Single plant traits such as green biomass, spike dry weight, biomass and nitrogen (N) transfer to grains are important traits for final grain yield. However, methods to assess these traits are laborious and expensive. Spectral reflectance measurements allow researchers to assess cultivar differences of yield-related plant traits and translocation parameters that are affected by different genetic material and varying amounts of available N. In a field experiment, six high-yielding wheat cultivars were grown with N supplies of 0, 100, 160 and 220 kg N ha^-1. Wheat canopies were observed spectrally throughout the grain-filling period and three spectral parameters were calculated. To describe the development of the vegetative plant parts (leaves + culms) and the spikes, plants were sampled four times during grain filling. Dry weights and the relative dry matter content were recorded for leaves + culms and spikes. The N status of the plants was assessed by measuring the total N concentration and by calculating the above-ground N uptake. In case of an equal and sufficient N supply of 160 kg ha-1, the water index, R₉₇₀/R₉₀₀, explained the biomass partitioning between spikes and vegetative plant parts. The NIR/NIR-based index, R₇₆₀/R₇₃₀ described nitrogen partitioning and the spikes dry matter with an r² of up to 0.89. The final grain dry matter was best assessed by spectral indices offering information about physiological maturity. Observing effects of various N supply, good correlations were found between spectral indices and single plant traits throughout grain filling but varied with N supply and development stage. The normalized difference vegetation index, NDVI, was strongly affected by the saturation effects of increased N concentration. The red edge inflection point, REIP, predicted plant traits with r² values up to 0.98.  However, in plants with advanced senescence, the REIP was less efficient in describing plant traits. The NIR-based index R₇₆₀/R₇₃₀ was closely related to yield-related plant traits at early grain filling. Compared to the REIP, the R₇₆₀/R₇₃₀ index was resistant to strong chlorophyll decays being able to predict plant traits at late grain filling, with r² values of up to 0.92. Spectral reflectance measurements may represent a promising tool to assess phenotypic differences in yield-related plant traits during grain filling.