Co-occurrence of organic and inorganic N sources influences asparagine uptake and internal amino acid profiles in white clover

Abstract

24Direct plant uptake of organic nitrogen (N) is important for plant N nutrition, but we lack knowledge of 25 how the concentration of external N forms (organic and inorganic) -influence organic N uptake and plant N status. We 26 investigated the uptake of the amino acid asparagine (Asn) in white clover in the presence of different nitrate (NO 3 -), 27Asn, and total N concentrations. White clover seedlings were for one week exposed to combinations of NO 3 - 28 µmol N kg -1 sand DW) and Asn (3-30 µmol N kg -1 sand DW), where after the Asn uptake rate was determined by 29 addition of 13 C 4 -Asn. Shoot and root Asn content and amino acid profiles were also analyzed. Increasing external NO 3 -30 and total N concentrations decreased 13 C 4 -Asn uptake rates and internal clover Asn content. Furthermore, total N 31 affected clover amino acid profiles from non-essential amino acids at low N doses to the dominance of essential amino 32 acids at increasing N doses. Asn uptake rate in white clover is reduced by increasing inorganic N, but not by increasing 33 organic N concentrations. Furthermore, plant amino acid profiles are likely to be a more sensitive indicator of N supply 34 and descriptor of the N status. 42of assimilating organic N into proteins is lower than that of inorganic N, mainly because of its carbon content. This 43 carbon bonus makes it more beneficial for plants to take up organic than inorganic N (8). We recently reported white 44 clover (Trifolium repens, cv. Rivendel) uptake of Asn in a sterile hydroponic solution (3) and in soil conditions at field 45 relevant concentrations (9). Other legumes such as alfalfa (Medicago sativa), alsike-clover (Trifolium hybridum L., cv. 46Stena), and red clover (T. pratense L., cv.Betty) have also been reported to take up organic N in laboratory or soil 47 studies (10, 11). Although the literature underlines the potential of amino acid absorption by legumes, the importance of 48 amino acid uptake is unclear. This uncertainty is due to a lack of knowledge on how the amino acid uptake is influenced 49 by the presence of other N forms that occur in soil simultaneously and can be acquired by N 2 -fixing legumes. To 3 50 advance our understanding on how much amino acids contribute to the legume N budget, plant growth should be 51 compared in soils that differ in N quality and quantity. 52Information on the co-occurrence of different N forms and their influence on root absorption of amino acids mainly 53 comes from studies with non-legume crop and tree species. One of the common findings is that uptake of amino acids is 54 increased, while the absorption of inorganic N is reduced in mixtures of different N sources. Perennial ryegrass (Lollium 55 perenne) exposed to a single and equimolar mixture of N sources (2 mM total N) doubled the uptake of glycine when 56 supplied with NO 3and NH 4 + compared to when supplied alone (12). Spring wheat (Triticum aestivum L. cv. Amaretto) 57 downregulated the assimilation of NO 3when exposed to a mixture of glycine (1 mM) and NO 3 -(1 mM...