*2.4. Determination of Mineral Contents*

The plant leaves were powdered to a fine powder using a small crusher after drying for five days at 50 ◦C. In a 50 mL crucible, 0.1 g of each sample powder was digested with 5 mL of concentrated HNO3 and 1 mL of HClO4 (70%). The mixture was then heated to 150–200 ◦C on a hot plate until the digest became semi-dried. The cooled sample was dissolved in deionized water to a total volume of 20 mL before the analysis [49,50].

The content of K was measured using a flame photometer (Model: FP6431, Shanghai Yidian Analysis Instrument Co., Ltd., Shanghai, China). The Ca and Mg contents were determined using an atomic absorption spectrophotometer (Model: 3110, Thermo Scientific, Oxford, UK). The concentrations of P3+ were determined using UV–Vis spectrophotometry (UV-2007, Shimadzu Global Laboratory Consumables Co., Ltd., Shanghai, China). Inductively coupled plasma mass spectrometry (iCAP Q ICP-MS, Thermo Fisher Scientific, Germany) was also used to determine the content of Fe2+. The ICP-MS operating conditions were as follows: the radio frequency (R.F.) power was 1550 W, the nebulizer gas flow was 1.01 L/min, the auxiliary gas flow was 0.8 L/min, and the cool gas drift was 14 L/min.
