Dynamics of nitrogen remobilization in defoliated Phleum pratense and Festuca pratensis under short and long photoperiods

The impact of photoperiod on the late and magnitude of N remobilization relative to uptake of inorganic N during the recovery of shoot growth after a severe defoliation was compared over 18 days in two temperate grass species, timothy (Phleum pratense L, cv. Bodin) and meadow fescue (Festuca pratensis Huds. cv. Salten). Plants were grown in flowing solution culture with N supplied as 20 mM NH4NO3 and pre-treated by extending the 11 h photosynthetically significant light period either by 1 h (short-day or SD plants) or 7 h (long-day or LD plants) of very low light intensity, during the 10 days prior to defoliation. Following a single severe defoliation, N-15-labelled NH4+ or NH4+ NO3- was supplied over a 20-day recovery period under the same SD and LD conditions. Changes in the relative contributions of remobilized N and newly acquired mineral N to shoot regrowth were assessed by sequential harvests. Both absolute and relative rates of N remobilization from roar and stubble fractions were higher in LD than SD plants of both species, with the enhancement more acute but of shorter duration in timothy than fescue. Remobilized N was the predominant source of N for shoot regrowth in all treatments between days 0 and 8 after cutting: on average more so for fescue than timothy, because the presence of NO3- reduced the proportional contribution of remobilized N to the regrowth of timothy but not of fescue. Net uptake of mineral N began to recover between days 4 and 6 after cutting, with NO3- uptake restarting 1 or 2 days earlier than NH4+ uptake, even when NH4+ was the only form of N supply. LD timothy plants supplied solely with NH; were slowest to resume uptake of mineral N. Supplying NO3- in addition to NH4+ after defoliation promoted shoot regrowth rate but not remobilization of N. Rates of regrowth (shoot dry weight production per plant) were not correlated with rates of N remobilization from stubble either over the short-term (days 0-8) or longer term (days 0-8), interpreted as evidence against a causal dependence of regrowth rate on N remobilization under these conditions. The results are discussed in relation to hypotheses for source/sink-driven rates of N remobilization and their interactions with mineral N uptake following defoliation.