Toxic action of zinc on growth and enzyme activities of rice oryza sativa L. Seedlings

Divalent nutrient cations: Friend and foe during zinc stress in rice

Zn deficiency is the most common micronutrient deficit in rice but Zn is also a widespread industrial pollutant. Zn deficiency responses in rice are well documented, but comparative responses to Zn deficiency and excess have not been reported. Therefore, we compared the physiological, transcriptional and biochemical properties of rice subjected to Zn starvation or excess at early and later treatment stages. Both forms of Zn stress inhibited root and shoot growth. Gene ontology analysis of differentially expressed genes highlighted the overrepresentation of Zn transport and antioxidative defense for both Zn stresses, whereas diterpene biosynthesis was solely induced by excess Zn. Divalent cations (Fe, Cu, Ca, Mn and Mg) accumulated in Zn-deficient shoots but Mg and Mn were depleted in the Zn excess shoots, mirroring the gene expression of non-specific Zn transporters and chelators. Ascorbate peroxidase activity was induced after 14 days of Zn starvation, scavenging H₂ O₂ more effectively to prevent leaf chlorosis via the Fe-dependent Fenton reaction. Conversely, excess Zn triggered the expression of genes encoding Mg/Mn-binding proteins (OsCPS2/4 and OsKSL4/7) required for antimicrobial diterpenoid biosynthesis. Our study reveals the potential role of divalent cations in the shoot, driving the unique responses of rice to each form of Zn stress.

Farmyard manures: the major agronomic sources of heavy metals in the Philippi Horticultural Area in the Western Cape Province of South Africa

Heavy metal toxicity in agro-ecosystems is a global problem. Recently, it has been indicated that the soils used for agriculture and the fresh produce grown on these soils in the Philippi Horticultural Area (PHA) contains heavy metals exceeding the maximum permissible concentrations thereof in South Africa. This study was therefore aimed at evaluating the concentrations of heavy metals in the soils and vegetables produced in the PHA, as well as to determine the major agronomic sources of these metals in this area. Cu, Pb, and Zn concentrations in the soils exceeded the maximum permissible concentrations of 6.6, 6.6, and 46 mg/kg, respectively. Cd, Pb, and Zn concentrations in the vegetables also exceeded the maximum permissible concentrations of 0.1, 0.5, and 40 mg/kg, respectively. The biggest agronomic contributors of these heavy metals to the soils in the PHA were found to be the farmyard manures. Knowing what the major sources of these heavy metals are, it is important to determine ways to mitigate the inputs thereof, as well as to remove existing concentrations from the soils without contaminating the groundwater resources in the area.

Peroxidase, phenolics, and antioxidative capacity of common mullein (Verbascum thapsus L.) grown in a zinc excess

Common mullein (Verbascum thapsus L.) is the dominant plant species at a disposal site polluted with metal from the hydrometallurgical jarosite zinc production process. Seeds collected at the site were germinated and plants were grown hydroponically under controlled conditions in a excess of Zn. Induction of total soluble POD activity in the root occurred at 1, 5, and 10 mM Zn, indicating Zn accumulation within the root. Accumulation of Zn in leaves was not accompanied by changes in POD activity, but resulted in gradual increase of total antioxidative capacity, which could be partly attributed to accumulation of soluble phenolics. The role of the phenolics/POD system in defense of V. thapsus against zinc is discussed.