Differential effects of elevated atmosphere CO2 concentration on root growth in association with regulation of auxin and cytokinins under different nitrate supply

An extensive root system is one of the highly desirable traits for plant growth and adaptation to environmental stresses which is affected by the availability of atmosphere CO2 and nitrogen (N) nutrition. The objectives of this study were to examine the interactive effects of elevated CO2 and nitrogen on root growth and underlying regulatory mechanisms related to hormones metabolism in tall fescue (Festuca arundinacea). Plants (cv. ‘Barlexas’) were hydroponically grown in nutrient solution with low N (0.25 mM) and moderate N (4 mM) conditions and exposed to ambient CO2 concentration (400 µmol·mol−1) or elevated CO2 concentration (800 µmol·mol−1) for 35 days. Elevated CO2 differentially promoted root growth under different nitrate levels. The longest root length, root biomass and all types of roots including the length, number, volume and surface area of both crown and lateral roots were enhanced by elevated CO2 under moderate nitrate but without significant effects on crown root number under low nitrate condition. The extent of elevated CO2-promotion on lateral root was greater under moderate nitrate than that under low nitrate condition. The improvement of elevated CO2 on root growth under moderate nitrate condition was related with increased content of indoleacetic acid (IAA) and isopentenyl adenosine (iPA) through up-regulating genes in auxin synthesis, cytokinin synthesis and down-regulating cytokinin degradation gene. Our results indicated that elevated CO2 had differential promotive effects on crown and lateral root growth in tall fescue under low and moderate nitrate conditions in association with the regulation of endogenous auxin and cytokinin metabolism.