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Fig. 2.4.15. Light dependence of C02 assimilation related to leaf weight in A sun and B shade leaves of beech, throughout the year. Only in spring do buds show respiration, which however rises sharply with bud break. With leaf emergence C02 is assimilated. Also, in autumn, only respiration is measured in buds. Sun leaves develop faster, but age faster than shade leaves. In sun leaves full development takes almost 2 months (until mid-June), they are fully active for only 1 month and start to yellow in August. (Schulze 1970)

relations and physiological activity by leaf shedding, so that at the end of the dry period the water conditions of a dry slope are more favourable than for C4 plants in the valley floors (Kappen et al. 1976).

With such a range of reactions the question remains whether there is a basic principle for the regulation of physiological activity. As the coupling of C02 assimilation to the atmosphere occurs via stomata and as a consequence water is lost, regulation balancing both processes might be feasible. It is striking that maximum photosynthetic rates are only rarely attained at most sites. Most of the time plants operate, in

Picea abies, Soiling

O-year-needles

1 -year-needles

August

September

Fig. 2.4.16. Light dependence of C02 assimilation based on leaf weight in A current year and B 1-year-old spruce needles throughout the year. Young needles develop later in the year than leaves of beech (see Fig. 2.4.15). Development requires about 5 weeks before they reach the highest rates of C02 assimilation. With the reinforcement of the cell wall which continues until the autumn of the current year the rate of C02 uptake per dry weight decreases. In contrast the rate of C02 assimilation in the 1-year-old needles is lower than in current year's needles, but is constant for about 5 months. Ageing occurs after 3-5 years. (Fuchs et al. 1977)

Picea abies, Soiling

1 -year-needles

September August

£Z

4T*

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