Aims Previous studies discovered unusual plasticity in root growth of Callistephus chinensis, in that fewer fine roots grow in pulse soil nutrient patches than in unfertilized patches. Our goal is to interpret this event via experimentation following the theory of plant physiological ecology. The experiment tested the following hypotheses: NO3–, NH4+ and method of application 1) significantly affect root growth and 2) significantly affect root hormones and 3) root hormones (auxin (IAA), abscisic acid (ABA), and cytokinin (zeatin riboside + zeatin) (CK (ZR + Z))) significantly affect root architecture (indicated by the length of the 1st order fine roots, the inter 1st order root length and the density of the 1st order fine lateral roots).
Methods The water culture approach was employed with treatments of two fertilizers (NO3– and NH4+), two methods of application (stable vs. pulse, i.e., pulse shift to high N concentration from low and mid concentration) and low, mid and high concentrations of the fertilizers (NO3–: 0.2, 1.0 and 18.0 mmol·L–1; NH4+: 0.2, 4.0 and 20.0 mmol·L–1). The experiment lasted 35 days after the treatments, the plants were harvested by the end of the experiment and all concerned parameters were measured including root mass, the above-mentioned root architecture parameters and the contents of root hormones.
Important findings The experimental treatments affected root mass and root hormones with different significances. The high concentrations of NO3– and NH4+ both greatly lowered the root mass and IAA. Values were lower in the NH4+ culture than in the NO3– one. Pulse shift treatment lowered both further. CK (ZR + Z) contents
did not respond to the concentrations and the methods of application, but to NO3– or NH4+; the latter led much lower CK (ZR + Z) contents than the former. Root mass had positive regression relationships with 1st order root length and the density of the fine lateral roots, had a linear regression relationship with IAA, a negative exponential relationship with CK (ZR + Z) and complicated relationships with ABA. The different hormones had different relationships with the root architecture parameters. They were all irrelevant with 1st order inter-root length, ABA had relationships with 1st order lateral root density only and IAA and CK (ZR + Z) both had significant regression relationships with 1st order root length and 1st order lateral root density. The results established physio-ecological path relationships of heterogeneous nutrient stimulation→changes in root hormone contents→changes in root architecture→root plastic growth. As to the unusual root growth of C. chinensis, we infer that the species had very low tolerance to temporal variation in the N environment, particularly NH4+.