Growth and branching of the taproot of young oak trees--a dynamic study

The growth and branching of the taproot of young oak trees werestudied on seedlings grown in root observation boxes over aperiod of 30–45 d. The development of shoots and rootswere recorded daily on a set of eight plants, and additionalobservations on the initiation of primordia were made on anotherset of 18 plants. Taproot growth was typically indeterminateand linear, with growth rates in the range of 1.5–2.5cm d^–1. In some cases, however, growth slowed down orstopped, and resumed a few days later. The growth of shootsand roots were not synchronized. Taproots branched in two ways:acropetal branches emerged from 4–8-d-old taproot tissues,and late branches emerged from older tissues (up to 30 d inour experiment). The latter appeared especially when taprootgrowth slowed down or stopped. New primordia were initiatedon tissues older than 1.4 d, and lateral roots emerged aftera minimal development duration of 2.3 d. These time-relatedparameters described the emergence of branching very effectivety,since they were quite stable over a wide range of taproot growthrates. However, emergence duration decreased slightly in fastgrowing taproots. Branching density tended to increase withthe taproot growth rate. Physiological significance and consequences for modelling rootsystem development are discussed. Key words: Quercus robur, root system, growth, branching, primordium initiation     

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

The effects of taproot blocking and pruning on the developmentof the early secondary roots (ESR) of rubber seedlings werestudied in root observation boxes under controlled conditions.During shoot flush, both the mean elongation rate and mean apicaldiameter of the ESR decreased regardless of treatments. Thereafter,the elongation rate of the ESR increased greatly when the taprootwas blocked, slightly for the control and scarcely for the prunedsystems in which fast growing regenerated roots developed. Thedifferences between treatments were related to the proportionof ESR which ceased growing. Following shoot arrest, the apicaldiameter of ESR increased greatly for blocked seedlings andto a lesser extent for pruned seedlings. Branching density ofthe ESR and elongation of tertiary roots were also higher forseedlings without a growing taproot. The dynamics of ESR response was not consistent with activeinhibition of their development by the growing taproot. Moreover,this response was dependent on concurrent development of shootand regenerating roots, hence competition processes were morelikely to be determining. In such an hypothesis, root elongationcan be limited by assimilate availability, but also by eachroot'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 parallelincrease in apical diameter and this may explain the relativeinertia of ESR to taproot alteration. Conversely, regeneratingroots could have a high growth potential because they were initiatedin a favourable context, thus their development competed stronglywith elongation of ESR. Key words: Hevea brasiliensis, root system, development, growth potential, root diameter, competition