The pivotal role of secondary nutrients and micronutrients in regulating fruit quality and root exudates metabolism profile of citrus.

Background and aims: Ensuring adequate and balanced nutrient supply is crucial for enhancing citrus fruit quality. Some plants could modulate nutrient bioavailability in soil through the enhancement of root exudation. The study aimed to reveal how citrus change its root exudates metabolism profile to adapt to the soil with secondary nutrients and micronutrients deficiency. Methods: Citrus sinensis Osbeck cv. Newhall ×Poncirus trifoliate (L.) Raf grown in Ganzhou citrus orchards was used to evaluate the effect of soil applied and foliar sprayed secondary nutrients and micronutrients (Ca, Mg, B, Fe, Zn, Se) on the mineral elements concentration, external and intrinsic quality of citrus. The hydroponic experiment was conducted with Poncirus trifoliate (L.) Raf grown either in full nutrient solutions or in specific nutrient shortage conditions (Ca, Mg, B) for 45 days to obtain root exudate and perform untargeted metabolomic analysis (UHPLC-MS/MS). Results: Adequate supplementation of secondary nutrients and micronutrients effectively enhanced the nutritional value and intrinsic quality (TSS/TA) of citrus. 791 features were identified in citrus root exudates. Some metabolites was identified as root exudates for the first time, such as hecogenin, bergaptol and limonin. In the absence of secondary nutrients and micronutrients, citrus roots will secrete more substances related to plant disease resistance, nutrient acquisition and plant-microbial interactions, such as resveratrol, cinnamaldehyde, scopoletin, and phloretin. Conclusion: These findings highlight the value of secondary nutrients and micronutrients in regulating fruit quality and root exudates metabolism profile of citrus. It reveal the strategies of citrus to mitigate the nutrient deficiency in soils. [ABSTRACT FROM AUTHOR]