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PLANT PHYSIOLOGY , Vol 105, Issue 3 831-837, Copyright © 1994 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Nitrogen Reserve Mobilization during Regrowth of Medicago sativa L. (Relationships between Availability and Regrowth Yield)
A. Ourry, T. H. Kim and J. Boucaud
Laboratoire de Physiologie et de Biochimie Vegetales, Institut National de la Recherche Agronomique, Institu de Recherche en Biologie Appliquee, Universite, 14032 Caen Cedex, France
An experiment was designed to study the role of N and C reserves on
regrowth of the shoots following defoliation of forage species. Starch and
N accumulation in root and crown tissue of nonnodulated Medicago sativa L.
were modified during regrowth by applying different levels of N and
different cutting heights. Plants were obtained with similar crown and root
dry weights, but having either low starch and high tissue N or high starch
and low tissue N. The plants were then submitted to a second defoliation
and supplied with optimal N nutrition, and N flow from reserve was
quantified using pulse-chase 15N labeling. Maximum yields following the
second regrowth were obtained from those plants having a high tissue N,
despite their low level of nonstructural carbohydrate. When N in the roots
and crown exceeded 5 mg N plant-1 at the beginning of regrowth, about 68%
was translocated to regrowing shoots. Highly significant correlations were
also found between the amounts of N available in roots and crown at the
beginning of regrowth and (a) the amount of N that was mobilized to new
tissues, (b) the amount of N taken up during the regrowth period, and (c)
the final shoot yield after 24 d of regrowth. No similar correlations were
found for plants that varied in their initial starch content of roots and
crown. It is suggested that N reserves were used mainly during the first 10
d after defoliation, and that the resulting aerial growth during this
period should be sufficient to restore N2 fixation and/or N uptake to
levels equal to those prior to defoliation. These data emphasize (a) the
importance of root N reserves in initiating and sustaining new shoot
growth, and (b) the need for a re-evaluation of the contribution of C
reserves to shoot regrowth.
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