Amino acid content in xylem sap of regrowing alfalfa (Medicago sativa L.): Relations with N uptake, N2 fixation and N remobilization

Abstract

During vegetative regrowth of Medicago sativa L., soil N, symbiotically fixed N 2 and N reserves meet the nitrogen requirements for shoot regrowth. Experiments with nodulated or non-nodulated plants were carried out to investigate the changes in N flows originating from the different N sources and in xylem transport of amino acids during regrowth. Exogenous N uptake, N 2 fixation and endogenous N remobilization were estimated by 15N labelling and amino acids in xylem sap were analysed. Removal of shoots resulted in great declines of exogenous N flows derived either from N 2 or from NHnNO 3 during the first week of regrowth, thereafter recovery increased linearly. Mineral N uptake as well as N 2 fixation occurred mainly between the 10th and 18th day after removal of shoots while exogenous N assimilation in intact plants remained at a steady level. Nitrogen remobilization rates in defoliated plants increased by at least three to five-fold, especially during the first 10 days following shoot removal. Compared to control plants, contents of amino acids in xylem sap, during the first 10 days of regrowth, were reduced by about 72% and 82% in NH4NO 3 grown and in N z fixing plants, respectively. Asparagine was the main amino acid transported in xylem sap of both treated plants. Its relative contents during this period significantly decreased from 75% to 59% and from 67% to 36% respectively in non-nodulated plants and in nodulated ones. This decline was accompanied by compensatory increase in the relative contents of aspartate and glutamine.

Root Nitrogen Remobilization and Ion Status of Two Alfalfa (Medicago sativa L.) Cultivars in Response to Salinity Stress

“…Source/sink relationships for N within the plant can be affected by farming practices such as periodical defoliation by cutting or grazing (Kim et al. 1993).…”

“…1991). Kim et al. (1993) reported that the N remobilization rate in defoliated plants is generally increased by up to three to five times during the first days after cutting.…”

“…Source/sink relationships for N within the plant can be affected by farming practices such as periodical defoliation by cutting or grazing (Kim et al 1993). Changes in N source/sink relationships for N during re-growth of non-nodulated alfalfa plants following removal of shoot alfalfa has been studied, and it has been reported that stems and taproots serve as the source organs and re-growing shoots as a strong sink in defoliated plants (Kim et al 1991).…”

Nitrogen assimilation in alfalfa: isolation and characterization of an asparagine synthetase gene showing enhanced expression in root nodules and dark-adapted leaves.

“…Moreover, when effective To whom correspondence should be addressed. E-mail vance004 Qmaroon.tc.umn.edu; fax 61 2-649-5058. nodules are removed from alfalfa, clover, and bird's-foot trefoil, Asn concentrations in xylem sap decrease significantly (Maxwell et al, 1984;Kim et al, 1993). Higher amounts of Asn are also formed in plant tissues that are stressed and/or carbon limited.…”

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

“…Vance and Heichel (1981) and Kim et al (1993a,b) demonstrated that the lower rates of carbohydrate synthesis observed during the first 10 d after cutting were associated with a reduction of N 2 fixation and mineral N uptake by roots of alfalfa. During this early stage of regrowth, the N requirements for new shoot growth mainly relies on taproot N reserves (Kim et al 1993a;Volenec et al 1996;Avice et al 1997a,b). However, there is a current lack of information concerning the impact of the initial N reserve status on the morphological aspects of alfalfa regrowth.…”