Aluminum (Al) toxicity is one of the main growth and yield limiting factors for barley grown on acid soils. Silicon (Si) ameliorates Al toxicity as well as it promotes the phenolic compounds production that have antioxidant or structural role. We evaluated the time-dependent kinetics of Al and Si uptake and the impact of Si on the production of antioxidant- or structural- phenols in barley cultivars at the short-term. Two barley cultivars with contrasting Al tolerance (Hordeum vulgare ‘Sebastian’, Al tolerant; and H. vulgare ‘Scarlett’, Al sensitive), exposed to either −Al (0 mM) or +Al (0.2 mM) nutrient solutions without Si (−Si) or with 2 mM (+Si) were cultured for 48 h. Aluminum and Si concentration decreased in plants at all harvest times when Al and Si were simultaneously supplied; this effect was more noticeable in ‘Scarlett’. Nevertheless, Si influenced the antioxidant system of barley irrespective of the Al tolerance of the cultivar, decreasing oxidative damage and enhancing radical scavenging activity, the production of phenolic compounds, and lignin accumulation in barley with short-term exposure to Al.

Silicon (Si) exerts beneficial effects in mitigating aluminum (Al) toxicity in different plant species. These include attenuating oxidative damage and improving structural strengthening as a result of the increased production of secondary metabolites such as phenols. The aim of this research was to evaluate the effect of Si on phenol production and composition in two barley cultivars under Al stress. Our conceptual approach included a hydroponic experiment with an Al-tolerant (Sebastian) and an Al-sensitive (Scarlett) barley cultivar treated with two Al doses (0 or 0.2 mM of Al) and two Si doses (0 or 2 mM) for 21 days. Chemical, biochemical and growth parameters were assayed after harvest. Our results indicated that the Al and Si concentration decreased in both cultivars when Al and Si were added in combination. Silicon increased the antioxidant activity and soluble phenol concentration, but reduced lipid peroxidation irrespective of the Al dose. Both barley cultivars showed changes in culm creep rate, flavonoids and flavones concentration, lignin accumulation and altered lignin composition in Si and Al treatments. We concluded that Si fertilization could increase the resistance of barley to Al toxicity by regulating the metabolism of phenolic compounds with antioxidant and structural functions.

Although silicon (Si) and lignin are accumulated on plant cell walls and both confer resistance to multiple biotic and abiotic stresses [¹,²], the impact of Si on lignin production in plants grown under phosphorus (P) stress still remains unknown. We evaluated the effect of Si on the lignin accumulation pattern and expression of lignin biosynthesis-related genes in wheat plants grown at different P levels. Two wheat cultivars differing in tolerance to P deficiency (Púrpura-sensitive and Fritz-tolerant) were hydroponically grown in a continuously aerated nutrient solution, as proposed by [³]. Ten days later, plants were treated with P (0, 0.01 or 0.1 mM) in combination with Si (0, 1 or 2 mM). Twenty-one days after the initiation of treatments, plants were harvested and lignin concentration, the lignin distribution pattern and the gene expression of phenylalanine ammonia lyase (TaPAL) and cinnamyl alcohol dehydrogenase (TaCAD) were analyzed in shoots. The lignin concentration of both wheat cultivars did not vary at different P doses; nevertheless, 2 mM Si increased lignin accumulation mainly at either 0 mM P (cv. Púrpura) or 0.01 mM P (cv. Fritz), with a more noticeable effect in Púrpura than in Fritz. This increase was in agreement with the stronger intensity of Safranine O staining observed after Si was added to both cultivars grown at sufficient or deficient P levels and supports previous findings showing the alleviative role of Si by increasing lignin production under stressful conditions [⁴,⁵,⁶,⁷,⁸]. Such an effect may be related to either increased hydrogen peroxide production or peroxidase activity in cell walls, as well as to the modulation of the activity and/or gene expression of some key enzymes involved in lignin biosynthesis. In this way, we also found that Si induced the expression of lignin biosynthesis genes. The up-regulation of TaPAL was detected in cv. Púrpura grown at low P levels, with a further increase observed in plants treated with Si. Similar to Si addition to P-stressed plants of cv., Fritz caused a 1.5-fold increase in the transcript level of TaPAL. Similarly, the expression level of TaCAD increased 1.7-fold as a result of the Si supply to both cultivars grown at low P. Overall, our results show that Si induced the biosynthesis of lignin in shoots of wheat plants grown under P stress. Acknowledgments. FONDECYT Regular Project N° 1201257 and FONDECYT Postdoctoral Project N° 3200901.

Phosphorus (P) deficiency is one of the major limiting factors of wheat production worldwide. Although silicon (Si) is known to improve plant growth under low phosphorus (P) conditions, the impact of Si supply on the nutritional quality of wheat grains at field conditions remains unclear. This study aimed to investigate the impact of Si fertilization on the healthy attributes and yield of grains of wheat plants grown under a P deficiency. A field experiment on an Andisol with low available P content never amended with Si fertilizer was conducted. Two wheat cultivars with contrasting tolerance to P deficiency (cv. Púrpura, sensitive to P deficiency; and cv. Fritz, tolerant to P deficiency) were used during two growing seasons. Three P doses (0, 200, and 400 mg P kg⁻¹ soil; as triple superphosphate) were applied in combination with three Si doses (0, 250, and 500 mg Si kg⁻¹ soil; as magnesium silicate). At the mature grain stage, Si and P concentration, total phenols, phenolic acids, radical scavenging activity, and yield components were evaluated. At both growing seasons, Si supply enhanced the grain P concentration of cv. Púrpura grown in the absence of P, whereas grain Si concentration increased in both cultivars along the gradient of P supply. Interestingly, increasing Si doses augmented the phenol concentration and antioxidant capacity in grains of cv. Púrpura grown without P in both growing seasons. In contrast, Si decreased grain phenol concentration of cv. Fritz under P deficiency. A slight increment of phenolic acids induced by 250 mg kg⁻¹ Si was also detected in grains of both wheat cultivars grown without P. A positive effect of Si fertilization on the grain yield of both wheat cultivars was also found. In the first growing season, Si added to P deficient plants increased grain yield by about 23% and 47% in cv. Púrpura and cv. Fritz, respectively. A significant increase in grain yield was also observed in both wheat cultivars in the second year of the field assay. Taken together, our finding showed that Si fertilization improved the P concentration, phenols production, antioxidant capacity and yield of wheat grains under low P conditions. In this way, Si fertilization could be used as a potential strategy to improve crop production in soils with low P availability. Acknowledgments. FONDECYT Regular Project N°1201257.

Current evidence shows that silicon (Si) can alleviate multiple plant stresses by inducing the antioxidant defense and phenolic metabolism of plants. Nevertheless, the mechanisms underlying these responses remain unclear. We investigated the Si effect on the phenolic metabolism of two barley cultivars differing in their tolerance to phosphorus (P) deficiency (cv. Sebastian, P-deficiency-tolerant and cv. Traveler, P-deficiency-sensitive). Plants were hydroponically grown with P (0, 0.01 or 0.1 mM P; applied as Na₂HPO₄) in combination with Si (0, 1 or 2 mM Si; applied as Na₂SiO₃). At harvest, total phenols, antioxidant capacity, individual phenolics, and the gene expression of enzymes involved in the synthesis of soluble phenolic compounds (including phenylalanine ammonia lyase (HvPAL) and chalcone synthase (HvCHS)) were analyzed in shoots. In cv. Sebastian grown without P, Si reduced both total phenols and antioxidant capacity to levels comparable to plants supplied with an optimal P dose. In contrast, increasing Si doses triggered an enhancement of total phenols and antioxidant ability in cv. Traveler cultivated in the absence of P. Seven flavonoids were identified, with the most relevant being derivatives of apigenin and lutonarin. Although we did not observe a clear effect of Si on the content of individual phenolics in cv. Sebastian exposed to P stress, an increment in the concentration of apigenin–pentoxide–hexoside was detected in cv. Traveler as a consequence of Si application to P-deficient plants. Differential expression of genes associated with the synthesis of phenolics was also induced by Si under P-stress. The transcript level of HvPAL diminished in cv. Sebastian and augmented in cv. Traveler due to Si supply under P limitation. Likewise, Si decreased HvCHS expression in cv. Sebastian grown without P, whereas an up-regulation of HvCHS was observed when Si was applied to cv. Traveler grown with low P. Acknowledgments. FONDECYT Regular Project N°1201257 and FONDECYT Postdoctoral Project N°3200901.