Ngb

Photon flux density (pmol nr2 s_1) Photon flux density (pmol nr2 s_1)

Fig. 2.4.8. A Dependence of C02 gas exchange on the light intensity, measured as quantum flux (pmol photons m"2 s"1). The y-axis corresponds to a positive value of C02 assimilation and a negative value for respiration. The figure shows the processes which occur in the region below light saturation (after Larcher 1994). B Dependence of C02 gas exchange on quantum fluxes of species differently adapted to shade. C Change in light-dependent photosynthesis with adaptation to the light climate in Solidago virgaurea. Clones that were pre-adapted to full light adapt their rate to the location (i.e. shade) compared with D clones that are adapted to the shaded location and do not change the rate of C02 assimilation. (Bjorkman 1981)

Photon flux density (pmol nr2 s_1) Photon flux density (pmol nr2 s_1)

Fig. 2.4.8. A Dependence of C02 gas exchange on the light intensity, measured as quantum flux (pmol photons m"2 s"1). The y-axis corresponds to a positive value of C02 assimilation and a negative value for respiration. The figure shows the processes which occur in the region below light saturation (after Larcher 1994). B Dependence of C02 gas exchange on quantum fluxes of species differently adapted to shade. C Change in light-dependent photosynthesis with adaptation to the light climate in Solidago virgaurea. Clones that were pre-adapted to full light adapt their rate to the location (i.e. shade) compared with D clones that are adapted to the shaded location and do not change the rate of C02 assimilation. (Bjorkman 1981)

• increased fluorescence (Björkman and Dem-mig-Adams 1994);

• increased release of heat (Björkman and Demmig-Adams 1994); or

• damage in PS II at the D1 protein (photoinhibition) with subsequent yellowing (Schäfer 1994).

2.4.3.2 Temperature

Starting from temperatures below 0°C where photosynthesis as well as respiration are at a complete standstill, respiration of leaf cells is generally activated earlier than photosynthesis with increasing temperatures. At low temperatures a range exists at which only respiration is measurable. Photosynthesis starts around -5°C (Fig. 2.4.10 A), reaches an optimum and decreases at high temperatures. In contrast, respiration increases exponentially over a wide range of temperatures and is only inhibited at very high temperatures. Net photosynthesis as the balance between these two processes shows an optimum shifted to slightly lower temperatures than the physiological process of photosynthesis. The interaction between respiration and photosynthesis is particularly obvious considering the common effect of light and temperature: At low

Adenocaulon bicolor cu t/3 "o cm

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