2.5. Determination of nitrate
Nitrate content was measured according to the method of
Cataldo et al. (1975). A total of 2.0 g lettuce samples (shoot FW)
were cut into small pieces and mixed with 10 mL deionized water.
After heating in a 80 ◦C water bath for 30 min, the precipitate were
removed by centrifuging at 13,000 × g for 10 min, and 0.2 mL of the
supernatant was mixed with 19 mL of 4 M NaOH and 0.8 mL of 5% (w/v) salicylic acid (in pure H2SO4), the nitrate content was measured
with a spectrophotometer (UV-1200, MAPADA, Shanghai,
China) at a wavelength of 410 nm.