*2.3. Analysis of Nitrite Content*

The nitrite (NO2 −) content was analyzed using a Promega Griess Reagent System assay kit (Promega, USA). Sweet potato leaves were homogenized on ice using a pestle and mortar in 25 mM HEPES buffer (pH 7.7), incubated for 20 min at 4 ◦C, and centrifuged for 10 min at 12,000 rpm. The NO2 − concentration was determined using the Griess Reagent System, according to the manufacturer's instructions. Briefly, 40 μL of each sample was added to 50 μL sulfanilamide solution in a 96-well microtiter plate. The microtiter plate was covered and incubated at room temperature for 10 min before 40 μL NED (N-(1-naphthyl) ethylenediamine) solution was added to each well. The plate was then incubated at room temperature for a further 10 min. The absorbance of nitrite was read at 540 nm using a plate reader.
