Observation on the morphological differentiation of flower buds of sweet cherries in a greenhouse

Abstract: 【Objective】The initiation time of flower bud differentiation and the speed of differentiation of sweet cherries were greatly influenced by variety characteristics, climatic factors and growing measures. The temperature and humidity conditions in a greenhouse were obviously different from those in natural environment, where the phenology of sweet cherries was advanced, and the flower bud differentiation was greatly affected. In this paper, the flower bud of sweet cherries in a greenhouse was observed, to confirm the exact time and characteristics of the morphological differentiation. It will provide a theoretical basis for improving the quantity and quality of flower buds.【Methods】The flower buds of‘Tieton'‘Hongdeng'and‘Крупноплодная'cultivars were collected at regular intervals in Changli and Laoting from March to August, 2015. Twenty-thirty flower buds were collected from the bouquet spurs, then the outer scales of flower buds were stripped off before they were fixed in the FAA solution quickly. After gradient dehydration and transparence, the flower buds were buried in paraffin. The flower buds were sliced at a thickness of 10μm, dyed with Safranin O and fast green FCF, and then sealed with neutral gum. The morphological differentiation of flower buds at each sampling time was observed by paraffin section. The phenology of sweet cherries and the temperature and humidity in the greenhouse were recorded, and the MH-WH01 recorder was suspended 1.5 meters high between the tree rows.【Results】The differentiation speed with two varieties was different, but the morphological characteristics were the same in each period. The morphological differentiation of flower buds was dividedinto 6 stages, including initial differentiation stage, floral primordium stage, sepal primordium stage, petal primordium stage, stamen primordium stage and the pistil primordium stage. At undifferentiating stage, the growing point of flower bud was thin, pointed, and covered with scales. At initial differentiation stage, the growth point became wide and flat, which marked the beginning of the differentiation period. In middle differentiation initiation stage, the basal protuberance of the bud formed at the base of growth cone. In late initial differentiation stage, the growing point continued to split, the top was widened and the crown was flattened and hemispherical. The initial differentiation stage lasted about 3-4 weeks. Then, the apical cells of the growing point continued to divide into a small bulge and gradually differentiated into the floral primordium. The growing point can differentiate into 1-7 floral primordia, mostly 2-4 floral primordia. The split cells surrounding the apical edge of the floral primordium continued to divide into the sepal primordium. Then, the cells in the inner side of sepal primordium differentiated into small protrusions, when the petal primordium and stamen primordium formed in sequence. In the end, a protuberance formed in the middle of the flower bud, which was the initial pistil, and then continued to split up and extend until the pistil formed. The flower buds of‘Tieton'began to appear in mid-March in Changli, and then morphological differentiation initiated. At April 6, the ratio of flower buds that began to differentiate reached 54.5%, and the pistil primordium was evident on May 25. The differentiation time lasted about 85 days. With both‘Hongdeng'and‘Крупноплодная', the flower bud differentiation initiated on late Marchand ended on late June, lasting about 90 days. The times of flowering and fruit maturity of‘Tieton'in Laoting were 15 days earlier than those in Changli. The morphological differentiation time was advanced correspondingly. Flower buds appeared on early March in Laoting. There were about 33.3% flower buds that differentiated on March 23, which was 10-15 days earlier than those in Changli. The temperature in the greenhouse had great influence on the differentiation speed of flower buds. In Laoting, the morphological differentiation began on mid-March and completed on late June, lasting about 100 days. The temperature in a greenhouse in Changli was higher, so the flower bud differentiation was faster than that in Laoting, lasting about 85 days only. Flower bud differentiation of‘Tieton'began in the stone-hardening period, while flower bud differentiation of‘Hongdeng'and‘Крупноплодная'began in the mature stage【.Conclusion】The morphological differentiation of flower buds was divided into 6 stages, including initial differentiation stage, floral primordium stage, sepal primordium stage, petal primordium stage, stamen primordium stage and pistil primordium stage. Flower bud differentiation of‘Tieton'began in stone-hardening period, lasting about 85 days.Flower bud differentiation of‘Hongdeng'and‘Крупноплодная'began in the mature stage, lasting about 90 days. The initiation time of flower bud differentiation cannot be determined according to the time of maturity, it should be judged by observing the differentiation characteristics of flower buds. The high temperature accelerated the morphological differentiation of flower buds. For sake of different varieties, the relationship between morphological differentiation time and maturity time was different, but that pattern was relative stable for the same variety.