Measurement of Chlorophyll (Chl) pigments and greenness
A puncher obtained uniform size leaf discs (1.2 cm diameters) to measure chlorophyll concentration. Chl pigments were extracted with 80% ice-cold acetone, and the absorbance of Chl a and Chl b was measured using an ultraviolet/visible spectrophotometer ( DU-730, Beck Man Coulter Inc., USA) according to the previous method (Feng et al., 2019). Maize leaf greenness was measured by spad-502 plus (Konika-Minolta company, Tokyo, Japan).
Photosynthetic gas exchange characteristics
Photosynthetic parameters, including net photosynthetic rate (P n), transpiration rate (T r), stomatal conductance (G s), and intercellular CO2concentration (Mutlu et al.), were analyzed by using a portable photosynthesis system (Li-6400; LI-COR Inc., Lincoln, NE, USA) at six different positions from each leaf and presented as average. All the parameters were measured before noon (between 9:00 AM to 11:00 AM) under a steady light intensity of 1000 µmol m−2 s−1, environment temperature 25°C, and a CO2 concentration of 400 (µmol mol−1).