Possibility of using a SPAD chlorophyll meter to establish a normalized threshold index of nitrogen status in different potato cultivars

Leaf SPAD values are thought to be an indicator of potato nitrogen (N) status; however, when making plant N diagnosis, no single threshold leaf SPAD value can be used for all potato cultivars because of cultivar variation. We conducted field experiments over 3?years to test whether the difference in SPAD values between the upper and lower leaves was consistent among potato cultivars, and whether this value responded to N application levels. The results showed cultivar affects the SPAD values of the fourth and eighth leaves (SPAD_L4 and SPAD_L8), but not SPAD_L4-8, the difference between SPAD_L4 and SPAD_L8. Moreover, the SPAD_L4-8 values responded well to N application levels. Therefore the SPAD_L4-8 could be applied as a general index of N status across different potato cultivars, and establishing a threshold SPAD_L4-8 value might be more useful in guiding N fertilization recommendations for different potato cultivars.