T

Fig. 1.1.7. Frost resistance and photosynthetic capacity (C02 uptake at saturating light and 1% C02 concentration) of one generation of spruce needles measured under identical conditions over the course of a year. Note that the annual fluctuation of photosynthesis shows the same dynamics as the chlorophyll content of the chloroplasts. Needles were taken from a 50-year-old spruce tree in the Botanical Garden in Munich. (After Senser and Beck 1979)

Fig. 1.1.7. Frost resistance and photosynthetic capacity (C02 uptake at saturating light and 1% C02 concentration) of one generation of spruce needles measured under identical conditions over the course of a year. Note that the annual fluctuation of photosynthesis shows the same dynamics as the chlorophyll content of the chloroplasts. Needles were taken from a 50-year-old spruce tree in the Botanical Garden in Munich. (After Senser and Beck 1979)

are significantly less efficient (measured in terms of their productivity) than in the frostsensitive state. Upon decrease of the stress they therefore undergo a dehardening process, in which they regain their former efficiency (Fig, 1.1.7). They thus adapt to their particular environmental conditions within the limits of their genetic disposition.

• Normally, plants react to stresses with a change of their metabolism (strain) which may be interpreted as an alarm signal. If this strain is tolerated, hardening occurs. If the strain exceeds the range of tolerance, the organism is injured and, if the damage overstrains the plant's capability of repair, it will not survive.

• Multiple stress and multiple stress responses:

Let us continue with the above example of frost hardening and dehardening. When subjected to subfreezing temperature, a large portion of the water in the plant freezes, also in frost-hardened tissue, and crystallised water is not available to the plant. Nevertheless, during bright winter days, leaves, for example, needles of conifers, are exposed to very high radiation intensities. The photosynthetic apparatus is energised, as the absorption of solar radiation does not depend on temperature, but this energy cannot be used because of frost dehydration of the tissue and the low temperatures. Various other possibilities of energy dissipation, for example, photorespiration or photosynthetic oxygen reduction (Mehler reaction), are, for the same reason, not functional (see Chap. 1.2.1.3). An increased energy dissipation via radicals would result in so-called oxidative stress, which can easily be observed by the destruction of pigments. Frost hardening thus not only requires development of tolerance to freeze-desiccation of the cells, but also of protective measures against the damaging effects of oxidative stress, and of strategies to avoid the formation of radicals. Multiple stress requires a multiple stress response and thus frost hardening and dehardening are not simple reactions, but concerted reaction complexes (often termed syndromes) where avoidance strategies and the development of tolerance are combined and usually cannot be clearly differentiated from one another.

Larcher (1987) developed a new concept of stress based on the concepts of Levitt and Selye, but incorporating the additional points mentioned above. Starting with the stress induced, a beneficial stress (eustress after Selye) and a detrimental stress (distress) are differentiated. In the reaction coordinate "time", this component is also included and shows that hardening is reversible and that increased stress or multiple stress can also overstrain even a hardened organism and damage it lethally. However, these additional modes of reaction are difficult to include in a two-dimensional diagram.

In Fig. 1.1.8, a three-dimensional model is shown where stress, strain (deformation) and time are shown as the three coordinates.

0 0

Post a comment