Aphid and caterpillar feeding drive similar patterns of induced defences and resistance to subsequent herbivory in wild cotton.

MAIN CONCLUSION: Our results indicate caterpillars and aphids cause similar levels of induced defences and resistance against caterpillars in wild cotton plants. These symmetrical effects are not consistent with patterns predicted by plant defensive signaling crosstalk and call for further work addressing the biochemical mechanisms underpinning these results. Plant-induced responses to attack often mediate interactions between different species of insect herbivores. These effects are predicted to be contingent on the herbivore's feeding guild, whereby prior feeding by insects should negatively impact subsequent feeding by insects of the same guild (induced resistance) but may positively influence insects of a different guild (induced susceptibility) due to interfering crosstalk between plant biochemical pathways specific to each feeding guild. We compared the effects of prior feeding by leaf-chewing caterpillars (Spodoptera frugiperda) vs. sap-sucking aphids (Aphis gossypii) on induced defences in wild cotton (Gossypium hirsutum) and the consequences of these attacks on subsequently feeding caterpillars (S. frugiperda). To this end, we conducted a greenhouse experiment where cotton plants were either left undamaged or first exposed to caterpillar or aphid feeding, and we subsequently placed caterpillars on the plants to assess their performance. We also collected leaves to assess the induction of chemical defences in response to herbivory. We found that prior feeding by both aphids and caterpillars resulted in reductions in consumed leaf area, caterpillar mass gain, and caterpillar survival compared with control plants. Concomitantly, prior aphid and caterpillar herbivory caused similar increases in phenolic compounds (flavonoids and hydroxycinnamic acids) and defensive terpenoids (hemigossypolone) compared with control plants. Overall, these findings indicate that these insects confer a similar mode and level of induced resistance in wild cotton plants, calling for further work addressing the biochemical mechanisms underpinning these effects.

Wetland plant species improve performance when inoculated with arbuscular mycorrhizal fungi: a meta-analysis of experimental pot studies.

The presence of arbuscular mycorrhizal fungi (AMF) in wetlands is widespread. Wetlands are transition ecosystems between aquatic and terrestrial systems, where shallow water stands or moves over the land surface. The presence of AMF in wetlands suggests that they are ecologically significant; however, their function is not yet clearly understood. With the aim of determining the overall magnitude and direction of AMF effect on wetland plants associated with them in pot assays, we conducted a meta-analysis of data extracted from 48 published studies. The AMF effect on their wetland hosts was estimated through different plant attributes reported in the studies including nutrient acquisition, photosynthetic activity, biomass production, and saline stress reduction. As the common metric, we calculated the standardized unbiased mean difference (Hedges' d) of wetland plant performance attributes in AMF-inoculated plants versus non-AMF-inoculated plants. Also, we examined a series of moderator variables regarding symbiont identity and experimental procedures that could influence the magnitude and direction of an AMF effect. Response patterns indicate that wetland plants significantly benefit from their association with AMF, even under flooded conditions. The beneficial AMF effect differed in magnitude depending on the plant attribute selected to estimate it in the published studies. The nature of these benefits depends on the identity of the host plant, phosphorus addition, and water availability in the soil where both symbionts develop. Our meta-analysis synthetizes the relationship of AMF with wetland plants in pot assays and suggests that AMF may be of comparable importance to wetland plants as to terrestrial plants.

Ecological and evolutionary consequences of tri-trophic interactions: Spatial variation and effects of plant density.

PREMISE OF STUDY: The factors driving variation in species interactions are often unknown, and few studies have made a link between changes in interactions and the strength of selection. METHODS: We report on spatial variation in functional responses by a seed predator (SP) and its parasitic wasps associated with the herb Ruellia nudiflora. We assessed the influence of plant density on consumer responses and determined whether density effects and spatial variation in functional responses altered natural selection by these consumers on the plant. We established common gardens at two sites in Yucatan, Mexico, and planted R. nudiflora at two densities in each garden. We recorded fruit output and SP and parasitoid attack; calculated relative fitness (seed number) under scenarios of three trophic levels (accounting for SP and parasitoid effects), two trophic levels (accounting for SP but not parasitoid effects), and one trophic level (no consumer effects); and compared selection strength on fruit number under these scenarios across sites and densities. KEY RESULTS: There was spatial variation in SP recruitment, whereby the SP functional response was negatively density-dependent at one site but density-independent at the other; parasitoid responses were density-independent and invariant across sites. Site variation in SP attack led, in turn, to differences in SP selection on fruit output, and parasitoids did not alter SP selection. There were no significant effects of density at either site. CONCLUSIONS: Our results provide a link between consumer functional responses and consumer selection on plants, which deepens our understanding of geographic variation in the evolutionary outcomes of multitrophic interactions.

Impact of weed control on arbuscular mycorrhizal fungi in a tropical agroecosystem: a long-term experiment.

Cover crop species represent an affordable and effective weed control method in agroecosystems; nonetheless, the effect of its use on arbuscular mycorrhizal fungi (AMF) has been scantily studied. The goal of this study was to determine root colonization levels and AMF species richness in the rhizosphere of maize plants and weed species growing under different cover crop and weed control regimes in a long-term experiment. The treatment levels used were (1) cover of Mucuna deeringian (Muc), (2) "mulch" of Leucaena leucocephala (Leu), (3) "mulch" of Lysiloma latisiliquum (Lys), (4) herbicide (Her), (5) manual weeding (CD), (6) no weeding (SD), and (7) no maize and no weeding (B). A total of 18 species of AMF belonging to eight genera (Acaulospora, Ambispora, Claroideoglomus, Funneliformis, Glomus, Rhizophagus, Sclerocystis, and Scutellospora) were identified from trap cultures. Muc and Lys treatments had a positive impact on AMF species richness (11 and seven species, respectively), while Leu and B treatments on the other hand gave the lowest richness values (six species each). AMF colonization levels in roots of maize and weeds differed significantly between treatment levels. Overall, the use of cover crop species had a positive impact on AMF species richness as well as on the percentage of root colonized by AMF. These findings have important implications for the management of traditional agroecosystems and show that the use of cover crop species for weed control can result in a more diverse AMF community which should potentially increase crop production in the long run.

Arbuscular mycorrhizal propagules in soils from a tropical forest and an abandoned cornfield in Quintana Roo, Mexico: visual comparison of most-probable-number estimates.

The present study was aimed at comparing the number of arbuscular mycorrhizal fungi (AMF) propagules found in soil from a mature tropical forest and that found in an abandoned cornfield in Noh-Bec Quintana Roo, Mexico, during three seasons. Agricultural practices can dramatically reduce the availability and viability of AMF propagules, and in this way delay the regeneration of tropical forests in abandoned agricultural areas. In addition, rainfall seasonality, which characterizes deciduous tropical forests, may strongly influence AMF propagules density. To compare AMF propagule numbers between sites and seasons (summer rainy, winter rainy and dry season), a "most probable number" (MPN) bioassay was conducted under greenhouse conditions employing Sorgum vulgare L. as host plant. Results showed an average value of 3.5 ± 0.41 propagules in 50 ml of soil for the mature forest while the abandoned cornfield had 15.4 ± 5.03 propagules in 50 ml of soil. Likelihood analysis showed no statistical differences in MPN of propagules between seasons within each site, or between sites, except for the summer rainy season for which soil from the abandoned cornfield had eight times as many propagules compared to soil from the mature forest site for this season. Propagules of arbuscular mycorrhizal fungi remained viable throughout the sampling seasons at both sites. Abandoned areas resulting from traditional slash and burn agriculture practices involving maize did not show a lower number of AMF propagules, which should allow the establishment of mycotrophic plants thus maintaining the AMF inoculum potential in these soils.

Genetic variation in the response of the weed Ruellia nudiflora (Acanthaceae) to arbuscular mycorrhizal fungi.

The main goal of this work was to test for plant genetic variation in the phenotypic plasticity response of the weed Ruellia nudiflora to arbuscular mycorrhizal (AM) fungi inoculation. We collected plants in the field, kept them under homogeneous conditions inside a nursery, and then collected seeds from these parent plants to generate five inbred lines (i.e., genetic families). Half of the plants of each inbred line were inoculated with AM fungi while the other half were not (controls); a fully crossed experimental design was then used to test for the effects of treatment (with or without AM fungi inoculation) and inbred line (genetic family). For each plant, we recorded the number of leaves produced and the number of days it survived during a 2-month period. Results showed a strong positive treatment effect (plastic response to AM fungi inoculation) for leaf production and survival. Moreover, in terms of survival, the treatment effect differed between genetic families (significant genetic family by treatment interaction). These findings indicate that the positive effect of AM fungi on plant survival (and potentially also growth) differs across plant genotypes and that such condition may contribute to R. nudiflora's capacity to colonize new environments.

Propagules of arbuscular mycorrhizal fungi in a secondary dry forest of Oaxaca, Mexico.

Plant cover loss due to changes in land use promotes a decrease in spore diversity of arbuscular mycorrhizal fungi (AMF), viable mycelium and, therefore, in AMF colonization, this has an influence in community diversity and, as a consequence, in its recovery. To evaluate different AMF propagules, nine plots in a tropical dry forest with secondary vegetation were selected: 0, 1, 7, 10, 14, 18, 22, 25, and 27 years after abandonment in Nizanda, Oaxaca, Mexico. The secondary vegetation with different stages of development is a consequence of slash and burn agriculture, and posterior abandonment. Soil samples (six per plot) were collected and percentage of AMF field colonization, extrarradical mycelium, viable spore density, infectivity and most probable number (MPN) ofAMF propagules were quantified through a bioassay. Means for field colonization ranged between 40% and 70%, mean of total mycelium length was 15.7 +/- 1.88 mg(-1) dry soil, with significant differences between plots; however, more than 40% of extracted mycelium was not viable, between 60 and 456 spores in 100 g of dry soil were recorded, but more than 64% showed some kind of damage. Infectivity values fluctuated between 20% and 50%, while MPN showed a mean value of 85.42 +/- 44.17 propagules (100 g dry soil). We conclude that secondary communities generated by elimination of vegetation with agricultural purposes in a dry forest in Nizanda do not show elimination of propagules, probably as a consequence of the low input agriculture practices in this area, which may encourage natural regeneration.

Establishment of Desmoncus orthacanthos Martius (Arecaceae): effect of inoculation with arbuscular mycorrhizae.

Inoculation with arbuscular mycorrhizal (AM) fungi has often promoted increased growth of plants but very little work has been done in the tropics to evaluate the effects of inoculation on the establishment and development of seedlings in forests. Desmoncus orthacanthos Martius is a scandent palm present both in early and late succession, and consequently can be used in restoration processes. A test was conducted to determine the effect of AM on the establishment of Desmoncus orthacanthos in tropical forest in the Yucatan Peninsula, Mexico. Thirty inoculated and 30 non-inoculated seedlings were introduced in two sites of different successional age, a mature forest and an eight-year old abandoned cornfield (acahual). Survival and growth parameters were evaluated after 12 months. Leaf area and phosphorus, but not height, were greater in inoculated than non-inoculated plants in the forest but not in the acahual. However, mycorrhizae had a clear effect on plant survival in both sites, with a threefold increase in survival of inoculated compared with non-inoculated plants bassed on an odds ratio. The results suggest that inoculation will be important to increase the establishment of this commercially important palm.

Propagules of arbuscular mycorrhizal fungi in a secondary dry forest of Oaxaca, Mexico

Plant cover loss due to changes in land use promotes a decrease in spore diversity of arbuscular mycorrhizal fungi (AMF), viable mycelium and, therefore, in AMF colonization, this has an influence in community diversity and, as a consequence, in its recovery. To evaluate different AMF propagules, nine plots in a tropical dry forest with secondary vegetation were selected: 0, 1, 7, 10, 14, 18, 22, 25, and 27 years after abandonment in Nizanda, Oaxaca, Mexico. The secondary vegetation with different stages of development is a consequence of slash and burn agriculture, and posterior abandonment. Soil samples (six per plot) were collected and percentage of AMF field colonization, extrarradical mycelium, viable spore density, infectivity and most probable number (MPN) ofAMF propagules were quantified through a bioassay. Means for field colonization ranged between 40% and 70%, mean of total mycelium length was 15.7 +/- 1.88 mg(-1) dry soil, with significant differences between plots; however, more than 40% of extracted mycelium was not viable, between 60 and 456 spores in 100 g of dry soil were recorded, but more than 64% showed some kind of damage. Infectivity values fluctuated between 20% and 50%, while MPN showed a mean value of 85.42 +/- 44.17 propagules (100 g dry soil). We conclude that secondary communities generated by elimination of vegetation with agricultural purposes in a dry forest in Nizanda do not show elimination of propagules, probably as a consequence of the low input agriculture practices in this area, which may encourage natural regeneration.

La vegetación secundaria con diferentes grados de desarrollo es consecuencia de prácticas agrícolas de rozatumba- quema y su posterior abandono. La remoción de la vegetación por cambios de uso de suelo promueve una disminución en la diversidad de esporas, micelio viable y por lo tanto de la colonización de los hongos micorrizógenos arbusculares (HMA), lo cual repercute en la diversidad de la comunidad y como consecuencia en su regeneración. Para evaluar los propágulos de HMA se seleccionaron nueve parcelas con vegetación secundaria con diferentes edades de abandono: 0, 1, 7, 10, 14, 18, 22, 25, 27 años, en la región de Nizanda, Oaxaca, México. Se recolectaron muestras de suelo (seis por parcela) y se cuantificó la colonización de campo, el micelio extrarradical, la densidad de esporas viables, así como la infectividad y el número más probable de propágulos infectivos (NMP). Los promedios de la colonización de campo fueron de 40 a 70 %, el promedio de la longitud de micelio total alcanzó 15.7 ± 1.88 mg-1 suelo seco con diferencias significativas entre parcelas, pero más del 40 % del micelio extraído no fue viable. Se encontraron entre 60 y 456 esporas en 100 g de suelo pero más del 64 % presentaron algún tipo de daño. Los valores de infectividad se encontraron entre 20 % y 50 %, mientras que el NMP presentó un promedio de 85.42 ±44.17 (100 g de suelo seco). Concluimos que las comunidades secundarias generadas por la eliminación de la vegetación con fines agrícolas en la selva baja caducifolia en Nizanda o presentan eliminación de propágulos, probablemente por el bajo impacto de la agricultura, lo cual indica que la regeneración natural es posible.

Establishment of Desmoncus orthacanthos Martius (Arecaceae): effect of inoculation with arbuscular mycorrhizae

Inoculation with arbuscular mycorrhizal (AM) fungi has often promoted increased growth of plants but very little work has been done in the tropics to evaluate the effects of inoculation on the establishment and development of seedlings in forests. Desmoncus orthacanthos Martius is a scandent palm present both in early and late succession, and consequently can be used in restoration processes. A test was conducted to determine the effect of AM on the establishment of Desmoncus orthacanthos in tropical forest in the Yucatan Peninsula, Mexico. Thirty inoculated and 30 non-inoculated seedlings were introduced in two sites of different successional age, a mature forest and an eight-year old abandoned cornfield (acahual).Survival and growth parameters were evaluated after 12 months. Leaf area and phosphorus, but not height, were greater in inoculated than non-inoculated plants in the forest but not in the acahual. However, mycorrhizae had a clear effect on plant survival in both sites, with a threefold increase in survival of inoculated compared with non-inoculated plants bassed on an odds ratio. The results suggest that inoculation will be important to increase the establishment of this commercially important palm.

La inoculación con hongos micorrizógenos arbusculares (AM)promueve un incremento en el crecimiento de las plantas, sin embargo poco trabajo se ha realizado en los trópicos para evaluar el efecto de la inoculación en el proceso de establecimiento de plántulas. Desmoncus orthacanthos Martius es una palmera trepadora que se distribuye tanto en etapas tempranas y tardías de la sucesión, por lo tanto puede ser empleada en procesos de restauración. Se realizó una prueba de establecimiento de D. orthacanthos en una selva tropical de la península de Yucatán, México. Treinta plántulas inoculadas y 30 no inoculadas se establecieron en dos sitios con diferentes etapas sucesionales: selva madura y un campo abandonado de maíz (acahual). Después de 12 meses el valor de área foliar y contenido de fósforo fue mayor en el tratamiento con micorrizas en la selva pero no en el acahual, sin embargo, el tratamiento con micorrizas mostró un claro efecto en la supervivencia de las palmeras en ambos sitios con un incremento tres veces mayor en la probabilidad de supervivencia de palmeras inoculadas comparadas con las no inoculadas de acuerdo al análisis de razón de disparidad (odds ratio). La inoculación podría ser importante para incrementar el establecimiento de esta palmera.

Mycorrhizal dynamics and dependence of desmoncus orthacanthos maritus (arecaceae) a native palm of the Yucatan peninsula Mexico

Se realizaron observaciones en una parcela (milpa) abandonada de ocho años de edad y en una selva madura para determinar la dinámica estacional y la recuperación de una perturbación en la asociación micorrícica arbuscular (MA) presente en Desmoncus orthacanthos Martius. En una prueba de invernadero se evaluó la dependencia micorrícica relativa (RMD) de esta especie. En el campo se seleccionaron cuatro etapas de crecimiento en esta palmera y se monitoreó durante un año las raíces y el suelo rizosférico de cinco individuos de cada etapa de crecimiento. Se detectó una dinámica estacional, pero no diferencia en la actividad de la asociación comparando la milpa abandonada y la selva madura. No se encontró correlación, durante el período de estudio, entre la humedad del suelo, densidad de esporas por gramo de suelo y colonización de raíces. El porcentaje de colonización fue mayor en ambos sitios durante la etapa de lluvias, este porcentaje fue igual de elevado en una temporada de secas en la milpa abandonada. La densidad de esporas fue mayor en la temporada de secas. Los valores calculados de RMD se encuentran entre 7.8 y 54.9 por ciento por ciento, con una mayor biomasa en las plantas micorrizadas comparadas con las no-micorrizadas usando concentraciones de fósforo similares a las presentes en el suelo de los sitios de estudio. Los resultados sugieren que esta especie es dependiente de la asociación micorrícica en las condiciones evaluadas y mantiene una actividad de AM relativamente alta incluso durante la temporada de secas. Los resultados sugieren que las practicas agrícolas empleadas en la zona de estudio permiten la recuperación del potencial de inoculo del suelo para el crecimiento de D. orthacanthos