Near-infrared reflectance spectroscopy allows rapid and simultaneous evaluation of chloroplast pigments and antioxidants, carbon isotope discrimination and nitrogen content in Populus spp. leaves

•NIRS is proposed for the evaluation of photoprotective and antioxidant compounds.•Seasonal measurements were performed on leaves of clones of the model plant Populus.•Calibration results were particularly accurate for total phenolics, Δ13C and N%.•VAZ, EI, neoxanthin and β-Car content calibration was also successful.•Multiple results on Populus leaves with a single measurement save time and money.