A study on the characteristics of dropped flower and fruit and their nutrient loss in Guanximiyou pomelo

Abstract: 【Objective】Pinghe county (24º02'-24º35'N, 116º54'-117º31'E) is located in the hilly region in Fujian Province, southern China, with acidic red soils, and has a subtropical oceanic monsoon climate with an annual mean air temperature and precipitation of 21.7 ℃ and 1 865.1 mm, respectively, which is beneficial for crop production. It is famous for Guanximiyou pomelo production, which has a planting area of 4.6×104 hm2 and a fruit yield of 176.6 ×104 t in China. However, there has been poor research about the characteristics of flower growth and nutrient loss due to flower and fruit abscission in pomelo trees.【Methods】In the flowering and fruit setting period from middle February to early April, 8-15 year old Guanximiyou pomelo trees in orchards in the dominant planting area of Banzai town, Ping-he county were selected as the experimental material to study the flower biological characteristics, including length, width, ratio of length to width, fresh weight, dry weight, water content, and nutrient concentrations in different flower classifications (Ⅰ-Ⅴ grade from bud to blooming). Dynamics of flower and fruitlet drop and biomass in falling flowers and fruit were traced, and the contents of nutrients including nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), and boron (B) were determined. The nutrient loss due to flower and fruit drop in pomelo trees was quantified. The results of the experiment will provide a scientific basis for nutrient management and rational fertilization in pomelo production during the flowering and fruiting period.【Results】Flower length, width, and ratio of length to width changed significantly with flower development. From Ⅰ to Ⅴ grade of flowers, the length increased from 8.1 to 22.5 mm, and the width from 5.6 to 22.8 mm, resulting in changes in the ratio of length to width. Besides, there were significant differences in fresh weight, dry weight, and water content between flower grades. The flower fresh weight increased from 467.9 to 2 977.1 mg from Ⅰ to Ⅴ grade, while the dry weight from 85.2 to 427.1 mg. Thus flower fresh weight increased faster than dry weight, resulting in gradual increase in flower water content from the initial bud to the blooming stage. The concentrations of N, Ca and B decreased gradually by 26.9%, 20.2% and 31.7% from grade Ⅰ to grade Ⅴ, respectively. In contrast, the concentration of K increased by 23.2% in grade Ⅴ compared with grade I flower. However, the concentrations of P, Mg, Fe, Cu, and Zn decreased first and then increased with the development of flower organs, while the concentrations of Mn maintained relative constant in different flower stages. The order of element concentrations was N > K> Ca > P > Mg > Zn > Fe > B > Mn > Cu during the flowering period. The numbers of flower and fruit drop and their biomass followed a trend of first rising and then declining and peaked at 41 d after flowering. For a single pomelo tree, the numbers of fallen flowers and fallen fruit, and their biomass reached 2080, 1264, and 636.8 g respectively. The concentrations of the macro-elements, N, P, K, Ca, and Mg, in the fallen flowers and fruit were 30.7-36.8, 3.3-3.8, 11.0-13.6, 3.7-4.4, and 1.8-2.3 mg · g^-1, respectively, while those of Fe, Mn, Cu, Zn, and B were 48.1-68.6, 30.0-37.3, 10.9-13.2, 38.8-69.6, and 40.2-53.6 mg·kg-1, respectively. The nutrient loss with the ,falling flowers and fruit was 21.6 g for N, 2.2 g for P, 7.7 g for K, 2.6 g for Ca, 1.3 g for Mg 36.4 mg for Fe, 21.6 mg for Mn, 7.5 mg for Cu, 33.2 mg for Zn, and 28.7 mg for B in a pomelo tree.【Conclusion】A large number of fallen flowers and fruit of pomelo plants not only consume a lot of nutrients but also affect the flower quality, fruit set, and fruit quality, and finally reduce the economic income of farmers. To prevent the biomass and nutrient loss due to excessive flower and fruit drop, the comprehensive management involving control of flower quantity and optimizing fertilization with both soil and foliar applications, should be applied especially during flowering period in‘Guanximiyou’pomelo orchards to improve its productivity and fruit quality.