In this study we report on the synthesis of silver nanoparticles (AgNPs) from the leaf extracts of Moringa oleifera using sunlight irradiation as primary source of energy, and its antimicrobial potential. Silver nanoparticle formation was confirmed by surface plasmon resonance at 450 nm and 440 nm, respectively for both fresh and freeze-dried leaf samples. Crystanality of AgNPs was confirmed by transmission electron microscopy, scanning electron microscopy with energy dispersive x-ray spectroscopy and Fourier transform infrared (FTIR) spectroscopy analysis. FTIR spectroscopic analysis suggested that flavones, terpenoids and polysaccharides predominate and are primarily responsible for the reduction and subsequent capping of AgNPs. X-ray diffraction analysis also demonstrated that the size range of AgNPs from both samples exhibited average diameters of 9 and 11 nm, respectively. Silver nanoparticles showed antimicrobial activity on both bacterial and fungal strains. The biosynthesised nanoparticle preparations from M. oleifera leaf extracts exhibit potential for application as broad-spectrum antimicrobial agents.

Several of the procedures necessary for cryopreservation of excised zygotic embryonic axes are known to be accompanied by emission of damaging levels of reactive oxygen species (ROS). These have been shown to be associated with shoot apical meristem necrosis, curtailing subsequent axis development to root production only, particularly for tropical/subtropical recalcitrant-seeded species. Here we report on the application of the principles of electrochemistry in the generation of strongly reducing, high pH cathodic water, by electrolysis of a solution containing calcium and magnesium chloride. The cathodic water in which Strychnos gerrardii axes were immersed for 30 min following dehydration, and importantly, after dehydration followed by cryopreservation, was shown to have strongly antioxidative properties in counteracting the damaging effects of ROS bursts and promoting shoot development. In a parallel experiment, axes of Boophane disticha exhibited enhanced total antioxidant activity when exposed to cathodic water both immediately following excision, and after flash-drying. For both species, the efficacious effects of cathodic treatment were manifest after the axes had been in culture for 4 h, suggesting that ROS were not quenched at source, but probably counteracted by enhancement of activity of endogenous antioxidants. Cathodic water therefore affords a non-toxic means of amelioration of oxidative, stress-related damage, which, coupled with the strongly fungicidal activity of the acidic, anionic water fraction, offers significant, and apparently non-injurious, advances towards successful cryopreservation of germplasm – and probably generally improved success of in-vitro-based procedures for plant tissues.

Studies to elucidate the biochemical basis of survival of excised embryonic axes (EAs) of recalcitrant seeds of Trichilia dregeana at different drying rates revealed significant differences between slow and rapid drying. Rapid drying allowed these EAs to survive dehydration to much lower water contents (WCs; ca. 0.31 g g⁻¹ dry mass basis with 73% germination) compared with slow drying, where 90% of the EAs lost viability at a WC of ca. 0.79 g g⁻¹. In EAs slowly dried within seeds, the levels of hydroxyl radical (three- to fivefold at WCs > 0.5 g g⁻¹) and lipid peroxidation (50% at similar WC) were significantly higher compared with those dried rapidly to comparable WCs. When EAs were dried slowly, enzymic antioxidant levels were not sustained and declined significantly with prolonged storage. In contrast, sustained activity of enzymic antioxidants was detected in rapidly dried EAs even at relatively low WCs. Furthermore, the greater decline in glutathione (GSH)/GSH disulphide ratio in EAs slowly dried within seeds compared with rapidly dried EAs and a shift in GSH redox potential to relatively more positive values in the EAs slowly dried within seeds was correlated with considerable viability loss. It is apparent from this study that greater retention of viability to lower WCs in rapidly dried EAs from recalcitrant seeds may at least be partly explained by the retention of functional antioxidant status. It is also suggested that the reduction of viability in rapidly dried EAs at very low WCs appears to be a non-oxidative process.

No abstract

Increased air pollution in a number of developing African countries, together with the reports of vegetation damage typically associated with acid precipitation in commercial forests in South Africa, has raised concerns over the potential impacts of acid rain on natural vegetation in these countries. Recalcitrant (i.e. desiccation sensitive) seeds of many indigenous African species, e.g. must germinate shortly after shedding and hence, may not be able to avoid exposure to acid rain in polluted areas. This study investigated the effects of simulated acid rain (rainwater with pH adjusted to pH 3.0 and 4.5 with 70:30, H2 SO4 :HNO3 ) on germination, seedling growth and oxidative metabolism in a recalcitrant-seeded African tree species Trichilia dregeana Sond., growing in its natural seed bank. The results suggest that acid rain did not compromise T. dregeana seed germination and seedling establishment significantly, relative to the control (non-acidified rainwater). However, pH 3.0 treated seedlings exhibited signs of stress typically associated with acid rain: leaf tip necrosis, abnormal bilobed leaf tips, leaf necrotic spots and chlorosis, reduced leaf chlorophyll concentration, increased stomatal density and indications of oxidative stress. This may explain why total and root biomass of pH 3.0 treated seedlings were significantly lower than the control. Acid rain also induced changes in the species composition and relative abundance of the different life forms emerging from T. dregeana's natural seed bank and in this way could indirectly impact on T. dregeana seedling establishment success.