Competition within the root system of rubber seedlings (Hevea brasiliensis) studied by root pruning and blockage

Abstract

The effects of taproot blocking and pruning on the development of the early secondary roots (ESR) of rubber seedlings were studied in root observation boxes under controlled conditions. During shoot flush, both the mean elongation rate and mean apical diameter of the ESR decreased regardless of treatments. Thereafter, the elongation rate of the ESR increased greatly when the taproot was blocked, slightly for the control and scarcely for the pruned systems in which fast growing regenerated roots developed. The differences between treatments were related to the proportion of ESR which ceased growing. Following shoot arrest, the apical diameter of ESR increased greatly for blocked seedlings and to a lesser extent for pruned seedlings. Branching density of the ESR and elongation of tertiary roots were also higher for seedlings without a growing taproot. The dynamics of ESR response was not consistent with active inhibition of their development by the growing taproot. Moreover, this response was dependent on concurrent development of shoot and regenerating roots, hence competition processes were more likely to be determining. In such an hypothesis, root elongation can be limited by assimilate availability, but also by each root's maximum growth rate in non-limiting conditions, i.e. growth potential. Since the latter is related to apical diameter, a significant acceleration of elongation required a parallel increase in apical diameter and this may explain the relative inertia of ESR to taproot alteration. Conversely, regenerating roots could have a high growth potential because they were initiated in a favourable context, thus their development competed strongly with elongation of ESR.