Manuel J. Reigosa

DO GERMINATION INDICES ADEQUATELY REFLECT ALLELOCHEMICAL EFFECTS ON THE GERMINATION PROCESS

Allelopathy is often demonstrated by testing allelochemical effects on germination using many germination indices to assess them. The objective of this work was to compare four common germination indices (GT, S, AS, and CRG) previously calculated with the same data in order to discuss their physiological meaning. Each index led to a different interpretation of allelochemical effect on germination. Considering all indices, the S index was the most sensitive, followed by the AS and CRG indices, which provided the same information. The GT index, which gave only a global germination interpretation, was not precise enough in describing allelochemical actions during germination. Nevertheless, none of the indices exactly reflected the germination process. Thus, simple comparisons between the control and the test at each exposure time were more appropriate.

The genus Acacia as invader: the characteristic case of Acacia dealbata Link in Europe

Abstract• We review current knowledge about the biology of the genus Acacia, and Acacia dealbata Link (silver wattle) in particular, as an invader in Europe, focusing on (i) the biology of the genus Acacia; (ii) biological attributes that are important for the invasiveness of the genus and A. dealbata; (iii) possible hypotheses for the invasion success; and (iv) control methods.• Several Australian wattles have been recorded as naturalized in Europe. Acacia has attained pest proportions in certain habitats and protected sites (notably coastal dunes, river courses, natural parks and biosphere reserves).• The spread of Acacia dealbata seems to be assisted by human interference such as soil disturbance and severe fires. The biological attributes favoring invasion by A. dealbata include tolerance of changing soil conditions, the ability to take advantage of environmental disturbance, phenotypic plasticity, vegetative reproduction, fire tolerance and allelopathic potential.• Different hypotheses explaining invasiveness and transition between invasion steps related to biological attributes as the key factor for A. dealbata invasion are discussed. Effects on the biodiversity of native flora are little understood and studies of suppression of autochthonous species are needed. It is desirable that further studies comparing Acacia at field sites in their native and exotic range should be done.• Understanding the biology of invasive wattles in Europe is the first step to an effective control method. Studies comparing plant invaders at field sites in their native and invaded areas seem to be most appropriate in order to be able to attack the most vulnerable stages.Résumé• Nous passons en revue les connaissances actuelles sur la biologie du genre Acacia et de Acacia dealbata Link, en particulier comme un envahisseur en Europe, en mettant l’accent sur : (i) la biologie du genre Acacia ; (ii) les attributs biologiques qui sont importants pour le caractère envahissant du genre et d’A. dealbata; (iii) les hypothèses possibles pour la réussite de l’invasion et (iv) les méthodes de contrôle.• Plusieurs acacias australiens ont été enregistrés comme naturalisés en Europe. Acacia a atteint la proportion de nuisible dans certains habitats et sites protégés (notamment des dunes côtières, des cours d’eau, des parcs naturels et des réserves de la biosphère).• La propagation de l’Acacia dealbata semble avoir été aidée par l’intervention humaine, telle que la perturbation des sols et de graves incendies. Les attributs biologiques favorisant l’invasion par A. dealbata incluent la tolérance aux changements des conditions du sol, la capacité à tirer profit des perturbations de l’environnement, la plasticité phénotypique, la reproduction végétative, la tolérance au feu, et le potentiel allélopathique.• Différentes hypothèses expliquant le caractère invasif et la transition entre les phases de l’invasion en relation avec les attributs biologiques comme facteur clé pour l’invasion d’A. dealbata sont discutées. Les effets sur la biodiversité de la flore sont mal compris et des études sur la suppression des espèces autochtones sont nécessaires. Il est souhaitable que d’autres études comparant Acacia sur le terrain dans leurs sites et dans des régions exotiques soient faites.• Comprendre la biologie des espèces d’Acacia invasives en Europe est la première étape d’une méthode de contrôle efficace. Les études comparant les envahisseurs végétaux sur le terrain dans leurs sites et dans les zones envahies semblent être plus appropriées afin d’être en mesure d’attaquer les étapes les plus vulnérables.

Effect of phenolic compounds on the germination of six weeds species

Low molecular weight phenolic compounds have been identified in fresh leaves and in soils in which leaves of five varieties of Capsicum annuum L. were decomposing. Six phenolic compounds were tested in laboratory bioassays for their allelopathic effects on germination and seedling growth of six weeds. Ferulic acid, gallic acid, p-coumaric acid, p-hydroxybenzoic acid, vanillic acid, and p-vanillin were bioassayed in concentrations of 10, 1, 0.1, and 0.01 mM. Equimolar mixtures containing all these phenolics were prepared at the final total concentration of 10, 1, 0.1, and 0.01 mM to test for possible interactive effects. Chenopodium album L., Plantago lanceolata L., Amaranthus retroflexus L., Solanum nigrum L., Cirsium sp. and Rumex crispus L. were the selected target weeds. The highest concentration of the compounds inhibited the germination of all these weeds, but lower concentrations had no effect or were stimulatory. However, effects varied with the weed species, the concentration of the compound tested and the compound itself. In assays with the mixture of phenolics we found evidence of some additive effects.

Allelochemical stress inhibits growth, leaf water relations, PSII photochemistry, non-photochemical fluorescence quenching, and heat energy dissipation in three C3 perennial species

In this study, the effect of two allelochemicals, benzoxazolin-2(3H)-one (BOA) and cinnamic acid (CA), on different physiological and morphological characteristics of 1-month-old C3 plant species (Dactylis glomerata, Lolium perenne, and Rumex acetosa) was analysed. BOA inhibited the shoot length of D. glomerata, L. perenne, and R. acetosa by 49%, 19%, and 19% of the control. The root length of D. glomerata, L. perenne, and R. acetosa growing in the presence of 1.5 mM BOA and CA was decreased compared with the control. Both allelochemicals (BOA, CA) inhibited leaf osmotic potential (LOP) in L. perenne and D. glomerata. In L. perenne, Fv/Fm decreased after treatment with BOA (1.5 mM) while CA (1.5 mM) also significantly reduced Fv/Fm in L. perenne. Both allelochemicals decreased ΦPSII in D. glomerata and L. perenne within 24 h of treatment, while in R. acetosa, ΦPSII levels decreased by 72 h following treatment with BOA and CA. There was a decrease in qP and NPQ on the first, fourth, fifth, and sixth days after treatment with BOA in D. glomerata, while both allelochemicals reduced the qP level in R. acetosa. There was a gradual decrease in the fraction of light absorbed by PSII allocated to PSII photochemistry (P) in R. acetosa treated with BOA and CA. The P values in D. glomerata were reduced by both allelochemicals and the portion of absorbed photon energy that was thermally dissipated (D) in D. glomerata and L. perenne was decreased by BOA and CA. Photon energy absorbed by PSII antennae and trapped by ‘closed’ PSII reaction centres (E) was decreased after CA exposure in D. glomerata. BOA and CA (1.5 mM concentration) decreased the leaf protein contents in all three perennial species. This study provides new understanding of the physiological and biochemical mechanisms of action of BOA and CA in one perennial dicotyledon and two perennial grasses. The acquisition of such knowledge may ultimately provide a rational and scientific basis for the design of safe and effective herbicides.

Allelopathic effects of Acacia melanoxylon R.Br. phyllodes during their decomposition

Abstract The development of phytotoxicity of phyllodes from Acacia melanoxylon R.Br. was studied during their decomposition in four different plots in which the dominant species of tree was Quercus robur L., A. melanoxylon, Pinus radiata D. Don. and Eucalyptus globulus Labill. The toxicity of the extracts was estimated by measuring the effects on the germination and growth of Lactuca sativa L. var. Great Lakes grains, after 1, 7, 15, 30, 180 and 365 days of decomposition in each soil. The toxicity of various leachates is shown and the influence of different microbiological conditions on the liberation of phytotoxins is discussed. Results indicate that residues from acacia produce a strong inhibitory effect at the beginning of the decomposition. This toxicity seems to affect germination at initial phases more than at end phases; the greatest effects are on growth. The effects are different depending on the soil where the decomposition takes place. Otherwise, the effects of soil (without any decomposing residues) were determined by using a direct bioassay. L. sativa, Trifolium repens and Dactylis glomerata germination rate and radicle growth were later measured. Soil from Acacia showed a strong toxic effect on L. sativa and D. glomerata radicle growth, and on germination of D. glomerata .

Phenotypic plasticity and acclimation to water deficits in velvet-grass: a long-term greenhouse experiment. Changes in leaf morphology, photosynthesis and stress-induced metabolites.

Summary Wild velvet-grass ( Hocus lanatus ) collected from a natural population along a drought gradient, previously demonstrated as genetically homogeneous, was studied for phenotypic plasticity and acclimation mechanisms to water deficits. A three months greenhouse experiment was designed with plants submitted to nine levels of water availability, three months being the usual length of summer drought in field conditions. Aboveground biomass, total soluble proteins, free proline, free polyamines, leaf morphology, stomatal and hair characteristics, and net photosynthetic rates were analysed, and their significance discussed. Moderate drought did not damage the velvet-grass, and the plants grew better than without water limitation. Water stress-typical responses were shown as time and severity dependent in all the measured parameters. Lowest water availability treatments induced significative increase in free proline and soluble protein contents, as well as reductions in leaf size and aboveground biomass. Spermidine contents rose significatively in the most severe water stress. Furthermore, velvet-grass acclimated progressively to long-term water limitation.

Whole Plant Response of Lettuce After Root Exposure to BOA (2(3H)-Benzoxazolinone)

The goal of our work was to expand the knowledge about plant stress response to the allelochemical 2(3H)-benzoxazolinone (BOA). We focused on physiological processes that are affected by this secondary metabolite. Physiological and biochemical characteristics of plants exposed to BOA help us to better understand its mode of action and open the gate to the use of allelochemicals as “natural” herbicides. Measurements on photosynthesis, fluorescence, water relations, antioxidant enzymes (superoxide dismutase, peroxidase), ATPases, and lipid peroxidation indicated that a phytotoxic effect follows BOA exposition. This effect was intense enough to interfere with plant growth and development and to produce “induced senescence.” Based on this, we propose a multifaceted mode of action for BOA with effects at different levels and in different parts of the plant.

The natural compound benzoxazolin-2(3H)-one selectively retards cell cycle in lettuce root meristems

Benzoxazolin-2(3H)-one (BOA) is a natural plant product that is phytotoxic to target plant species, inhibiting germination and growth and causing oxidative damage. We investigated its effects on the root meristems of seedlings of lettuce (Lactuca sativa) by means of light and transmission electron microscopy, flow cytometry, and conventional determination of mitotic index. Flow cytometry analyses and mitotic index showed a retard of cell cycle in BOA-treated meristems with selective activity at G2/M checkpoint.

Allelopathic Effects of Tree Species on Some Soil Microbial Populations and Herbaceous Plants

The allelopathic potential of four tree species on soil microbial populations and some herbaceous plants (two understory species and one general biotest species) was investigated. Effects of three nonindigenous tree species, Eucalyptus globulus Labill, Pinus radiata D.Don and Acacia melanoxylon R.Br., on microorganisms participating in the cycle of nitrogen were evaluated, comparing them with those produced by the autochthonous Quercus robur L. Influence of the trees on Lactuca sativa L., Dactylis glomerata L. and Trifolium repens L. was also checked in bioassays. Cell numbers of Nitrosomonas sp. were negatively affected by Acacia and Eucalyptus stands, mainly during spring, when flowers are especially abundant on the ground. Proteolytic microorganisms were also negatively affected by Eucalyptus and Pinus stands, whilst Quercus stand did not show any toxicity. Soil bioassays showed clear inhibitory effects on germination and growth of understory plants, particularly soils from Eucalyptus and Acacia stands. The greatest effects had the soil from Acacia stand, which was phytotoxic during the whole period of germination and growth of understory plants. Allelopathic phenomena could be, at least partially, responsible of the low species diversity in the understory of the nonindigenous tree stands.

Differential responses to allelopathic compounds released by the invasive Acacia dealbata Link (Mimosaceae) indicate stimulation of its own seed

Invasion by exotic species constitutes a major threat to natural ecosystems. One of the most invasive species of current concern in Europe is Acacia dealbata Link (Mimosaceae), the expansion of which is reducing populations of native species, and hence biodiversity. To investigate the potential involvement of allelopathic mechanisms in this process, we evaluated the germination and seedling growth performance of Lactuca sativa L. (Compositae), Zea mays L. (Gramineae), Dactylis glomerata L. (Gramineae), Arabidopsis thaliana (L.) Heynh. (Cruciferae) and A. dealbata, in the presence of mixtures of throughfall and litter leachate and of aqueous soil extracts collected under A. dealbata at different times of the year. Each test species responded differently to the solutions being assayed suggesting that A. dealbata allelochemicals may affect a variety of physiological growth parameters. In most cases the observed effects were stimulatory rather than inhibitory, which may have long-term deleterious effects on native plant populations since premature growth during periods of poor resources can be detrimental. Interestingly, increased radical growth of A. dealbata was detected with either extract suggesting a positive autoalleloapthy effect.