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Days in shade (3.6 mol photons nr2 d_1)

Fig. 1.2.10. Dynamic changes in the xanthophyll pools of young leaves of cotton in strong and weak light. The left-hand diagram shows the effects of transferring plants from shade into full sun light; the right-hand diagram the transfer from sunlight into shade. Note that the pools of lutin and also neoxanthin change relatively little in comparison with the total pool of violaxanthin, antherax-anthin and zeaxanthin. (Bjork-man and Demming-Adams 1994)

Spring 1995 Summer 1994

Spring 1995 Summer 1994

Control Strong drought stress

Fig. 1.2.11. Use of absorbed light energy in leaves of the Pacific madrone (Arbutus menziesii) in full sunlight. Blue Dissipation as heat; white photosynthetic C02 assimilation; light blue all additional uses including photorespiration and the Mehler reaction. (After Osmond et al. 1997)

Control Strong drought stress

Fig. 1.2.11. Use of absorbed light energy in leaves of the Pacific madrone (Arbutus menziesii) in full sunlight. Blue Dissipation as heat; white photosynthetic C02 assimilation; light blue all additional uses including photorespiration and the Mehler reaction. (After Osmond et al. 1997)

transferred to zeaxanthin is released as heat; it is still an open question whether the formation of zeaxanthin from violaxanthin is the cause of the changes in membrane leading to NPQ, or whether the changes run in parallel. The physiological importance of the xanthophyll cycle is also shown in the selective and reversible accumulation of the three xantho-phylls involved in the photosynthetic membranes during longer exposure to excess light (Fig. 1.2.10).

If one considers all physiological reactions for adaptation to, and avoidance of, excess light, and all the possibilities for repair in PS II, it is not surprising that many reactions and mechanisms are more or less attuned to each other. However, not all plants have all these possibilities available to the same extent; there are plants in which, for example, the xanthophyll cycle does not play an important role because these pigments only occur in very small concentrations and do not accumulate even after longer stress. That the removal of excessive energy can become a problem is shown by the various paths for the de-activation of absorbed light energy in the chloroplast (Fig. 1.2.11).

During drought stress (closed stomata and therefore secondary light stress through lack of C02), only a very small proportion of the light energy is used for photosynthesis and almost all available energy must be disposed of safely. It is obvious that one single mechanism is not sufficient for this.

Another argument for the use of many mechanisms for adaptation is the large variation in light intensities to which plants are subjected; e.g. with changing cloud cover or with light flecks on forest floors, light intensity can vary by a factor of 103 within seconds and plants must be able to cope with this stress (see Fig, 1.2.2 and Chap. 2.4, Fig. 2.4.9).

Summary

1. Light is the indispensable precondition for all plant life. Plants adapt to light intensities at all levels of organisation, from the molecular to the morphological, as light environments vary on earth not only in regions, but also in time. Continuously changing light conditions, or extremely low or high light intensities, together with other stressors, for example, lack of water, heat or cold, are particularly challenging to plants. Plants in those environ ments display very dynamic, adaptive behaviour.

2. The stressor "weak light" means lack of energy. Plants have adapted to this situation by developing different life forms (lianas, epiphytes as light parasites) and by adjusting the positions of their leaves (positive phototrophic reaction). A weakly developed mesophyll, but very large stacks of thylakoids in chloroplasts, are features of the anatomical and ultrastructural design of shade leaves. Shade leaves are characterised at the molecular level by a relatively low density of PS II reaction centres, but a correspondingly large area of antenna complexes.

3. Stress through high light intensity damages plants acutely because of over-excitation and formation of chemical radicals. These radicals easily react with oxygen to form reactive oxygen species (ROS). ROS destroy membranes, including proteins and other components (e.g. chlorophyll). Plants possess detoxification systems for ROS (see Chap. 1.3.5.3).

4. Because of the very substantial damage arising from over-excitation, there are many different mechanisms for avoidance and adaptation. Adaptation at the morphological level is explained in Chapter 2 "Autoecology". Many plants deal with high light energy by reactions which alter the position of the leaf. These positions may be permanent or dynamic. Sun leaves have a highly developed mesophyll, often with several layers of palisade cells. Chloroplasts are able to position themselves on the sides of the cell according to the light intensity, moving from the position with the stronger to the weaker radiation, and vice versa. The thylakoid system of sun leaves is not well developed, but consists of many photosynthetic reaction centres and a smaller antenna area (particularly the LHC II is more weakly developed). Dissociation of peripheral antenna from PS II, and a partial association with PS I, is often observed, resulting in a better balanced electron flow.

5. At the biochemical level strong light produces a change in the rate constant of energy flow in favour of heat (non-photochemical quenching). If this mechanism does not suffice, the absorbed light energy is disposed of via zea-xanthin, which is produced in the xanthophyll cycle from violaxanthin. At the same time, particularly because of closed stomata in heat or drought, light energy is dissipated with the help of photorespiration. This process pro duces C02 which is then photosynthetically assimilated.

6. If energetisation still exceeds the capacity of these quenching systems, Dl, one of the core proteins of PS II, is degraded. The resulting inhibition of photosynthesis is reversible as long as the capacity of the specific repair system, the "Dl cycle", is sufficient. This reversible inhibition is called photoinhibition. If strain on the Dl cycle is persistent, "photodestruction" occurs with strong bleaching of the chloroplast.

7. Light energy can also be absorbed by pigments in the epidermis, as well as by the photosynthetic apparatus. Light can be reflected by the surface of the epidermis. A hairy tomentum can increase the proportion of reflected light.

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