NITROGEN RESERVE MOBILIZATION DURING REGROWTH OF MEDICAGO-SATIVA L - RELATIONSHIPS BETWEEN AVAILABILITY AND REGROWTH YIELD

An experiment was designed to study the role of N and C reserves on re growth of the shoots following defoliation of forage species. Starch a nd N accumulation in root and crown tissue of nonnodulated Medicago sa tiva L. were modified during regrowth by applying different levels of N and different cutting heights. Plants were obtained with similar cro wn 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 N-15 labeling. Maximum y ields following the second regrowth were obtained from those plants ha ving a high tissue N, despite their low level of nonstructural carbohy drate. 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 o f 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 af ter 24 d of regrowth. No similar correlations were found for plants th at varied in their initial starch content of roots and crown. It is su ggested that N reserves were used mainly during the first 10 d after d efoliation, and that the resulting aerial growth during this period sh ould be sufficient to restore N-2 fixation and/or N uptake to levels e qual to those prior to defoliation. These data emphasize (a) the impor tance of root N reserves in initiating and sustaining new shoot growth , and (b) the need for a re-evaluation of the contribution of C reserv es to shoot regrowth.