Atzmon, N., Department of Horticulture, Faculty of Agriculture, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot, 76100, Israel
Salomon, E., Institute of Horticulture, The Volcani Center, P.O. Box 6, Bet-Dagan, 50250, Israel
Reuveni, O., Institute of Horticulture, The Volcani Center, P.O. Box 6, Bet-Dagan, 50250, Israel
Riov, J., Department of Horticulture, Faculty of Agriculture, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot, 76100, Israel

The effect of stimulating and inhibitory factors on the development of lateral roots was studied in Pinus pinea seedlings grown in a nutrient solution. In an intact root system, only 10-15% of the emerging lateral roots continue to elongate. Removing the tap root tip increased the number and length of the elongating lateral roots but the total number of the lateral roots remained unchanged. The timing of removal of the tap root tip seems to be an important factor in lateral root development. The earlier the removal of the tap root tip, the more lateral roots will elongate. Regardless of the remaining length of the tap root following removal of the tap root tip, lateral root elongation will occur particularly in the youngest lower segment. Removal of cotyledons had a greater inhibitory effect on lateral root development than on tap root growth. The total number of lateral roots and the number of elongating lateral roots were greatly reduced by removal of cotyledons, particularly in seedlings with an intact root system. A decrease in the dry weight of the root system paralleled the reduction in lateral root development. The data indicate that in P. pinea seedlings, factors stimulating root branching originate in the shoot whereas inhibitory factors originate in the tap root tip. Lateral root elongation seems to be the limiting factor in the development of a branched root system. © 1994 Springer-Verlag.