Peter Schopfer

Effects of Ionising Radiation

Ultraviolet, visible and near infrared radiation leads to electronic excitation in absorbing molecules. Excited molecules are able to perform certain photochemical reactions, e.g. photosynthetic electron transport or the photochemical transformations of phytochrome discussed earlier.

Physiology of Movement

This type of movement is rare in higher plants. Only rhizomes, i.e. subterranean shoot axes, growing more or less horizontally permit free movement between sites. As shown by the monopodial rhizome of Paris quadrifolia (Fig. 31.1), the flowering shoots appear in the rhizome axes at different sites from year to year. In the course of time the rhizome dies behind them; in front — at the vegetative apex — it continues to grow. Similar “wandering movements” are also observed in other groups of plants, e.g. in some liverworts or the rhizomes of ferns. These forms of movement have hardly been investigated physiologically.

Photosynthesis as a Chloroplast Function

The sun is the universal source of energy in the biosphere. During the nuclear-fusion processes occurring in the sun, matter is changed into energy (e.g. 4 protons → helium nucleus + 2 positrons + 4.5 · 10−12 J) which is emitted in the form of electromagnetic radiation (h v) into space. The energy emission of solar radiation corresponds, as a first approximation, to the continuous emission spectrum of a black body at 5800 K. Because of scattering and selective absorption of quanta in the earth’s atmosphere the spectrum of the sun is modified (Fig. 12.1) so that the energy flux is reduced from 1.4 (the solar constant) to ≤0.9 kW · m−2 (sea level). Approximately half this energy is within the 300–800 nm spectral band (“optical window” of the atmosphere; Fig. 12.1) which is in the centre of the photochemically active radiation band (approximately 100–1000 nm).

Physiology of Hormone Action

Multicellular organisms consist of a large number of specialised organs and tissues which are assembled to form a functional unit. Coordination of various parts of the organism is achieved by chemical messenger substances, for which the term hormone is used. In animals, hormones are usually made in special glands and distributed, via the bloodstream, within the organism. Thus, they reach the target and responsive tissues which are ready to react, and there trigger specific regulatory processes (Fig. 23.1 a). This classical concept of hormones originally developed for animals was extended to multicellular plants after the discovery of a “growth substance” (auxin) in oat coleoptiles. There were good reasons for this: Auxin is secreted by the coleoptile tip, which itself is not able to grow, and is then transported basipetally into the growth regions of the organ; there it regulates cell extension (see Fig. 7.3). There is an apparent analogy in this instance between the phytohormone auxin and typical animal glandular hormones.

Inhibition of Cell Expansion by Rapid ABP1-Mediated Auxin Effect on Microtubules? A Critical Comment.

Critical analysis of a recent article raises questions regarding the inhibition of cell expansion by rapid ABP1-mediated auxin effect on microtubules.

The Cell as a Morphological System

The biochemistry and structure of the eucyte is, within the eukaryotes as a group, much more uniform than one would assume after 3000 million years of evolution. The obvious uniformity of the cell structure within the animal and plant kingdom leads to the conclusion that the basic cellular structure of Precambrian flagellates, possibly the point from which the genetic evolution of the animal and plant kingdom started, was brought to such perfection that only small improvements could be made to the cell during the years of subsequent evolution. Evolution is, therefore, not primarily a characteristic of the cell; rather, the advances of evolution were achieved by the creation of multicellular systems with differentiation and division of labour.

Physiology of Development

Living systems must be regarded as systems in a continuous state of development. This statement applies to single cells as well as multicellular systems. Therefore, in order to fully characterise an organism it is necessary to look at its complete ontogeny (individual development) and not only at certain phases of ontogeny.

C4 Plants and CAM Plants

The photosynthetic efficiency of a plant can be defined by the amount of organic matter accumulated under optimal environmental conditions per unit area and time. Ecological conditions of the habitat, to which the plant is adapted, play an important role. This applies particularly to light (see Fig. 14.6). Sun plants, able to utilise the highest natural light fluxes, generally have a particularly high photosynthetic efficiency. However, locations with high light fluxes often experience high temperatures, which stimulate photorespiration (see p. 236), and large water deficits, necessitating a high diffusion resistance for gases at the stomata in order to reduce water loss (see p. 237). The latter is also important in salt-rich locations, where the high osmotic potential (low water potential) of the solution in the soil induces water stress. In general, both conditions prevent optimal utilisation of light by photosynthesis.

Development of Chloroplasts

The central question in developmental physiology is the connection between genetic information and morphology (specific form):