Dynamique du carbone organique du sol et de l’azote dans une chronoséquence de plantation de Acacia auriculiformis A. Cunn. ex Benth. (Fabaceae), à Bambou-Mingali (République du Congo)

Dans le contexte international de réchauffement global, plusieurs initiatives sont prises par les pays, les organismes internationaux pour apporter une réponse appropriée qui permettrait de limiter la hausse de la température à 1,5° C, comme le recommande le rapport 1,5° du GIEC. La République du Congo est un pays fortement engagé dans les questions de lutte contre les changements climatiques à travers la gestion durable de ses forêts, la mise en place des plantations domaniales et industrielles, à base d’espèces endogènes et exotiques à croissance rapide dont de Acacia auriculiformis A. Cunn. ex Benth. Pour évaluer l’impact des plantations de Acacia auriculiformis A. Cunn. ex Benth sur la restitution du carbone et de l’azote au sol, une étude portant sur la dynamique du carbone et de l’azote organique du sol dans les plantations de la réserve foncière de Bambou-Mingali a été réalisée. Des échantillons de sol ont été prélevés sur les horizons 0-15 cm et 15-30 cm, d’une chronoséquence de 2, 3, 4, 5, 6, 7, et 8 ans de celles-ci ainsi que sur une savane pure, conformément aux directives du GIEC qui encouragent de collecter les échantillons sol entre 0 et 30 cm de profondeur. L’objectif général de ce travail a été d’évaluer le stock de carbone organique du sol sur une chronoséquence de sept âges de plantations de Acacia auriculiformis A. Cunn. ex Benth.  In the international context of global warming, several initiatives are taken by countries, international organizations to provide an appropriate response that would limit the rise in temperature to 1.5 ° C, as recommended by the 1.5 report of the IPCC. The Republic of the Congo is a country strongly committed to issues of the fight against climate change through the sustainable management of its forests, the establishment of state and industrial plantations, based on endogenous and exotic species with rapid growth, including Acacia auriculiformis A. Cunn. ex Benth. To assess the impact of Acacia auriculiformis A. Cunn plantations. ex Benth on the return of carbon and nitrogen to the soil, a study on the dynamics of carbon and organic nitrogen in the soil in the plantations of the Bambou-Mingali land reserve was carried out. Soil samples were taken on the 0-15 cm and 15-30 cm horizons, with a chronosequence of 2, 3, 4, 5, 6, 7, and 8 years of these as well as on a pure savanna, in accordance with IPCC guidelines which encourage collecting soil samples between 0 and 30 cm depth. The general objective of this work was to assess the stock of organic carbon in the soil over a chronosequence of seven planting ages of Acacia auriculiformis A. Cunn. ex Benth.. The soil samples collected by horizon and by plot were sieved, dried and conditioned in plastic bags for laboratory determination of carbon and total nitrogen by the Walkey and Black and Kjeldahl methods, respectively. The results obtained revealed an age effect on the nitrogen (P = 0.07619), carbon (P = 0.0050) and carbon stock (P = 0.0067) contents. The nitrogen and carbon contents as well as the soil organic carbon stocks were higher in the 8-year-old plantation with averages of 1.25 ± 0.33 mgN respectively. g-1; 15.87 ± 6.08 mgC.g-1 and 28.70 ± 9.48 t.ha-1 and lower in the 6-year-old plot respectively at 0.93 ± 0.05 mgN.g-1; 7.87 ± 0.58 mgC.g-1 and 15.57 ± 1.31 t.ha-1. The study also revealed a horizon effect on carbon and nitrogen contents with respective P-values of 0.0019 and 0.00015. The H0 horizon (0-15 cm) presented the highest stocks whatever the age of the plantation with an average of 32.98 ± 6.56 t.ha-1 in the 8-year-old plot against 16, 46 ± 1.13 t.ha-1 in the 6-year-old plot and on the H1 horizon (15-30 cm) an average of 24.42 ± 11.26 t.ha-1 in the 8-year-old plot against 14 , 69 ± 0.83 t.ha-1 in the 6-year-old plot. Also the C / N ratio is influenced by the age (P-value = 0.008) of the plantation and not by the horizon (P-value = 0.1243). Whatever the parameter considered, it emerges that age influences the return of carbon and nitrogen to the soil. Thus, there is an accumulation of carbon in the plantations of the Acacia auriculiformis chronosequence

Phase change-related variations of dome shape in Eucalyptus urophylla x Eucalyptus grandis shoot apical meristems

International audienceShoot apical meristem (SAM) domes derived from five different outdoor and in vitro sources of juvenile and mature Eucalyptus urophylla x Eucalyptus grandis akin genotypes were compared. Overall measurements of SAM dome height H and diameter D ranged from 2 to 35 µm and 20 to 80 µm, notwithstanding significant differences according to the various physiological origins of plant material investigated. SAM domes from the mature trees ?Mat? were taller than those from the rejuvenated ministock plants ?Rej?; from the in vitro microcuttings ?IVM? of the same clone, and also from the in vitro juvenile seedlings ?IVJ?, whereas outdoor seedlings ?Juv? SAM domes exhibited intermediate height. SAM domes from the rejuvenated material ?Rej?, from the in vitro mature ?IVM? and juvenile ?IVJ? origins were also narrower than those from the outdoor seedlings ?Juv? and to lesser extent than those from the mature trees ?Mat?. Overall the mature source ?Mat? displayed bigger and somehow sharper hemispherical domes than those from ?Rej? and ?Juv?, physiologically more juvenile, or those from the in vitro origins ?IVM ? and ?IVJ? which looked flatter and smaller. SAM dome height, diameter D and H/D values varied also significantly according to shoot apex status. More specifically, H, D and H/D SAM differences between the five origins were not significant when SAM were in their more organogenic stage, to become more salient as leaf structures started to elongate and to differentiate. This was particularly obvious for mature tree ?Mat? SAM dome shapes which showed at this stage much higher H/D values than the other SAM sources. Additionally, a shape index S used for characterizing more accurately dome shape confirmed these trends. These observations provide additional arguments to the view that juvenility in trees become more and more time and space restricted as ageing increases in the course of time during the ontogenetical process, to be ultimately confined to the more organogenic phases of SAM, from which shoot characteristics derive

Characteristics of Acacia mangium shoot apical meristems in natural and in vitro conditions in relation to heteroblasty

PDF version of the authors can be published in January 2013International audienceMorphological and histocytological characteristics of Acacia mangium shoot apical meristems (SAMs) were assessed in natural and in vitro conditions in relation to heteroblasty. In the natural environment, SAMs with a mature-phyllode morphology were much bigger, contained more cells with larger vacuolated area, or vacuome, and lower nucleoplasmic ratios than those from the juvenile type (Juv). In these latter, nuclei appeared more voluminous, evenly and lightly stained, with clearly distinguishable nucleolei and less abundant chromocenters. In vitro, where reversions from mature to juvenile morphological traits do occur unpredictably, heteroblasty was less obvious in the SAM characteristics examined. In vitro SAMs corresponding to the juvenile and mature types showed similarities with outdoor Juv SAMs, but could be distinguished from these latter by a much larger vacuome that might be induced by the culture conditions. These findings encourage pursuing the investigations at the chromatin and nucleolus level in SAM zones where heteroblasty-related differences have been detected

Variations of DNA methylation in Eucalyptus urophylla×Eucalyptus grandis shoot tips and apical meristems of different physiological ages.

International audienceGlobal DNA methylation was assessed by high-performance liquid chromatography (HPLC) for the first time in Eucalyptus urophylla×Eucalyptus grandis shoot tips comparing three outdoor and one in vitro sources of related genotypes differing in their physiological age. The DNA methylation levels found were consistent with those reported for other Angiosperms using the same HPLC technology. Notwithstanding noticeable time-related fluctuations within each source of plant material, methylation rate was overall higher for the mature clone (13.7%) than for the rejuvenated line of the same clone (12.6%) and for the juvenile offspring seedlings (11.8%). The in vitro microshoots of the mature clone were less methylated (11.3%) than the other outdoor origins, but the difference with the juvenile seedlings was not significant. Immunofluorescence investigations on shoot apices established that the mature source could be distinguished from the rejuvenated and juvenile origins by a higher density of cells with methylated nuclei in leaf primordia. Shoot apical meristems (SAMs) from the mature clone also showed a greater proportion and more methylated cells than SAMs from the rejuvenated and juvenile origins. The nuclei of these latter were characterized by fewer and more dispersed labeled spots than for the mature source. Our findings establish that physiological ageing induced quantitative and qualitative variations of DNA methylation at shoot tip, SAM and even cellular levels. Overall this DNA methylation increased with maturation and conversely decreased with rejuvenation to reach the lower scores and to show the immunolabeling patterns that characterized juvenile material nuclei