Root morphology, configuration and nitrogen absorption kinetics of cassava cultivars with different nitrogen efficiencies

Objectives The root morphological configuration and nitrogen (N) absorption kinetics were compared between N efficient and inefficient cultivars of cassava under low N stress, which would help to illustrate the key indicator for the N efficiency and provide reference for cassava breeding and production.

Methods A pot experiment was conducted in the solar tunnel of National Key Laboratory, Guangxi University. A two-factor (cultivar × N level) split-plot design was employed. The two cassava cultivars were N-efficient SC10 and inefficient SC205, which were grown under N stress (no nitrogen application, N0) and normal N supply (applying N 55.2 mg/kg soil, N1). At 60 days after emergence of cassava, the whole root system was taken out of pot carefully and washed by water. The root images were taken using EPSON 2000 root scanner and analyzed using WinRHIZO PRO root analysis software. The plant was divided into root, stem and leaf and weighed, and the N contents were assayed. Root lamination test was conducted inside the large root observation system. The nitrogen absorption kinetics was tested using modified exhaustion method. The root morphology and configuration characteristics and the NO₃^–-N uptake kinetic parameters of the two cultivars were compared.

Results Compared with N1, the dry weight of plant and N accumulation in plant were significantly decreased in N0. The decrease in SC10 were by 37.4% and 46.2%, respectively, and those in SC205 were by 69.4% and 74.1%, respectively, significantly higher decrease was in SC205 than in SC10. Under the N0 condition, the N accumulation in roots, stems and leaves of SC10 were all significantly higher than those of SC205, and the N accumulation in whole plant of SC10 was152.5% higher of that of SC205. The decrease of the total root length, root surface, fine root length of SC10 were by11.0%, 10.0% and 20.4%, respectively, whereas those of SC205 were by 35.9%, 27.7% and 50.2%, respectively in N0. Significantly higher decrease was in SC205 than in SC10. Low N condition induced root growth to deeper soil. Under N0 condition, SC10 developed a root system of inverted triangle in shape, with a maximum depth of 180 cm, while SC205 had a root system of elliptical in shape, with maximum depth of 130 cm. N uptake kinetics experiment showed that the roots of SC10 had higher affinity to NO₃^– than SC205, with the Km value of 3.27 mmol/L in SC10 and 7.87 mmol/L in SC205.

Conclusions The root of N-efficient cassava cultivar has higher affinity to NO₃^– than inefficient cultivar. Under N stress, N efficient cultivar could develop a relatively larger and deeper root system, which could accumulate more nitrogen in root and aboveground parts of plant, and thus alleviating N stress.