Lumichrome and riboflavin are novel molecules from rhizobial exudates that stimulate plant growth. Developmental changes elicited by lumichrome at very low nanomolar concentrations (5 nM) include early initiation of trifoliate leaves, expansion of unifoliate and trifoliate leaves, increased stem elongation and leaf area, and consequently greater biomass accumulation in monocots and dicots. However, higher lumichrome concentration (50 nM) depressed root development and reduced growth of unifoliate and second trifoliate leaves. Applying either 10 nM lumichrome, 10 nM ABA, or 10 ml of infective rhizobial cells (0.2 OD600) to roots of monocots and dicots for 44 h produced identical effects, which included decreased stomatal conductance and leaf transpiration in Bambara groundnut, soybean and maize, increased stomatal conductance and transpiration in cowpea and lupin, and elevated root respiration in maize (19% by rhizobia and 20% by lumichrome). Extracellular exudation of lumichrome, riboflavin and IAA was greater in N2-fixing rhizobia than non-fixing bacteria, indicating their role as symbiotic signals. Xylem concentration of lumichrome in cowpea and soybean was greater in plants inoculated with infective rhizobia and treated with lumichrome (61.2 µmol lumichrome.ml-1 sap), followed by uninoculated plants receiving lumichrome (41.12 µmol lumichrome.ml-1 sap), and lowest in uninoculated, lumichrome-free plants (26.8 µmol lumichrome.ml-1 sap). Overall, soybean showed greater xylem concentration of lumichrome and a correspondingly increased accumulation in leaves relative to cowpea. As a result, soybean exhibited dramatic developmental changes than cowpea. Taken together, lumichrome and riboflavin secreted by soil rhizobia function as environmental cues for sensing stress. The fact that exogenous application of ABA to plant roots caused the same effect as lumichrome on stomatal functioning suggests molecular cross-talk in plant response to environmental stimuli.

This study investigated the effect of gibberellic acid (GA₃) on the growth of tomato seedlings and heavy metal accumulation within seedlings tissue irrigated with acid mine water (AMW). Three experimental treatments were administered using a completely randomized design with five replicates. The experimental treatments included were gibberellic acid + acid mine water (GA₃ + AMW), acid mine water (AMW), and tap water. Seedlings were irrigated directly in pots with 400 mL of 100% AMW at two-day intervals 21 days after planting. Drenching of the seedlings with GA₃ was done every 24 h for eight consecutive days from 28 days after planting. Results on the physicochemical analysis showed high concentrations of heavy metals (HMs) in AMW compared to tap water and the experimental treatment significantly affected the measured plant growth parameters. Tomato plants irrigated with AMW alone were shorter (4.00 cm) than plants irrigated with tap water (14.00 cm), while plants treated with AMW and GA₃ were much taller (16.50 cm) than the latter (control). Moreover, HM accumulation differed among the three treatments. Seedlings that received AMW with no GA₃ accumulated more HMs (Cd, Cr, Cu, Ni, and Zn) in their roots, stems, and leaves while plants treated with GA₃ had a decrease in the accumulation and distribution of HMs in the different plant tissues (roots, stems, and leaves) relative to AMW alone and the plants irrigated with tap water alone. The study revealed that GA₃ boosted the growth of tomato seedlings irrigated with AMW and also altered HM accumulation with the tissues of the seedlings.

Nutrient-poor soils coupled with micronutrient deficiency among many rural resource-poor communities remain a challenge in sub-Saharan Africa. Nutrient-poor soils can be managed through various soil amendment or fertilisation strategies. Micronutrients can be supplied through plants. The study was aimed at determining the symbiotic nitrogen fixation of cowpea as well as the contribution of inter-cropping under varying levels of nitrogen, phosphorus, and potassium (NPK) fertilisation. In addition, the amount of micronutrients supplied by cowpea and amaranth were determined. The experiment was laid out in a 2 × 4 factorial treatment structure in a completely randomised design, with inter-cropping (cowpea and amaranth) and fertiliser (control, 25%, 50%, and 100% of the recommended NPK levels) as treatment factors with four replications. Symbiotic N₂ fixation of cowpea decreased from 341–448 kgN.ha⁻¹ to 77–91 kgN.ha⁻¹ for the first year and 557–227 kgN.ha⁻¹ to 92−164 kgN.ha⁻¹ for the second year with fertilisation. The iron and zinc nutritional yield increased (61–210 g.ha⁻¹ for first year and 304–867 g.ha⁻¹, for second year), proportional to fertiliser application to both crops. The research shows the benefits of leguminous crops in soil nutrient fertility and inorganic fertilisation with inter-cropping in managing micronutrient deficiency to meet the nutritional needs of rural communities.

Aspalathus linearis (Burm. F.) R. Dahlg., commonly referred to as Rooibos tea, grows naturally in nutrient-poor, sandy, acidic soils (pH 3–5.3) with high aluminum concentration ranging from 110 to 275 μg Al g⁻¹in the Cederberg’s mountainous areas in South Africa. Earlier studies found significant differences in Al concentration in organs of A. linearis, with roots having higher amounts (1262–4078 μg Al g⁻¹), suggesting that the plant is capable of accumulating excess Al in acidic soils. Identification of the mineralogical constituents of organs of A. linearis using X-ray diffraction (XRD) analysis revealed the presence of an Al–Si complex (aluminosilicate or hydroxyaluminosilicate (HAS) species) in the shoot and root, possibly to internally ameliorate Al toxicity. In addition, A. linearis has specialized cluster roots that exude Al-chelating organic acid ligands such as citric, malic, and malonic acids. Organic acids can bind strongly to Al in the plant and rhizosphere to reverse its phytotoxic effects to the plants. Field and glasshouse studies revealed significant differences in pH between rhizosphere and nonrhizosphere soils of A. linearis and also showed that roots of the plant release OH⁻and HCO₃⁻anions to raise rhizosphere pH possibly to immobilize Al through complexation. Furthermore, A. linearis is easily infected by arbuscular mycorrhizae (AM) fungi, but mycorrhizal associations are known to inhibit transport of metallic cations into plant roots. These features of A. linearis are perceived as good indicators for bioremediation; and the plant could, therefore, be a suitable candidate for phytoremediation technologies such as phytoaccumulation, phytostabilization, and phytodegradation. The environmental and economic implications of the potential of A. linearis to bioremediate Al-contaminated soils are briefly discussed. Furthermore, this review briefly highlights future studies investigating the utilization of the shoot of A. linearis as adsorbent for the removal of trace and (or) heavy metal from aqueous solutions.

Rooibos (Aspalathus linearis) plants are cultivated for the production of the Rooibos tea. In this work, biosorption process of Pb(II) from aqueous solution on Rooibos shoot powder (RSP) was studied in a batch process. The effect of experimental parameters such as solution pH (2–9), contact time (30–180 min) and initial Pb(II) concentration (25–400 mg L⁻¹), adsorbent dosage (0.2–2.1 g), and temperature (25–80°C) on biosorption process was examined. In an attempt to determine the biosorption capacity and rate of Pb(II) removal, isotherm and kinetic data were modeled using different equations. To this end, the biosorption process by RSP followed the Langmuir model for Pb(II) ions (R² = 0.9907), with maximum monolayer adsorption capacity of 18.90 mg g⁻¹. Kinetic data were properly fitted with the pseudo-second order kinetic model, and it can be seen that the calculated q_e (cal) value (11.31 mg g⁻¹) agrees with the experimental value (12.80 mg g⁻¹). Furthermore, the biosorbent showed good removal of Pb(II) from industrial wastewaters.