Adenocaulon bicolor o

10 12 14 16 Time of day (h)

| Fig. 2.4.9. A Change with time of a light fleck in a shaded habitat and B delay in C02 fixation in Adenocaulon bicolor. The period of C02 uptake is longer than the duration of the light fleck, i.e. reduction equivalents are stored and, after the light fleck has passed, these are used in dark reactions for C02 reduction. (Pearcy and Pfitsch 1994)

light intensities the temperature optimum is shifted to lower temperatures (Fig. 2.4.10 B) with the consequence that plants are able to operate at low light intensity during the cool hours of the morning as well as in the optimum range of their potential net photosynthesis during midday, at high temperatures and full light.

The temperature optimum of net photosynthesis is not at all constant, for example, it changes with season (Lange et al. 1975 a). Similarly, the size of plant leaves is "adapted" to the radiation conditions so that the temperature (see Chap. 2.1) remains in the physiologically optimum range.

Even though plants operate most of the day in the photosynthetically optimum range there are two critical ranges of temperature in photosynthesis (see also Chap. 1.3):

• Low temperature is not a factor damaging photosynthesis, but frost together with high light intensities leads to photoinhibition. This effect was first observed in Alpine sites, which is not surprising as the C02 concentration decreases with increasing height and thus increases the imbalance between light flux and

Photosynthesis at high C02

Photosynthesis at natural CO2 Net CO2 assimilation Leaf respiration

| Fig. 2.4.10. A Schematic presentation of the dependence of photosynthesis, respiration and net C02 gas exchange on temperature. Activation of photosynthesis leads to an exponential increase in C02 uptake. Increasing temperature also increasingly inhibits photosynthesis. Interaction between activation and inhibition leads to an apparent optimum of C02 uptake that finally leads to a lower C02 uptake at high temperatures. The rate of net assimilation at increased temperature is balanced in part by the exponential rise in respiration. The temperature optimum for respiration is higher than that of photosynthesis (after Larcher 1994). B Interaction between light and temperature on photosynthesis (positive value on the y-axis) and respiration (negative value on the y-axis) leads to a decrease in the optimal temperature range at lower light intensities, i.e. for the lichen Ramalina maciformis cool mornings with low light are as much "optimum" as are high temperatures at full light. (Lange et al. 1977)

Atriplex 21 % 02

Atriplex 21 % 02

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