Multiple stresses occurring with boron toxicity and deficiency in plants.

Boron (B), an essential nutrient for plants, participates in many physiological processes, with emphasis its role in the formation of the plant's cell wall. In soil, the range between deficiency and toxicity of B is very narrow as compared to other nutrients, which makes its management in agriculture very difficult, as it depends on the soil and environmental conditions. B stress simultaneously acts with others (extreme temperatures, excess of light, high concentration of CO2, drought, salinity or heavy metal contamination, etc.). The effects of these other stresses could increase the sensitivity of plants to B toxicity or deficiency. The simultaneous combination (B stress × other abiotic stresses) is a complex interaction that should be analyzed in detail if the resistance of crops to climate change is needed. This article reviews the response of plants when facing a combination of B stress with other stresses, and compares this response with the individual stresses. Also, in the last few years, the role of B has been described in multiple plant functions that can improve its resilience to specific stresses. Thus, this article also analyses in what conditions the application of B can be efficient for the improvement of the plant's response to other stresses.

Physiological, Nutritional and Metabolomic Responses of Tomato Plants After the Foliar Application of Amino Acids Aspartic Acid, Glutamic Acid and Alanine.

Agriculture is facing a great number of different pressures due to the increase in population and the greater amount of food it demands, the environmental impact due to the excessive use of conventional fertilizers, and climate change, which subjects the crops to extreme environmental conditions. One of the solutions to these problems could be the use of biostimulant products that are rich in amino acids (AAs), which substitute and/or complement conventional fertilizers and help plants adapt to climate change. To formulate these products, it is first necessary to understand the role of the application of AAs (individually or as a mixture) in the physiological and metabolic processes of crops. For this, research was conducted to assess the effects of the application of different amino acids (Aspartic acid (Asp), Glutamic acid (Glu), L-Alanine (Ala) and their mixtures Asp + Glu and Asp + Glu + Ala on tomato seedlings (Solanum lycopersicum L.). To understand the effect of these treatments, morphological, physiological, ionomic and metabolomic studies were performed. The results showed that the application of Asp + Glu increased the growth of the plants, while those plants that received Ala had a decreased dry biomass of the shoots. The greatest increase in the growth of the plants with Asp + Glu was related with the increase in the net CO2 assimilation, the increase of proline, isoleucine and glucose with respect to the rest of the treatments. These data allow us to conclude that there is a synergistic effect between Aspartic acid and Glutamic acid, and the amino acid Alanine produces phytotoxicity when applied at 15 mM. The application of this amino acid altered the synthesis of proline and the pentose-phosphate route, and increased GABA and trigonelline.

Arbuscular mycorrhizal symbiosis improves tolerance of Carrizo citrange to excess boron supply by reducing leaf B concentration and toxicity in the leaves and roots.

This study explores the possibility of using mycorrhization as a novel technique for diminishing the negative effects of boron (B) in the nutrient solution on seedlings of Carrizo citrange rootstock plants. For this, an experiment was planned for studying the physiological (gas exchange and chlorophyll fluorescence parameters), morphological (vegetative growth parameters), nutritional (organic solutes, carbohydrates) and oxidative stress responses of seedlings that were either mycorrhized (+AM, Rhizophagus irregularis; previously known as Glomus intraradices) or not mycorrhized (-AM), and irrigated with water containing different concentrations of B (0.5, 5 and 10 mg L-1). It was observed that an excess of B in the nutrient solution decreased the vegetative growth in both +AM and -AM plants, but this decrease was greater in -AM plants. Mycorrhized plants (+AM) under high B concentration accumulated less B in the leaves, and had a smaller reduction of net assimilation rate of CO2 and lower MDA concentration than non-mycorrhized plants. Thus, it can be concluded that mycorrhization increased the tolerance to high boron concentration in the irrigation water of citrange Carrizo seedlings by reducing both the B concentration in the plant tissue and the B toxicity in the physiological processes. The study of organic solutes and carbohydrates also pointed to a different response model between +AM and -AM plants that could be related to the different tolerance observed between these plants.

Response of three citrus genotypes used as rootstocks grown under boron excess conditions.

In citrus, the effects of an excess of boron (B) are conditioned by the type of rootstock. In the present work, the morphological, physiological and biochemical responses of seedlings from three citrus genotypes, commonly used as rootstocks in citriculture. In particular, Citrange Carrizo (CC), Citrus macrophylla (CM) and sour orange (SO) seedlings were treated with an excess of B (10 mg L-1) in the nutrient solution in order to determine the relative tolerance and to understand the possible mechanisms that make a rootstock more tolerant than the others. To assess these responses, different parameters were measured in plants, such as vegetative growth, B concentration in leaves, stems and roots, gas exchange and chlorophyll fluorescence, the concentration of osmolytes and the activity of enzymes related to the antioxidant system. The results showed, according to the growth parameters, that the SO rootstock was the most tolerant to an excess of B; while CC was the most sensitive. This result was due to the fact that SO plants accumulated less B in leaves, as its roots have a great capacity of restricting the uptake and transport of B towards the aerial part. Moreover, SO is suggested to diminish B toxicity risk through its antioxidant system, since it presented high activity of ascorbate peroxidase (APX) and superoxide dismutase (SOD), as well as high accumulation of quaternary ammonium compounds (QACs).