Study on the Relationship Between Cell Wall Components of Pericarp and Fruit Cracking in Litchi

Abstract

【Objective】By comparing the differences in cell wall components among litchi varieties with different susceptibility to fruit cracking, the study aims to investigate the effects of exogenous application of auxin analogue 2,4-D on the cell wall composition of pericarp and fruit cracking, and to further explore the relationship between the cell wall synthesis metabolism and fruit cracking incidence.【Methods】The changes in carbohydrate contents of two easily cracked litchi varieties ('Guiwei' and 'Nuomici') and two not easily-cracked litchi varieties ('Huaizhi' and 'Lingfengnuo') after 75 days of flowering were compared, and the tissue structure and lignin distribution of the pericarps at the 60-85 d after flowering were observed. Fruit clusters of 'Nuomici' in two orchards were dipped into auxin analog 2,4-D, and the soluble sugar contents in the pericarps were determined by GC-MS. The starch and structural carbohydrate contents of the samples were detected by using a spectrophotometer. Transmission electron microscopy and lignin staining were used to observe the tissue structure of litchi pericarp.【Result】The contents of hexose including fructose, glucose and galactose in the pericarp of cracking susceptible variety 'Nuomici' were significantly lower than those of low-cracking varieties 'Huaizhi' and 'Lingfengnuo'. A complicate relationship between pectin contents and fruit cracking susceptibility was found. In the pericarp of 'Nuomici', the pectin content was only around 1.7 mg/g. And the fruit was bigger and susceptible to cracking. While in the cracking susceptible smaller fruit variety 'Guiwei', the pericarp contained high pectin (4.0 mg/g). 2,4-D treatment only temporarily increased the fruit setting rate of litchi in the early stage, but it had no significant impact on the final fruit setting rate. 2,4-D treatment could lead to an increase in the number of layers of pericarp cells and the blurring of the contour of inner pericarp cells; Compared with the control, the treatment with 40 mg/L 2,4-D resulted in an increase in pectin content, thickening of cell walls, and an increase in fruit cracking rate.【Conclusion】There are significant differences in the contents of carbohydrates in the pericarps of litchi varieties with different susceptibility to fruit cracking. Pectin may play a dual role in litchi fruit cracking. The development of pericarp requires dynamic changes in pectin synthesis and degradation. Pectin might play dual role in the cracking of litchi fruit. High pectin in the pericarp results in low expansion, smaller fruit size and higher cracking rate; while low pectin results in higher pericarp expansion, larger fruit size and higher cracking rate. 2,4-D treatment show no significant effect on the final fruit setting rate but will increase the risk of fruit cracking.

Keywords

• litchi (litchi chinensis sonn.)
• fruit cracking
• pericarp
• soluble sugar
• cell wall component
• 2,4-dichlorophenoxyacetic acid