Effects of low nitrogen stress on the physiological and morphological traits of roots of different low nitrogen tolerance maize varieties at seedling stage

Nitrogen (N) is a key nutrient that influences the growth, yield and quality of maize. In order to determine the effects of low N stress on the physiological and morphological traits of roots of maize at seedling stage, a hydroponic experiment of four maize varieties was carried out. The experiment consisted of two low N tolerant varieties (‘ZH311’ and ‘CD30’), two low N sensitive varieties (‘XY508’ and ‘SB2’) and three N treatments N concentration at normal of 15 mmol·L^-1 (B3), low N stress at 0.05 mmol·L^-1 (B1) and low N stress at 0.5 mmol·L^-1 (B2). Root morphological indexes (root length, root diameter, root number, root volume, root surface area), bleeding intensity and activities of nitrate reductase (NR), glutamine synthelase (GS) and glutamate dehydrogenase (GDH) were measured on the 7th and 14th days after low N stress. The results showed that the bleeding intensity and activities of NR, GS and GDH of the root system of maize varieties with different low N tolerance at seedling stage declined under low N stress treatments B1 and B2. There were remarkable differences among the maize varieties in terms of the above indexes. The decreasing ranges of the above indexes of the low N tolerant varieties (29.8% and 8.7%, 46.9% and 39.6%, 7.3% and 4.4%, 31.3% and 19.8%) were less than those of the low N sensitive varieties (37.0% and 27.5%, 68.8% and 56.6%, 24.5% and 18.7%, 60.7% and 42.7%). The NR and GDH activities of the low N tolerant varieties were respectively 1.4 and 1.35 times those of the low N sensitive varieties under B1 treatment. The effects of low N stress on the growth of aboveground system of maize seedling were greater than those on the belowground system. This significantly decreased shoot dry weight, significantly increased root-to-shoot ratio and decreased root number. Root-to-shoot ratio of low N sensitive varieties increased by 81.6% and 25.4% and root number declined by 22.2% and 31.1% against those of the low N-tolerant varieties (61.0% and 21.1%, 19.8% and 19.4%) under B1 and B2 treatments, respectively. With increasing of low N stress degree, root length of low N tolerant varieties increased and root diameter decreased. This resulted in root elongation and thinning, increasing N absorption area and responsiveness to low N stress. Compared with low N sensitive varieties, low N tolerant varieties had better root morphology and stronger root physiological activity to respond to low N stress. The low N tolerant maize varieties also had relatively stable growth and better adaptability to low N stress. Adaptability to low N stress of low N tolerant maize varieties increased, while that of low N sensitive varieties decreased with prolonged low N stress time.