Could Leaf SPAD Values (Chlorophyll index) Compliment Nitrogen, Phosphorus, Potassium, Calcium, Sulfur, Magnesium, and Iron Nutrient Status in Romaine Lettuce (Lactuca sativa)

Measurement of SPAD values (relative chlorophyll index) gives instant, repetitive, non-destructive estimation of plant nitrogen status. A study was conducted to verify the technique in Romaine lettuce (Lactuca sativa) for the nutritional status of elements nitrogen (N), phosphorus (P), potassium (K), calcium  (Ca), sulfur (S), magnesium (Mg) and iron(Fe). The treatments consisted of N, P, K, Ca, S, Mg or Fe deprived solutions plus a control with complete modified Hoagland nutrient solution. SPAD value measurement of N, P, K, Ca, S, Mg and Fe deficient plant leaves at different stages of development, leaf position on the plant and physiological age corresponded significantly with the visual nutrient symptoms as compared to normal. The SPAD value of N deficient matured leaves and, S and Fe deficient young leaves declined with the physiological age of the plant. Whereas, the SPAD value of the entire K deficient plants had higher SPAD value than normal. This may give a false indication of normal plants. Therefore, SPAD value needs to be used with caution, especially in K deficient leaves, which appeared darker green with a SPAD value in the ascending order of 35 matured < 37 young < 39 old < 45 recently matured leaves, while normal leaf tissue SPAD values were 28 young < 30 matured < 33 old < 34 recently matured leaves. Greener leaf pigmentation does not always represent healthy plant. There was no significant difference in the SPAD value at early P deficient compared to normal leaves. SPAD value could be used to monitor and manage the nutritional status of Romaine lettuce provided the measurements are specific to leaf position, area of the leaf surface and the physiological age of the leaf. Significant reduction in growth was noted in N, P, K, Ca, S, Mg and Fe deprived plants, with dry mass declined by 18%, 3%, 29%, 9%, 16%,  and 17%, respectively, compared to normal.