Rice responses to silicon addition at different Fe status and growth pH. Evaluation of ploidy changes

Modulation of plant photosynthetic processes during metal and metalloid stress, and strategies for manipulating photosynthesis-related traits

Metals constitute vital elements for plant metabolism and survival, acting as essential co-factors in cellular processes which are indispensable for plant growth and survival. Excess or deficient provision of metal/metalloids puts plant's life and survival at risk, thus considered a potent stress for plants. Chloroplasts as an organelle with a high metal demand form a pivotal site within the metal homeostasis network. Therefore, the metal-mediated electron transport chain (ETC) in chloroplasts is a primary target site of metal/metalloid-induced stresses. Both excess and deficient availability of metal/metalloids threatens plant's photosynthesis in several ways. Energy demands from the photosynthetic carbon reactions should be in balance with energy output of ETC. Malfunctioning of ETC components as a result of metal/metalloid stress initiates photoinhiition. A feedback inhibition from carbon fixation process also impedes the ETC. Metal stress impairs antioxidant enzyme activity, pigment biosynthesis, and stomatal function. However, genetic manipulations, nutrient management, keeping photostasis, and application of phytohormones are among strategies for coping with metal stress. Consequently, a comprehensive understanding of the underlying mechanisms of metal/metalloid stress, as well as the exploration of potential strategies to mitigate its impact on plants are imperative. This review offers a mechanistic insight into the disruption of photosynthesis regulation by metal/metalloids and highlights adaptive approaches to ameliorate their effects on plants. Focus was made on photostasis, nutrient interactions, phytohormones, and genetic interventions for mitigating metal/metalloid stresses.

Silicon mitigates iron deficiency in two energy cane cultivars by modulating physiological and nutritional mechanisms

Introduction

Alkaline soils with iron (Fe) deficiency are found in many regions of the world, and the use of silicon (Si) can mitigate the damages caused by such deficiency. The aim of this study was to evaluate the effect of Si in mitigating a moderate deficiency of Fe in two energy cane cultivars.

Methods

Two experiments were performed, one with the VX2 cultivar and the other with the VX3 cultivar of energy cane, which were cultivated in pots with sand and a nutrient solution. In both experiments, treatments followed a factorial scheme 2x2, designed based on the sufficiency and deficiency of Fe, being combined with the absence or presence of Si (2.5 mmol L^-1), disposed in a randomized blocks design with six replicates. In the condition of Fe sufficiency, plants were cultivated in a solution containing 368 µmol L^-1 of Fe, while plants cultivated under deficiency were initially submitted to cultivation with a 54 µmol L^-1 concentration of Fe for 30 days, and later, with Fe complete omission for 60 days. The supply of Si was carried out by applying 15 fertirrigations with Si (via root and leaf) during the initial stage of seedling development, and after transplanting, the nutrient solution was added daily (via root).

Results and discussion

Both cultivars of energy cane were sensitive to Fe deficiency in the absence of Si, impairing its growth by causing stress and pigment degradation, thus reducing the photosynthesis efficiency. The supply of Si mitigated the damages caused by Fe deficiency in both cultivars, by increasing Fe accumulation in new and intermediate leaves, stem, and roots in the VX2 cultivar, and in new, intermediate, and old leaves and stem in the VX3 cultivar, which in turn reduced stress and favored both the nutritional and photosynthesis efficiency, while increasing the dry matter production. Si by modulating physiological and nutritional mechanisms, mitigates Fe deficiency in two energy cane cultivars. It was concluded that Si can be used as a strategy to improve growth and nutrition of energy cane in environments that are susceptible to Fe deficiency.

Silicon in action: Between iron scarcity and excess copper

Essential micronutrients belonging to the transition metals, such as Fe and Cu, are indispensable for plant growth and stress tolerance; however, when present in excess, they can become potentially dangerous producers of reactive oxygen species. Therefore, their homeostases must be strictly regulated. Both microelement deficiencies and elevated concentrations of heavy metals in the soil are global problems that reduce the nutritional value of crops and seriously affect human health. Silicon, a beneficial element known for its protective properties, has been reported to alleviate the symptoms of Cu toxicity and Fe deficiency stress in plants; however, we are still far from a comprehensive understanding of the underlying molecular mechanisms. Although Si-mediated mitigation of these stresses has been clearly demonstrated for some species, the effects of Si vary depending on plant species, growing conditions and experimental design. In this review, the proposed mechanistic models explaining the effect of Si are summarized and discussed. Iron and copper compete for the common metal transporters and share the same transport routes, hence, inadequate concentration of one element leads to disturbances of another. Silicon is reported to beneficially influence not only the distribution of the element supplied below or above the optimal concentration, but also the distribution of other microelements, as well as their molar ratios. The influence of Si on Cu immobilization and retention in the root, as well as Si-induced Fe remobilization from the source to the sink organs are of vital importance. The changes in cellular Cu and Fe localization are considered to play a crucial role in restoring homeostasis of these microelements. Silicon has been shown to stimulate the accumulation of metal chelators involved in both the mobilization of deficient elements and scavenging excess heavy metals. Research into the mechanisms of the ameliorative effects of Si is valuable for reducing mineral stress in plants and improving the nutritional value of crops. This review aims to provide a thorough and critical overview of the current state of knowledge in this field and to discuss discrepancies in the observed effects of Si and different views on its mode of action.

Impact of Silicon on Plant Nutrition and Significance of Silicon Mobilizing Bacteria in Agronomic Practices

Globally, rejuvenation of soil health is a major concern due to the continuous loss of soil fertility and productivity. Soil degradation decreases crop yields and threatens global food security. Improper use of chemical fertilizers coupled with intensive cultivation further reduces both soil health and crop yields. Plants require several nutrients in varying ratios that are essential for the plant to complete a healthy growth and development cycle. Soil, water, and air are the sources of these essential macro- and micro-nutrients needed to complete plant vegetative and reproductive cycles. Among the essential macro-nutrients, nitrogen (N) plays a significant in non-legume species and without sufficient plant access to N lower yields result. While silicon (Si) is the 2nd most abundant element in the Earth’s crust and is the backbone of soil silicate minerals, it is an essential micro-nutrient for some plants. Silicon is just beginning to be recognized as an important micronutrient to some plant species and, while it is quite abundant, Si is often not readily available for plant uptake. The manufacturing cost of synthetic silica-based fertilizers is high, while absorption of silica is quite slow in soil for many plants. Rhizosphere biological weathering processes includes microbial solubilization processes that increase the dissolution of minerals and increases Si availability for plant uptake. Therefore, an important strategy to improve plant silicon uptake could be field application of Si-solubilizing bacteria. In this review, we evaluate the role of Si in seed germination, growth, and morphological development and crop yield under various biotic and abiotic stresses, different pools and fluxes of silicon (Si) in soil, and the bacterial genera of the silicon solubilizing microorganisms. We also elaborate on the detailed mechanisms of Si-solubilizing/mobilizing bacteria involved in silicate dissolution and uptake by a plant in soil. Last, we discuss the potential of silicon and silicon solubilizing/mobilizing to achieve environmentally friendly and sustainable crop production.

Priming With Silicon: A Review of a Promising Tool to Improve Micronutrient Deficiency Symptoms

Priming consists of a short pretreatment or preconditioning of seeds or seedlings with different types of primers (biological, chemical, or physical), which activates various mechanisms that improve plant vigor. In addition, stress responses are also upregulated with priming, obtaining plant phenotypes more tolerant to stress. As priming is thought to create a memory in plants, it is impairing a better resilience against stress situations. In today's world and due to climatic change, almost all plants encounter stresses with different severity. Lots of these stresses are relevant to biotic phenomena, but lots of them are also relevant to abiotic ones. In both these two conditions, silicon application has strong and positive effects when used as a priming agent. Several Si seed priming experiments have been performed to cope with several abiotic stresses (drought, salinity, alkaline stress), and Si primers have been used in non-stress situations to increase seed or seedlings vigor, but few has been done in the field of plant recovery with Si after a stress situation, although promising results have been referenced in the scarce literature. This review pointed out that Si could be successfully used in seed priming under optimal conditions (increased seed vigor), to cope with several stresses and also to recover plants from stressful situations more rapidly, and open a promising research topic to investigate, as priming is not an expensive technique and is easy to introduce by growers.

Impact of the application of monosilicic acid to grapevine (Vitis vinifera L.) on the chemical composition of young red Mencia wines

Impact of applying monosilicic acid to grapevines during ripening on chemical composition of Mencía red wines in an area where fungal diseases during summer are common was examined. The foliar application of monosilicic acid to grapevines led to a less oxidized wine, with lower levels of acetic acid, acetaldehyde, ethyl acetate and diacethyl; this should be considered as positive from a sensory point of view. Wines made with silicon-treated grapes also contained lower levels of gluconic acid and glycerol, which are chemical markers of wines made with botrytized grapes, as well as higher contents of total phenols, anthocyanins and tannins. Furthermore, the contents of several mid-chain alcohols were higher (p < 0.05) in wines made with grapes from silicon-treated plants.

Beneficial Effect of Root or Foliar Silicon Applied to Cucumber Plants under Different Zinc Nutritional Statuses

Zinc (Zn) is an essential micronutrient involved in a large variety of physiological processes, and its deficiency causes mainly growth and development disturbances, as well as oxidative stress, which results in the overproduction and accumulation of reactive oxygen species (ROS). A possible environmentally friendly solution is the application of silicon (Si), an element that has shown beneficial effects under abiotic and biotic stresses on many crops. Si could be applied through the roots or leaves. The aim of this work is to study the effect of Si applied to the root or shoot in cucumber plants under different Zn statuses (sufficiency, deficiency, and re-fertilization). Cucumber plants were grown in hydroponics, with 1.5 mM Si applied at the nutrient solution or sprayed on the leaves. During the different Zn statuses, SPAD index, fresh weight, ROS, and Si, Zn, P, Cu and B mineral concentration were determined. The results suggested that Si application had no effect during sufficiency and deficiency periods, however, during re-fertilization foliar application of Si, it showed faster improvement in SPAD index, better increment of fresh weight, and a decrease in ROS quantity, probably due to a memory effect promoted by Si previous application during the growing period. In summary, Si application to cucumber plants could be used to prepare plants to cope with a future stress situation, such as Zn deficiency, due to its prompt recovery after overcoming the stress period.