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Light fleck

4 ^mol 1590 ^mol photons m"2 s"1 photons m"2 s"1

Fig. 1.2.2. Avoidance of light stress by changing the leaf angle. In the shade plant Oxalis oregana, the lowering of leaflets upon exposure to a light fleck, and then return to the normal day position after the light fleck moves away, are shown. (After Bjork-man and Powles 1981)

Fig. 1.2.3. Minimisation of light stress by chloroplast movement. Surface view of leaf cells of the moss Fuñaría hygrometrica. A Position of the chloroplast on the horizontal cell surface in weak light. B Position on the side walls of cells under high light. (Nultsch 1996)

found at the edges of clearings in a forest. Lianas and epiphytes are light parasites avoiding the scarcity of light within the stand by using trees as support, and thus capturing resources which are then available for the development of their own leaves and flowers in the canopy of the supporting plant.

It is different for surplus of light: leaves often avoid excessive radiation by adopting a parallel position to the incident light (see also Chap. 2.1, Fig. 2.1.14). The hanging leaves of Eucalyptus are well known, forming the "shade-less forests" in Australia. The vertical position of leaves has been shown to be an important avoidance strategy for tropical plants in high tropical mountains (Fig. 1.2.1).

The position of the leaves of many plants changes during the day, affecting the angle of the incoming light and thus the intensity of incoming radiation. Such leaves often have pulvini (particularly Leguminosae), which enable plants to change the position of their leaves or pin nules (see also Fig. 2.1.14). The reaction of the North American wood sorrel to a short-term light stress of strong intensity caused by a sun ray ("light fleck") is shown in Fig. 1.2.2. The sensor for the light intensity is possibly a pigment absorbing blue light.

Avoidance of light stress in shade plants can also be observed at the cellular level. In many algae and mosses, but also in some cormophytes, chloroplasts move to the lesser irradiated, vertical surfaces of the cell and turn their front side towards the incoming light. In weak light they are found in positions with their larger surface exposed to the incident light (Fig, 1.2.3).

Besides such short-term adaptations (reaction time within minutes), the development of leaves is also affected by the light environment, with the formation of sun and shade leaves (Fig. 1.2.4 and Table 1.2.1).

Sun leaves are usually small, but thick, because they possess a well-developed mesophyll, frequently with several layers of palisade cells.

Acer saccharum (sugar maple), Aceraceae Prosopis (mesquite tree), Mimosaceae

Acer saccharum (sugar maple), Aceraceae Prosopis (mesquite tree), Mimosaceae

"ig. 1.2.4. Anatomical adaptation of leaves to the light environment. A The anatomical structure of leaves from light and shade positions in the crown of sugar maple is shown. The weaker the light intensity, the less developed is the photosynthetic parenchyma. B The mesquite tree has hanging leaves of equifacial anatomical structure which only absorb diffuse solar light when the sun is at its highest. The vacuoles of the thick-walled epidermal cells are filled with protective pigments. Psychotria, a relative of coffee, grows in deep shade in tropical rain forests in very low light. Lens-shaped cells in the upper epidermis are very characteristic and could gather light for the weakly developed mesophyll. UE Upper epidermis; LE lower epidermis; PP palisade parenchyma; SP spongy parenchyma; M mesophyll; Hy hypodermis; VB vascular bundle; SC substomatal cavity. (After Larcher 1994)

Table 1.2.1. Comparison of characteristics of sun and shade leaves of beech (Fagus sylvatica) and ivy (Hedera helix), and the range of adaptation of various characters. (Larcher 1994)

Characteristic Fagus sylvatica Hedera helix

Table 1.2.1. Comparison of characteristics of sun and shade leaves of beech (Fagus sylvatica) and ivy (Hedera helix), and the range of adaptation of various characters. (Larcher 1994)

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