LATERAL ROOT-FORMATION IN PINE-SEEDLINGS .2. THE ROLE OF ASSIMILATES

The role of assimilates in lateral root development was studied in Pin us pinea seedlings grown in a nutrient solution. Seedlings were treate d with (CO2)-C-14 for 2 h following removal of the tap root tip at var ious times prior to the application of (CO2)-C-14 Or removal of a diff erent number of cotyledons at one time. In seedlings with intact root systems most of the radioactivity accumulated in the lower section of the root containing the tap root apex. When the tap root tip was remov ed, the pattern of radioactivity accumulation along the root was affec ted by the presence and the stage of lateral root development. Removin g the tap root tip of young seedlings (with no lateral roots) resulted in an almost equal distribution of radioactivity along the root. Abou t 50% of the total radioactivity was found in the section showing the highest lateral root growth. Removing the tap root tip of mature seedl ings (with lateral roots in the upper section) resulted in an immediat e increase in the radioactivity accumulation in the upper section. Whe n lateral roots appeared in the middle section, the pattern of radioac tivity distribution was similar to that found in root decapitated youn g seedlings. Removal of cotyledons of mature seedlings somewhat increa sed the transport of radioactivity to the lower root section at the ex pense of the radioactivity in the lateral roots of the upper section. The present study suggests that competition within the root system bet ween the tap root apex and the lateral roots may play an important rol e in determining the morphology of the root system.