Nondestructive diagnosis of nitrogen status in cauliflower using a Dualex® leaf-clip sensor: calibration of an NBI-based critical curve across three seasons

Abstract Traditional assessment of plant nitrogen (N) relies on destructive chemical analysis, whereas optical sensors provide rapid, nondestructive estimates of crop N status. This study calibrated and validated a fluorescence/transmittance-based leaf-clip sensor (Dualex®) for diagnosing N status in cauliflower (Brassica oleracea var. botrytis) with Barcelona cultivar in a loam soil classified as Torripsamments over three seasons (2012–2014) in Agoncillo (La Rioja, Spain). Field trials comprised four N availability treatments in a randomized complete block design. At multiple dates, we measured leaf chlorophyll (Chl), epidermal flavonols (FLAV) and the Nitrogen Balance Index (NBI = Chl/FLAV) with Dualex®, together with plant biomass and total N in leaves and heads. The Dualex® NBI was linearly related to leaf total N concentration, enabling the derivation of a non-linear NBI–biomass critical curve analogous to critical N dilution models. For biomass ≥1 Mg ha−1, the critical curve correctly classified 92% of non-deficit treatments and 79% of deficit treatments (R2 = 0.8). Proposed NBI sufficiency ranges were 34–36 (≈1 Mg ha−1, ≈50% ground cover), 31–33 (≈2 Mg ha−1, button stage), and 27–29 (≈4 Mg ha−1, pre-harvest). These results demonstrate that Dualex® NBI can support in-season N management in cauliflower, reducing the need for destructive sampling.