Dynamics of root growth stimulation in Nicotiana tabacum in increasing light intensity.

Light intensity is crucial for plant growth. In this study, the hypothesis was tested whether a sudden increase in light intensity leads to an immediate increase of root growth. Seedlings of Nicotiana tabacum grown in agar-filled Petri dishes were subjected to light intensities of 60 and 300 micromol m(-2) s(-1), respectively. Seedling biomass, sucrose, glucose and fructose concentration as well as primary root growth increased significantly with light intensity. The dynamics of the increase in root growth were analysed here in more detail. In transition experiments from low to high light intensities, root growth increased by a factor of four within 4 d, reaching the steady-state level measured in plants that were cultivated in high-light conditions. The distribution of relative elemental growth rates along the root growth zone retained a constant shape throughout this transition. During the first three hours after light increase, strong growth fluctuations were repeatedly observed with the velocity of the root tip cycling in a sinusoidal pattern between 120 and 180 microm h(-1). These dynamic patterns are discussed in the context of hydraulic and photosynthetic acclimation to the altered conditions. Experiments with externally applied sucrose and with transgenic plants having reduced capacities for sucrose synthesis indicated clearly that increasing light intensity rapidly enhanced root growth by elevating sucrose export from shoot to root.