The employment of SPAD index for N status diagnosis of crops has been largely studied in recent years. Our objective was to evaluate the employment of SPAD index for nitrogen status diagnosis of Japanese cucumber plants (Taisho hybrid) under protected cultivation. The randomized block design was used, arranged in a 2x5 factorial design, with two nitrogen sources (ammonium sulfate and ammonium nitrate) and five nitrogen doses (0; 75; 150; 300 and 600 kg ha(-1)), with four replications. At the early flowering stage, total chlorophyll concentration, SPAD index and total nitrogen concentration in the fourth fully expanded leaf from the apex of Japanese cucumber plants were evaluated. No significant effects of nitrogen sources and of the interaction between nitrogen sources and nitrogen doses were found under the evaluated characteristics. However, total chlorophyll concentration, SPAD index and total nitrogen concentration in Japanese cucumber leaves increased following a quadratic model depending on the nitrogen doses. The estimated critical levels of total chlorophyll concentration, SPAD index and total nitrogen concentration on Japanese cucumber (Taisho hybrid) leaves responsible for the highest fruit yield were 6.01 mg g(-1) of fresh matter, 52.33 units and 4.78 dag kg(-1) of dry matter, respectively. Significant linear and positive correlation between the evaluated characteristics were observed. The SPAD index can be used for nitrogen status diagnosis of Japanese cucumber plants.
