Several studies have demonstrated Si absorption in monocotyledon and dicotyledon species. Regarding Si accumulation, studies in monocotyledons have identified Si deposition around the cell wall, cuticle layer bulliform cells, silica cells and endodermal cells. In previous studies with different citrus species there were evidence of Si accumulation, however no information on Si deposition can be found. Therefore, in this study, Si was applied (0 and 1000 mg L⁻¹) to the roots of two citrus species, ‘Delta’ Valencia (Citrus sinensis) and ‘Nules’ Clementine (Citrus reticulata). Si accumulation were investigated in new, and old leaves and roots. Si deposition was investigated through scanning electron microscopy using energy dispersive analysis X-ray, environmental scanning electron microscopy and light microscopy. Si accumulation was significantly higher in Si treated leaves comparatively to the control in both young and mature leaves. Meanwhile, Si accumulation increased with leaf age. Additionally, Si accumulation was significantly higher in roots than in leaves. With respect to Si deposition, granules were identified in the epidermal cells through SEM and ESEM studies. The light microscopy identified the presence of Si granules in the surface and around the outer cell surface forming the cuticle-silica double layer of the lower epidermis in Si treated plants. Silica deposit were not found on the abaxial leaf surface. The findings suggest a passive uptake in citrus species.

This paper reviews the threat of citrus pathogens during citrus production, with a focus on two pre-harvest diseases, citrus leaf spot, caused by Alternaria alternata (Fr.) Keissl. (1912) and brown rot, caused by Phytophthora citrophthora (R.E. Sm. and E.H. Sm.) Leonian, (1906) as well as green and blue mold post-harvest disease, caused by Penicillium digitatum (Pers.) Sacc. and P. italicum Wehmer, (1894), respectively. Furthermore, it reviews the role of soluble silicon, Si nutrition in biotic stress mitigation and potential mitigation mechanisms. Previous studies on the use of Si fertilizers have focused on high accumulator Si crops. These have demonstrated the potential of Si to reduce the occurrence of biotic stresses, which takes place through both physical and biochemical mechanisms. However, few studies have demonstrated the potential of Si to mitigate biotic stress in citrus, or the mechanisms involved. There is a clear need for studies on the impact of Si on various stress biochemical pathways in plants generally, and specifically for citrus due to the huge loss caused by pre- and post-harvest pathogens. This will assist in deepening our understanding of the pathophysiology which is essential to develop resistant cultivars.

Low-temperature stress is an important limiting factor affecting citrus growth and fruit yields. Therefore, increasing citrus cold stress tolerance may enhance the growth, yield, and quality of citrus production in marginal areas. The objective of this study was to determine the efficacy of silicon (Si) fertilizer application on cold-tolerance enhancement in citrus. Two citrus cultivars (Delta and Nules) were subjected to Si fertilization (control, 1000 mg L⁻¹) and cold-stress temperature treatments (control and 0 °C for 72 h) using a 2 × 2 × 2 factorial treatment structure with six replications. Leaf gas exchange and chlorophyll fluorescence parameters, such as net photosynthetic rate (A), stomatal conductance (g_s), transpiration rate (Tr), internal CO₂ concentration (C_i), intrinsic water-use efficiency (iWUE), minimal fluorescence (F_o), maximum fluorescence (F_m), maximum quantum efficiency of PSII primary photochemistry of dark-adapted leaves (F_v/F_m), maximum quantum efficiency of PSII primary photochemistry of dark-/light-adapted leaves (F’_v/F’_m), electron transport rate (ETR), non-photochemical quenching (NPQ), and the relative measure of electron transport to oxygen molecules (ETR/A), were measured. The application of Si drenching to trees that were subsequently exposed to cold stress reduced g_s, Tr, and C_i but improved iWUE and F_o in both cultivars compared to the Si-untreated trees. In addition, specific adaptation mechanisms were found in the two citrus species; NPQ and ETR were improved in Si-treated Valencia trees, while A, F_m, and ETR/A were improved in Clementine trees under chilling stress conditions. The current research findings indicate the potential of Si application to enhance cold stress tolerance in citrus, which can provide a strategy for growing citrus in arid and semi-arid regions that may experience cold stress. Overall, after the application of silicon drenching, the cold-sensitive citrus Valencia cultivar became as cold-tolerant as the cold-tolerant Clementine cultivar.

South African soils generally lack native Bradyrhizobium strains that nodulate and fix atmospheric nitrogen (N₂) in soybeans (Glycine max L.). It is therefore very important to inoculate soybeans with products that contain effective Bradyrhizobium strains as active ingredients. In this study, a field experiment was conducted on two bioclimatic zones in South Africa during the 2019/2020 season to assess the effect of Bradyrhizobium japonicum strain WB74 inoculant formulation on nitrogen fixation, growth and yield improvement in soybeans. The first bioclimatic zone was characterized by a sandy clay loam soil, whereas the second bioclimatic zone has a sandy loam soil. The results showed that inoculation of soybeans with both peat and liquid formulations of Bradyrhizobium japonicum WB74 increased nitrogen uptake, which resulted in yield increase. The amount of N fixed was measured as ¹⁵N isotopes and increased with all treatments compared to the uninoculated control in both liquid and peat inoculant formulations. In bioclimatic zone A, slightly better results were obtained using the liquid formulation (1.79 t ha⁻¹ for liquid compared to 1.75 t ha⁻¹ for peat treatments), while peat formulations performed better in bioclimatic zone B (1.75 t ha⁻¹ for peat compared to 1.71 t ha⁻¹ for liquid treatments). In both areas higher yields were obtained with the formulations used in this study compared to the registered standards (treatment T3). The findings in this study provide vital information in the development and application of formulated microbial inoculants for sustainable agriculture in South Africa.

Lucerne is regarded as the best legume crop for forage to be cultivated in South Africa. It is commonly used to produce good quality hay. It also plays an important role in soil conservation, regeneration, and crop rotation systems as it supplies substantial amounts of nitrogen to succeeding crops through symbiotic N₂ fixation, which makes it the preferable choice for intensive forage production systems. Fertilizer in liquid inoculant formulations has demonstrated to contribute growth and yield increase for leguminous crops. Therefore, the aim of this paper was to determine the effects of Sinorhizobium meliloti liquid formulation inoculation on the growth, yield, and nitrogen content in lucerne. The strain RF14 (Sinorhizobium meliloti) was collected from the Agricultural Research Council at Roodeplaat (Plant Health and Protection located (East), Pretoria (South Africa). The liquid inoculant contained 6.5 × 10⁹ viable cells mL⁻¹. According to the Kooen–Gieger climatic classification, the experiments were conducted on two different climatic zones. The first site was in Bronkhorspruit (Blesbokfontein farm) in the Gauteng province and the second was in Hartbeesfontein (Rietfontein Farm) in the Northwest province. The results showed that lucerne inoculation with liquid inoculant formulation of Sinorhizobium meliloti significantly increased nodule number, size, growth, and yield in both bioclimatic zones. The significantly increased were compared to the negative control. The Sinorhizobium meliloti inoculant increased nitrogen accumulation in all inoculated treatments compared to the control. The finding of this research provides important information on the impact of rhizobium microbial inoculant application in the improvement of soil fertility through nodule formation. In addition, seed vigor improvement was translated in overall growth and yield increase in lucerne plants.

Common bean (Phaseolus vulgaris L.) ranks among the most produced and consumed legume crops and contains essential macro- and micronutrients. Grain yield of the food crop is markedly decreased by poor management, especially a lack of additional essential nutrient elements through the application of fertilizers. In addition to the application of fertilizers, scholarly research and crop farmers have shown that the use of biofertilizer inoculants improves the yield of legume crops. The objective of this research study was to assess the effectiveness of peat-based Rhizobium tropici sp. UD5 on the growth, yield, and nitrogen concentration of common bean. The peat inoculant contained 6.5 × 10⁹ viable cells/g. The experiment was conducted in two climatic zones, as described by the Koppen–Gieger climatic classification system. Treatments involved the peat-based inoculant Rhizobium tropici (T0 = 0 g without inoculation, T1 = 250 g of peat inoculant of strain UD5 for 50 kg seeds, T2 = 500 g of inoculant of strain UD5, and T3 = 200 g of comparative peat inoculant). The results indicated that common-bean-inoculated formulation of R. tropici sp. strain UD5 increased the following parameters compared to the controls: plant height (T1 = 18.22%, T2 = 20.41%, and T3 = 19.93% for bioclimatic zone 1; T1 = 16.78%, T2 = 20.71%, and T3 = 19.93% for bioclimatic zone 2), root length (T1 = 13.26%, T2 = 21.28%, and T3 = 19.38% for zone 1; T1 = 15.06%, T2 = 23.70%, and T3 = 19.20% for zone 2), number of nodules (T1 = 1162.57%, T2 = 1166.36%, and T3 = 1180.30% for zone 1; T1 = 1575%, T2 = 1616.5%, and T3 = 1608.25% for zone 2), size of nodules (T1 = 224.07%, T2 = 224.07%, and T3 = 208.33% for zone 1; T1 = 166.4%, T2 = 180%, and T3 = 140% for zone 2), and yield (T1 = 40.49%, T2 = 47.10%, and T3 = 45.45% for zone 1; T1 = 62.16%, T2 = 54.05%, and T3 = 58.55% for zone 2). R. tropici sp. UD5 peat inoculant formulation also increased the nitrogen concentration in leaves compared to the control (T1 = 3.75%, T2 = 1.12%, and T3 = 8.72%) in both bioclimatic zones. The findings of this study provide significant information on the positive effect of R. tropic UD5 strain peat inoculant application in the improvement of plant growth, development, and yield through the formation of nodules.