Water-based root exudates of Molinia caerulea (L.) Moench disrupt root nitrogen metabolism in Quercus petraea (Matt.) Liebl. seedlings with a fast negative effect on budburst

Abstract Key message Molinia caerulea (L.) Moench has been observed to significantly reduce budburst in Quercus petraea (Matt.) Liebl. seedlings through water-based M. caerulea root exudates. This suggests direct allelopathic effects between the two species. In terms of nutrient uptake, oak roots primarily take up nitrogen in the forms of ammonium and glycine. Interestingly, the application of root exudates from M. caerulea doubled the nitrate uptake in oak roots. Moreover, gene sets involved in nitrogen metabolism within oak roots exhibited strong deregulation when treated with M. caerulea root exudate, indicating that the interaction between these two plant species alters the nitrogen metabolism in the oak roots. Context Oak regeneration encounters numerous impediments, including interactions with Molinia caerulea (L.) Moench, an understory grass species widespread in temperate forests. Besides competition for resources, our previous work suggested that the interaction between oak and M. caerulea may involve allelopathic processes. Aim This study tested the hypothesis that M. caerulea affects the budburst dynamics, early growth of oak seedlings, and root N uptake transport and assimilation systems. Methods Potted oak seedlings ( Quercus petraea (Matt.) Liebl.) were watered with M. caerulea root exudates or water for 6 weeks during April and May 2021. The capacity of oak seedlings to take up nitrogen in its main molecular forms was characterized by an influx analysis of isotopically labeled ( 15 N) nitrate, ammonium, and glycine on excised roots using the “teabag” technique. Concomitantly, targeted transcriptomics were carried out to monitor changes in gene expression related to nitrogen metabolism. Results The treatment resulted in reduced budburst rates (− 50%), together with an early reduction in height increment. Kinetics of N influx rates revealed that ammonium, and to a lesser degree glycine, were the predominant forms in which N was taken up by roots. Application of M. caerulea root exudate barely increased the ammonium influx rate and had no effect on the glycine influx rate. By contrast, the nitrate influx rate, despite its low values, doubled after application of the exudate. Targeted transcriptomic analysis revealed a regulatory shift in gene sets associated with key mechanisms underlying N uptake, assimilation, and long-distance transport in oak roots. Conclusion Interactions between a perennial grass, M. caerulea , and a woody species, Q. petraea , do not only rely upon direct competition for resources. Allelopathic processes must be taken into consideration when designing regeneration operations.