Stable diurnal growth rhythms modulate root elongation of Arabidopsis thaliana

Arabidopsis root growth kinetics were investigated with high temporal and spatial resolution in combination with detailed statistical analysis to resolve presence of diurnal modulation of root tip displacement. In particular, high resolution video imaging was used to monitor root-tip displacement of Arabidopsis thaliana seedlings over several days. Root growth kinetics were sampled and statistically analyzed in two different photoperiods: long day LD (16 h-8 h) and equal LD (12 h-12 h) light. Diurnal root elongation kinetics exhibited five highly reproducible phases, one of these being a maximum of root growth rate displayed 1-2 h after the light on phase. Then, during the later part of the light period, root growth rate decreased. Several hours before darkening root elongation rates started to increase, with a profound decrease immediately after darkness. Subsequent to this dark-induced reduction in root growth rate the remaining part of the night was characterized by increasing growth activity. Together, tip elongation, in Arabidopsis roots is modulated by strong diurnal rhythms that are maintained in both photoperiods used and also in continuous illumination.