Low nitrogen supply inhibits root growth but prolongs lateral root lifespan in cotton

Primary and lateral roots are the main components of the root system in cotton, and their development and lifespan are critical for the uptake of water and nutrients (e.g., nitrogen). However, the responses of the daily growth dynamics and lifespan of cotton roots to low nitrogen are poorly understood. In this study of the whole growth period of cotton plants, two cotton cultivars (K836 and K837) were grown in pots in a greenhouse with 138 mg·kg-1 nitrogen fertilizer as the control (normal nitrogen) and no nitrogen fertilizer as the low nitrogen treatment. The growth of primary and lateral roots was investigated using the in situ root phenotyping platform RhizoPot. The root responses of both cultivars to low nitrogen were consistent except for the correlation between lateral root diameter and length on the 11th d after lateral root emergence. Root growth was significantly inhibited under low nitrogen, manifested as substantial reductions in root length, surface area, volume, and dry matter weight, which decreased by 23.91–26.88 %, 34.92–37.59 %, 32.71–37.94 %, and 17.92–21.62 %, respectively, in comparison with the control. Low nitrogen treatment reduced the diameter of primary roots by shortening the duration of its rapid accumulation, by 7.54–10.92 d. The smaller diameter of lateral roots was owing to low nitrogen treatment increasing the proportion of finer roots compared to the control. The effect of low nitrogen on the thickening rate of lateral roots showed stage-specific differences accompanied by elongation of lateral roots. In the early stage, the lateral root length of the low nitrogen treatment was longer than that of the control because lateral root elongation was higher, while the opposite pattern was observed in the later stage. Low nitrogen treatment significantly increased the lifespan of both the primary and secondary lateral roots, by 3.84–3.93 d and 4.95–5.46 d, respectively. Detailed studies revealed that low nitrogen treatment induced the greatest increase in the survival of lateral roots with larger diameters. Collectively, the results of this study indicate low nitrogen treatment not only reduced primary root thickness, but also resulted in shorter and thinner lateral roots with a longer lifespan relative to the normal nitrogen treatment.