5. Conclusions
Our findings explain the acceleration of senes- cence in annual monocarpic C
3
plants under CO
2
enrichment as a process driven by the differing effects of CO
2
on nutrients, in particular nitrogen, in vegetative and generative tissues. We postulate
that CO
2
enhances flag leaf senescence in barley crops and probably leaf senescence in mono-
carpic C
3
species in general by a sequence of several processes induced by CO
2
: 1 CO
2
reduces the nitrogen demand of green tissues by alter-
ations of the photosynthetic apparatus, i.e. lower RubisCO contents, lower contents of enzymes of
the PCO-cycle. Thus, nitrogen uptake does not keep pace with carbon acquisition during vegeta-
tive growth. 2 CO
2
increases the number of diaspores. Thus, the sink size for nutrients during
seed ripening is enlarged. 3 During grain filling or seed ripening in general there is a higher
demand for nutrient redistribution to the seeds since CO
2
enrichment does not reduce the nitro- gen demand of the diaspores. Thus, a nitrogen
deficiency is induced during the switch from vege- tative to generative growth. 4 This deficiency
acts as a trigger to induce leaf senescence in order to release nutrients from vegetative tissues accord-
ing to the nutrient salvage function of senescence, although the demand of the seeds can not be
accomplished due to restricted nutrient pools.
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