Senescence and nitrogen remobilization at plant level as affected by carbon and nitrogen source variation
Final Report Abstract
Atmospheric carbon dioxide enrichment has been shown to improve nitrogen use efficiency (NUE) of non-legume C3 crops. This effect is based on several physiological mechanisms, e.g. an optimization of nitrogen (N) allocation within the photosynthetic apparatus; a reduced uptake of mineral N due to reduced transpiration, or a potential competition for reduction equivalents between nitrate reduction and CO2 assimilation in the chloroplasts. As a consequence, CO2 enrichment creates green leaves with lower N contents which means a reduction in N source strength of senescing leaves for sinks such as kernels. At the same time, green leaves in CO2-rich atmospheres are fully functional in terms of CO2 assimilation and in so far differ from leaves grown under insufficient soil N supply. Thus, variation both in [CO2] and soil N supply was involved to test these hypotheses: (i) atmospheric CO2 enrichment reduces [N] in green leaves and enhances leaf senescence. (ii) This creates an imbalance in sourcesink relations for N during grain/kernel filling. (iii) Increased nitrogen supply via the roots does not mitigate CO2 enrichment effects on leaf [N] and senescence. (iv) Lines differing in N remobilization during leaf senescence will differ in NUE but will not show principally different responses to CO2 enrichment. Experiments were performed in a climate chamber system with full control of climatic conditions and [CO2]. Climate in the chambers tracked ambient conditions, and [CO2] was adjusted to either 380 or 550 µmol mol-1. The system served as platform for joint experiments with other groups of Research Unit 948, in particular TP1, TP3, TP4, and TP7. The crops involved were Brassica napus cv. Mozart, a spring cultivar, the winter oilseed rape lines Apex and Capitol and hybrids Uluru and SLM1107, Hordeum vulgare cv. Golden Promise and Bambina, and Triticum aestivum cv. Certo and its transgenic line 24-31. The main conclusions derived from the experimental results in close cooperation with partners from the Research Unit and published in peer reviewed scientific journals are: (i) CO2 enrichment in general decreased leaf [N] and caused faster leaf senescence, thus altering the sink-source balance for N during grain/kernel filling. (ii) In B. napus cv. Mozart, the combination of surplus N supply and high [CO2] led to severe nutrient imbalances with late outbranching and lower N redistribution towards kernels. (iii) In experiments with winter oilseed rape lines and hybrids, such strong disturbances of the redistribution pattern could not be confirmed, irrespective of the rooting volume provided in different experiments. Rather, CO2 enrichment increased branching of the influorescences, silique number and kernel yield. (iv) Analyses of N redistribution in oilseed rape revealed that N remobilization from senescing leaves is effective, i.e. shed leaves contained only approx. 15% of the N concentration found in green leaves. Rather, at final harvest, large amounts of N were still found in stems and branches and in empty siliques. (v) The cereal species under investigation exhibited no uniform response of leaf senescence to CO2 enrichment (enhanced in barley but not so in wheat), but both had higher C/N ratios in grains under high CO2 irrespective of soil N supply. Overall, creating N deficiency by means of either low soil N supply or growing plants under high CO2 allowed to track signalling of the senescence process triggered by different physiological processes. Transcriptome analysis of samples from joint experiments with oilseed rape has been performed for variation of N supply as yet but deserves further investigation with regard to CO2 enrichment effects.
Publications
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(2011). Growth, senescence and water use efficiency of spring oilseed rape (Brassica napus L. cv. Mozart) grown in a factorial combination of nitrogen supply and elevated CO2. Env. Exp. Bot. 72: 284-296
Franzaring, J., Weller, S., Schmid, I., Fangmeier, A.
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(2012). Allocation and remobilisation of nitrogen in spring oilseed rape (Brassica napus L. cv. Mozart) as affected by N supply and elevated CO2. Env. Exp. Bot. 83: 12-22
Franzaring, J., Gensheimer, G., Weller, S., Schmid, I., Fangmeier, A.
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(2012). Senescence-specific alteration of hydrogen peroxide levels in Arabidopsis thaliana and oilseed rape spring variety Brassica napus L. cv. Mozart. J. Integr. Plant Biol. 54: 540–554
Bieker, S., Riester, L., Stahl, M., Franzaring, J., Zentgraf, U.
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Effects of CO2 enrichment and drought on photosynthesis, growth and yield of an old and a modern barley cultivar. J. Agron. Crop Sci.
Schmid, I., Franzaring J., Müller M., Brohon N., Calvo O.C., Högy P., Fangmeier A.
