Objectives The physiological disorder known as “translucency disease” is prevalent in the pineapple cultivar ‘Tainong17’ (Golden Diamond), leading to a decline in fruit quality and significantly impacting the quality improvement and efficiency of the pineapple industry. Through field investigations, we studied the causes and mechanisms of translucency disease in pineapples.

Methods Using a multi-point sampling method, topsoil samples were collected from diseased and healthy fields in 14 pineapple orchards across five cities and counties in Hainan. These samples were analyzed for total N content, moisture content, and bulk density. Additionally, samples of diseased and healthy pineapple plants and fruits were collected. The fresh weight of the fruits was measured, and the N, Ca, and B contents, as well as the levels of hormones such as trans-zeatin (ZT), jasmonic acid (JA), and abscisic acid (ABA), were analyzed in the leaves, stems and fruits. Relationships between nutritional and physiological indicators and disease incidence were explored.

Results The total soil N, bulk density, and moisture content of the sick fields were 51.0%, 11.3%, and 14.8% higher than those in healthy fields, respectively. Compared to normal fruits, the contents of soluble solids, soluble sugars, and vitamin C in sick pineapple fruits were significantly reduced by averages of 12.5%, 12.0%, and 20.4%, respectively, the titratable acid contents were increased by 8.7%, resulting in a 19.5% decrease in the sugar-to-acid ratio and a significant decline in quality. Compared to normal pineapples, the fresh weight of sick fruits was significantly increased by 17.8%, but the dry matter content was reduced by an average of 17.5%. The N content in leaves and stems of diseased pineapples increased by 48.7%, and 48.3%; the Ca content in fruits decreased by 25.6%, while in stems and leaves increased significantly. The total B absorption and fruit B content in sick pineapples were reduced by 13.5% and 64.1%, respectively, although the leaf B content was higher than in normal pineapples. The levels of ZT, JA, ABA in fruits, and ABA in leaves of sick pineapples were reduced by 23.9%, 23.9%, 15.5%, and 38.1%, respectively, compared to normal pineapples. The structural equation modeling (SEM) analysis of factor relationships revealed that soil N had a positive impact on leaf N content and the incidence of translucency disease in pineapples, while it had a negative impact on fruit Cd content, ABA levels, and dry weight (P<0.05). Leaf N content had a positive impact on translucency disease and a negative impact on fruit ABA content, fruit Ca content, and fruit dry weight. Excessively high soil N content and an imbalance in N and Ca nutrition in the plant are closely related to the physiological changes and translucency disease in pineapple plants.

Conclusion Excessively high soil nitrogen content can lead to an imbalance in nitrogen metabolism in the plant, promoting root development and the transfer of photosynthetic products and nutrients to the fruits, while significantly reducing endogenous hormone levels. This, in turn, inhibits the fruit's competition for calcium and boron, leading to the decomposition of protopectin in the cell wall, decreases mechanical strength of the cell wall, and increases incidence of translucency disease in pineapples.