Assessing post‐anthesis nitrogen uptake, distribution and utilisation in grain protein synthesis in barley ( Hordeum vulgare L.) using 15 N fertiliser and 15 N proteinogenic and non‐proteinogenic amino acids

We investigated the effects of nitrogen (N) availability during the vegetative phase on (a) post‐anthesis N uptake and (b) its translocation into ears in barley plants grown in a greenhouse at two levels of N: low (50 mg N kg −1 sand) and optimal N supply (150 mg N kg −1 sand). Plants in the two N treatments were fertilised with the same amount of labelled 15 N [50 mg 15 N kg −1 sand at 10% 15 N exc (N excess , i.e. N exc , is defined as the abundance of enriched stable isotope minus the natural abundance of the isotope) applied as 15 NH 4 15 NO 3 ] 10 days after anthesis (daa). In a separate experiment, the uptake and transport into ears of proteinogenic and non‐proteinogenic amino acids were studied to determine whether a relationship exists between amino acid transport into ears and their proteinogenic nature. Plants were fed with either 15 N‐α‐alanine, a proteinogenic amino acid, or 15 N‐α‐aminoisobutyric acid, a non‐proteinogenic amino acid. Both these amino acids were labelled at 95.6% 15 N exc . Results showed that N accumulations in stems, leaves and especially in ears were correlated with their dry matter (dm) weights. The application of 150 mg N kg −1 sand significantly increased plant dm weight and total N accumulation in plants. During their filling period, ears absorbed N from both external (growth substrate) and internal (stored N in plants) sources. Nitrogen concentration in ears was higher in optimal N‐fed plants than in low N‐fed plants until 10 daa, but from 21 to 35 daa, differences were not detected. Conversely, 15 N exc in ears, leaves and stems was higher in low N‐fed plants than in optimal N‐fed plants. Ears acted as strong sink organ for the post‐anthesis N taken up from the soil independently of pre‐anthesis N nutrition: on average, 87% of the N taken up from the soil after anthesis was translocated and accumulated in ears. Low N‐fed plants continued to take up N from the post‐anthesis N fertiliser during the later grain‐filling period. The increase of pre‐anthesis N supply rate led to a decrease in the contribution of nitrogen derived from post‐anthesis 15 N‐labelled fertiliser (N dff ) to total N in all aboveground organs, especially in ears where 44% and 22% of total N originated from post‐anthesis N uptake in low N‐fed and optimal N‐fed plants, respectively. The experiment with labelled amino acids showed that there was greater transport of proteinogenic amino acid into the ear (50% of total 15 N) than non‐proteinogenic amino acid (39%). However, this transport of the non‐proteinogenic amino acids into ear suggested that the transport of N compounds from source (leaves) to sink organs (ear) might not be intrinsically regulated by their ability to be incorporated into storage protein of ears.