Stomatal density was calculated by counting number of stomata
from the flag leaf images taken with a Dino-Lite digital microscope
(AM411 series with ver. 1.4.1, Vidy Precision Equipment Co. Ltd,
Wuxi, China) without damaging the leaves. The image of calibration
sample was also saved to calculate the actual area of the image
using AxioVision SE64 software (Rel. 4.4.3, Carl Zeiss Microscopy,
Jena, Germany).