Effects of recommended fertilization on fruit quality and pulp texture in mandarin orange ‘Satsuma mandarin' and ‘Nanfeng tangerine'

Abstract：【Objective】China is one of the most important citrus cultivation countries with the largest total cultivated area and production in the world. However, fruit quality declines with planting time, such as worse chewy characteristics, higher content of acids, less juice, abnormal shape, and separated juice sacs. The poor pulp mastication reflected by a coarse feeling in the mouth with rough fiber residue after patient chewing reduces texture quality of citrus. The fruit pulp mastication character is an important factor to evaluate fruit quality of citrus, and is related mainly with dietary fiber, especially pectin. Cell wall is composed of cellulose, hemicellulose, pectin, and sometimes lignin. Pectin is one of the important compounds that harden the cell wall. When it is degraded by enzymes during fruit maturation, tissue softens gradually. Cellulose and hemicellulose are also degraded by cellulase during this period.The fruit pulp mastication characteristics can be quantified using a texture analyzer that measures the shearing force required to slice through the fruit segment. Fertilization exerts significant effect on the chewiness of fruit pulp. In this study, the effects of fertilization on the quality and texture of citrus fruit were examined.【Methods】Two citrus materials, ‘Satsuma mandarin' (20 years old, in Yichang, Hubei province, with recommended fertilization treatment from 2013 to 2016) and‘Nanfeng tangerine' (6 years old, in citrus cultivation test field of College of Resources and Environment, Huazhong Agricultural University, in Wuhan, Hubei province, with recommended fertilization treatment in 2016) , fertilizedwith different compositions of chemical fertilizers were used. One was recommended fertilization treatment (OPT, with N, P2 O5, K2 O rates of 0.64, 0.28, 0.38 kg·plant^-1) , in which the ratios of N, P2 O5 and K2 O applied were 1.05: 1.05: 1: 1.5, 0.9: 0.9: 1: 1.5 and 0.2: 1.2: 1: 1.5 in flower bud differentiation period (early March) , fruit set period (early May) , fruit expanding period (late July) and mature period (midOctober) , respectively. Another treatment was the conventional fertilization treatment, serving as the control with N, P2 O5 and K2 O rates of 0.71, 0.29 and 0.43 kg/plant, respectively. N application in flower bud differentiation period (early March) , fruit set period (early May) , and fruit expanding stage (late July) was at a ratio of 4.09: 4: 1. P was all applied in the fruit expanding stage, and K applied in budbreak period and fruit expanding period at a ratio of 1: 2.57. Fruit texture characteristics (shear work, shear force, hardness, springiness, chewiness, and resilience) and the contents of insoluble dietary fiber and pectin in different parts of citrus fruit were measured to show the fruit pulp mastication characteristics.【Results】The shear force, hardness, springiness, gumminess, chewiness, resilience, and the contents of hemicellulos, lignin and protopectin in juice sacs were higher in Nanfeng tangerine fruit than in Satsuma mandarin fruit, which suggests that the Satsuma mandarin is better than Nanfeng tangerine in pulp mastication characteristics. Compared with the conventional fertilization or the control, the recommended fertilization treatment had the following effects. (1) The treatment significantly increased the fruit redness, yellowness, brightness, and the soluble solid content, decreased the amount of titratable acids, fruit weight and pericarp thickness in Satsuma mandarin, and improved the appearance quality and the commercial value. The effects were similar in Nanfeng tangerine, but the differences did not reach significant level. (2) The recommended fertilization significantly reduced the maximum shear force, chewiness, hardness, springiness and gumminess but increased resilience and cohesiveness in Satsuma mandarin. These results indicated that the fruit chewiness was improved more in Satsuma mandarin than in Nanfeng tangerine. (3) The recommended fertilization significantly reduced hemicellulos content in juice sac and membrane, and protopectin content of cell walls in Nanfeng tangerine, and also significantly reduced cellulose content, soluble pectin in juice sac, and cell wall components in Satsuma mandarin. All these changes suggested that the fruit pulp mastication characteristics were improved by the recommended fertilization treatment.【Conclusion】The contents of insoluble dietary fiber and pectin in different parts of citrus fruit decreased and thus pulp mastication characteristics were improved by the recommended fertilization in Satsuma mandarin, but there were no significant effects in Nanfeng tangerine. However, further study is needed to evaluate the long-term effects of the recommended fertilization.