Direct acquisition of organic N by white clover even in the presence of inorganic N

Abstract

Aim This study was conducted to answer the question of whether clover can absorb asparagine in the presence and absence of inorganic nitrogen, as well as to determine the resulting concentration of post-uptake compounds closely involved in asparagine metabolism. Methods Clover was grown at two asparagine concentrations (10 μM and 1 mM) supplied in both the absence and presence of ammonium nitrate. Using dual-labeled 13 C 15 N-asparagine, the uptake rate was analyzed via bulk 15 N and 13 C excess and the detection of intact 13 C 15 N-asparagine in white clover. ResultsThe results from the two methods indicated greater utilization of 13 C 15 N-asparagine in the 10 μM treatment than in the 1 mM treatment. The 13 C 15 Nasparagine uptake rate was higher when 13 C 15 N-asparagine was provided alone than when it was supplemented with inorganic nitrogen. Up to nine times lower uptake rates were obtained when intact 13 C 15 N-asparagine was measured than when bulk 15 N and 13 C excess were analyzed. The labeled amino acids that are closely related to 13 C 15 N-asparagine metabolism (aspartic acid, glutamic acid and glutamine) were detected in clover roots and shoots. Conclusions Using two different methods, white clover's potential to absorb intact asparagine, even in the presence of inorganic nitrogen, was confirmed. The dual-methodology approach employed in this study demonstrates how the post-uptake metabolism can affect quantification of amino acid uptake.

Newly depolymerized large organic N contributes directly to amino acid uptake in young maize plants

“…In fertile soil, crop N uptake is viewed as being dominated by the inorganic N forms (Schimel & Bennett, ; Hill & Jones, ), although most plants have the capacity for organic N uptake (Chapin et al ., ; Kielland et al ., ; Näsholm et al ., ; Paungfoo‐Lonhienne et al ., ). Most knowledge about plant organic N uptake comes from studies based on the addition of individual amino acids including glycine (Näsholm et al ., ), alanine (Hill & Jones, ), asparagine (Czaban et al ., 2016a, ), glutamate (Jones et al ., ), mixtures of amino acids (Forsum et al ., ; Jämtgard et al ., ; Sauheitl et al ., 2009b) or short peptides (Paungfoo‐Lonhienne et al ., ; Hill et al ., 2011a; Soper et al ., ). These studies confirm direct plant uptake of organic N, but further indicate that the microbial competition for these small organic N compounds may reduce the importance of organic N uptake for crop N nutrition (Biernath et al ., ; Jones et al ., ; Hill & Jones, ).…”

“…Uptake of organic N is determined mainly using (carbon‐13 ( 13 C), 15 N) dual‐labelled compounds with the uptake estimated based on the ratio of bulk 13 C and 15 N isotope uptake (Näsholm et al ., ), and the intact uptake of the added compound confirmed by compound‐specific isotope analysis (Näsholm et al ., ; Sauheitl et al ., 2009a; Czaban et al ., 2016a). Although these methods mainly have been used for individual amino acids, the application in studies of larger dual‐labelled organic N is conceivable, despite the inclusion of several challenges.…”

Plant–microbe competition: does injection of isotopes of C and N into the rhizosphere effectively characterise plant use of soil N?

“…Commonly, direct unmodified amino acid uptake by roots is estimated by the use of dual-labelled organic forms of N with correlated co-location of C and N labels, in the same proportions as in the supplied amino acid, considered to be evidence of intact amino acid uptake Weigelt et al, 2005;Quinta et al, 2015;Wilkinson et al, 2015). In a much smaller number of investigations, compound-specific recovery of isotopic labels in plant tissues has been used to estimate intact uptake (Persson & N€ asholm, 2001;Persson et al, 2006;Sauheitl et al, 2009a;Warren, 2012;Czaban et al, 2016). The recovery of the supplied dual-labelled amino acid in plants by compound-specific methods probably provides the most reliable proof of intact amino acid uptake (Persson & N€ asholm, 2001;Warren, 2012;Czaban et al, 2016).…”

“…In a much smaller number of investigations, compound-specific recovery of isotopic labels in plant tissues has been used to estimate intact uptake (Persson & N€ asholm, 2001;Persson et al, 2006;Sauheitl et al, 2009a;Warren, 2012;Czaban et al, 2016). The recovery of the supplied dual-labelled amino acid in plants by compound-specific methods probably provides the most reliable proof of intact amino acid uptake (Persson & N€ asholm, 2001;Warren, 2012;Czaban et al, 2016). However, in both bulk and compound-specific methods, any interpretation is hindered by the rapid loss of amino acid C in plant and microbial respiration, large plant and soil pools of C relative to tracers, and high background levels of the most frequently used C isotope, 13 C (N€ asholm et al, 2009b;Sauheitl et al, 2009a;Warren, 2012;Wilkinson et al, 2014;Moran-Zuloaga et al, 2015;Quinta et al, 2015).…”

“…However, in both bulk and compound-specific methods, any interpretation is hindered by the rapid loss of amino acid C in plant and microbial respiration, large plant and soil pools of C relative to tracers, and high background levels of the most frequently used C isotope, 13 C (N€ asholm et al, 2009b;Sauheitl et al, 2009a;Warren, 2012;Wilkinson et al, 2014;Moran-Zuloaga et al, 2015;Quinta et al, 2015). The separation of C and N labels due to rapid postuptake transformation of amino acids by plants is particularly problematic for quantification using compound-specific methods, with comparisons showing lower estimates of uptake than by bulk tissue IRMS (N€ asholm et al, 2009b;Sauheitl et al, 2009a;Warren, 2012;Czaban et al, 2016).…”