CO2 enrichment enhances flag leaf senescence in barley due to greater grain nitrogen sink capacity

Senescence is a highly regulated process which is under genetic control. In monocarpic plants, the onset of fruit development is the most important fa ctor initiating the senescence process. During senescence, a large fraction of plant nutrients is reallocated away from vegetative tissues into genera tive tissues. Senescence may therefore be regarded as a highly effective sa lvage mechanism to save nutrients for the offspring. CO2 enrichment, beside s increasing growth and yield of C-3 plants, has often been shown to accele rate leaf senescence. C-3 plants grown under elevated CO2 experience altera tions in their nutrient relations. In particular their tissue nitrogen conc entrations are always lower after exposure to elevated CO2. We used a monoc arpic C-3 crop - spring barley (Hordeum vulgare cv. Alexis) - grown in open -top field chambers to test the effects of CO2 enrichment on growth and yie ld, on nitrogen acquisition and redistribution, and on the senescence proce ss in flag leaves, at two applications of nitrogen fertilizer. CO2 enrichme nt (650 vs. 366 mu mol mol(-1)) caused an increase both in biomass and in g rain yield by 38% (average of the two fertilizer applications) which was du e to increased tillering. Total nitrogen uptake of the crops was not affect ed by CO2 treatment but responded solely to the N supply. Nitrogen concentr ations in grains and straw were significantly lower (-33 and -24%) in plant s grown at elevated CO2. Phenological development was not altered by CO2 un til anthesis. However, progress of flag leaf senescence as assessed by chlo rophyll content, protein content and content of large and small subunit of RubisCO and of cytochrome b559 was enhanced under elevated CO2 concentratio ns by similar to 4 days. We postulate that CO2 enhanced flag leaf senescenc e in barley crops by increasing the nitrogen sink capacity of the grains. ( C) 2000 Elsevier Science B.V. All rights reserved.