Evaluation and Determination of Lettuce (Lactuca sativa L.) Nitrogen Status Using Chlorophyll Meter under Greenhouse Conditions

Leaves Chlorophyll content can be used as an indicator to determine nitrogen status in plants. The soil–plant analyses development (SPAD) meter is one of the most widely used tools in crop nitrogen management studies. The current research was aimed to investigate crop nitrogen status, introduce a standardized index called the normalized chlorophyll meter index (NSPAD) and find its relationship with nitrogen nutrition in lettuce. For this purpose, lettuce was cultivated in the research greenhouse of the College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran during two periods. Different nitrogen fertilizer treatments were applied at six levels with three replications. The values of dry matter, nitrogen concentration and chlorophyll were measured seven times during the growth period. Nitrogen nutrition indices (NNI) and NSPAD were calculated for the first period and the NSPAD-NNI relationship was extracted. Then the obtained relationship was verified by the data of the second cultivation. The results showed that the chlorophyll meter readings and nitrogen concentration have a positive correlation with the increase in the level of fertilizer application. Also, the validation results of NSPAD-NNI relationship showed a good performance with R2, MBE and RRMSE values equal to 0.79, -0.01 and 11.41%, respectively. Therefore, the developed relationship can be used as a fast, non-destructive and reliable approach for sustainable nitrogen fertilizer management of lettuce grown under greenhouse conditions.