Fungal endophytes from cactus Cereus jamacaru in Brazilian tropical dry forest: a first study

Endophytic fungi live within the healthy tissues of plants and can promote host species tolerance to different environmental stresses. However, most studies have been of plants in humid environments, and there are few reports of the benefits of such associations for plants of extreme environments. Our aims were to identify endophytic fungi using morphological taxonomy, to explore richness and estimate species frequency in a cactus, C. jamacaru, in Brazilian tropical dry forest (Caatinga). We thus identified 59 taxa, corresponding to 69.7 % of the total number of isolates; the other 30.3 % were sterile mycelia. Cladosporium cladosporioides and Fusarium oxysporum were the species most commonly isolated, followed by Acremonium implicatum, Aureobasidium pullulans, Trichoderma viride, Chrysonilia sitophila, and Aspergillus flavus. Forty-seven species are recorded for the first time as endophytic fungi of cacti, and 18 others as endophytes for Brazil; this suggests that C. jamacaru harbors a highly diverse fungal community as measured by a diversity index. However, species accumulation curves suggest that our study still underestimates endophyte diversity because it does not provide an exhaustive sample. To our knowledge, this is the first report of endophytic fungi from C. jamacaru in tropical dry forests.     

Growth and chemical defense in relation to resource availability: tradeoffs or common responses to environmental stress?

One aspect of plant defense is the production of constitutive secondary compounds that confer toxicity on herbivores and pathogens. The purpose of this study was to compare patterns of plant tissue toxicity across gradients of irradiance and nutrient content. We measured the potential toxicity (1/LC50) of extracts of six species of herbaceous Asteraceae grown under controlled conditions of temperature (25 degrees C), humidity (80%), photoperiod (16 h/day), in a range of concentrations of a modified Hoagland hydroponic solution (full-strength, 1/5 dilute, 1/10 dilute, and 1/50 dilute) and under two different light intensities (250 and 125 micromol/m2/s). The plants grew from seed for 42 days post-germination, and randomly chosen plants were harvested each 7 days. We did a general measure of potential phytochemical toxicity using an alcohol extraction of secondary compounds followed by brine shrimp (Artemia sp.) bioassay. Contrary to the carbon/nutrient balance hypothesis, tissue toxicity generally increased with decreasing irradiance and nutrient levels, so that plants whose growth was most restricted had tissues that were most toxic, although there were species-specific differences in this trend.     

Ecophysiological performance of Calotropis procera: an exotic and evergreen species in Caatinga, Brazilian semi-arid

To better understand the proliferation of Calotropis procera in a semi-arid region of northeastern Brazil (Caatinga), we designed two experiments to determine which ecophysiological characteristics contribute to the species adaptive success. The first experiment was conducted with young plants under greenhouse conditions and three water regimes. The second experiment was conducted with adult plants under field conditions subject to regional seasonality. Young plants exhibited a high tolerance to water deficits, mainly because of their strong stomatal control, which was observed before any biochemical alterations in leaf metabolism. Only under full suspension of irrigation did the plants show a reduction in relative water content. Under field conditions, adult plants showed a high resilience to the semi-arid environment with respect to gas exchange and other measured biochemical parameters, including photosynthetic pigment, soluble sugars, amino acids and protein content, even under the low soil water availability of the dry season. This season featured high photosynthetically active radiation, low relative humidity and high temperatures, and thus exposed plants to extreme differences between leaf and air temperatures. Calotropis procera remains green throughout the year, indicating that it has developed several means of tolerating the semi-arid climate. Furthermore, this species maintains a high photosynthetic rate despite reduced stomatal conductance, which increases its water use efficiency, a fundamental characteristic for survival in this ecosystem.     

Do plant species with high relative growth rates have poorer chemical defences?

1. Most theories of plant strategies assume the presence of certain 'trade-offs'. One such evolutionary trade-off assumes a decrease in growth rate with increasing investment in chemical defences in species adapted to different levels of habitat fertility.
2. To test this hypothesis, we grew 31 herbaceous species of Asteraceae under controlled conditions of temperature (25 °C), humidity (80%), light (500 μmol m^–2 s^–1) and photoperiod (16 h day^–1) in a modified Hoagland hydroponic solution. The plants grew from seed for 35 days post-germination and were harvested at 14, 21, 28 and 35 days. Relative growth rate (RGR) was calculated as well as a general measure of potential phytochemical toxicity (LC₅₀) using an alcohol extraction of secondary compounds followed by Brine Shrimp bioassay and an assay of total phenolics.
3. The interspecific correlation between RGR and the potential phytochemical toxicity was weak and non-significant ( r _S = 0·12, P = 0·53). The correlation between RGR and total phenolics was weak, positive but significant ( r _S = 0·40, P = 0·03).
4. These results suggest that such an evolutionary trade-off does not exist in this group of Asteraceae.     

Germination of Prosopis juliflora (Sw.) D.C. seeds at different osmotic potentials and temperatures

The effects of osmolytes, osmotic potential (Ψ_s), temperature, and their interactions on the germinability, germination rate, and other germination parameters of the invasive shrub Prosopis juliflora, which grows in the semiarid environmental conditions of the Caatinga in northeast Brazil, were evaluated. To study the effects of polyethylene glycol (PEG) and NaCl stress and temperature on germination, two separate experiments were carried out at the Plant Ecophysiology Laboratory of the Federal University of Pernambuco in 2011. The overall germinability decreased significantly with increases in both PEG (one‐way anova , F_4,75 = 111.21, P ≤ 0.001) and NaCl (one‐way anova , F_4,75 = 12.82, P ≤ 0.001); however, the effects were more accentuated with PEG than NaCl. The PEG‐treated seeds maintained their germinability, even when they were subjected to a Ψ_s = ?1 MPa after being rinsed and allowed to germinate on deionized water. In contrast, NaCl‐treated seeds usually lost their ability to germinate; this fact was possibly linked to the accumulation of Na⁺ and Cl^? in the cells, which may have contributed to a loss of membrane function that led to the death of the embryos. Although numerous studies describing seed germination in the presence of osmolytes have been conducted, studies that show the interactions between osmolytes, osmotic potentials, and temperature are scarce. The present study is the first to describe these interactions for P. juliflora seeds.