Effects of initial N reserve status and residual leaf area after cutting on leaf area and organ establishment during regrowth of alfalfa

Nitrogen reserves in remaining organs and residual leaf area after cutting have long been recognized as key factors during alfalfa (Medicago sativa L.) regrowth. The present work examined which morphological components were influenced by contrasting initial N status and residual leaf area during 29 d of regrowth after cutting at 15 cm height. Two levels of initial N reserves (high and low) and of residual leaf areas (not or completely defoliated) were combined in four treatments. Both factors affected shoot dry matter (DM) production at the end of regrowth. The increase in DM of new organs observed with high N reserves resulted from a combination of short- and long-term effects on plant morphology; i.e., (1) an increase in the rate of axis appearance from the crown in the early regrowth stage (day 0–9) and (2) an increase in individual leaf area (ILA) all along the regrowth. Compared with completely defoliated plants, plants with a residual leaf area at cutting exhibited an increased rate of axillary stems appearance all along the regrowth. Then the architecture of plants with a residual leaf area was more branched than the one of defoliated plants. This increase in branching was always associated with smaller ILA. Hence, differences in plant leaf area were only significant in the early growth stage. This suggested that differences observed in new stems DM at the end of the regrowth were established by day 9 and remained unchanged in late regrowth. Our results clearly showed that initial N reserve status and residual leaf area both significantly modify the dynamic of leaf area establishment and new organ growth of alfalfa. Key words: Medicago sativa L., defoliation, morphology, N storage, stems regrowth