Threshold values of canopy reflectance indices and chlorophyll meter readings for optimal nitrogen nutrition of tomato

Abstract

Sustainable and optimal economic N management requires correct and timely on-farm assessment of crop N status to detect N deficiency or excess. Optical sensors are promising tools to assess crop N status throughout a crop or at critical times. The ability of optical sensor measurements of canopy reflectance (Crop Circle ACS 470) and leaf chlorophyll (SPAD 502 chlorophyll meter) to assess crop N status was evaluated weekly throughout an indeterminate tomato crop. Strong linear relationships with the optical sensor measurements were obtained, throughout most of the crop, for both (a) crop N content for ranges of 1.5-4.5%, and (b) the nitrogen nutrition index (NNI) for ranges of 0.4-1.3. The relationships of the optical sensor measurements to crop NNI were generally equal to or slightly better than with crop N content. Indices based on reflectance in the red, the normalised difference vegetation index (NDVI) and the red vegetation index (RVI), were the best predictors of crop N status in terms of goodness of fit, earliness and maintenance of relationships throughout the crop. SPAD chlorophyll readings and reflectance indices based on reflectance in the green, the normalised difference vegetation index on greenness (GNDVI) and the green vegetation index (GVI), were good indicators of crop N status for most of the crop, but with lower goodness of fit in the latter part of the crop. The linear relationships between sensor indices and readings with NNI or crop N content, each week, demonstrated the potential for using proximal canopy reflectance indices such as NDVI and RVI, and chlorophyll meter for monitoring crop N status of indeterminate tomato crops. Threshold values for optimal crop N nutrition for canopy reflectance indices and for chlorophyll meter readings were derived for each day of measurement from the relationships between optical sensor measurements and NNI by solving for NNI = 1. The threshold values obtained for each index and type of measurement varied during the crop cycle. The approach developed for determining threshold values from NNI can facilitate on-farm use of optical sensors for monitoring crop N status, by enabling assessment of whether crop N status is excessive, deficient or adequate.

Propuesta para la determinación del contenido de clorofila en hojas de tomate

“…Existe una clara correlación entre las unidades SPAD (Análisis del Desarrollo de la Planta en el Suelo, por sus siglas del inglés Soil Plant Analysis Development) y la concentración de clorofila y nitrógeno total en las hojas de tomate, según autores como Rodríguez et al (1998), Padilla et al (2014), Camen et al (2017). Los valores SPAD se obtienen en función del principio que considera que parte de la luz que llega a la hoja es absorbida por la clorofila y la parte que se refleja entra en contacto con la celda SPAD-502 (Minolta Camera Co.…”

“…El contenido de clorofila y absorción de nitrógeno se han correlacionado con las unidades SPAD en diversas condiciones ambientales en gramíneas (Hidema et al, 1992;Piekielek y Fox, 1992), con distintos sustratos y aportes nutricionales en tomate (Rodríguez et al, 1998) y según la tolerancia de distintos híbridos de tomate al estrés salino (Camen et al, 2017). Autores como Wood et al (1992) y Padilla et al (2014), indican que el SPAD-502 permite hacer seguimiento al contenido de nitrógeno y clorofila durante el desarrollo del cultivo lo que facilita el manejo nutricional de la planta. Rodríguez et al (1998), concluye que el estado fenológico del cultivo afecta el valor de las unidades SPAD lo que puede explicarse por la actividad fisiología de la hoja que varía en el tiempo y tiene directa relación con el color de la hoja desde su emergencia a la senescencia.…”

“…Por esta razón, se propone una metodología, rápida y no destructiva, para determinar el estado nutricional de la planta, según el contenido de clorofila y nitrógeno en la hoja, con el medidor de clorofila SPAD-502. La metodología propuesta es una modificación a la indicada por Padilla et al (2014).…”

New Trends in the Fertigation Management of Irrigated Vegetable Crops

“…The SPAD produces a dimensionless index positively correlated to the concentration of chlorophyll in the leaf blade, which is in turn related to the concentration of N in plant tissues. Once the values of N sufficiency are assessed, the corresponding SPAD index value can be used to support N fertilization [104][105][106][107]. Some authors have found variability in the SPAD index when the culture was carried out in the presence of abiotic stress [106].…”

Nitrogen Related Diffuse Pollution from Horticulture Production—Mitigation Practices and Assessment Strategies

“…Petiole sap testing involves taking leaf petioles and collecting the sap, which is then tested for nitrate and/or potassium using portable meters [119]. Chlorophyll meters are used to measure the "greenness" of individual leaves and to monitor nitrogen status after proper calibration against the crop with proper fertilization [120,121]. Monitoring the soil N status by collecting soil samples, or placing suction cups beyond the root system to collect soil solution, also constitute efficient mitigation practices, as they provide information regarding the N stored in the soil and/or moving beyond the roots' depth.…”