Mobilization of nitrogen in swards of Trifolium subterraneum L. during regrowth after defoliation

Abstract

SUMMARYThe net mobilization of nitrogen compounds (total N, soluble protein, amino N, membrane-bound protein) from different plant parts after defoliation, and the extent to which mobilized nitrogen contributed to new leaf growth, were studied in small swards of Trifolium subterraneum L. cv. Seaton Park (subterranean clover) either completely dependent on nitrogen fixation (-N treatment) or receiving abundant nitrate at defoliation ( + N treatment).Net mobilization of nitrogen from nodulated roots (75 % of mobilized nitrogen) and branches (25 % of mobilized nitrogen) accounted for most of the nitrogen in new leaf growth for up to 5 d in -N swards and for at least 2 d in +N swards. In -N swards, nodules degenerating as a result of defoliation were a major source of mobilized nitrogen for 2 d (50 % of mobilized nitrogen), after which the fine roots became the largest net contributor of mobilized nitrogen. Overall, a net 17% of the nitrogen present in residual organs was mobilized.Soluble protein was the major form of mobilized nitrogen for which assays were performed, accounting for 50 % of the decline in total N in the -N treatment. The relative loss of soluble protein from sward components declined in the order, nodules (47 % over 5 d), fine roots (44 % over 9 d), branches (38 % over 9 d) and taproots (37 % over 5 d). The amino N pool increased sharply in size for two days after defoliation, possibly indicating breakdown of soluble protein in excess of the nitrogen demand for regrowth or to provide a respiratory substrate.It was concluded that, in plants of adequate nitrogen status, leaf growth was unlikely to be nitrogen-limited during the first few days after defoliation.

Nitrogen redistribution from the roots in post-anthesis plants of spring wheat

“…In this experiment,cv Sport redistributed 46.3% and WL 25.4% of the N amount from the roots to the grains or other aboveground parts. Previous investigations have shown that plant roots generally tend to redistribute <30% of their N amount (Culvenor et al,1989;Culvenor and Simpson 1991;Peoples and Dalling, 1988;Volenec et al, 1996). In the present study, the contribution of N from the roots to the grains at maturity was 24.3% in cv Sport and 8.7% in WL.…”

“…in wheat plants (Cooper et al, 1986;Simpson et al, 1982), and in perennial ryegrass and pasture legumes in regrowth after cutting (Culvenor and Simpson 1991; Volenec et al, 1996). N mobilisation can be considered as a general process in forage species (Corre et al, 1996).…”

“…Since there is a high N turnover in the roots and N redistribution from the roots (Culvenor and Simpson 1991;Ourry et al 1988;Volenec et al, 1996) it is likely that N is withdrawn from the decaying root parts and distributed to active growing plant parts. But some N will remain in the dead root part to at least the same concentration as in this experiment 5 mg g −1 dry weight.…”

“…N mobilisation can be considered as a general process in forage species (Corre et al, 1996). To meet the demand of the aboveground regrowth in forage species, about half of the N amount in roots and crowns at cutting time (Culvenor and Simpson, 1991;Ourry et al 1988;Volenec et al, 1996) and up to three quarters of the N amount from tap roots (Volenec et al, 1996) is mobilised and distributed in two to three weeks after cutting. The decreased N concentration in the roots for cv WL indicates that there was a considerable turnover of N in the roots.…”

Effects of simulated and insect herbivory on nitrogen and protein precipitable phenolic concentrations of two legumes

“…Lower N content may be due to N mobilization for leaf production; greater herbivory intensities result in fewer leaves to photosynthesize, so the plant allocates resources to produce more leaves (Culvenor and Simpson 1991;Cooper et al 2014). Alternatively, the reduction in N content could be the result of nutrient stress (Bryant et al 1993).…”