Changes in radiation capture and use in response to the nitrogen status of durum wheat cultivars at different developmental stages

Context: Cereal nitrogen economy throughout the whole crop cycle is a critical driver of biomass and grain production, but while some periods of N deficiency have a detrimental impact on grain yield, others exert no effect. An appropriate indicator of the N nutritional status of a crop is the N nutrition index (NNI). Objective: i. to quantify NNI from mid tillering until anthesis; ii. to analyze its effects on the capture and use of radiation and biomass production; iii. to highlight cultivar variability in these traits. Methods: A three-year field trial was carried out in a Mediterranean climate to assess NNI of four durum wheat (Triticum turgidum ssp. durum Desf.) cultivars grown under three N fertilization rates (0, 80 and 160 kg N ha-1) from mid tillering until anthesis, and analyze its effects on radiation capture (LAI and fraction of intercepted radiation, FIPAR) and use (RUE, Radiation Use Efficiency) Results: The level of N shortage imposed with the N0 treatment increased from a moderate shortage at tillering (NNI = 0.86 on average) to a maximum N stress at anthesis (NNI = 0.45). The relationship between NNI and RUE was linear until booting, but became curvilinear at anthesis.The relative response of LAI to NNI variation was greater than that observed for RUE, but it did not affect proportionally the FIPAR because the LAI values above 2 observed after tillering already assured an FIPAR between 75% and 92%,. Cultivar Aureo was characterized by the greater NNI (frequently above 1), N uptake, and N allocated to pseudo-stems across all developmental stages. Conclusion: All the cultivars shared common relationships between NNI and RUE, LAI and FIPAR, implying that differences between cultivars in RUE, LAI and FIPAR were partly due to their different NNIs and not to a different response to NNI. RUE was more sensitive to NNI variation than radiation interception expressed as FIPAR, but using LAI as a proxy for radiation interception gives opposite results. Implication: Lines with high potential grain protein percentages may be selected early in the season using NNI above 1 as selection criterium. Cultivars, such as Aureo, which are able to store large quantities of N in their vegetative tissues and attain high NNI values at anthesis are penalized in terms of their RUE during the period in which the potential grain number is determined.