Changes in root architecture and endogenous hormone levels in alfalfa under phosphorus stress

To explain changes in plant growth, root architecture, and endogenous hormones of different root-type alfalfa to phosphorus (P) stress (non-stressed control, 1.00 mmol·L⁻¹ KH₂PO₄, versus P stress treatment, 0.01 mmol·L⁻¹ KH₂PO₄). We used the sand culture of rhizomatous-rooted Medicago sativa ‘Qingshui’ (QS), tap-rooted M. sativa ‘Longdong’ (LD), and creeping-rooted M. varia ‘Gongnong No. 4’ (GN) to study the performance with contrasting degrees of P tolerance. After 34 days of P stress treatment, values for plant height, leaf area, specific root length, specific surface area, zeatin (ZT) level, and P content were decreased in response to stress treatment. In contrast, the contents of indole-3-acetic acid (IAA), abscisic acid (ABA), and gibberellin (GA₃), as well as the root/shoot ratio, total root length, taproot length, root angle, and the number of root tips and lateral roots, increased in the year and next year, respectively. The topological index and fractal dimension (FD) were small, while the fractal abundance (FA) and the average link length were large. The root branching patterns were dichotomous branching. The amplitude of this response of plant biomass, plant height, leaf length, leaf area, root/shoot ratio, total root length, number of root tips, root angle, specific surface area, and FD in GN of the year and next year was much smaller than those in LD and QS under low-P stress. From the point of view, GN was more tolerant than LD and QS under P condition.