trunks, stems or roots.

Many plant parts can adapt to tolerate cold, but fruits and roots have little ability to acclimate or develop cold tolerance.

Cold injury to roots of plants in exposed containers is a common occurrence and usually is not evident until the plant is stressed by higher temperatures.

Leaf and stem tissue will not survive ice formation inside the cells (result of rapid freeze) but many plants can adapt to tolerate ice formation between cells.

One type of winter injury is plant desiccation or drying out.

This is characterized by marginal or leaf tip burn in mild cases and totally brown leaves in severe cases.

Desiccation occurs when dry winds and solar radiation result in the loss of more water from the leaves than can be absorbed and/or transported by a cold or frozen root system.

Root systems in the landscape are seldom frozen in Florida, but potting media in small containers in north Florida can be frozen for several consecutive hours.

One WHAT TO DO BEFORE THE FREEZE

Homeowners can take steps to help acclimate plants to cold temperatures and to protect plants from temperature extremes.

These steps range from selection of a proper planting site to alteration of cultural practices.

Planting Site Selection

The microclimate of a location is determined by factors such as elevation, landform, surface reflectivity, soil properties, degree of canopy cover, proximity of structures or plants, and the general solar heat exchange model.

Temperature fluctuation can differ from one location to another, even within a residential landscape.

Thus, existing microclimates and/or possible modifications of microclimates should be considered when choosing the planting site for cold sensitive plants.

Tender plants should be planted in a site with good air drainage, and not in a low area where cold air settles.

Arranging plantings, fences, or other barriers to protect tender plants from cold winds improves cold protection, especially from advective freezes.

Poorly drained soils result in weak, shallow roots which are susceptible to cold injury.

Proper Plant Nutrition

Plants grown with optimal levels and balance of nutrients will tolerate cold temperatures better and recover from injury faster than plants grown with suboptimal or imbalanced nutrition.

Late fall fertilization of nutrient deficient plants or fertilization before unseasonably warm periods can result in a late flush of growth which is more susceptible to cold injury.

Plants in Florida landscapes should be fertilized four times per year.

Landscape plants in north and north central Florida should be fertilized in March, June, September, and December.

Plants in south and south central Florida should be fertilized in February, May, August, and November.

One to 1½ pounds (454 to 681 grams) of 6-6-6 or 8-8-8, or ½ pound (227 grams) of 12-4-8 or 16-4-8 should be applied per 100 square feet (9 square meters) of planting area for the first three applications per year.

A decrease in the amount of fertilizer applied in the fall is necessary because plant nutrient consumption declines during the colder season.

Plants grown in colder portions of the state require one-third to one-half the standard fertilization rate in the fall, and two-thirds the standard rate should be applied in the warmer sections of Florida.

Shading

Tree canopy covers can reduce cold injury caused by radiational freezes.

Plants in shaded locations usually go dormant earlier in the fall and remain dormant later in the spring.

Tree canopies elevate minimum night temperature under them by reducing radiant heat loss from the ground to the atmosphere.

Shading from early morning sun may decrease bark splitting of some woody plants.

Plants that thrive in light shade usually display less winter desiccation than plants in full sun.

But plants requiring sunlight that are grown in shade will be unhealthy, sparsely foliated, and less tolerant of cold temperatures.

Windbreaks

Fences, buildings, and temporary coverings, as well as adjacent plantings, can protect plants from cold winds.

Windbreaks are especially helpful in reducing the effects of short-lived advective freezes and their accompanying winds.

Injury due to radiational freezes is influenced little by windbreaks.

The height, density, and location of a windbreak will affect the degree of wind speed reduction at a given site.

Water Relations

Watering landscape plants before a freeze can help protect plants. A well watered soil will absorb more solar radiation than dry soil and will reradiate heat during the night. This practice elevated minimum night temperatures in the canopy of citrus trees by as much as 2°F (1°C). However, prolonged saturated soil conditions damage the root systems of most plants.

Other Cultural Practices

Avoid late summer or early fall pruning which can alter the plant hormonal balance resulting in lateral vegetative budbreak and a flush of growth. This new growth is more susceptible to cold injury. Healthy plants are more resistant to cold than plants weakened by disease, insect damage, or nematode damage. Routine inspection for pests and implementation of necessary control measures are essential. Contact your County Extension Office for information on pest identification and recommended controls.

Methods of Protection

Plants in containers can be moved into protective structures where heat can be supplied and/or trapped. Containers that must be left outdoors should be protected by mulches and pushed together before a freeze to reduce heat loss from container sidewalls. Leaves of large canopy plants may be damaged if crowded together for extended periods. Heat radiating from soil surfaces warms the air above the soil or is carried away by air currents. Radiant heat from the soil protects low growing plants on calm cold nights, while tall, open plants receive little benefit. Radiant heat loss is reduced by mulches placed around plants to protect the roots. For perennials, the root system is all that needs to be protected since the plants die back to the ground annually.

Coverings protect more from frost than from extreme cold. Covers that extend to the ground and are not in contact with plant foliage can lessen cold injury by reducing radiant heat loss from the plant and the ground. Foliage in contact with the cover is often injured because of heat transfer from the foliage to the colder cover. Some examples of coverings are: cloth sheets, quilts or black plastic. It is necessary to remove plastic covers during a sunny day or provide ventilation of trapped solar radiation. A light bulb under a cover is a simple method of providing heat to ornamental plants in the landscape.

WHAT TO DO DURING A FREEZE

Ornamental plants can be protected during a freeze by sprinkling the plants with water. Sprinkling for cold protection helps keep leaf surface temperatures near 32°F (0°C) because sprinkling utilizes latent heat released when water changes from a liquid to a solid state. Sprinkling must begin as freezing temperatures are reached and continue until thawing is completed. Water must be evenly distributed and supplied in ample quantity to maintain a film of liquid water on the foliage surfaces. Irrigation for several days may water soak the soil resulting in damaged root systems and/or plant breakage due to ice build up. Consult Extension Circular 348, Sprinkler Irrigation for Cold Protection, for more technical information on this subject.

WHAT TO DO AFTER THE FREEZE

Water Needs

Plant water needs should be checked after a freeze. The foliage could be transpiring (losing water vapor) on a sunny day after a freeze while water in the soil or container medium is frozen. Apply water to thaw the soil and provide available water for the plant. Soils or media with high soluble salts should not be allowed to dry because salts would be concentrated into a small volume of water and can burn plant roots.

Pruning

Severe pruning should be delayed until new growth appears to ensure that live wood is not removed. Dead, unsightly leaves may be removed as soon as they turn brown after a freeze if a high level of maintenance is desired. Cold injury may appear as a lack of spring bud break on a portion or all of the plant, or as an overall weak appearance. Branch tips may be damaged while older wood is free of injury. Cold injured wood can be identified by examining the cambium layer (food conducting tissue) under the bark for black or brown coloration. Prune these branches behind the point of discoloration. Florida homeowners enjoy a vast array of plant materials and often desire a tropical or semitropical appearance to their landscapes. Plants are often planted past their northern limit in Florida, although microclimates differ dramatically.

Tropical and subtropical plants can be used effectively in the landscape, but they must be protected or replaced when necessary. A combination of tender and hardy plants should be planted in order to prevent total devastation of the landscape by extremely cold weather.

Footnotes

1. This document is ENH1, one of a series of the Environmental Horticulture Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date June 1990. Reviewed October 2003. Visit the EDIS Web Site at http://edis.ifas.ufl.edu. 2. Dewayne L. Ingram, professor and former extension horticulturist; Thomas H. Yeager, associate professor and extension woody horticulturist, Environmental Horticulture Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville FL 32611.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.

• Let’s face it, at some point we have fallen in love with a tropical palm that has become an integral part of our landscape.

• We know going in that it should never have been planted to begin with.

• Still we persist hoping that the past doesn’t repeat itself in the remainder of our lifetimes.

• Here are some statistics that might make us cringe but are factual nonetheless.

• The temperatures may vary a few degrees depending on colder or warmer microclimates.

• All temperatures were recorded at Tampa International Airport and are the lowest during that particular freeze event.

• In addition, a total number of consecutive nights below freezing are listed.

Review

• Nutrients, Vitamins, and Hormones

• Plant Hormones

• Hormonal Interactions

• Plant Movements

• Internal Stimuli

• External Stimuli

• Photoperiodism

• Temperature and Growth

• Dormancy and Quiescence

• “Why plumeria go dormant”

• This is a subject that has attached to it, many falsehoods from some very prominent and serious plumeria growers. We have never subscribed to the theory plumeria go dormant to protect themselves from the cold. The following is our reasons:

• The fact is the plumeria grower’s biggest losses are during this period of time in our area/latitude. So if plumeria go dormant as a protection from cold, why are so many trees affected so adversely by the cold?

• The answer is very simple: Plumeria in areas of their native habitat go dormant when the day length shortens. Why? Because it is the “DRY” season in this equatorial part of the world. If plumeria did not loose their leaves it would continue transpiration, i.e. loss of water through the leaves leading to total dehydration then imminent death

• Length of day is called “photo period”. So when the day length shortens (i.e. photo period) in autumn and winter plumeria go dormant. Because of plumeria genetics and physiology this dormancy is prevalent anywhere in the world a plumeria is exposed to sunlight.

• Much study has been done over the past 20 years on various tropical plants, succulents, cacti, true palms and plumeria showing their inability to “harden off” prior to winter as do “woody evergreens, coniferous evergreens and deciduous trees” native to our latitude. My personal experience and association with the Southern California Nursery Industry for over fifty years and growing plumeria for over thirty six years has made this fact very obvious.

• The following science is an important fact about tropical plants:

• “Plants from tropical and subtropical regions tend to have more saturated fatty acids in their membranes than those from temperate regions (our latitude). Plants which acclimate to low temperatures increase the proportion of unsaturated fatty acids in their membranes.”

• In Southern California and most latitudes in the US our plumeria become much stressed during this dormancy period. The plant is genetically expecting hot dry conditions but cold and rain is prevalent.

• Plumeria cultivated in out of doors conditions should be in the “peak of health” to best survive the conditions of dormancy in our latitude. This becomes the grower’s responsibility. We at Kimis hope to be of help so please read all the articles on our websites.

• 2) The dendrogram on the left clusters plant species by chemical similarity; each of the four main chemical groups is

• indicated with a different color. This tree does not depict descent relationships, just degree of chemical similarity.

• On the right, the evolution of these chemical types is reconstructed on a phylogeny of the plants (this does depict

• inferred evolutionary relationships). The colors correspond to the chemical groups on the left, and the gray branches

• indicate uncertainty in character reconstruction. What does a comparison of these two figures tell us about the

• evolution of plant secondary chemistry?

• a) The four groups of chemically similar species each constitutes a distinct evolutionary lineage

• b) The group colored “black” has the most advanced chemical defenses

• c) The red (3) and blue (1) chemical groups are most distantly related

• d) The chemical groups have each been gained and/or lost multiple times in evolution

Detection Of Herbicide Injury Using Remote Sensing

Introduction

• Current assessment of herbicide injury is visual rating

• Human assessment is prone to variations in perception in color and intensity in damage (Nilsson, 1995)

• Also, sunlight intensity, fatigue, and experience can cause visual rating to have a high error rate (Nilsson, 1995)

• In a survey conducted by AgrEvo, 8 out of 10 farmers stated that herbicide injury lead to a crop loss

An Alternative

• Remote sensing can assess injury without human error interfering

• Remote sensing allows for early and continuous monitoring of injury

Proof

• Stressed plants have greater red and blue reflectance (Nilsson, 1995)

• Persimmon reflectance was greatest in the violet and green spectra after treatment with diuron (Carter, 1993)

• Injury can be detected as early as 2 days after treatment with spectroscopy (Adcock et al., 1990)

Benefits

• Precise evaluation of new herbicides

• “Live evaluation”

• Creation of a standard rating system

NASA

R NASA (Stennis Space Center) has created a hand-held instrument for detecting plant stress

R The instrument determines plant chlorophyll content through fluorescence

RIt measures how much far-red or near-infrared light is re-emitted by the plant (plant is irradiated with short wavelength light, either blue or green)

R Monitor physiological effects of plant stress - nutrition, biological influences, herbicides, and others

R Patent is pending