Abstract: A field pot experiment was conducted using Xinmai26 (XM26) with weak resistance to late spring coldness and Yannong19 (YN19) with strong resistance to late spring coldness. Low temperature (2℃ and −2℃) stress treatment was carried out in intelligent ultra-low temperature light incubator during the anther differentiation period of young wheat spike differentiation, and 10℃ was used as CK control group. The length, width and thickness of the superior and inferior caryopsis of wheat were analyzed by sampling every 7 days after anthesis. At the same time, the microstructures of pericarp, endosperm cells and large and small starch grains were observed by paraffin sectioning technique, so as to explore the microstructures of caryopsis development in wheat under the late spring coldness during the anther differentiation period. The results showed as follows: (1) the length, width and thickness of wheat caryopsis decreased with the increase of cold stress (2℃→−2℃). The length, width and thickness of wheat caryopsis decreased by 1.17%−4.41%, 1.57%−10.22% and 1.42%−9.40%, respectively, at the mature stage. The ratios of length/width, length/thickness and width/thickness of inferior grain caryopsis were higher than that of superior grain caryopsis in all treatments at maturity stage. (2) The degradation rate of early caryopsis pericarp was slowed down. Meanwhile, the development of starch granules in caryopsis endosperm cells of two wheat cultivars was inhibited. (3) The circumference and area of large and small starch grains in wheat superior and inferior caryopsis were decreased by 8.17%−14.66%, 14.08%−17.98%, 0.94%−5.00% and 2.59%−10.03% in endosperm cells of dominant caryopsis at 28 days after anthems. The circumference and area of large and small starch grains decreased by 3.08%−10.31%, 6.59%−8.70%, 3.17%−6.39% and 11.85%−16.17% in endosperm cells of inferior grain caryopsis. The circumference and area of large starch grains decreased more in dominant grain caryopsis, while the circumference and area of small and medium starch grains decreased more in inferior grain caryopsis. In conclusion, late spring coldness during the anther differentiation period can slow down the degradation of pericarp cells of early wheat caryopsis development, inhibit the development of starch grains in endosperm cells, reduce the circumference and area of large and small starch grains in endosperm cells, and thus reduce the length, width and thickness of wheat caryopsis, resulting in the decrease of grain storage capacity of wheat, and ultimately reduce wheat grain weight.

Key words: Wheat, Late spring coldness, Superior and inferior, Caryopsis, Amyloplast