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Abstract
Nitrogen (N) is an essential nutrient for plant growth and development and is especially important in the production of high quality leafy green vegetables. In this experiment, leaf N concentration, chlorophyll concentration (Chl) and weight above fresh matter (AFM) of romaine lettuce (Lactuca sativa L. var. longifolia) were estimated by correlations between in situ SPAD and atLEAF readings. Lettuce was grown in high tunnels during 42 days and was irrigated at five nitrogen levels: 0, 4, 8, 12 and 16 mEq·L-1 of NO3-, based on the Steiner nutrient solution. The N concentration, Chl concentration and AFM were determined in the laboratory, while SPAD and atLEAF readings were measured in situ weekly. SPAD readings had high, positive and significant linear correlations with N (R2 = 0.90), Chl (R2 = 0.97) and AFM (R2 = 0.98); atLEAF readings had a similar linear correlation with N (R2 = 0.91), Chl (R2 = 0.92) and AFM (R2 = 0.97). Besides, SPAD and atLEAF readings had high, positive, and significant linear correlation (R2 = 0.96). Thus, SPAD and atLEAF meters can be used to non-destructively and accurately estimate the N status of lettuce, in a reliable and quick manner during the crop production cycle. In addition, atLEAF is currently more affordable than SPAD.
 
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 In press - Online First. Article has been peer reviewed, accepted for publication and published online without pagination. It will receive pagination when the issue will be ready for publishing as a complete number (Volume 47, Issue 3, 2019). The article is searchable and citable by Digital Object Identifier (DOI). DOI link will become active after the article will be included in the complete issue.
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Highlights
Nitrogen (N) is an essential nutrient for plant growth and development
The N concentration, Chl concentration and above fresh matter (AFM) were determined in the laboratory, while SPAD and atLEAF readings were measured in situ weekly
The Chl concentration of the leaf is closely related to N concentration in the plant (Zebarth et al, 2002), monitoring plant Chl and N concentrations during production could be used as a management strategy to enhance plant growth, yield and marketability (Gitelson et al, 2003; Peng and Yuan, 2017)
Summary
Nitrogen (N) is an essential nutrient for plant growth and development. It is found in amino acids, nucleic acids, proteins and in chlorophyll (Chl). Portable nondestructive equipment has been used successfully with some plant species to quickly estimate the leaf Chl or N concentrations in situ (Loh et al, 2002; Abdelhamidg et al, 2003; Padilla et al, 2018a) and enable repeat measurements from the same leaf during a production cycle (Kalaji et al, 2017; Yamamoto et al, 2002). The SPAD 502 Plus Chlorophyll Meter (Konica Minolta®, Japan) is used for rapid and non-destructive determination of relative leaf Chl concentration (Gianquinto et al, 2004). It has been reported that the SPAD readings are related to the concentration of foliar
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