Stomatal and non-stomatal limitations in savanna trees and C4 grasses grown at low, ambient and high atmospheric CO2.

By the end of the century, atmospheric CO2 concentration ([CO2]a) could reach 800 ppm, having risen from ∼200 ppm ∼24 Myr ago. Carbon dioxide enters plant leaves through stomata that limit CO2 diffusion and assimilation, imposing stomatal limitation (LS). Other factors limiting assimilation are collectively called non-stomatal limitations (LNS). C4 photosynthesis concentrates CO2 around Rubisco, typically reducing LS. C4-dominated savanna grasslands expanded under low [CO2]a and are metastable ecosystems where the response of trees and C4 grasses to rising [CO2]a will determine shifting vegetation patterns. How LS and LNS differ between savanna trees and C4 grasses under different [CO2]a will govern the responses of CO2 fixation and plant cover to [CO2]a - but quantitative comparisons are lacking. We measured assimilation, within soil wetting-drying cycles, of three C3 trees and three C4 grasses grown at 200, 400 or 800 ppm [CO2]a. Using assimilation-response curves, we resolved LS and LNS and show that rising [CO2]a alleviated LS, particularly for the C3 trees, but LNS was unaffected and remained substantially higher for the grasses across all [CO2]a treatments. Because LNS incurs higher metabolic costs and recovery compared with LS, our findings indicate that C4 grasses will be comparatively disadvantaged as [CO2]a rises.

In vitro antitumor effect of a lignan isolated from Combretum fruticosum, trachelogenin, in HCT-116 human colon cancer cells.

The use of natural products in therapeutics has been growing over the years. Lignans are compounds with large pharmaceutical use, which has aroused interest in the search for new drugs to treat diseases. The present study evaluated the cytotoxicity of (-)-trachelogenin, a dibenzylbutyrolactone type lignan isolated from Combretum fruticosum, against several tumor and non-tumor cell lines using the MTT assay and its possible mechanism of action. (-)-Trachelogenin showed IC50 values ranging of 0.8-32.4µM in SF-295 and HL-60 cell lines, respectively and IC50 values >64µM in non-tumor cell lines. (-)-trachelogenin persistently induced autophagic cell death, with cytoplasmic vacuolization and formation of autophagosomes mediated by increasing LC3 activation and altering the expression levels of Beclin-1.

Does the Growth Differentiation Balance Hypothesis Explain Allocation to Secondary Metabolites in Combretum apiculatum , an African Savanna Woody Species?

The growth differentiation balance hypothesis (GDBH) provides a framework that predicts a trade-off between costs of secondary metabolites (SMs) relative to the demand for photosynthate by growth. However, this hypothesis was developed using empirical evidence from plant species in northern boreal and temperate systems, leaving its applicability to species under different abiotic and biotic conditions questionable and generalizations problematic. The objective of this study was to investigate whether the GDBH explains allocation to SMs in the deciduous African savanna woody species C. apiculatum along a 6-point N gradient. The cornerstone prediction of the GDBH, i.e., the parabolic response in SMs along the N gradient, was not observed, with secondary metabolism showing compound-specific responses. Quercetin, myricetin, and kaempferol glycoside concentrations, all produced via the same pathway, responded differently across the N gradient. Flavonol glycoside, cinnamic acid, and quercetin glycoside concentrations decreased as N increased, which provides partial support for the carbon nutrient balance hypothesis. Simulated herbivory had no effect on photosynthesis, decreased foliar N and consequently increased C:N ratio, but did not induce an increase in SMs, with condensed tannins and flavonol glycosides being unaffected. Defoliated plants at low N concentration compensated for lost biomass, which suggests a tolerance response, but as predicted by the limiting resource model, plants at higher N concentration were evidently C limited and thus unable to compensate. Our results show that the GDBH does not explain allocation to SMs in C. apiculatum, and suggest that mechanistic explanations of plant allocation should consider the integrative defensive effect of changed SMs.

Photosynthetic and morphological acclimation of seedlings of tropical lianas to changes in the light environment.

UNLABELLED: • PREMISE OF THE STUDY: Few studies have analyzed the physiological performance of different life stages and the expression of ontogenetic niche shifts in lianas. Here, we analyzed the photosynthetic and morphological acclimation of seedlings of Stigmaphyllon lindenianum, Combretum fruticosum, and Bonamia trichantha to distinctive light conditions in a tropical dry forest and compared their response with the acclimation response of adult canopy lianas of the same species. We expected acclimation to occur faster through changes in leaf photochemistry relative to adaptation in morphology, consistent with the life history strategies of these lianas.• METHODS: Seedlings were assigned to the following light treatments: high light (HH), low light (LL), sun to shade (HL), and shade to sun (LH) in a common garden. After 40 d, HL and LH seedlings were exposed to opposite light treatments. Light response curves, the maximum photosynthetic rate in the field (Amax), and biomass allocation were monitored for another 40 d on leaves expanded before transfer.• KEY RESULTS: Photosynthetic responses, Amax, and biomass of Stigmaphyllon and Combretum varied with light availability. Physiological characters were affected by current light environment. The previous light environment (carryover effects) only influenced Amax. Morphological characters showed significant carryover effects. Stigmaphyllon showed high morphological and physiological plasticity. Sun-exposed seedlings of this liana increased stem biomass and switched from self-supporting to climbing forms.• CONCLUSIONS: Acclimation in seedlings of these lianas is consistent with the response of adult lianas in the canopy in direction, but not in magnitude. There was no evidence for ontogenetic niche shifts in the acclimation response.

Chemometrics and vibrational spectroscopy as green tools for mine phytoremediation strategies.

This study describes the use of near infrared (NIR) spectroscopy in combination with chemometrics to characterise Combretum erythrophyllum plant material to determine differences in the chemical profiles of samples harvested from mine contaminated areas and those of natural populations. The chemometric computation of near infrared vibrational spectra was used to generate principal component analysis and partial least squares models. These models were used to determine seasonal differences in the chemical matrices of samples harvested from the mine sites with different levels of contamination. Principal component analysis scatter plots illustrated clustering of phenolic profiles of samples depending on whether they originated from contaminated or uncontaminated soils. A partial least squares model was developed to link the variations in the chemical composition and levels of contamination in all samples collected in the same season (autumn). The levels of total soluble phenolic compounds in leaf extracts of C. erythrophyllum were measured using the Folin-Ciocalteau assay. Data analysis of the samples revealed that plants harvested from mine sites, particularly in summer, produced a higher level of phenolic compounds than those of the natural population, thereby displaying a good correlation with the chemometric models.