Accumulation dynamics of biomass, Ca and Mg nutrient in the aboveground newborn organs of Guanximiyou pomelo tree

Abstract: 【Objective】Guanximiyou pomelo (Citrus grandis) is a famous and popular Citrus speciesnative to Pinghe County, Fujian Province, China. As an essential geographical product, it has a cultivation history of more than 500 years. With the rapid development of the pomelo industry, farmers apply largely and unreasonably nitrogen (N), phosphorus (P) and potassium (K) fertilizers for pursuit of high economic benefits, resulting in unbalance tree nutrients and poor soil quality in pomelo orchards. Also, calcium (Ca) and magnesium (Mg) are important mineral elements and have multiple physiological functions for the plant growth, yield formation and fruit quality of pomelo trees. However, less attention has been paid upon them and therefore deficiency of the two elements is common in pomelo orchards. To solve the problem Ca and Mg deficiency in Guanximiyou pomelo production, the present study investigated the dynamic changes in growth and development of the aboveground newborn organs and revealed the characteristics of Ca and Mg requirements in different growth periods of pomelo tree. The results will help to improve the quality and production efficiency of pomelo.【Methods】In 2019 and 2020, a field experiment was conducted in Pinghe County of Fujian province. Ten- year- old red flesh pomelo trees were selected as the materials, and the samples of the aboveground new organs were collected 5 times, including the spring shoot germination stage (SSGS), flowering stage (FS), young fruit stage (YFS), fruit expanding stage (FES), and fruit maturity stage (FMS), and their biomass and Ca and Mg uptake in leaves from shoots with (L+F) or without fruit (L-F), branch with (B+F) or without fruit (B-F), fruit (including pulp and peel), and fallen flowers and fruit (FFF) were measured.【Results】The biomass of aboveground newborn organs of the pomelo tree increased gradually with the growth process. In 2019 and 2020, the mean apparent biomass was 17.12 kg·plant^-1 at the harvest, and the biomass of leaf, branch, and fruit accounted for 24.14% , 3.89% , and 71.97% , respectively. The biomass was mainly distributed in L-F (23.19%), B-F (3.56%), and pulp (44.54%) organs, and the biomass of fallen flowers and fruit reached 0.57 kg · plant^-1 . At harvest across 2019 and 2020, the mean Ca concentration in L+F, L-F, B+F, B-F, pulp and peel were 29.63, 34.33, 30.32, 37.69, 2.08 and 8.80 g·kg^-1 , respectively, and the Ca concentration in FFF was 3.50 g·kg^-1 . The mean Mg concentration in L+F, L-F, B+F, B-F, pulp and peel was 2.43, 2.65, 1.60, 3.17, 0.91 and 1.19 g·kg^-1 , respectively, and the Mg concentration in FFF was 1.81 g·kg^-1 . In general, except for the Ca concentration in the pulp that decreased and Mg concentration in B-F that increased, the Ca concentration in each organ showed a gradually increase with time, and the Mg concentration decreased gradually. Ca and Mg concentration at harvest showed a trend of B-F ＞ L-F ＞ B+F ＞ L+F ＞ peel ＞ pulp. Also, except for the Mg accumulation in leaf that decreased at harvest stage, the accumulation of Ca and Mg in all organs increased gradually with the growth process, and the percentage of Ca and Mg in leaf and branch organs decreased but increased in fruit. They were mainly distributed in the peel and L-F. As a whole, the mean apparent accumulation of Ca and Mg were 223.58 and 25.34 g·plant-1 at harvest in 2019 and 2020, respectively. Ca accumulation in leaf, branch, and fruit accounted for 63.10%, 11.03%, and 25.87%, respectively, and Mg accumulation accounted for 42.75%, 7.92%, and 49.33%, respectively. At a planting density of 750 plant·hm^-2 , the annual aboveground newborn organs of full bearing Guanximiyou pomelo trees required Ca 169.18 kg·hm-2 and Mg 19.78 kg·hm^-2 . However, the nutrient loss or return of Ca and Mg from the fallen flowers and fruit reached 1.50 kg·hm^-2 and 0.77 kg·hm^-2 , respectively.【Conclusion】This study systematically quantified the characteristics of biomass and Ca and Mg nutrients in the aboveground newborn organs, and calculated the apparent Ca and Mg requirement in full bearing pomelo trees. These results suggested that Ca and Mg nutrients should be supplied timely and application of Ca and Mg containing fertilizers should be improved for Guanximiyou pomelo production in Pinghe County.