CSIRO PUBLISHING www.publish.csiro.au/journals/fpb Functional Plant Biology, 2005, 32, 321–334 Contribution of initial C and N reserves in Medicago sativa recovering from defoliation: impact of cutting height and residual leaf area Fr´ ed´ eric Meuriot A , Marie-Laure Decau A , Annette Morvan-Bertrand A , Marie-Pascal Prud’Homme A , Franc ¸ois Gastal B , Jean-Claude Simon A , Jeffrey J. Volenec C and Jean-Christophe Avice A,D A UMR INRA / UCBN 950, Ecophysiologie V´ eg´ etale, Agronomie and Nutritions NCS Institut de Biologie Fondamentale et Appliqu´ ee, Universit´ e, 14032 Caen Cedex, France. B UEPF INRA, Unit´ e d’Ecophysiologie des Plantes Fourrag` eres, Domaine du Chˆ ene, Route de Saintes, 86600 Lusignan, France. C Department of Agronomy, Lilly Hall of Life Sciences, 915 W. State St, Purdue University, West Lafayette, IN 47907-2054, USA. D Corresponding author. Email: avice@ibfa.unicaen.fr Abstract. We studied the effects of stubble carbon / nitrogen (C / N) reserves or residual leaf area (RLA) on the contribution of taproot C / N reserves to shoot regrowth of Medicago sativa L. after cutting. The study assessed the effects of two cutting heights (6 and 15 cm), two RLAs (0 or 100%), and two initial C / N reserve levels (high N or low N) on forage production, nitrogen (N) distribution, and C / N reserve dynamics within stubble and taproot. Alfalfa forage production was mainly affected by the initial taproot C / N reserve levels. However, stubble initial organic reserves (and to a lesser extent the RLA) were also of particular importance during early regrowth. The increase of cutting height led to increased stubble C / N supply to regrowing shoots, which partly offset the negative effect on forage production and on taproot C / N reserve depletion. Unlike taproot reserves, the positive contribution of stubble organic reserves to shoot C / N supply was effective for a single defoliation–regrowth cycle. Alfalfa management strategies that increase cutting height (and RLA) during the penultimate harvest in autumn should be considered in cold regions with significant winter stress in order to improve alfalfa winter survival and persistence, as well as spring herbage regrowth. Keywords: alfalfa, amino acids, leaf area, N partitioning, reserve mobilisation, starch, sucrose, vegetative storage protein. Introduction Alfalfa (Medicago sativa L.) is a herbaceous perennial legume of great agricultural importance, which has been grown for centuries as forage for ruminant livestock (Bolton et al. 1972; Hill 1987). However, the area sown to alfalfa has been significantly reduced in recent years, mainly because of difficulties associated with two interacting characteristics: forage productivity, and stand persistence (Lemaire and Allirand 1993). Optimisation of these two agronomic parameters has long been pursued at the field level (Graber et al. 1927; Hodgkinson 1969; Brink and Marten 1989). However, results are generally observational and could be improved by a detailed study of carbon and nitrogen resource Abbreviations used: DM, dry matter; ELISA, enzyme-linked immuno-sorbent assay; HN, high nitrogen; LA, leaf area; LN, low nitrogen; RLA, residual leaf area; TNC, total non-structural carbohydrates, VSP, vegetative storage protein. distribution and utilisation at the whole-plant level (Philippot et al. 1991; Lemaire and Allirand 1993). During post-cutting regrowth, the synthesis of new photosynthetic shoot tissues requires the mobilisation of C and N reserves previously stored in perenniating organs (Hodgkinson 1969; Avice et al. 1996a; Volenec et al. 1996). In alfalfa, the taproot represents the major storage organ. In addition to its soil-anchoring role, this organ accumulates high concentrations of C and N compounds, which sustain initial shoot growth in spring as well as shoot regrowth after cutting (Volenec et al. 1996, 1998). For example, total non-structural carbohydrates (TNC), consisting mainly of soluble sugars and starch, can reach © CSIRO 2005 10.1071/FP04151 1445-4408/05/040321