Temporal and structural relations within bark and trunk in Hevea brasiliensis Muell. Arg. (Euphorbiaceae): physiological maturity index of bark and latex vessels.

In Africa and particularly in Cote d'Ivoire, the exploitation of Hevea brasiliensis clones having strong vegetative growth based on the criterion of vigour adopted so far has raised physiological and physical problems characterized by a high rate of tapping panel dryness and wind damage. A study was conducted on three clones of Hevea brasiliensis (PB 235, GT 1 and PR 107) belonging respectively to classes of fast, moderate and slow vegetative growth, in order to determine the right moment for an exploitation which would minimize those drawbacks. Trunk measurements, bark collecting and histological sections followed by laticifers counting made on rubber trees aged from one to fifteen years, have enabled to describe the process of establishment of bark and laticifers. The intensity of development and thickening of the bark and the rate of laticifer emission are described respectively by distinct hyperbolic and logistic sigmoid functions. However, their temporal evolution is strongly marked by an irreversible decrease of the whole process of formation, whatever the clone, from six years after planting. Furthermore, the density of laticifers per mm2 switches, whatever the clone studied, from a number greater than 5, the first six years (5-8 < Δlv <2) to 0.35 the next 25 years (2 < Δlv < 0.4). This evolution which is very significant the first six years varies relatively little the rest of time. These results show six years after planting a major phenologic phenomenon, like a physiological maturity, which occurs within the tree. This study has allowed identifying good indicators for determining the age and/or the time when plantations should be tapped in Hevea brasiliensis. These relationships have certain and practical interests insofar as they will allow to determine the maturity of exploitation for plantations which age is unknown by using only a bark gauge to measure bark thickness and a measuring tape to measure the girth.