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Floods

1 Higher surface temperatures and

2 changes in precipitation cause floods in some places, anddroughts in others.

A rise in sea level and severe rain are two causes of floods.

Long term changes in the hydrological cycle due to highertemperatures (more evaporation; melting of ice, moreintense rain fall) contribute to more floods.

More water vapor in the atmosphere, because theatmosphere is heating up, can cause floods to happen moreregularly.

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What causes Floods?• Floods are an excess of water

that covers land that isnormally dry.

• This can be due dam or leveefailures, more rain than thelandscape can dispose of,torrential rains caused bystorms, rapid snow melts, or ablocked river.

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The two main Floods• Flash floods occur when heavy

rainfall persists for only a shorttime period (usually only a fewhours), yet can cause majordamage and death due to theirsudden arrival.

• Riverine floods occurwhen water rises aboveits natural banks, oftencaused by snowmelts incombination withprolonged and heavyprecipitation.

• Riverine floods take days,weeks, or months to riseto its max and return tonormal, much longer thanit takes a flash flood.

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When do they occur?• Floods can occur any time of the year.

• Floods occur primarily in the spring season due to iceand snow melting and frequent storms.

• In some countries, there are monsoon seasons, atime of great rain, when floods often occur, after a dryseason, when drought conditions occur.

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Where do Floods often occur?

• Floods occur all overthe world, except forin Antarctica.Highlands are alsounlikely to be struckby flooding.

• They often occur inurban areas wherewater cannot draininto the soil properly.

• Floods usually affectfloodplains, flat areasnear large bodies ofwater.

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Major River Floods in 2002

The areas in red are where river floods have occurred this year. c o r sformazione GenHORT

Before and During a Flood in St.Louis, Missouri

This image shows theThis image shows thenegative effects of livingnegative effects of livingon a flood plain.on a flood plain.

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Dangers of Floods

• Deadly and destructive (considered natural disaster)

• Human activities worsen floods

• Bangladesh- One of the world’s most densely populatedcountries.

• Great floods are now occurring every 4 years instead of every50 years because of deforestation, overgrazing (pascoloeccessivo), and unsustainable farming.

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Destruction by Floods• Floods can cause major structural damage to buildings,

kill plants and animals, destroys habitats, and removessoil.

• Floods cause many deaths and drown many plants andcrops, and hundreds of millions of dollars in damage.

• Many nutrients in topsoil are brought downstream ordeposited in the ocean.

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Hazards of Floods• There are 6 categories to tell

how severe floods are in aregion.

• They are depth of water,duration, velocity, rate of rise,frequency, and seasonality.

• Frequency is how often floodsoccur in an area, whichdepends greatly on thetopography and climate.

• Seasonality is the time of yearfloods often occur.

• Depth is simply how deep thewater is.

• Duration is how long the waterlasts for.

• Velocity is how quick the wateris moving; the faster it movesthe more damage done.

• Rate of rise is how quickly thedepth of the water increases.(important when givingwarnings to evacuate areas)

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How often do they occur?• Floods can occur anytime of

the year.• It is difficult to predict when

floods will occur in manyregions. It may be decadesbetween floods in certainareas.

• Certain regions flood severaltimes a year (5 or more).

• There are an average of 100 to150 extreme floods annually,world-wide.

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Weather that causes Floods

• Floods are caused by melting snow andice, severe storms, hurricanes, tropicalstorms, monsoons, or prolonged orsudden rain fall.

• El Niño is a major contributor to floodingglobally.

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Monsoons, Floods andDroughts

• Monsoons are seasonal winds that bring heavy rains onepart of the year and dry air the other. They occur in partsof Africa, Australia, Asia, and South America causingfloods in the wet season, and droughts in the dryseason.

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Other Facts• Floods account for 40% of all deaths caused by

natural disasters.• In an average year, floods account for about 200

deaths and $2 billion of damage in the U.S.alone.

• In 1937 floods removed 300 million tons oftopsoil in the Ohio Valley.

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(1) Injury in morphology and anatomy by O2deficiency：growth↓, leaf yellowish (nutritiondeficiency，epinasty (piegatura verso il basso),tissue degradation caused by Eth

(2) Injury in metabolism by O2 deficiency，photosynthesis↓, stomatal block, inhibition of CO2entrance. Anaerobic respiration↑ toxicants:alcohol, acetaldehyde, lactate, H2S

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(3) Nutrition disorder： absorption ↓，soil N, P, K, Ca loss but H2S, Fe, Mn ↑,microelements poison.

(4) Changes in plant hormones：IAAand CTK ↓ , release of Eth in shoot.

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AnoxiaFlood-tolerant species (hydrophytes) possessa range of adaptations that enable them eitherto tolerate stresses or to avoid them.These adaptations can be grouped into twomain categories:

1) Morphological2) Physiological

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1) Structural adaptations (morphological) a. Aerenchyma

b. Special organsAdventitious roots

Stem elongation

Hypertrophied lenticels

Pneumatophores

2) Physiological adaptations a. Anaerobic respiration b. Malate production

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• Mechanism of flood resistance– Well-developed aerenchyma– More lenticles– More adventitious root

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HormonesHormonal changes, especially the concentration of ethylene,initiate structural adaptations.

Ethylene stimulates cellulase activity in the cortical cells, withthe subsequent collapse and disintegration of cell walls.

In addition to aerenchyma development, ethylene has beenreported to stimulate the formation of adventitious roots, justabove the anaerobic zone when these plants are flooded.

These roots are able to function normally in an aerobicenvironment.

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Maize (corn)

- Shift carbohydrate metabolismfrom respiration to anaerobicglycolysis

- Protein synthesis affected:results in selective synthesis of~10-20 proteins

-mRNAs for other proteins therebut not translated well!

There are enzymes associatedwith glycolysis and fermentation.

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Protein synthesis in aerobic versus anoxic maize root tips.5-hour labeling with 3H-leucine and 2-D gel electrophoresis.

Aerobic Anoxic

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Adventitious Roots

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Stem Elongation•stimulated by submergence

•in some aquatic andsemiaquatic plants

-Floating Heart (Nymphoidespeltata) right

-Wild Rice (Oryza sativa)

-Bald Cypress (Taxodiumdistichum).

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Root aerenchyma c o r sformazione GenHORT

Pneumatophores

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Bald Cypress

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Mangroves

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Red mangrove (Rhizophoraspp.) grows on arched rootsin tropical and subtropicaltidal swamps (paludi dimaree).

These have numerous smalllenticels above the tide levelwhich terminate in long, air-filled, submerged roots.

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Oxidative Stress

• Oxidative stress results from conditionspromoting the formation of active oxygenspecies that damage or kill cells

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Oxidative stress

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Environmental factors thatcause oxidative stress

• air pollution (increased amounts of ozone or sulfurdioxide)

• oxidant forming herbicides e.g. paraquat dichloride• heavy metals• drought• heat and cold stress• wounding• UV light• intense light that stimulate photoinhibition

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Reactive oxygen species (ROS)

• Formed during certain redox reactions• during incomplete reduction of oxygen• or oxidation of water by the mitochondrial

or chloroplast electron transfer chain• Single oxygen, hydrogen peroxide,

superoxide anion, hydroxyl andperhydroxyl radicals

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Ozone and oxidative stress

• Hydrocarbons and oxides of nitrogen (NO,NO2) and sulfur (SOx) react with solar UVradiation to generate ozone (O3).

• Ozone is a highly reactive oxidant

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The negative effects of ozoneon plants

• decreased rates of photosynthesis• leaf injury• reduced growth of shoots and roots• accelerated senescence• reduced crop yield

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Ozone damage

• alters ion transport• increases membrane permeability• inhibits H+-pump activity• collapses membrane potential• increases Ca2

+ uptake from the apoplasm• Oxidative damage to biomolecules

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Resistance to ozone

• Utilizes either avoidance or tolerance• Avoidance involves physically excluding

the pollutant by closing the stomata, theprincipal site at which ozone enters theplant

• Tolerance - biochemical responses thatinduce or activate the antioxidant defencesystem and possibly also various repairmechanisms

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Tolerance to oxidative stress

Stress conditions, antioxidan ts and antioxidant enzymes

Antioxidant or antioxidant enzyme Stress condition

Anionic peroxidases Chilling, high CO 2

Ascorbate peroxidase Drought, high CO 2, high light intensity, ozone, paraquat Catalase Chilling

Glutathione Chilling, drought, !-irradiation, heat stress, high CO 2, ozone, SO 2

Glutatione reductase Chilling, drought, high CO 2, ozone, paraquat

Polyamines Deficiency of K, P, Ca, Mg, Mn, S, or B; drought, heat, ozone

Superoxide dismutase Chilling, high CO2, high light, increase d O 2, ozone, paraquat, SO 2

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