PHYSIOLOGIA PLANTARUM 97; 185-193. 1996 Copyrigh, e Fh,siotogia Ptama^ur, 1996 Printed in Denmark-a!i righis reserved ISSN 0031-9317 Minireview A role for nitrogen reserves in forage regrowth and stress tolerance J. J. Volenec, A. Ourry and B, C, Joern Volenec, J, J,, Ourry, A. and Joern, B, C, 1996, A role for nitrogen reser\es in forage regrowth and stress tolerance. - Physiol. Plant, 97; 185-193. Carbohydrate accumulation and utilization during shoot regrowth after defoliation and winter has been studied extensively in most species used as forage. However, re- cent woik suggests that N reserves found in vegetative tissues also are important for defoliation tolerance and winter hardiness. Results suggest that these N reserves con- stitute an altemative N source used when N; fixation and/or mineral N uptake are re- duced. " N labelling experiments indicate that a large proportion of herbage N is de- rived from N reserves mobilized from stem bases or roots to developing leaves and shoots. Amino acids and specific proteins (i,e, vegetative storage proteins, VSPs) are deposited in roots and stem bases and, in the case of VSPs, are degraded rapidly after defoliation. Identification and characterization of VSPs will increase our understand- ing of the role N reserves play in stress toierance and may lead to innovative ap- proaches for improving forage persistence and productivity. Key words — Alfalfa, carbohydrate reserves, clover, defoliation tolerance, Lolium, Medicago, ryegrass, Trifolium, vegetative storage protein, winter hardiness, /. J. Volenec (corresponding author, e-mailjvolenec@dept.agry.ptirdue.edu) andB. C Joern, Dept of Agronomy, Purdue Univ., West Lafayette, IN' 47907-1150, USA; A. Ourry. Labortoire de Physiologie et de Biochimie Vegetales. Institute de Recherche en Biologie Applique, Univ. Caen. F-14032, Caen, Cedex. France. Int od f drate reserve accumulation and forage stress tolerance. Accurate understanding of the mechanisms controlling A major challenge facing the sustainability of forage pro- spring growth, herbage regrowth after defoliation, and duction systems is poor plant persistence induced by winter survival should lead to innovative approaches for stressful environmental conditions - a problem aggra- improving yield and persistence of forage species, vated by the current emphasis on intensive harvest man- agement. Genetic selection of forages continues to be the Abbreviations - TNC, total nonstructural carbohydrate; VSPs, primary mechanism by which we increase yield and vegetative storage proteins; WSC, water-soluble carbohydrate, stress tolerance. However, as in many crop species, fu- ture improvement through genetic manipulation will de- ,> . . j .. . ^ -I. I.- u • K • , ,!,-• Carbohydrate reserves pend on new insights into basic physiological and bio- chemical plant processes that control these agronomic ^"'^^ legume species features. To appreciate where research on reserve N pools may This review is an outgrowth of our belief that avail- lead us, it is useful to consider the conventional wisdom ability and mobilization of reserve N pools are essential regarding forage carbohydrate reserves. No other facet for herbage growth in spring, herbage regrowth after of forage physiology has been studied more extensively harvest, and ultimately the stirviva! of perennial species than the role of carbohydrate reserves in herbage re- used as forage, pasture, and biofuels. Our goal is to stim- growth, winter hardiness, and plant persistence, Graber ulate critical reassessment of current views that focus et al, (1927) were among the first to report that taproot nearly exclusively on the association between carbohy- total nonstructural carbohydrate (TNC) levels declined Received 6 June, 1995; revised 25 September, 1995; in fmal form 2 November, 1995 Physiol. Plant. 97, 1996 185