Variations micrométriques des tiges de jeunes plants de Acacia senegal (L.) Wildd dans différents régimes hydriques

L’influence des régimes d’alimentation hydrique sur les variations du diamètre des tiges et la réponse de jeunes plants de Acacia senegal (L.) Wildd après une réhydratation a été appréciée par la méthode des capteurs. 1-En condition d’alimentation hydrique optimale, Acacia senegal (L.) Wildd subit des variations nycthémérales: déshydratation au cours de la journée et double récupération hydrique pendant la nuit et en début de matinée. 2-L’état hydrique du sol détermine les variations d’amplitude des diamètres des jeunes plants qui sont d’autant plus importantes que la contrainte est sévère. 3-Les micro-variations des diamètres déterminent la croissance des jeunes plants qui est d’autant plus importante que le traitement est humide. 4-La réhydratation dans les traitements hydriques modéré et sévère se traduit par une reprise de la croissance avec des récupérations nocturnes en condition modérée.Mots clés: Acacia senegal, capteurs, traitements hydriques, variations de diamètre des tige

Proline and glycine betaine accumulation in two succulent halophytes under natural and experimental conditions

[EN] Proline (Pro) and glycine betaine (GB) contents were determined in two Mediterranean halophytes, Plantago crassifolia and Inula crithmoides, to assess their possible role in salt tolerance of both taxa. Plant material was collected in a littoral salt marsh under different environmental conditions, and from plants subjected to salt treatments in a growth chamber. Relative growth inhibition by NaCl indicated that I. crithmoides is more salt-tolerant than P. crassifolia, in agreement with the distribution of the two species in nature. Field and laboratory data confirmed GB as the major osmolyte responsible for osmotic adjustment in I. crithmoides, but with only a minor role if any as osmoprotectant in the salt tolerance of P. crassifolia. Under natural conditions, Pro contents were very low in both taxa, but increased to levels high enough to contribute significantly to osmotic balance when plants were artificially treated with 450 600mM NaCl higher salt concentrations than those they would normally encounter in their natural habitats. These data suggest that halophytes possess built-in mechanisms, such as accumulation of additional osmolytes, to rapidly adapt to increasing salinity levels in their natural ecosystems; for example, those expected to be caused by climate change in salt marshes in the Mediterranean region.This work was funded by a grant to O.V. from the Spanish Ministry of Science and Innovation (Project CGL2008-00438/BOS), with contribution from the European Regional Development Fund. COST Action FA0901: "Putting Halophytes to work - From Genes to Ecosystems" supported M. N. Grigore for his stay in Valencia within the frame of a Short-Term Scientific Mission. A. Tifrea was funded by the Erasmus fellowship programme for her stay in Valencia to carry out her Master Thesis.Pardo-Domenech, L.; Tifrea, A.; Grigore, M.; Boscaiu, M.; Vicente, O. (2016). Proline and glycine betaine accumulation in two succulent halophytes under natural and experimental conditions. Plant Biosystems - An International Journal Dealing with all Aspects of Plant Biology. 150(5):904-915. https://doi.org/10.1080/11263504.2014.990943S904915150

Facilitation or Competition? Tree Effects on Grass Biomass across a Precipitation Gradient

Savanna ecosystems are dominated by two distinct plant life forms, grasses and trees, but the interactions between them are poorly understood. Here, we quantified the effects of isolated savanna trees on grass biomass as a function of distance from the base of the tree and tree height, across a precipitation gradient in the Kruger National Park, South Africa. Our results suggest that mean annual precipitation (MAP) mediates the nature of tree-grass interactions in these ecosystems, with the impact of trees on grass biomass shifting qualitatively between 550 and 737 mm MAP. Tree effects on grass biomass were facilitative in drier sites (MAP≤550 mm), with higher grass biomass observed beneath tree canopies than outside. In contrast, at the wettest site (MAP = 737 mm), grass biomass did not differ significantly beneath and outside tree canopies. Within this overall precipitation-driven pattern, tree height had positive effect on sub-canopy grass biomass at some sites, but these effects were weak and not consistent across the rainfall gradient. For a more synthetic understanding of tree-grass interactions in savannas, future studies should focus on isolating the different mechanisms by which trees influence grass biomass, both positively and negatively, and elucidate how their relative strengths change over broad environmental gradients. © 2013 Moustakas et al

Seed Germination Strategies of Mediterranean Halophytes Under Saline Condition

The study of the ecological strategies adopted by seed plants to ensure their success in different environments is closely related to germination ecology. This implies a careful knowledge of ecophysiology of seeds and, therefore, also of interaction between plants and the complexity of external factors. In particular, the environmental conditions of the area where a plant grows and produces seeds represent the main factors that influence successful seedling establishment. The physical-chemical features of habitats, and therefore their heterogeneity, affect the behavior of seeds in different ways. In addition to the timing of seed production, they can induce or terminate dormancy and/or germination and influence the germination pattern of different seeds in the same plant and so the composition and dispersal of soil seed banks. Salinity is a major abiotic stress affecting growth and plant productivity worldwide, constituting one of the main topics of study in the field of plant physiology. Halophytes are the plants that have the availability to survive and develop in different types of saline habitats. In this chapter, we consider some examples to illustrate the main adaptive strategies used by the seeds of halophytes on ecophysiological perspectives to survive in habitats affected by high levels of salinity. The focus is on the species that live in the brackish or salt coastal areas of the Mediterranean Basin. On these environments, the salt stress may act synergistically with intense anthropic pressure, generating profound alterations in the ecosystem and threatening the survival of the plant species very sensitive to the effects of climate change also. The results show the main diverse strategies, such as dormancy cycling, seed heteromorphism, and recovery capacity, from saline shock, favoring the chances of seed survival. The interaction between temperature and salinity during germination was also discussed assessing its crucial role as an ecological strategy

Short Term Effects of Burning and Grazing on the Forage Quality of a <em>Heteropogon contortus</em>- Dominated Savanna in the Southwest of Madagascar

An experimental study of the effects of burning and grazing was carried out in Sakaraha area (SW Madagascar). The objectives were to evaluate the combined or separate effects of burning and grazing on the forage quality of a Heteropogon contortus- and Poupartia caffra-dominated savanna. Three burning treatments (no burning, early burning and late burning) and two grazing treatments (grazed, ungrazed) were compared. The forage quality was assessed by studying the grazing value and the forage value. A diachronic study showed (a) burning increased the pasture value and grazing decreased it, (b) burning and grazing could increase the rates of forage value parameters. Both index values (pasture and forage values) had overall the same evolution after burning

Valeur pastorale des herbages en région soudanienne, le cas des parcours sahéliens du Nord-Sénégal

Pasture Value of Herbages in Soudanian Region : The Case of Sahelian in North-Senegal. The pasture value of sahelian rangelands, which is characterized by a more or less continuous herbaceous layer and a discontinuous cover of shrubs and trees, was investigated from the pastoral composition and the global quality index. The observations consisted on floristic sampling in the shade and in the open part of the plots. Frequential analysis technics applied to floristic data allowed to highlight a great floristic diversity in the forage spectrum of the shaded rangelands compared to the open. Good fodder plants were scare in the tree shelter where their input in the herbaceous layer are very limited. The global quality index of the rangelands is largely more important in the unshaded zones, because of the Poaceae predominance

Phénologie de la nodulation d'Acacia raddiana en milieu naturel

L'association d'Acacia tortilis (Forssk.) Hayne subsp. raddiana (Savi) Brenan avec les bactéries du sol à croissance rapide de la famille des rhizobiacées joue un rôle important dans l'amélioration de la nutrition minérale de cette plante. Cependant, les caractéristiques de cette symbiose ont été très peu étudiées en conditions naturelles. Ce travail a été conduit au Sénégal sur des peuplements naturels d'Acacia raddiana de deux sites du Ferlo sénégalais : Souilène et Dahra-Djoloff. La répartition dans l'espace et les variations saisonnières des nodules présents sur le système racinaire de surface d'Acacia raddiana (0-1 m) ont été étudiées en relation avec les conditions hydriques du sol. À cet effet, des prélèvements de sol et des extractions ont été effectués à différentes périodes de l'année. Le maximum de nodules a été rencontré entre 2 et 3 mètres du tronc, et à une profondeur comprise entre 25 et 75 cm. Une biomasse importante de nodules apparemment très fixateurs d'azote a été produite pendant la saison pluvieuse. Ces nodules ont une durée de vie courte, et apparaissent desséchés pendant la majeure partie de l'année. La densité des nodules apparaît corrélée à la biomasse racinaire, mais également à l'humidité du sol de la rhizosphère.The association between Acacia tortilis (Forssk.) Hayne subsp. raddiana (Savi) Brenan and soil bacteria with rapid growth belonging to the Family Rhizobiaceae plays an important role in the improvement of the mineral nutrition of this plant. However, the characteristics of this symbiotic relationship have been very little studied under natural conditions. This work has been conducted in Senegalese natural populations of Acacia raddiana in two sites: Souilène and Dahra-Djoloff (Ferlo). The spatial distribution around tree and the seasonal variations of nodules present on the root system of Acacia raddiana (0-1 m) were studied under different soil moisture conditions. For that, extraction of soil samples was made at the different periods of the year. The maximum number of nodules was obtained between 2 and 3 m far from the tree trunk, and between 25 and 75 cm soil depth. A significant nodule biomass apparently more efficient in N fixation was obtained during the rainy season. These nodules have short life span, and appeared dried during the greater part of the year. The density of the nodules correlated significantly with the root biomass, and also with the soil moisture content of the rhizosphere. nodulation, root, natural population, soil water budget, acacia tortilis (forssk.), hayne subsp. raddiana (savi), brena

Influence d'Acacia raddiana sur la structure et le fonctionnement de la strate herbacée dans le Ferlo sénégalais

Les observations réalisées sur les interactions arbre/herbe dans le Sahel sénégalais (Ferlo, 280 mm de précipitations annuelles) montrent qu'Acacia raddiana augmente la richesse floristique, modifie le cycle phénologique et accroît la production et l'immobilisation minérale de la strate herbacée située sous son couvert. L'impact positif de l'arbre sur la strate herbacée est principalement dû à l'amélioration des disponibilités en eau et à l'augmentation de la fertilité du sol.Studies on tree and grass interactions in the Sahelian zone of Senegal (Ferlo, 280 mm of annual rainfall) show that Acacia raddiana increases the floristic richness, modifies the phenological cycle and improves the phytomass and the inorganic status of the understory herbaceous layer.These benefic effects are mainly due to the soil enrichment (water balance and fertility)