Analysis of boron nutrition status in soils and trees and its relationship with fruit granulation in ‘Guanximiyou' pomelo

Abstract: 【Objective】Honey pomelo of‘Guanximiyou' (Citurs grandis) is one of the important citrus varieties in China, and the largest production and export region is in Pinghe County (24°02′-24°35′N, 116°53′-117°31′E) , Fujian Province, which is known as "the hometown of honey pomelo". At present, the annual yield of honey pomelo is about 120×104 t and the planting area was more than 5×104 hm².But, there are some problems caused by excessive application of chemical fertilizers and unreasonable nutrient ratio of fertilization in honey pomelo orchards, which results in the quality reduction. The serious problem of juice sac granulation, which reduces the edible quality and commercial value of honey pomelo, is an urgent problem to be solved. Boron (B) is an essential nutrient element for plants and plays important physiological and biochemical roles in the flowering and fruit development of citrus. In pomelo production, farmers often unreasonably apply and/or overuse B fertilizer. However boron is a micro-element, whose critical ranges in soils and plants are very narrow between deficiency and excess.Excessive B can cause toxicity to plants. In order to provide a theoretical and practical basis for rational application of B containing fertilizers, the B status in soils, leaves and fruits of honey pomelo in orchards in Pinghe County and its relationship with fruit quality were investigated.【Methods】Relative paired samples of soils, leaves and fruits were collected from 314 10-15 year old pomelo orchards in 10 main production towns including Wenfeng, Qiling, Luxi, Jiufeng, Nansheng, Guoqiang, Banzai, Shange, Xiazhai and Xiaoxi in Pinghe County, Fujian Province. Representative samples of each orchard were collected from five individual trees and soil. In order to show the B status of different soil depths, a total of 144 soil samples at depths of 0-20 cm, 20-40 cm, and 40-60 cm were collected from 48 orchards with age above 15 years. All the soil samples were taken 10 cm inward from the canopy drip line for avoiding fertilization ditch. Soil available boron was extracted with boiling water and determined with curcumin colorimetry. The total boron content in leaves and fruits was determined by curcumin colorimetry after drying and ashing. The total content of calcium (Ca) in fruit was determined by atomic absorption spectrometry. Four to five fruit were taken from each orchard for the determination of fruit granulation rate.【Results】In the sampled pomelo orchards, the soil available B content ranged from0.05 to 4.61 mg∙kg-1, with an average value of 0.77 mg∙kg-1. Based on the classification, the percentage of low (<0.5 mg ∙kg-1) , adequate (0.5-1 mg ∙kg-1) and high (>1 mg ∙kg-1) was 43.57%, 33.54% and22.89%, respectively. The content of available B in soils of different town was quite different, with a coefficient of variation of 88.31%. The percentage of soils with low level of available B in Wenfeng and Guoqiang town was 68.75% and 65.38%, respectively, while that in Qiling and Jiufeng town was64.71% and 35.00%, respectively. Additionally, the content of available B in different soil layers decreased with soil depth, reflecting an obvious surface accumulation characteristic. The available B contents in 0-20 cm, 20-40 cm and 40-60 cm soil layers were 0.59, 0.40 and 0.33 mg∙kg-1, respectively. It was significantly higher in 0-20 cm soil layer than in 20-40 cm and 40-60 cm soil layers, but there was no significance between 20-40 cm and 40-60 cm layers. Besides, the total B content in leaf ranged from9.61 to 252.02 mg∙kg-1, with an average value of 72.47 mg∙kg-1. The percentage of low (<15 mg∙kg-1) , adequate (15-50 mg∙kg-1) and high (>50 mg∙kg-1) was 0.32%, 24.84% and 74.84%, respectively. The proportion of pomelo orchards with high leaf B reached 94.12% and 80% in Luxi and Jiufeng town, respectively. The proportion of orchards with high leaf B was significantly higher than the percentage of orchards with high available soil B, indicating excessive use of B fertilizer as foliar spray. The total B content of fruit (or pulp) ranged from 1.13 to 57.50 mg∙kg-1 with an average of 14.07 mg∙kg-1. Compared with fruit samples with low level B content (<8 mg∙kg-1) , the incidence of severe juice sac granulation in samples with middle (8-16 mg∙kg-1) and high (>16 mg∙kg-1) B level increased by 31.87% and31.89%, respectively. Correlation analysis showed that a significant negative correlation between fruit B content and fruit Ca/B ratio (y = 304.81 x-0.63, r = 0.77**) . Our results also showed that the proportion of severe juice sac granulation (granulation rate > 40%) decreased with the increase in Ca/B ratio.【Conclusion】Taken together, the risk of juice sac granulation increases as fruit B content increases in orchards in Pinghe County. The incidence of juice sac granulation can be reduced through increasing fruit Ca/B ratio by applying calcium fertilizer. For sustainable production of honey pomelo in Pinghe County, the excessive use of B containing fertilizer as foliar spray should be avoided while calcium containing fertilizer application should be increased.