Changes in hormonal balance and meristematic activity in primary root tipson the slowly rotating clinostat and their effect on the development of the rapeseed root system

The morphometry of the root system, the meristematic activity and the level of indole-3-acetic acid (IAA), abscisic acid (ABA) and zeatin in the prima ry root tips of rapeseed seedlings were analyzed as functions of time on a slowly rotating clinostat (1 rpm) or in the vertical controls (1 rpm). The fresh weight of the root system was 30% higher throughout the growth period (25 days) in clinorotated seedlings, Morphometric analysis showed that the increase in biomass on the clinostat was due to greater primary root growt h, earlier initiation and greater elongation of the secondary roots, which could be observed even in 5-day-old seedlings. However, after 15 days, the growth of the primary root slowed on the clinostat, whereas secondary roots still grew faster in clinorotated plants than in the controls. At this tim e, the secondary roots began to be initiated closer to the root tip on the clinostat than in the control. Analysis of the meristematic activity and de termination of the levels in IAA, ABA and zeatin in the primary root tips d emonstrated that after 5 days on the clinostat, the increased length of the primary root could be the consequence of higher meristematic activity and coincided,vith an increase in both IAA and ABA concentrations. After 15 day s on the clinostat, a marked increase in IAA, ABA and zeatin, which probabl y reached supraoptimal levels, seems to cause a progressive disturbance of the meristematic cells, inducing a decrease of primary root growth between 15 and 25 days. These modifications in the hormonal balance and the perturb ation of the meristematic activity on the clinostat were followed by a loss of apical dominance, which was responsible for the early initiation of sec ondary roots, the greater elongation of the root system and the emergence o f the lateral roots near the tip of the primary root.