The Diurnal Physiological Change in Root of Paulownia Elongate

The growth rhythm of "slow in the day and fast at night" due to the growth inhibition by high light and temperature in the day has been fully documented. However, whether plants still follow this growth rhythm under the condition of no such daytime environment stress remains unclear. Here, we examined the diurnal changes of plant biomass, fine root morphology, fine root chemical composition, root respiration and root acid phosphatase activity of Paulownia elongate in a growing house without apparent environmental stress. Results showed the root acid phosphatase activity (243.50 ± 43.30 μm/g/h, 151.48 ± 8.37 μm/g/h, p < 0.01), weight-based root respiration rate (266.25 ± 21.30 nmol/g/s, 179.43 ± 62.97 nmol/g/s, p = 0.08) and length-based root respiration rate (1.55 ± 0.32 nmol/m/s, 0.95 ± 0.28 nmol/m/s, p = 0.06) in the day were higher than those at night, which is contrary to the aforementioned growth rhythm. No difference in shoot and root biomass was observed between day and night, which might be attributed to the large biomass difference among individual plants masking the tiny day-night biomass-based plant growth. Our study suggests that the generally observed “fast growth at night and slow growth in the day” could be a plastic growth response to the diurnal environmental changes rather than a reflection of the interior plant growth rhythm.