Nitrogen (N) fertilization strategies aim to limit environmental pollution by improving potato crop N use efficiency. Such strategies may use indicators for the assessment of in season crop N status (CNS). Leaf polyphenolics (flavonoids) content appears as a valuable indicator of CNS. Because of their absorption features in the UV part of the spectrum, flavonoids compounds can be measured by rapid and non-destructive optical methods generating chlorophyll fluorescence. The objective of this research was to compare the use of leaf flavonoids content as potential indicators for the evaluation of CNS with other recognized plant-based indicators such as chlorophyll content measured by transmittance, reflectance, or fluorescence.

Trials were conducted in Belgium in 2010 on two potato cultivars: Charlotte and Bintje. The experiments included five increasing N rates for each cultivar. Leaf flavonoids content was determined using two devices, the Dualex and Multiplex (Force-A, Paris, France). Leaf chlorophyll content was measured with a SPAD/HNT chlorophyll-meter (Yara, Oslo, Norway), a Cropscan radiometer (Cropscan, Rochster, USA) and Dualex and Multiplex devices. The measurements were made periodically during the potato growth cycle. Plant tissue samples were collected and analyzed allowing the calculation of the biomass N concentration and the N nutrition index. The indicators performances were evaluated by ANOVA and orthogonal contrast (SAS software) on the basis of three criteria: 1) sensitivity to N (indicators strongly related to N doses); 2) stability of the diagnosis (absence of interaction with the date of measurements) and 3) earliness of the diagnosis (early assessment of relevant CNS).

Neither flavonoids nor chlorophyll content of the leaf, considered alone, was able to address successfully all requirements. The combination of leaf flavonoids content to leaf chlorophyll content as ratios suggests that these indicators could be used as valuable tools to assess the potato CNS.