Samuel Obouayeba, Eric Francis Soumahin, Koffi Mathurin Okoma, Angelo Evariste Badou N’guessan, Lancina Fanlégué Coulibaly , Kouablan Edmond Koffi, and Régis Lacote, from the different institute of the Côte d’Ivoire. wrote a research article about,Temporal and Structural Relations in Hevea brasiliensis: Indicators of Bark and Latex Vessel Maturity. entitled,Temporal and structural relations within bark and trunk in Hevea brasiliensis Muell. Arg. (Euphorbiaceae): Physiological maturity index of bark and latex vessels.This research paper published by the International Journal of Biosciences (IJB). an open access scholarly research journal on Biosciences . under the affiliation of the International Network For Natural Sciences | INNSpub. an open access multidisciplinary research journal publisher.
Abstract
In Africa and particularly in Côte 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 sixyears 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 ofexploitation for plantations which age is unknown by using only a bark gauge to measure bark thickness and a measuring tape to measure the girth.
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Introduction
The animal and/or plant living kingdom is governed by a certain number of relations whom knowledge allows to better exploit the potentialities of that kingdom. Rubber tree is not an exception to that rule, all the more since its yielding mode is particular from the other vegetables from which yielding are fruits, tubers, roots (Templeton, 1969). Moreover, in that species, maturity is not exteriorized; at least, it’s not obvious. Rational and optimal exploitation of Hevea brasiliensis require so, more than in the major part of its homologous of the plant kingdom, knowledge and control of the setting and functioning of the main organs involved in the processing of cis-polyisoprene, the source of natural rubber production (Jacob et al., 1988; Sekha, 1989), the main resource expected from rubber tree cultivation.
Therefore, every part of rubber tree (roots, trunk, branches and leaves) is concerned by these investigations, as laticifers are found in these organs at any age (Meunier, 1912; Bobilioff, 1923; Riches and Gooding, 1952; Dickenson, 1969; Gomez, 1982, 1975; Hébant and Fay, 1980). However, the works of Bobilioff (1923), Gomez (1982), Gomez (1975) and Compagnon (1986) have shown that the trunk of rubber tree (2.50 m above the ground) is the part which has the highest latex extraction yield and which is easier to manage. The presence of latex at any age in the bark of the trunk rises up the problem of the ideal moment to start tapping. This tapping moment put on appearance an important character as Gomez (1975) has shown that laticifers switch from juvenility to old age, via maturity. But as rubber yielding is highly energy and photosynthetate demanding (Le Bras, 1953; Templeton, 1969; Wycherley, 1976), the exploitation of the rubber-producing tissue should be made, without damage on the physiological state of the trees, only at maturity of this tissue and laticifers that it bears. Indeed, the current criterion for first tapping that is, arbitrary and based on vigour (Compagnon, 1986) and which allow, to exploit rubber trees having 50 cm girth at one meter above the ground level has some deficiencies. Fast-growing clones (PB 235, IRCA 18, etc.) show physico-physiological constraints, such as dry tapping panel dryness and wind damage (Premakumari, 1991; CIRAD-CP, 1993, Obouayeba and Boa, 1993; Jacob et al., 1994; Dian 1993; Dian et al., 1995) more important than those of moderate-growing or slow-growing clones (GT 1, PB 217, PR 107, etc.). The recent works of Obouayeba et al., (2000a) shown that fast-growing clones are precociously exploited (tapped). But Templeton (1969), Gohet (1996) have already indicated that precocious exploitation is prejudicial to further productions. Indeed, precocity in tapping provokes a strong reduction in radial vegetative growth during tapping (Obouayeba and Boa, 1993; Obouayeba et al., 2002), and a high rate of tapping panel dryness (Dian, 1993), Dian et al. (1995) leading to less and less sustained yielding from the 5th exploitation campaign (Templeton, 1969; Ouattara, 1998).
The research of criteria more relevant than the current one becomes then necessary; and is justified. Indeed, Obouayeba et al. (2000a,b) have already determined the age, notably the tapping at 6 years after planting as the criterion susceptible to solve this problem (Obouayeba et al., 2002). Furthermore, it is not excluded that some relations, between time and structural organs of the trunk can also contribute to elucidate the problem, allowing thus to determine, for an unknown age, the maturity of the bark and/or those organs.
To solve this problem and make the exploitation of rubber tree more efficient, a study on some structural and temporal relations at the level of the bark and trunk of rubber tree has been carried out. The present paper sums up the results of this study which concerned three clones of Hevea brasiliensis; PB 235; GT 1 and PR 107, grown on the whole rubber cultivation area of Côte d’Ivoire.
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