THE DETERMINATION OF RELATIVE ELEMENTAL GROWTH-RATE PROFILES FROM SEGMENTAL GROWTH-RATES - A METHODOLOGICAL EVALUATION

Relative elemental growth rate (REGR) profiles describe spatial patter ns of growth intensity; they are indispensable for causal growth analy ses. Published methods of REGR profile determination from marking expe riments fall in two classes: the profile is either described by a seri es of segmental growth rates, or calculated as the slope of a function describing the displacement velocities of points along the organ. The latter technique is usually considered superior for theoretical reaso ns, but to our knowledge, no comparative methodological study of the t wo approaches is currently available. We formulated a model REGR profi le that resembles those reported from primary roots. We established th e displacement velocity profile and derived growth trajectories, which enabled us to perform hypothetical marking experiments on the model w ith varying spacing of marks and durations of measurement. REGR profil es were determined from these data by alternative methods, and results were compared to the original profile. We find that with our model pl otting of segmental relative growth rates versus segment position prov ides exact REGR profile estimations, if the initial segment length is less than 10% of the length of the whole growing zone, and if less tha n 20% of the growing zone is displaced past its boundary during the me asurement. Based on our analysis, we discuss systematic errors that oc cur in marking experiments.