Petiole sap nitrate concentration to assess crop nitrogen status of greenhouse sweet pepper

Abstract

Vegetable production requires improved nitrogen (N) management practices. Monitoring petiole sap nitrate concentration ([NO3−–N]) is a simple and cheap method to evaluate crop N status. The sensitivity of petiole sap [NO3−–N] to assess crop N status of sweet pepper was evaluated. Three sweet pepper crops were grown in different cropping seasons, each with an autumn-winter growing period. The crops commenced in 2014, 2016, and 2017. Combined fertigation and drip irrigation frequently applied (every 1–4 days) complete nutrient solution throughout each crop. The crops were grown in a greenhouse in soil. Five N treatments as N concentrations were applied throughout each crop: N1 (2.0–2.4 mmol L−1); N2 (5.3–6.2 mmol L−1); N3 (9.7–12.6 mmol L−1); N4 (13.1–16.1 mmol L−1); N5 (16.7–20.0 mmol L−1). These corresponded to very deficient, deficient, conventional, excessive and very excessive N supply. Petiole sap [NO3−–N] was determined every 1–2 weeks and related to Nitrogen Nutrition Index (NNI), which was used as an indicator of crop N status. For each of the N treatments in each crop, petiole sap [NO3−–N] was relatively constant throughout the crop. The relationship between petiole sap [NO3−–N] and NNI, for pooled data from the three pepper crops, was described by (a) the polynomial equation NNI=−1.10E−07×Sap2+0.000473×Sap+0.5514 with an R2 of 0.84, and (b) the segmented linear equations NNI=0.00034×Sap+0.572 and NNI = 1.04, with an R2 of 0.83. Sufficiency values for maximum growth of sweet pepper were obtained by (a) solving the polynomial equation for NNI = 1.0, and (b) using the intercept value of the horizontal line of the segmented linear regression. The corresponding sufficiency values for the duration of a complete crop cycle were 1441 and 1367 mg NO3−–N L−1, respectively. A sufficiency value of 1400 mg NO3−–N L−1 was rounded-off and suggested for the duration of a complete crop cycle of greenhouse-grown sweet pepper in SE Spain. The relationships between petiole sap [NO3−–N] and NNI, and the derived sufficiency values for the flowering and early fruit growth, and harvest phenological stages were similar to those determined for the entire crop. Petiole sap [NO3−–N] is a sensitive and effective method to monitor crop N status of sweet pepper.