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"55 10

0 200 400 600

CO2 concentration (ppm)

0 200 400 600

CO2 concentration (ppm)

| Fig. 1.1.1. Limitation of photosynthesis by C02 and light. The rate of photosynthesis of a sorghum leaf (Sorghum sudanense) is shown at different light intensities and C02 concentrations in air. (After Fitter and Hay 1987)

If the dosage is inappropriate, stress is caused, as is obvious with the effects of the following factors: light (weak light, strong light), temperature (cold, heat), water (drought, flooding), nutrients (lack of ions, over-fertilisation, salt stress), carbon dioxide and oxygen (photosynthesis, respiration/photorespiration, oxidative stress, anaerobiosis; Fig. 1.1.3). Optimal intensities and concentrations of these may also differ not only for individual organisms, but also for particular organs of the same organism.

Environmental noxae are stress factors which trigger stress reactions when applied in any concentration or intensity: UV-B, ozone, ionising radiation, xenobiotics, heavy metals and aluminium. In this context, electrical and strong magnetic fields can also be considered as stress factors.

Endogenous stress may also occur, for example, by separating an organ from its water supply, as is the case during ripening of seeds and the desiccation of embryo and endosperm.

"physiological normal type", maximising its physiologically achievable performance.

Plants almost never find the optimal quantities or intensities of all essential abiotic factors (Fig. 1.1.1). Thus the "physiological normal type" is rather the exception and deviation from the rule. It is very important to realise that growth is only one of many reactions of a plant to its environment. Flowering and fruiting determine the plant's success in reproduction and propagation and might equally be used as a measure of the plant's reaction to the environment. The value of the factors might, in this case, change but the principal behaviour would be similar.

Deviations from the physiological normal type are regarded as reactions to suboptimal or damaging quantities or intensities of environmental factors, i.e. situations for which we use the term stress. Thus stress and reactions caused by it (stress reactions) can be used as a measure of the strength of the stress on a scale of intensity, ranging from deficiency to excessive supply. Environmental factors deviating from the optimal intensity or quantity for the plant are called stress factors. The optimal quantity can, in fact, be zero, e.g. with xenobiotics. Stress factors which could potentially influence the plant are listed in Fig. 1.1.2.

Biotic iā€” stress factors

Exogenous stress

ā€” Infection Herblvory

1ā€” Competition

Heat

Water

Abiotic stress factors

Cold r Chilling L I

Drought

Flooding (hypoxia)

Chemical stress

Mechanical stress

L Ionising radiation

Mineral salts (deficiency, over-supply)

Pollutants

(heavy metals, pesticides) L Gaseous toxins r Wind Soil movement Submergence

Other stresses

Electrical fields

Magnetic fields

| Fig. 1.1.2. Biotic and abiotic environmental factors creating stress for plants

Specific and Unspecific Reactions to Stress

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