Recommended Literature

• Aarts MGM, Fiers MWEJ (2003) What drivesplant stress genes? Trends Plant Sci 8:99–102

• Bohnert HJ, Nelson DL, Jensen RG (1995)Adaptations to environmental stress. PlantCell 7:1099–1111

• Couzin J (2002) Breakthrough of the year.Small RNAs make big splash. Science 298:2296–2297

• http://www.ambion.com/hottopics/rnaiRNA interference

• http://www.nature.com/nature/fow/000316.html RNA interference

• Larcher W (ed) (2003) Physiological plantecology, 4th edn (chapter: Plants understress). Springer, Berlin Heidelberg New York,pp 345–450

• Robertson D (2004) VIGS vectors for gene si-lencing: many targets, many tools. Annu RevPlant Biol 55:495–519

• Stokes T (2003) DNA-RNA-protein gang to-gether in silence. Trends Plant Sci 8:53–55

• Taiz L, Zeiger E (eds) (2002) Plant physiology,3rd edn (chapter: Stress physiology). SinauerAssociates, Sunderland, MA

• Waterhouse PM, Wang M-B, Finnegan EJ(2001) Role of short RNAs in gene silencing.Trends Plant Sci 6:297–301

1.1.1

Abiotic and Biotic EnvironmentsCause Stress

The environment affects an organism in manyways, at any time. To understand the reactionsof a particular organism in a certain situation,individual external influences, so-called environ-mental factors, are usually considered separately,if at all possible. Environmental factors can beof abiotic and biotic nature. Biotic environmen-tal factors, resulting from interactions withother organisms, are, for example, infection ormechanical damage by herbivory or trampling,as well as effects of symbiosis or parasitism.Abiotic environmental factors include tempera-ture, humidity, light intensity, the supply ofwater and minerals, and CO2; these are the pa-rameters and resources that determine thegrowth of a plant. Many other influences, whichare only rarely beneficial to the plant (wind asdistributor of pollen and seeds), or not at allbeneficial or are even damaging (ionising raysor pollutants), are also classified as abiotic fac-tors. The effect of each abiotic factor dependson its quantity. With optimal quantity or inten-sity, as may be provided in a greenhouse, theplant grows “optimally” and thus achieves its

Plants are bound to places. They, therefore, have to beconsiderably more adaptable to stressful environments andmust acquire greater tolerance to multiple stresses thananimals and humans. This is shown very clearly by the lim-itations in the distribution of particular types of vegetation,for example, the tree line on mountains. Extremely highlight intensities, mechanical stress by wind, frequent peri-ods of frost, and a winter period of many months have lefttheir marks on these spruces and pines at about 3000 maltitude on Mt. Hood in Oregon (USA). Photo E. Beck

1.1Environment as Stress Factor: Stress Physiology of Plants

“physiological normal type”, maximising itsphysiologically achievable performance.

Plants almost never find the optimal quanti-ties or intensities of all essential abiotic factors(Fig. 1.1.1). Thus the “physiological normaltype” is rather the exception and deviation fromthe rule. It is very important to realise thatgrowth is only one of many reactions of a plantto its environment. Flowering and fruiting deter-mine the plant’s success in reproduction andpropagation and might equally be used as ameasure of the plant’s reaction to the environ-ment. The value of the factors might, in thiscase, change but the principal behaviour wouldbe similar.

Deviations from the physiological normaltype are regarded as reactions to suboptimal ordamaging quantities or intensities of environ-mental factors, i.e. situations for which we usethe term stress. Thus stress and reactionscaused by it (stress reactions) can be used as ameasure of the strength of the stress on a scaleof intensity, ranging from deficiency to excessivesupply. Environmental factors deviating from theoptimal intensity or quantity for the plant arecalled stress factors. The optimal quantity can,in fact, be zero, e.g. with xenobiotics. Stress fac-tors which could potentially influence the plantare listed in Fig. 1.1.2.

If the dosage is inappropriate, stress iscaused, as is obvious with the effects of the fol-lowing factors: light (weak light, strong light),temperature (cold, heat), water (drought, flood-ing), nutrients (lack of ions, over-fertilisation,salt stress), carbon dioxide and oxygen (photo-synthesis, respiration/photorespiration, oxidativestress, anaerobiosis; Fig. 1.1.3). Optimal intensi-ties and concentrations of these may also differnot only for individual organisms, but also forparticular organs of the same organism.

Environmental noxae are stress factors whichtrigger stress reactions when applied in any con-centration or intensity: UV-B, ozone, ionisingradiation, xenobiotics, heavy metals and alumi-nium. In this context, electrical and strong mag-netic fields can also be considered as stress fac-tors.

Endogenous stress may also occur, for exam-ple, by separating an organ from its water sup-ply, as is the case during ripening of seeds andthe desiccation of embryo and endosperm.

8 Environment as Stress Factor: Stress Physiology of Plants

50

Light limited

0

40

30

20

10

Rate

of C

O2 a

ssim

ilatio

n (

µm

ol m

-2 s

-1)

200 400 6000

CO2 concentration (ppm)

310 Wm-2

62 Wm-2

CO2 limited

� Fig. 1.1.1. Limitation of photosynthesis by CO2 andlight. The rate of photosynthesis of a sorghum leaf (Sor-ghum sudanense) is shown at different light intensities andCO2 concentrations in air. (After Fitter and Hay 1987)

Infection

Herbivory

Competition

Heat

ColdChilling

Frost

Drought

Flooding (hypoxia)

Light

UV

Ionising radiation

Mineral salts (deficiency, over-supply)

Pollutants(heavy metals, pesticides)

Gaseous toxins

Wind

Soil movement

Submergence

Electrical fields

Magnetic fields

Temperature

Water

Radiation

Chemical

stress

Mechanical

stress

Other

stresses

Biotic

stress

factors

Abiotic

stress

factors

Exogenous

stress

� Fig. 1.1.2. Biotic and abiotic environmental factorscreating stress for plants

Usually, an organism is subjected to severalstress factors, e.g. lack of water and heat, or a“secondary” stress factor follows a “primary”one: When the plant lacks water and closes itsstomata, internal CO2 deficiency occurs whenthe plant is illuminated, and as a further conse-quence oxidative stress ensues. Combination ofseveral stress factors is the normal case and isreferred to as multiple stress.

1.1.2

Specific and Unspecific Reactionsto Stress

An organism that is stressed, for example, byelevated temperature, not only increases its me-tabolic rate, but other reactions occur which areusually not observed in the unstressed organism,

a Specific and Unspecific Reactions to Stress 9

� Fig. 1.1.3. Life processes of an organism described as a function of an (abiotic) environmental factor. The relativegrowth rate, R, may be used as a measure of life processes:

R � �biomass

�t� 1

biomass

or take place only to a very small degree. An ex-ample of this is the formation of “heat shockproteins” (see Chap. 1.3.4.2). The modificationof the basic metabolism could be interpreted asan unspecific reaction, whilst the production ofheat shock proteins would be considered a spe-cific stress reaction of the organism. The differ-entiation of these two components of a stress re-action is based on the findings of Hans Selye(1973), a Canadian general practitioner, who, inthe 1970s, summarised the various complexes ofstress reactions of human beings as follows:“Everything which endangers life causes stressreactions and adaptive reactions. Both types ofreactions are partly specific and partly unspeci-fic.” Contrary to plants there is, in humans, alsoa strong psychic-humoral stress component. Theconcept of both components of the stress reac-tion is complicated by the fact that even the spe-cific reactions often lack specificity: The above-mentioned heat shock proteins also assist thefolding of proteins during synthesis and afterdenaturing (see Chap. 1.3.4.2), not only by high-temperature stress, but also under other stresses.They are produced in high amounts, for exam-ple, under stress by xenobiotics (e.g. heavy me-tals). This does not exclude that there are in ad-dition more specific responses by which an or-ganism differentiates between stress by heat andby heavy metals (see Chap. 1.7.5).

There is yet another facet to the question ofspecificity of stress reactions which is describedby the term cross-protection. Previous droughtstress or salt stress (osmotic stress) is known toharden plants against temperature stress, andparticularly cold stress (Fig. 1.1.4). Is this an un-specific stress response? The apparent lack ofspecificity of the adaptation is explained, on the

one hand by considering the physiological ef-fects of salt and drought stress on cells and, onthe other, the effects of frost. All three factorslead to a partial dehydration of cells (in an ivyleaf at –7 �C, ca. 90% of the total leaf water isfrozen, forming ice, and thus is no longer avail-able as free water; see Fig. 1.3.25). This causesproblems with the stability of biomembranes inparticular, as the lipid bilayers are stabilised byso-called hydrophobic interactions, which aredisturbed if the availability of water, or the ionconcentration at the surface of membranes, isdrastically changed (see also Chap. 1.3.5.2). Iftoo much water is removed from the aqueousenvironment of the biomembranes (by evapora-tion or freezing), the concentration of solutes in-creases, e.g. in the cytosol or the chloroplaststroma. Increase in the ion concentrations inturn changes the charges at the surface of mem-branes, and as a consequence the membrane po-tentials. This usually leads to destabilisation ofmembrane structure. High charge densities,however, not only result from water deficiency,but also from excessive salt concentration. Ageneral reaction to stress is the synthesis of hy-drophilic low molecular protectants, so-calledcompatible solutes (sugars, sugar alcohols andcyclitols, amino acids and betaines, see Chaps.1.5.2.6 and 1.6.2.3), which replace water at themembrane surfaces and dislodge the ionic com-pounds upon loss of cellular water. Productionof compatible solutes requires, of course, synthe-sis of respective enzymes, triggered by stress.Synthesis of these enzymes is often preceded bysignals transmitted by certain phytohormones –particularly abscisic acid (ABA) or the stresshormone jasmonate, but also ethylene, maytransiently change their concentration. One ex-

10 Environment as Stress Factor: Stress Physiology of Plants

-10

LT

50 (

°C)

1 20

Duration of salt treatment (days) Duration of salt treatment (days)

Salt treated plants- 9

- 8

- 7

- 6

- 5

- 4543

30

AB

A (

nm

ol g

-1 d

ry w

eig

ht)

1 20

Salt treatedplants

25

20

15

10

5

0543-1- 2

NaClControl

Control

A B

� Fig. 1.1.4. Frost hardeningthrough salt treatment.Cuttings of potato plants(Solanum commersonii Dun Pl458317) were grown inMurashige-Skoog mediumto which NaCl was added(100 mM final concentration).A Frost hardiness of plantsand B ABA content of plants.(After Ryu et al. 1995)

ample of such cross-protection is induction offrost hardening in wild potatoes by salt stress(Fig. 1.1.4). Potato plants treated with NaCl areable to tolerate lower temperatures than un-treated controls. A transient increase in ABAconcentration mediates this hardening reaction.

1.1.3

Stress Concepts

Based on physical principles, Levitt (1980) pub-lished a theoretical understanding of stress reac-tions that is applicable to all groups of organ-isms, as illustrated by an abstract experiment.It is known as the physical stress concept(Fig. 1.1.5). A body is deformed if it is stretchedby a force (stress); this deformation is at firstreversible (“elastic”), but upon intensifying theforce it becomes irreversibly (“plastic”) de-formed and finally breaks. The change in thebody caused by the force is called strain. Theforce required to produce a unit of change is theelastic modulus, M. In this sense, elasticity doesnot mean expansion in the sense of maximumelastic deformation. The modulus of elasticity Mcorresponds in principle to �, the elastic modu-lus of a cell wall, which is a measure of the cellwall’s flexibility (see, e.g., Fig. 1.5.2)

M � forcedeformation

� stressstrain (1.1 a)

According to this relation, M is also a mea-sure of the resistance of the system to an exter-nally applied force on the system.

In biological systems, stress is not commonlya single physical force affecting the organism,but a load from many individual environmen-tal factors. Primarily, metabolic processes arechanged or deformed. The concept by Levittconvincingly explains the relation of stress andstrain, but it can be applied to biological sys-tems only to a limited extent, as the following,biologically important parameters are lacking:

• Time factor: In a physical system, the amountof stress equals the strength of stress; in abiological system, the amount of stress is theproduct of the intensity of stress and dura-tion of stress. For example, if one cools thetropical ornamental Saintpaulia ionantha(African violet) for a short time (6 h) to 5 �C

and then returns it to the original tempera-ture, some of the metabolic reactions maychange their rates in accordance with theiractivation energy (Q10), but the increase ordecrease in metabolite pools is not changedso dramatically that the plant is damaged.However, if the plant is left for a longer peri-od (48 h) at 5 �C, metabolic chaos results, asindividual metabolite pools empty whilstothers grow disproportionally. The plant isdamaged, in other words: Elastic strain haspassed into plastic strain (Fig. 1.1.6).

• Repair: Plastic change or deformation is notcompletely irreversible. In most cases, the or-ganism is able to repair the damage, if it isnot too severe. One example is DNA repairafter damage by UV irradiation. Plastic straincan change to elastic strain (see Box 1.2.4).Because of the open life form of plants, “re-pairs” can also be accomplished through pre-mature senescence or shedding of damaged

a Stress Concepts 11

Stress

avoidance

Strain

avoidance

Stress

tolerance

Stress, strain and damage

Normal

condition

0

Elastic

deformation

(reversible)

1

Plastic

deformation

(irreversible)

2

Overly

stress of

the system

3

A B C

Three types of strain resistance (stress resistance)

� Fig. 1.1.5. The physical stress concept of Levitt (1980)

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