Photosynthesis increases with increasing irradiation. This is shown in Fig. 2.4.8 A, where C02 assimilation is a positive value and respiration negative. With increasing irradiation, dark respiration of mitochondria decreases and light-induced respiration increases. As light increases, a compensation point is reached at which C02 exchange is zero and above which net C02 uptake takes place eventually becoming saturated with increasing light. The immediate, short-term response of photosynthesis to the changing light is generally reversible.
In contrast to these short-term and reversible responses to radiation is adaptation involving structural modifications to the leaf (Chap. 22.214.171.124). Sun and shade leaves represent the extreme responses to light; however, there are many intermediate stages. Leaves of shade plants adapted to low light conditions usually have a large leaf area per leaf weight and a lower compensation point than those species adapted to high light intensities (Fig. 2.4.8 B). A shade leaf is damaged in strong light, shed and replaced by a new leaf adapted to the new light conditions. This may be observed in house plants, when they are put in the open air in spring. Some, but not all, plants are able to adapt to light conditions (Fig. 2.4.8 C,D). For obligate shade plants light dependence of photosynthesis does not change, even if the plant is grown at a sunny site. Specific adaptation to light may also be observed between different populations of a species. In Fig. 2.4.8 C, D clones of Solidago which originate from sunny and shady sites are shown. Increased photosynthesis with full light utilisation only occurs in the "sun clone".
There are two ecologically important situations in which adaptation to light is required: At extremely shady sites where sun flecks only allow a short time to assimilate C02 and in dry areas of the earth where plants close their sto-mata because of lack of water. Plants at shady sites (e.g. in the herbaceous layer of a forest) receive during the day, and also with the season, often very high light intensities for short periods of time, so-called sun flecks (Fig. 2.4.9; Pearcy and Pfitsch 1994). Photosynthesis of these plants responds to these sun flecks with higher rates than would be expected from the light response curve, as the acceptor for C02 is not limiting whilst the carboxylase may be fully activated. In addition, the pools of ATP and NADP formed during the sun fleck may be still utilised for C02 fixation after the light has faded so that dark fixation of C02 continues at low light intensity. As a result, a short light fleck causes a longer period of C02 uptake. Using stable isotopes (r)13C) it was shown that about 50% of the dry matter production of these plants was assimilated in the extremely short periods of sun flecks.
In dry areas of the earth plants have about 50 mol photons m~2 day-1 to process without sufficient water to open the stomata. Thus a situation arises in the leaf in which PS I and PS II are not able to release electrons to NADPH because no C02 enters the stomata and thus C02, the substrate for reduction, is inadequate. The C02 concentration in the mesophyll is around the compensation point. The consequences of such a "bombardment" with photons are (see Chap. 1.2):
Photon flux density
Net C02 assimilation
. Photorespiration Rphot
0 400 800 1200
Photon flux density (pmol nr2 s-1)
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