With the maturity of the leaves, the main changes of the green leaves are the increase in the number of chloroplasts, and the shape of the chloroplasts gradually takes on a spindle shape, and the color shows from light green to dark green. The same is true for the changes of the yellow leaf parts. Its chloroplast number gradually increases and its internal structure gradually completes, including the increase of grana stacking and the decrease of osmophilic granules. The appearance color of the chloroplast was from bright yellow to dark yellow. Secondly, at the same stage, the chloroplast morphology of the green leaves was normally spindle-shaped, with a large number of grana stacked into grana lamellae, neatly arranged, and a large number of starch granules (Fig. 1A-D, I-L, Q-T). The chloroplasts in the yellow leaves were nearly round, with incomplete structural development, such as no obvious stacking of grana, and a large number of osmophilic granules aggregated (Fig. 1 E-H, M-P, U-X).
Cut the yellow leaves and green leaves in 3 stages into 2mm × 2mm size, fixed with 2.5 % glutaraldehyde solution, and rinsed 3 times with 0.1 M phosphate buffer (pH 7.0) 3 times for 15 min each time, fixed with 1% osmic acid for 4 h, and rinsed with phosphate buffer for 3 times. Conventional gradient ethanol series dehydration is treated with the mixture of embedding agent and acetone with a volume ratio of 1/1 and 3/1. The pure 812 resin was embedded, polymerized at 70 ℃ for 8 hours, sliced with an ultrathin slicer at 60-80 nm, and stained with uranyl acetate and lead citrate. The images were collected to analyze and compare the internal changes of leaves at different stages and positions.