LATERAL ROOT-FORMATION IN PINE-SEEDLINGS .1. SOURCES OF STIMULATING AND INHIBITORY SUBSTANCES

The effect of stimulating and inhibitory factors on the development of lateral roots was studied in Pinus pinea seedlings grown in a nutrien t solution. in an intact root system, only 10-15% of the emerging late ral roots continue to elongate. Removing the tap root tip increased th e number and length of the elongating lateral roots but the total numb er of the lateral roots remained unchanged. The timing of removal of t he tap root tip seems to be an important factor in lateral root develo pment. The earlier the removal of the tap root tip, the more lateral r oots will elongate. Regardless of the remaining length of the tap root following removal of the tap root tip, lateral root elongation will o ccur particularly in the youngest lower segment. Removal of cotyledons had a greater inhibitory effect on lateral root development than on t ap root growth. The total number of lateral roots and the number of el ongating lateral roots were greatly reduced by removal of cotyledons, particularly in seedlings with an intact root system. A decrease in th e dry weight of the root system paralleled the reduction in lateral ro ot development. The data indicate that in P. pinea seedlings, factors stimulating root branching originate in the shoot whereas inhibitory f actors originate in the tap root tip. Lateral root elongation seems to be the limiting factor in the development of a branched root system.