How to Measure Stress on Plants?
Stress as such, of course, cannot be measured as it is only effective in the interaction between environmental factors and organisms. However, the strain, as caused by the action of stress on an organism, can be measured. Switching protein synthesis from the normal protein metabolism to the formation of heat shock proteins is a measurable parameter for particular strains (see Chap. 22.214.171.124) which might be well bearable or only just bearable. However, ecologists working in the field do not usually have access to a laboratory for molecular biological research. Therefore, it must suffice to quantify the strain by a measure of survival, i.e. the degree of damage incurred. Several, usually unspecific quantitative assays have been developed. In addition, non-invasive techniques can be employed to measure specific strains, e.g. on photosynthesis via chlorophyll fluorescence (Lichtenthaler and Miehe 1997). This is particularly practicable if the parameter in question is the "weak point" of the entire reaction complex. Unspecific methods usually allow only the determination of the limits of tolerance, e.g. the so-called LD50, i.e. at which dosage 50% of the cells are killed. The LD50 value, however, does not provide information about the degree of alterations within the range of tolerance. Box 1.1.1 shows the most frequently employed methods for quantification of damage after stress treatment.
A completely different strategy for strain detection is the molecular analysis of stress-induced gene expression or suppression of expression. DNA, representing genes from the organisms of interest, preferentially from those whose genome has been sequenced, can be spotted on so-called chips or microchips. Then the plant is subjected to a stressful treatment and the mRNA is isolated and labelled by coupling to a dye. This sample is then hybridised with the DNA on the chip. Binding of the colour-coded RNA shows which genes have undergone strong expression (and produced much RNA). If the position of the individual genes on the chip is known, stress-induced gene expression can be assessed.
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