Plant Environment: Temperature

• Temperature

Plant Environment: Temperature

– Biological activities of most plantsoccur within a range of temperatures

• Cool-season plants (peas,apples)

– Temperature classification:

• Intermediate-season plants(tomato, peach)

• Warm-season plants (melons)

c o r sformazione GenHORT

• Temperature influences most plant activities:


– Plant growth rate– Maturation– Fruit ripening– Seed germination– Crop quality


• Temperature and plant growth


– Plant growth results from enzymatic reactionsand is influenced by temperature

Plantgrowthrate

temperature0 15 30 50

• Optimum temperature for most plants 20-30oC;grower can adjust growth rate by adjustingtemperature


• Temperature and plant growth


– daytime and nighttime temperature influencegrowth rates of plants

• Most plants grow better if temperature changesduring the day than if temperature constant

• Some plants grow taller if day temperature is greaterthan night temperature

• Shorter, compact plants are produced if nighttemperature exceeds day temperature


• Temperature and plant maturation


– Base temperature depends on location, crop, moisture, soiland cultivar

• Degrees days = average of daily minimum andmaximum temperatures minus base temperature

– Assume base temperature of 40oF and a given day withhigh of 75o and low 45oF. Degree days for that day = 20.

– Harvest scheduled when degree days approachesoptimum for the crop (e.g. peas = 120-170 after bloom,apples 1400-2800 after bloom)


• Growers calculate degree days to schedule harvests– time to harvest also influenced by temperature

• Temperature and quality of crops


– Sugar concentration in crops influenced bytemperature

• Under cool conditions, starch is converted to sugar

• Under warm conditions, sugar in plants converted tostarch

– Crops harvested under cool temperature sweeter

» harvest and store sweet corn in cool temperatures tomaintain sweetness

» Cooler temperature produce brighter red flowers andfruits, because anthocyanins formed from sugars


• Soil temperature


– Influences seed germination, root growth, wateruptake

• Roots slow water uptake in low soil temperatures;plants in cold soils may wilt because not gettingenough water

• Cool-season plant will not germinate if soiltemperature too cold or too warm (15-20oC ideal)

• Cuttings root (talee) faster if rooting medium isheated (20-27oC ideal for many species)

• Cold soil may slow germination and encouragerooting;


• Temperature stress


– Low temperature effects• Chilling injury: plants damaged by low temperature

but ice crystals did not form– Tropical plants particularly sensitive to cool temperatures

Symptoms include: lesions, discoloration, defoliation,wilting, poor keeping quality (in fruit like bananas)

• Freezing injury: damage caused by freezing of waterinside plant

» In winter sun can warm southwest side of trunkthawing tissue; after sunset tissue freezes and causescell death (sunscald)

» Dead bark dries in Spring and peels off, exposingwood

» Orchards (frutteti) paint trunks white to reflect wintersunlight




• Freezing injury:– Low temperature effects

Sunscald Frost crack c o r sformazione GenHORT



– Winter hardiness• Winter hardiness refers to plants ability to tolerate or

avoid freezing damage– Freeze-tolerant plants allow water to move outside cells

and freeze in extracellular spaces; require gradualexposure to cold (acclimation)

– Freeze avoidance achieved by allowing some water toleave cells which increases solute concentration in cells;requires lower temperature to cause cellular freezing

– Freeze avoidance also achieved by remaining low toground so covered by insulating snow




– High-temperature effects

• Southwest trunk of thin trees such as apples canoverheat and kill vascular cambium;

• Fruits and leaves can also become sunscalded,producing yellowish areas or dead tissue

• Many desert plants (e.g. cactus) have adaptation toprevent heat stress such as whitish hairs to reflectsunlight and provide shade to stems




– Hardening off plants• Actively growing plant are more susceptible to

temperature stress than dormant plants– Plants can become hardened or acclimated to temperature

changes– Important to slowly decrease temperature of seedlings

before planting outdoors


Adaptation (genotypic) – Acclimatization(phenotypic)

• to increase the probability of survival of a certaingenotype in its habitat

• ADAPTATION has to be heritable, so we cannotspeak about adaptation of one individuum, insuch cases it is better to use the word

• ACCLIMATIZATION (acclimation, hardening),which means individual adjustment to anenvironmental stress taking place in response toenvironmental conditions. Example: hardening ofplants exposed to low temperatures


TOLERANCE:

1) physiological: endurance of a plant ismeasured in culture with no competition andonly one altered variable at a time

2) ecological: response of a plant is examinedunder field conditions


Climatic boundaries1) CHILLING INJURY:• tropical species suffer at temperatures of 6-10 °C for

leaves; so-called chill sensitive plants (Coffea arabica,Oryza sativa, Citrus, Musa, Passiflora). Flowers and fruits arealso sensitive.

• cellular dysfunctions, increase in membranepermeability, stopping of enzymatic activity.

Chilling injury of plants orfruits (mitochondrial injury)


2) FREEZING INJURY AND CRYOPROTECTION:

• frost resistance is genetically determined• hardening of plants to freezing temperatures is a long

and stepwise process, while de-hardening can last only2-4 days

• hardening temperatures of +5 to 0°C enable a plant towithstand a moderate frost

• in winter, frost tolerance can increase in 1-2 days, witha full effect in 10 days

• root tolerance is lesser than that of aerial parts


• ice crystals are not lethal if formed in the apoplast

• rapid rate of thawing causes osmotic problems

• photosynthesis is the most affected by freezing, mainly thephotophosphorylation mechanisms are affected

• soluble sucrose, when freezing temperatures occur, protectsmembrane, trisaccharides are more effective than di-andmonosaccharides

• sugar alcohols are also effective (mannitol, inositol, sorbitol,glycerol)

• amino acids: accumulation of proline, increase from 2-4 % to 60%.


3) SUMMER WARMTH:

• arctic species suffer from temperatures by 10 °C higherthan is the summer temperature in their habitats

• They do not suffer from heat injury, but the problem isrelated to carbon disequilibrium in the plants

• These plants waste their carbohydrate reserves insummer and therefore are weak in the next spring

• Heat injury occurs in most plants at temperatureshigher than about 40 oC.

• Extreme adaptation to high temperatures is found inthermophilous Cyanobacteria (70 to 85 oC) andArchaea (up to 95 oC)


Periodicity• Absorption rate of radiant energy in a place depends on

its position in face of the Sun.• Available energy changes periodically.• The fluctuations of energy supply are reflected in

periodical temperature fluctuations• This periodicity affects all phenomena on the Earth,

hence also any organism’s life.

- Biological processes reflect climatic rhythms- Growth periodicity synchronised with climatic rhythms


Climatic rhythmsDIURNAL CHANGES – (in plants)1. day-night light changes (photoperiodicity)2. day-night temperature changes (thermoperiodicity) - temperature changes when changes radiant energy input,

temperature maxima differ in dependence of this input.- close to the equator: small differences in photoperiod while around

the tropics: differences only of about 2 hours. Therefore diurnaltemperature fluctuations are more important than the small seasonalones

- wide temperature fluctuations especially in the mountains, bothtropical and temperate ones

SEASONAL CHANGES – at higher latitudes day and night lengthschange dramatically during the year

- vegetative processes can be suppressed (dryness, cold) c o r sformazione GenHORT

Activity rhythms

• Diurnal light rhythm strongly influences plants.• Day-night temperature differences influence

germination – evolutionary adaptation• Best plant development when night temperature is

by about 10-15°C colder than day temperature• For cacti and desert plants 20°C difference

optimal, for temperate zone plants 5 to 10°Cdifference optimal

• Exceptions:- tropical plants: only 3°C differences


Growth synchronisation withclimatic rhythms

• tropics: limited by decreased water availability indry periods

• others: activity synchronized with the vegetativeperiod, photo- and thermoperiodism

• latitudes > 40° = days are longer than nightsduring the whole growing season with productionof new shoots, leaves and flowers,

• a plant species is well acclimatized if the growingseason is fully utilized, without risk of injury in theunfavourable season


Most organisms are adapted to environmentaltemperature:

1. Psychrophiles (< 20 °C)2. Mesophiles (~ 20-35 °C)3. Thermophiles ( ~35-70 °C)4. Hyperthermophiles (70-110 °C)

Two well studied acclimation responses are:1. the Heat Shock response2. Cold acclimation


Cold Acclimation (CA) involves:

• Increased accumulation of small solutes– retain water and stabilize proteins– proline, glycine betaine, trehalose

• Altered membrane lipids, to lower gelling temp.

• Changes in gene expression [antifreeze proteins,proteases, RNA-binding proteins]


Freezing Stress

• Freezing injury iscaused by lowtemp.<0℃

• Supercooling


• Intercellular crystallization– Ice crystals form between cells.

• Intracellular crystallization– Ice crystals form in the cell.


• Freezing injury:

– Direct: injury by crystal formation– Indirect: dehydration– Membrane injury



• Strategies of increasing plant freezing tolerance:

– Lower water content– Reduce photosynthesis– Increase ABA/GB– Dormant– Increase osmolytes

All the above can be attained by cold hardening(acclimation)


Chilling Stress

• Caused by low temp. > 0℃• Damage

– Membrane phase– Root water absorption ability– Dysfunction of respiration,– Dysfunction of metabolism


• Mechanism of chilling injury: membranephase transition:

LC phase→→Gel phase


Liquid crystalline phase. The typical phase in biological membranes.The lipids have both lateral motion and kinetic energy and containmembrane proteins

Gel phase. The membrane lipids have less kinetic energy and lateralmotion than in the liquid crystalline phase resulting in a regular spacingbetween the acyl tails

• Strategies of improving plant chillingtolerance:

– Increase IUFA (index of unsaturated fattyacid), which leads to the decrease of phasetransition temperature.

– Synthesis of chilling-tolerant isoenzymes.


Heat Stress

• The typical response to heat stress is adecrease in the synthesis of normalproteins, accompanied by an acceleratedtranscription and translation of newproteins known as heat shock proteins(HSPs)


Heat shock proteins


Heat shock

• may arise in leaves– when transpiration is insufficient– when stomata are partially or fully closed and

irradiance is high• may arise in germinating seedlings

– When the soil is warmed by the sun• may arise in organs with reduced capacity

for transpiration e.g. fruits


Resistance or sensitivity ofplants to heat stress

• Duration• Severity of the stress• Susceptibility of different cell types• Stage of development


Classes of HSPs

Protein class Size (kDa) Location

HSP100 100-114 cytoplasm

HSP90 80-94 cytoplasm, ER HSP70 69-71 ER, cytoplasm, mitochondria

HSP60 10-60 chloroplasts, mitochondria smHSP 15-30 cytoplasm, chloroplast, ER, mitochondria


28oC 40oC 45oC 45oC

Soybean seedlings.

Thermotolerant growth of soybean seedlings following a heat shock.


Heat Stress (or Heat Shock) Response

• Discovered in Drosophila• Specific response to temperatures ~10-15oC

above normal growth temperature• Ubiquitous• Conserved• Rapid• Transient• Dramatic change in pattern of protein synthesis• most HSPs are chaperones (chaperonins) that promote

protein re-folding and stability• HSP induction mediated by a bZIP factor


Heat stress effects on protein synthesis in soybeanseedlings (J. Key).

Joe Key


Heat stress effects on protein synthesis in soybean seedlings (J. Key).

Heatstress/shockprotein synthesisin thecyanobacteriumSynechococcus.


Generalized order of events in heatshock response

Initial events (phase I):

1. Inhibition of protein synthesis2. Inhibition of transcription & RNA

processing3. Induction of new hs (hsp) mRNAs


Phase II

4 Partial restoration of protein synthesis,mainly translation of hsp mRNAs

5 Accumulation of hsp (heat shock proteins)

6 Gradual resumption of normal cellularprotein synthesis

7 Decline in hsp synthesis


Thermotolerance

• Enables organisms to survive high temp.

• A sub-lethal heat shock allows organisms to survive alethal treatment (acclimation).

• Production of hsps correlate with acquiredthermotolerance.

• Some mutants (yeast) and transgenic plants withaltered expression of certain hsps don’t showthermotolerance.


Heat Shock Proteins (hsp)• ~100, ~90, ~70, and ~60 kDa

• Low molecular weight (LMW) hsp: ~27, ~20-22,~15-18 kDa

• all induced within 30 min.

• more LMW hsp in plants

• 2-Dimensional gel electrophoresis and molecularcloning indicates most hsps are families ofrelated proteins, particularly hsp70 and the LMWhsps c

o r sformazione GenHORT

HSP functions

• A LMW hsp is ubiquitin, which marks proteins to be degraded

• hsp90, hsp70, and hsp60 involved in protein folding: "molecular chaperones”

• hsp100 may promote translation of hspmRNAs


HSP70, a chaperonin• Essential gene• Homologues found in cytoplasm, mitochondria, and

chloroplasts• function in protein assembly in normal (non-stressed)

cells, hydrolyze ATP• Constitutive and heat-induced (cytoplasmic) forms

– the heat-induced form first appears in the nucleolus,then goes to cytoplasm (may protect pre-ribosomesfrom heat stress)

• Also, some hsp70s are light-induced; chloroplast hsp70helps protect PSII from light/heat damage in Chlamy


HSP60 (cpn60)• first one termed "molecular chaperone“

• in eucaryotes, only in mitochondria and plastids

• exists as abundant complex with two subunits of 61 and 60 kDa (ATPase)

• facilitates folding/ assembly of other proteins


LMW HSPs• highly heat-induced• 4 nuclear gene families:

1. Class I cytoplasmic2. Class II cytoplasmic3. Chloroplast localized4. Endomembrane localized (ER)

• found in organelles only in plants• function mostly unknown• aggregate in vivo into "heat shock granules"


HSP regulation

• most work on LMW hsp in plants• induction is mainly transcriptional but

also translational control• genes induced coordinately, but not

equally in all tissues• light can also induce some LMW hsps


Heat-shock transcription factor (HSF)

• studied mostly in animals and yeast• Contains leucine zipper motifs• Binds DNA• Activity is induced by heat, and

phosphorylation• Activity repressed by HSP70


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Plant Environment: Temperature