Leaf color. At 0, 9, 12, and 12 d at 5 or 10 °C plus 24 h at 20 °C external color was measured on the upper epidermis using a chromameter (CR 400 series, Minolta Co., Japan) operating with a C illuminant and 11-mm-diameter light projection tube aperture. The chroma meter was calibrated with a white standard tile. The CIELAB values L* (lightness), a* and b* were measured. The results are presented as hue angle (90° hue angle = yellow; 180° = green), which was calculated from the measured a* and b* values using the formula h=arc tangent (b*/a*)