Abstract

Silicon (Si) is an element that can improve the growth and development of rice plants in water-deficient environments because it is an enzymatic stimulant, signaling for production of antioxidant compounds. Therefore, the aim of this study was to examine the relationship between water deficiency and the effect of Si on two rice cultivars whose seeds were treated with dietholate. The experimental design was fully randomized with three replicates, and treatments were organized in a 3x2x2x4 factorial arrangement: three water soil conditions (50% and 100% of soil water retention capacity (WRC) and complete submergence in a water blade of 5.0 cm); two cultivars (IRGA 424 RI and Guri INTA CL); two sources of Si (sodium metasilicate and potassium metasilicate); and four rates of Si (0; 4.0; 8.0 and 16 g L^-1). Chlorophyll a and b, leaf area and shoot and root dry weight increased at higher rates of Si under the three soil water regimes. There was an increase in superoxide dismutase and guaiacol peroxidase enzyme activity in the cultivars at higher rates of Si, reducing lipid peroxidation caused by water deficiency. Therefore, Si did indeed attenuate water deficiency stress in rice plants emerging from seeds treated with dietholate.

Keywords:
water deficiency; rice; lipid peroxidation

Resumo

O silício (Si) é um elemento que pode proporcionar melhor crescimento e desenvolvimento às plantas de arroz cultivadas em ambientes com deficiência hídrica, por ser um estimulador enzimático, promovendo sinalização para produção de compostos antioxidantes. Assim, o objetivo deste trabalho foi estudar a relação entre deficiência hídrica e o efeito do Si em duas cultivares de arroz tratadas com dietholate. O delineamento experimental utilizado foi inteiramente casualizado, com três repetições e os tratamentos arranjados em esquema fatorial 3x2x2x4, sendo os fatores: três condições hídricas do solo (50%, 100% da capacidade de retenção de água no solo (CRA) e lâmina d’água de 5,0 cm), duas cultivares (IRGA 424 RI e Guri INTA CL), duas fontes de Si (metassilicato de sódio e metassilicato de potássio) e quatro doses de Si (0; 4,0; 8,0 e 16 g L^-1). O índice de clorofila a e b, a área foliar e o teor de massa seca da parte aérea e raiz aumentaram com o aumento das doses de Si nas três condições hídricas do solo. Houve um aumento na atividade das enzimas superóxido dismutase e guaiacol peroxidase nas cultivares estudadas à medida que as doses de Si aumentaram, diminuindo a peroxidação de lipídios, causada pela restrição hídrica. Portanto, o Si atenua o estresse por déficit hídrico em plantas de arroz emergentes de sementes tratadas com dietholate.

Palavras-chave:
déficit hídrico; arroz; peroxidação de lipídeos

1. Introduction

Rice (Oryza sativa L.) is one of the most consumed cereals in the world and is a basic human food. Average production in Brazil reached 11.6 million metric tons over the last five years. The state of Rio Grande do Sul is the biggest Brazilian producer, with average production of 8,000 metric tons over this period (CONAB, 2019COMPANHIA NASCIONAL DE ABASTECIMENTO – CONAB, 2019. Acompanhamento da Safra Brasileira Grãos, v. 6 Safra 2018/19 - Sexto levantamento. Brasília: CONAB, pp. 1-6.).

Weeds are considered the main factor limiting rice production (Antigua et al., 1990ANTIGUA, G., COLON, C. and GARCIA, J., 1990. Utlizacion del herbicida Arrozan en el control de malezas del arroz en Cuba. Ciencia y Tecnica en la Agricultura - Arroz, vol. 13, no. 1-2, pp. 135-144.), and clomazone, Gamit trademark, is one of the most widely used herbicides, inhibiting carotenoid synthesis and impairing photosynthesis. Clomazone is tolerated by the rice crop up to the rates specified, depending on the soil type (Sanchotene et al., 2010SANCHOTENE, D.M., KRUSE, N.D., AVILA, L.A., MACHADO, S.L.O., NICOLODI, G.A. and DORNELLES, S.H.B., 2010. Efeito do protetor dietholate na seletividade de clomazone em cultivares de arroz irrigado. Planta Daninha, vol. 28, no. 2, pp. 339-346. http://dx.doi.org/10.1590/S0100-83582010000200013
https://doi.org/10.1590/S0100-8358201000... ). However, to control the main grasses competing with the rice, higher rates are usually applied during the crop cycle and can be phytotoxic, impairing post-germination seedling growth and tiller production (Karam et al., 2003KARAM, D., CARNEIRO, A.A., ALBERT, L.H., CRUZ, M.B., COSTA, G.T. and MAGALHÃES, P.C., 2003. Seletividade da cultura do milho ao herbicida clomazone por meio do uso de dietholate. Revista Brasileira de Milho e Sorgo, vol. 2, no. 1, pp. 72-79. http://dx.doi.org/10.18512/1980-6477/rbms.v2n1p72-79.
http://dx.doi.org/10.18512/1980-6477/rbm... ).

For this reason, dietholate, Permit trademark, is widely used to protect seeds, seeking to reduce the phytotoxic effects of clomazone on seedlings when it is applied at a rate above the level tolerated by the crop (Ferhatoglu et al., 2005FERHATOGLU, Y., AVDIUSHKO, S. and BARRET, M., 2005. The basic for safening of clomazone by phorate insecticide in cotton and inhibitors of cytochrome P450s. Pesticide Biochemistry and Physiology, vol. 81, no. 1, pp. 59-70. http://dx.doi.org/10.1016/j.pestbp.2004.09.002.
http://dx.doi.org/10.1016/j.pestbp.2004.... ). Dietholate is an organophosphate compound that inhibits cytochrome P-450 mono-oxygenase, the agent that hydroxylates clomazone inside the cell cytoplasm, rendering it toxic to the rice plant (Sanchotene et al., 2010SANCHOTENE, D.M., KRUSE, N.D., AVILA, L.A., MACHADO, S.L.O., NICOLODI, G.A. and DORNELLES, S.H.B., 2010. Efeito do protetor dietholate na seletividade de clomazone em cultivares de arroz irrigado. Planta Daninha, vol. 28, no. 2, pp. 339-346. http://dx.doi.org/10.1590/S0100-83582010000200013
https://doi.org/10.1590/S0100-8358201000... ). However, in plants this enzyme detoxifies certain compounds (Yun et al., 2005YUN, M.S., YOGO, Y., MIURA, R., YAMASUE, Y. and FISCHER, A.J., 2005. Cytochrome P-450 monooxygenase activity in herbicide-resistant and - susceptible late watergrass (Echinochloa phyllopogon). Pesticide Biochemistry and Physiology, vol. 83, no. 2-3, pp. 107-114. http://dx.doi.org/10.1016/j.pestbp.2005.04.002.
http://dx.doi.org/10.1016/j.pestbp.2005.... ) and when inhibited by the dietholate, the activation of antioxidant enzymes such as superoxide dismutase (SOD), peroxidases (POD) and catalase (CAT) can also be inhibited. These antioxidant enzymes play an important role in removing excess of reactive oxygen species (ROS) from the cell, produced during periods of biotic and abiotic stresses and impairing photosystems and lipid peroxidation, rupturing membranes and causing cell leakage and other effects, resulting in cell collapse and death (Silva et al., 2019SILVA, K.S., TABALDI, L.A., ROSSATO, L.V., BASILIO, V.B., CAVICHIOLI, B.M. and MACHADO, S.L.O., 2019. Contents of pigments and activity of antioxidant enzymes in rice plants pre-treated with sodium nitroprusside and exposed to clomazone. Planta Daninha, vol. 37, pp. 1-10. http://dx.doi.org/10.1590/s0100-83582019370100032.
http://dx.doi.org/10.1590/s0100-83582019... ).

Research has shown that these deleterious effects caused by dietholate can be triggered by water deficiency (Mauad et al., 2011MAUAD, M., CRUSCIOL, C.A.C. and FILHO, H.G., 2011. Produção de massa seca e nutrição de cultivares de arroz de terras altas sob condição de déficit hídrico e adubação silicatada. Semina, vol. 32, no. 3, pp. 939-948. http://dx.doi.org/10.5433/1679-0359.2011v32n3p939.
http://dx.doi.org/10.5433/1679-0359.2011... ). To reduce these effects, biostimulants containing some essential minerals and phytohormones have been used (Inoue et al., 2012INOUE, M.H., ARAÚJO, T.D.C., MENDES, K.F., BEN, R. and CONCIANI, P.A., 2012. Eficiência do dietholate e bioestimulantes isolados e associados no tratamento de sementes de algodoeiro adensado com clomazone aplicado em pré-emergência. Revista de Ciências Agro-ambientais, vol. 10, no. 2, pp. 163-172.). The success of these biostimulants, recommended as possible regulators of metabolic activity in plants, is usually due to mineral elements such as Si (Mauad et al., 2011MAUAD, M., CRUSCIOL, C.A.C. and FILHO, H.G., 2011. Produção de massa seca e nutrição de cultivares de arroz de terras altas sob condição de déficit hídrico e adubação silicatada. Semina, vol. 32, no. 3, pp. 939-948. http://dx.doi.org/10.5433/1679-0359.2011v32n3p939.
http://dx.doi.org/10.5433/1679-0359.2011... ). Which is an enzymatic stimulant (Taiz et al., 2017TAIZ, L., ZEIGER, E., MOLLER, I.M. and MURPH, A., 2017. Fisiologia e desenvolvimento vegetal. 6ª ed. Porto Alegre: Artmed Editora S.A., 120p.). And can affect plant growth and development by initiating signaling for production of antioxidant compound and osmoprotectants, such as proline and glycine betaine (Etesami and Jeong, 2017ETESAMI, H. and JEONG, B.R., 2017. Review and future prospects on the action mechanisms in alleviating biotic and abiotic stresses in plants. Ecotoxicology and Environmental Safety, vol. 147, pp. 881-896. http://dx.doi.org/10.1016/j.ecoenv.2017.09.063. PMid:28968941.
http://dx.doi.org/10.1016/j.ecoenv.2017.... ). Glycine betaine protects thylakoid membranes, maintaining photochemical efficiency in photosynthesis (Ashraf and Foolad, 2007ASHRAF, M. and FOOLAD, M., 2007. Roles of glycine betaine and proline in improving plant abiotic stress resistance. Environmental and Experimental Botany, vol. 59, no. 2, pp. 206-216. http://dx.doi.org/10.1016/j.envexpbot.2005.12.006.
http://dx.doi.org/10.1016/j.envexpbot.20... ). Proline acts to stabilize proteins and protects membranes against the deleterious effects of ROS (Sharma and Dubey, 2005SHARMA, P. and DUBEY, R.S., 2005. Modulation of nitrate reductase activity in rice seedlings under aluminium toxicity and drought stress: role of osmolytes as enzyme protectant. Journal of Plant Physiology, vol. 162, no. 8, pp. 854-862. http://dx.doi.org/10.1016/j.jplph.2004.09.011. PMid:16146311.
http://dx.doi.org/10.1016/j.jplph.2004.0... ).

There are no conclusive studies on the effectiveness of biosimulants in mitigating the stresses caused by the use of dietholate, neither if they are correlated or not with water deficiency. The aim of this study was to evaluate the possible effects of Si in attenuating water deficiency stress in plant grown from rice seeds treated with dietholate.

2. Material and Methods

The experiment was conducted in the greenhouse (6 x 20 m, headroom 5 m) from February to April 2018. The experimental design was fully randomized with three replicates, organized in a 3x2x2x4 factorial arrangement: three soil water regimes (50% and 100% of soil water retention capacity (WRC) and a 5 cm water blade); two cultivars (IRGA 424 RI and Guri INTA CL); two sources of Si a) sodium metasilicate (composition: Na₂O≅ 28%; SiO₂ ≅ 27%; Fe ≅ 0.02%), and b) potassium metasilicate (composition: N≅ 3%; P₂O₅≅ 2%; K₂O≅ 15%; SiO₂≅ 25%); and four rates of Si applied to the seed in-furrow (0; 4.0; 8.0 and 16 g L^-1).

Each experimental unit consisted of a pot containing 500 g dystrophic sandy red-yellow argisol. The basic fertilization with NPK macro and micronutrients was applied prior to sowing, based on the results of the chemical soil analysis, using a ROLAS recommendation table (Official Soil Analysis Laboratory Network) for irrigated rice. All treatments received the same basic fertilization condition. Ten seeds treated with dietholate seed protector (Permit Star®) at a rate of 6 mL per kg seed were sown in each pot at a depth of two centimeters, and after emergence the six most similar seedlings in each pot were retained.

The method described in Spatt (2018)SPATT, L.L., 2018. Desempenho fotossintético e eficiência de herbicidas em Urochloa plantaginea (Poaceae) submetidas às diferentes condições hídricas do solo e doses de nitrogênio. Universidade Federal de Santa Maria, Santa Maria, 15 p. Dissertação de Mestrado em Agrobiologia. was used to maintain water conditions at 50% and 100% of soil water retention capacity (WRC) and for daily irrigation. Treatments related to soil water regimes were begun 15 days after sowing, simulating water ingress conditions in an irrigated rice crop. Evaluations of rice plant physiological and morphological variables were carried out when the plants were in vegetative stage V10.

Shoot lipid peroxidation was evaluated using the method described by El-Moshaty et al. (1993)EL-MOSHATY, F.I.B., PIKE, S.M., NOVACKY, A.J. and SEHGAL, O.P., 1993. Lipid peroxidation and superoxide productions in cowpea (Vigna unguiculata) leaves infected with tobacco rings virus or southern bean mosaic virus. Journal of Physiological and Molecular Plant Pathology, vol. 43, no. 2, pp. 109-119. http://dx.doi.org/10.1006/pmpp.1993.1044.
http://dx.doi.org/10.1006/pmpp.1993.1044... ; guaiacol peroxidase (POD) enzyme activity was determined using the method described in Zeraik et al. (2008)ZERAIK, A.E., SOUZA, F.S., FATIBELLO-FILHO, O. and LEITE, O.D., 2008. Desenvolvimento de um spot test para o monitoramento da atividade da peroxidase em um procedimento de purificação. Quimica Nova, vol. 31, no. 4, pp. 731-734. http://dx.doi.org/10.1590/S0100-40422008000400003.
http://dx.doi.org/10.1590/S0100-40422008... ; and the spectrophotometric method was used to determine superoxide dismutase (SOD) activity according to Giannopolitis and Ries (1977)GIANNOPOLITIS, C.N. and RIES, S.K., 1977. Purification and quantitative relationship with water-soluble protein in seedlings. Plant physiology, vol. 59, no. 2, pp. 315-318. http://dx.doi.org/10.1104/pp.59.2.315. PMid:16659840.
http://dx.doi.org/10.1104/pp.59.2.315... .

Chlorophyll a and b were evaluated using a chlorophyll meter (ClorofiLOG^®, Falker) on the middle third of the fourth leaf of three plants per treatment, and leaf area (cm²) was measured using an AM350 meter (ADC BioScientific). To evaluate plant dry weight, three plants were removed from each pot, placed in a porous paper bag and dried in a forced air oven at 65° C until a constant dry weight (g) was reached.

Response variables were subjected to analysis of variance, and factors analyzed by the Tukey test at 0.05 error probability, except for the silicon rates that were fitted to polynomial models using Sisvar® 5.3 statistical software (Ferreira, 2014FERREIRA, D.F., 2014. Sisvar: A guide for its bootstrap procedures in multiple comparisons. Ciência e Agrotecnologia, vol. 38, no. 2, pp. 109-112. http://dx.doi.org/10.1590/S1413-70542014000200001.
http://dx.doi.org/10.1590/S1413-70542014... ).

3. Results and Discussion

Shoot and root dry weight variables showed significant interaction for all factors (water regime x cultivar x rate of Si x source of Si). Lipid peroxidation showed interaction between water regime x cultivar x rate of Si, with no effect for the source of Si in the F-test (p <0.05). Thus, for this variable regression models and tables were obtained for the means of the two sources. For peroxidase (POD) enzyme activity and leaf area, there was an interaction between the rate of Si and the soil water regime, with the cultivar and source of Si showing no effect in the F-test (p <0.05). Thus, for these variables, regression models and tables were obtained for the means of the two cultivars and two sources. Chlorophyll a and b showed interactions between the soil water regime x rate of Si x source of Si, but there was no cultivar effect in the F-test (p <0.05). Thus, regression models and tables were obtained for the means of the two cultivars. For superoxide dismutase (SOD) enzyme activity, there was a significant effect only for the rate of Si, with no effect for the cultivar, source of Si and soil water regime in the F-test (p <0.05). Thus, regression models were obtained for the means of the cultivars, sources of Si and soil water regimes.

In general, the shoot dry weight variable was higher for complete submergence (water blade), which is the ideal condition for the development of rice plants (Ismail et al., 2012ISMAIL, A.M., JOHNSON, D.E., ELLA, E.S., VERGARA, G.V., BALTAZAR, A.M., 2012. Adaptation to flooding during emergence and seedling growth in rice and weeds, and implications for crop establishment. AoB PLANTS, v. 2012, pp. 1-18. http://dx.doi.org/10.1093/aobpla/pls019
http://dx.doi.org/10.1093/aobpla/pls019... ). Si showed a positive effect on shoot dry weight under the three soil water regimes, increasing the dry weight of both cultivars (Table 1). Si boosts growth by reinforcing the absorption of mineral nutrients, possibly by a kinetic relationship with the absorption process for some nutrients, improves the absorption of nitrogen, phosphorus and potassium in rice (Lee et al., 2010LEE, S.K., SOHN, E.Y., HAMAYUN, M., YOON, J.Y. and LEE, I.J., 2010. Effect of silicon on growth and salinity stress of soybean plant grown under hydroponic system. Agroforestry Systems, vol. 80, no. 3, pp. 333-340. http://dx.doi.org/10.1007/s10457-010-9299-6.
http://dx.doi.org/10.1007/s10457-010-929... ; Pati et al., 2016PATI, S., PAL, B., BADOLE, S., HAZRA, G.C. and MANDAL, B., 2016. Effect of silicon fertilization on growth, yield, and nutrient uptake of rice. Communications in Soil Science and Plant Analysis, vol. 47, no. 3, pp. 284-290. http://dx.doi.org/10.1080/00103624.2015.1122797.
http://dx.doi.org/10.1080/00103624.2015.... ; Crooks and Prentice, 2017CROOKS, R. and PRENTICE, P., 2017. Extensive investigation into field based responses to a silica fertilizer. Silicon, vol. 9, pp. 301-304. https://doi.org/10.1007/s12633-015-9379-3.
https://doi.org/10.1007/s12633-015-9379-... ).

The only statistical difference was for the shoot dry weight variable under the soil water regime of 50% WRC and at 8.0 and 16 g L^-1 sodium metasilicate. The IRGA 424 RI cultivar had a higher mean shoot dry weight than Guri INTA CL. Conversely, under the water blade at a rate of 16 g L^-1 sodium metasilicate, the Guri INTA CL showed a higher mean shoot dry weight (Table 2).

Regression analysis on the rates of Si showed that, on average, the shoot dry weight of the two cultivars was influenced by the rates of potassium metasilicate. For the IRGA 424 RI cultivar, under all three water regimes (50 and 100% WRC and the water blade), shoot dry weight increased respectively by 66, 120 and 65% at the higher rate of potassium metasilicate (16 g L^-1) compared to plants not supplemented with Si (0 g L^-1) (supplementary data). For the Guri INTA CL cultivar, the water blade did not significantly increase dry weight at the potassium metasilicate rates used (Supplementary Material). However, at 50 and 100% WRC, shoot dry weight increased by 144 and 54% with the addition of 16 g L^-1 potassium metasilicate compared to plants not supplemented with Si (0 g L^-1) (supplementary data). In terms of shoot dry weight for both IRGA 424 RI and Guri INTA CL under all three water regimes (50 and 100% WRC and water blade), sodium metasilicate at a rate of 16 g L^-1 increased the respective dry weight by 166, 86 and 43% for the IRGA 424 RI cultivar and 107, 52 and 92% for the Guri INTA CL cultivar, compared to plants not supplemented with Si (0 g L^-1) (supplementary data).

Si positively affected shoot dry weight under all water regimes, and despite the fact that water restriction impaired rice plant growth in terms of some parameters, Si did improve the plant’s physiological growth processes, corroborating the work of Lima et al. (2011)LIMA, M.A., CASTRO, V.F., VIDAL, J.B. and FILHO, J.E., 2011. Aplicação de Si em milho e feijão-de corda sob estresse salino. Ciência Agronômica, vol. 42, no. 2, pp. 398-403. http://dx.doi.org/10.1590/S1806-66902011000200019.
http://dx.doi.org/10.1590/S1806-66902011... . Habibi (2014)HABIBI, G., 2014. Silicon supplementation improves drought tolerance in canola plants. Russian Journal of Plant Physiology: a Comprehensive Russian Journal on Modern Phytophysiology, vol. 61, no. 6, pp. 784-791. http://dx.doi.org/10.1134/S1021443714060077.
http://dx.doi.org/10.1134/S1021443714060... , working on the effects on some physiological characteristics of canola plants (Brassica napus L. cv. Okapi) observed that plants grown under drought stress for 25 days and treated with Si exhibited higher shoot and root dry weight, compared to those not treated with Si under the same drought stress conditions. The exogenous supplementation of Si proves to be beneficial for plants particularly under abiotic stress conditions (Manivannan and Ahn, 2017MANIVANNAN, A. and AHN, Y.-K., 2017. Silicon Regulates Potential Genes Involved in Major Physiological Processes in Plants to Combat Stress. Frontiers of Plant Science, vol. 8, pp. 1346. http://dx.doi.org/10.3389/fpls.2017.01346. PMid:28824681.
http://dx.doi.org/10.3389/fpls.2017.0134... ). Silicon nutrition resulted in the improvement of growth and development (Eneji et al., 2008ENEJI, A.E., INANAGA, S., MURANAKA, S., LI, J., HATTORI, T., AN, P. and TSUJI, W., 2008. Growth and nutrient use in four grasses under drought stress as mediated by silicon fertilizers. Journal of Plant Nutrition, vol. 31, no. 2, pp. 355-365. http://dx.doi.org/10.1080/01904160801894913.
http://dx.doi.org/10.1080/01904160801894... ; Soundararajan et al., 2014SOUNDARARAJAN, P., SIVANESAN, I., JANA, S. and JEONG, B.R., 2014. Influence of silicon supplementation on the growth and tolerance to high temperature in Salvia splendens. Horticulture, Environment and Biotechnology, vol. 55, no. 4, pp. 271-279. http://dx.doi.org/10.1007/s13580-014-0023-8.
http://dx.doi.org/10.1007/s13580-014-002... ; Zhang et al., 2015ZHANG, Q., LIU, J., LU, H., ZHAO, S., WANG, W., DU, J. and YAN, C., 2015. Effects of silicon on growth, root anatomy, radial oxygen loss (ROL) and Fe/Mn plaque of Aegiceras corniculatum (L.) Blanco seedlings exposed to cadmium. Environmental Nanotechnology, Monitoring & Management, vol. 4, pp. 6-11. http://dx.doi.org/10.1016/j.enmm.2015.04.001.
http://dx.doi.org/10.1016/j.enmm.2015.04... ), increase in yield (Epstein, 1999EPSTEIN, E., 1999. Silicon. Annual Review of Plant Biology, vol. 50, no. 1, pp. 641-664. http://dx.doi.org/10.1146/annurev.arplant.50.1.641. PMid:15012222.
http://dx.doi.org/10.1146/annurev.arplan... ), abiotic stress tolerance (Ma, 2004MA, J.F., 2004. Role of silicon in enhancing the resistance of plants to biotic and abiotic stresses. Soil Science and Plant Nutrition, vol. 50, no. 1, pp. 11-18. http://dx.doi.org/10.1080/00380768.2004.10408447.
http://dx.doi.org/10.1080/00380768.2004.... ; Zhu et al., 2004ZHU, Z., WEI, G., LI, J., QIAN, Q. and YU, J., 2004. Silicon alleviates salt stress and increases antioxidant enzymes activity in leaves of salt-stressed cucumber (Cucumis sativus L.). Plant Science, vol. 167, no. 3, pp. 527-533. http://dx.doi.org/10.1016/j.plantsci.2004.04.020.
http://dx.doi.org/10.1016/j.plantsci.200... ; Liang et al., 2007LIANG, Y., SUN, W., ZHU, Y.G. and CHRISTIE, P., 2007. Mechanisms of silicon mediated alleviation of abiotic stresses in higher plants: a review. Environmental Pollution, vol. 147, no. 2, pp. 422-428. http://dx.doi.org/10.1016/j.envpol.2006.06.008. PMid:16996179.
http://dx.doi.org/10.1016/j.envpol.2006.... ; Muneer et al., 2014MUNEER, S., PARK, Y.G., MANIVANNAN, A., SOUNDARARAJAN, P. and JEONG, B.R., 2014. Physiological and proteomic analysis in chloroplasts of Solanum lycopersicum L. under silicon efficiency and salinity stress. International Journal of Molecular Sciences, vol. 15, no. 12, pp. 21803-21824. http://dx.doi.org/10.3390/ijms151221803. PMid:25431925.
http://dx.doi.org/10.3390/ijms151221803... ), and management of macro and micro nutrients (Tripathi et al., 2014TRIPATHI, D.K., SINGH, V.P., GANGWAR, S., PRASAD, S.M., MAURYA, J.N. and CHAUHAN, D.K., 2014. Role of silicon in enrichment of plant nutrients and protection from biotic and abiotic stresses. In: P. AHMAD, M.R. WANI, M.M. AZOOZ and L.S.P. TRAN, eds. Improvement of crops in the era of climatic changes. New York, NY: Springer, pp. 39-56.).

In general, there were no differences in root dry weight in relation to the source of Si under soil water regimes of 50 and 100% WRC for the two sources of Si. Under the water blade, sodium metasilicate resulted in higher root dry weight in the IRGA 424 RI cultivar, whereas the Guri INTA CL cultivar exhibited higher root dry weight when supplemented with potassium metasilicate (Table 1).

With regard to soil water regimes, it can be seen that under water deficiency (50% WRC), root dry weight dropped significantly compared to the water blade (ideal condition) (Table 1).

Comparing the two cultivars, in general there was a significant different in root dry weight for plants grown under the water blade. Of the eight treatments combined with the water blade, 62% exhibited higher root dry weight for the Guri INTA CL cultivar compared to the IRGA 424 RI cultivar (Table 2). The absorption and transportation of Si in plants is a complex process which involves influx and efflux Si transporters belonging to aquaporin family with specific selectivity properties. For instance, the high Si accumulator like rice consists of low silicon rice 1 (Lsi1) transporter, a nodulin 26-like intrinsic protein (NIP) in roots (Manivannan and Ahn 2017MANIVANNAN, A. and AHN, Y.-K., 2017. Silicon Regulates Potential Genes Involved in Major Physiological Processes in Plants to Combat Stress. Frontiers of Plant Science, vol. 8, pp. 1346. http://dx.doi.org/10.3389/fpls.2017.01346. PMid:28824681.
http://dx.doi.org/10.3389/fpls.2017.0134... ). According to Ma et al. (2007)MA, J.F., YAMAJI, N., MITANI, N., TAMAI, K., KONISHI, S., FUJIWARA, T., KATSUHARA, M. and YANO, M., 2007. An efflux transporter of silicon in rice. Nature, vol. 448, no. 7150, pp. 209-212. http://dx.doi.org/10.1038/nature05964. PMid:17625566.
http://dx.doi.org/10.1038/nature05964... , genotypic differences in the accumulation of Si in rice plants could be related to the difference in the expression of Si transporter genes in the roots.

In the regression analysis on Si rates for the means of root dry weight, it was observed that in the IRGA 424 RI cultivar, increasing the rate of potassium metasilicate resulted in a linear increase in dry weight under the three water regimes (50 and 100% WRC and water blade), with respective increases of 166, 316 and 296% at the highest rate (16 g L^-1) compared to plants not supplemented with Si (0 g L^-1) (supplementary data). In contrast, for the rates of sodium metasilicate, in IRGA 424 RI there was a quadratic relationship under the water blade, peaking at a rate of 12.5 g L^-1. Under the other two water regimes, there was no significant polynomial (supplementary data). For the Guri INTA CL cultivar at water blade, increasing the rates of both sources of Si (sodium metasilicate and potassium metasilicate) resulted in a quadratic increase in root dry weight, peaking at 16 and 15 g L^-1 sodium metasilicate and potassium metasilicate, respectively. The other water regimes did not produce any significant fits in the polynomial models (supplementary data).

This corroborates the work of Faria Junior et al. (2009)FARIA JÚNIOR, L.A., CARVALHO, J.G., PINHO, P.J., BASTOS, A.R.R. and FERREIRA, E.V.O., 2009. Produção de matéria seca, teor e acúmulo de Si em cultivares de arroz sob doses de Si. Ciência e Agrotecnologia, vol. 33, no. 4, pp. 1034-1040. http://dx.doi.org/10.1590/S1413-70542009000400013.
http://dx.doi.org/10.1590/S1413-70542009... , who observed an increase in root dry weight in proportion to increases in rates of Si in the Conai and Curinga cultivars. Hattori et al. (2005)HATTORI, T., INANAGA, S., ARAKI, H., NA, P., MORITA, S., LUXOVÁ, M. and LUX, A., 2005. Application of silicon enhanced drought tolerance in Sorghum bicolor. Physiologia Plantarum, vol. 123, no. 4, pp. 459-466. http://dx.doi.org/10.1111/j.1399-3054.2005.00481.x.
http://dx.doi.org/10.1111/j.1399-3054.20... reported that under dry conditions, sorghum plants (Sorghum bicolor (L.) Moench) treated with Si showed improved root growth compared to plants not treated with Si. Sávio et al. (2011)SÁVIO, F.L., SILVA, G.C., TEIXEIRA, I.R. and BORÉM, A., 2011. Produção de biomassa e conteúdo de silício em gramíneas forrageiras sob diferentes fontes de silicato. Semina: Ciências Agrárias, vol. 32, no. 1, pp. 103-110. http://dx.doi.org/10.5433/1679-0359.2011v32n1p103.
http://dx.doi.org/10.5433/1679-0359.2011... reported that Brachiaria brizantha and Panicum máximum responded positively to Si in terms of dry weight production.

With regard to mean leaf area under the different water regimes, the highest availability of water in the soil (water blade) resulted in higher rice plant leaf area for the two cultivars and sources of Si, regardless of the rate of Si (Figure 1). Growth in plants is initiated primarily by the multiplication of meristematic cells and the subsequent elongation of these cells, and water plays an important part in cell elongation under the effect of turgor pressure (Woodruff and Meinzer, 2011WOODRUFF, D.R. and MEINZER, F.C., 2011. Size-dependent changes in biophysical control of tree growth: The role of turgor. In: F.C. MEINZER, B. LACHENBRUCH and T.E. DAWSON, eds. Size-and age-related changes in tree structure and function. Netherlands: Springer, pp. 363-384. http://dx.doi.org/10.1007/978-94-007-1242-3_14.
http://dx.doi.org/10.1007/978-94-007-124... ). That is why drought stress reduces plant growth and impairs crop yield (Boyer, 1982BOYER, J.S., 1982. Plant productivity and environment. Science, vol. 218, no. 4571, pp. 443-448. http://dx.doi.org/10.1126/science.218.4571.443. PMid:17808529.
http://dx.doi.org/10.1126/science.218.45... ).

Figure 1
Means for rice plant leaf area at different Si rates for the soil water regime factor. Santa Maria, RS, 2019. *Means not followed by the same letter were statistically different for soil water regimes in the Tukey test (p≤0.05). WRC: soil water retention capacity.

In the regression analysis for the effects of Si rates on leaf area under the three soil water regimes (50 and 100% WRC and water blade), there was a linear increase in leaf area associated with increased rates of Si, with an increase of 81% at 50% WRC (Y=03951x + 6.682; R²=0.932), 39% at 100% WRC (Y=0.4168x + 17.85; R²= 0.949) and 51% for the water blade (Y= 0.9841x + 31.574; R²= 0.966) at the highest Si rate (16 g L^-1) compared to plants not treated with Si (0 g L^-1), showing the beneficial effect of Si, even in cases of water deficiency (50% WRC). It is possible that this effect was the result of an increase in the relative moisture content in the leaf brought about by the Si, increasing turgor pressure inside the cells, promoting cell expansion and boosting leaf area, corroborating the results of Kim et al. (2012)KIM, Y.-H., KHAN, A.L., SHINWARI, Z.K., KIM, D.-H., WAQAS, M., KAMRAN, M. and LEE, I.-J., 2012. Silicon treatment to rice (Oryza sativa L. cv.‘Gopumbyeo’) plants during different growth periods and its effects on growth and grain yield. Pakistan Journal of Botany, vol. 44, pp. 891-897..

With regard to the means of chlorophyll a and b under the different water regimes and sources of Si, in general the chlorophyll readings were higher in submerged plants (water blade), possibly because of the increased production or lower degradation of pigments. There was no significant difference between the sources of Si for chlorophyll a and b (Table 3).

A regression analysis between rates of Si and chlorophyll a for the potassium metasilicate source showed that the highest rate (16 g L^-1) resulted in respective linear increments of 26 and 40% in chlorophyll a in plants grown at 100% WRC and under the water blade, compared to plants not supplemented with Si (0 g L^-1). At 50% WRC, there was a quadratic increase in chlorophyll a, peaking at a rate of 13 g L^-1 potassium metasilicate (Figure 2a). For the sodium metasilicate under water regimes of 50 and 100% WRC, there were respective increases of 30 and 47% in chlorophyll a at a rate of 16 g L^-1 compared to plants not supplemented with Si (0 g L^-1). However, under the water blade, there was a quadratic increase peaking at a rate of 13 g L^-1 sodium metasilicate (Figure 2b).

Figure 2
Relationship between the concentrations of chlorophyll a and b in the IRGA 424 RI and Guri INTA CL at means cultivars under different soil water regimes and for different Si rates and sources (sodium metasilicate (b and d) and potassium metasilicate (a and c)). Santa Maria, RS, 2019.

The regression analysis between rates of Si and chlorophyll b for the sodium metasilicate shows a direct linear relationship with the water regimes of 50 and 100% WRC and a quadratic relationship under the water blade, peaking at a rate of 12.5 g L^-1 (Figure 2c). For the potassium metasilicate, chlorophyll b showed a linear increase under the water blade, and a quadratic increase at 50% WRC, peaking at a rate of 12.4 g L^-1. At 100% WRC, there was no significant fit for the polynomial models (Figure 2d).

Note that with the addition of Si the levels of chlorophyll a and b increased under all three water regimes, possibly because Si boosts pigment levels (Gong et al., 2005GONG, H., ZHU, X., CHEN, K., WANG, S. and ZHANG, C., 2005. Silicon alleviates oxidative damage of wheat plants in potsunder drought. Plant Science, vol. 169, no. 2, pp. 313-321. http://dx.doi.org/10.1016/j.plantsci.2005.02.023.
http://dx.doi.org/10.1016/j.plantsci.200... ; Souza, 2008SOUZA, J.V. 2008. Aplicação de silicato de potássio via foliar no milho: fotossíntese, crescimento e produtividade. Uberlândia: Universidade Federal de Uberlândia, 33 p. Monografia.). Furthermore, Si is related to adjustments in the chloroplast ultrastructure, and is beneficial in maintaining chlorophyll levels (Ahmed et al., 2014AHMED, M., ASIF, M. and HASSAN, F.U., 2014. Augmenting drought tolerance in sorghum by silicon nutrition. Acta Physiologiae Plantarum, vol. 36, no. 2, pp. 473-483. http://dx.doi.org/10.1007/s11738-013-1427-2.
http://dx.doi.org/10.1007/s11738-013-142... ). In our study, the pigment level results are related to the production of biomass, since plants exposed to Si show higher pigment levels with a higher potential to absorb light energy, and therefore produce more biomass.

There is no consensus in the literature on how Si influences photosynthetic pigment levels. This could be related to the different experimental conditions in individual studies. Shen et al. (2010)SHEN, X., ZHOU, Y., DUAN, L., LI, Z., ENEJI, A.E. and LI, J., 2010. Silicon effects on photosynthesis and antioxidant parameters of soybean seedlings under drought and ultraviolet-B radiation. Journal of Plant Physiology, vol. 167, no. 15, pp. 1248-1252. http://dx.doi.org/10.1016/j.jplph.2010.04.011. PMid:20713250.
http://dx.doi.org/10.1016/j.jplph.2010.0... reported that applying Si to soybean leaves under water deficiency conditions increased chlorophyll levels in irrigated plants, but this treatment did not alter chlorophyll levels in non-irrigated plants. However, Pei et al. (2010)PEI, Z.F., MING, D.F., LIU, D., WAN, G.L., GENG, X.X., GONG, H.J. and ZHOU, W.J., 2010. Silicon improves the tolerance to water deficit stress induced by polyethylene glycol in wheat (Triticum aestivum L.) seedlings. Journal of Plant Growth Regulation, vol. 29, no. 1, pp. 106-115. http://dx.doi.org/10.1007/s00344-009-9120-9.
http://dx.doi.org/10.1007/s00344-009-912... reported that applying Si to the leaves of wheat increased the chlorophyll level only in plants undergoing water deficiency, not differentiating irrigated plants. In strawberry (Braga et al., 2009BRAGA, F.T., NUNES, C.F., FAVERO, A.C., PASQUAL, M., CARVALHO, J.G. and CASTRO, E.M., 2009. Características anatômicas de mudas de morangueiro micropropagadas com diferentes fontes de Si. Pesquisa Agropecuária Brasileira, vol. 44, no. 2, pp. 128-132. http://dx.doi.org/10.1590/S0100-204X2009000200003.
http://dx.doi.org/10.1590/S0100-204X2009... ) and rice plants (Ávila et al., 2010ÁVILA, F.W., BALIZA, D.P., FAQUIN, V., ARAÚJO, J.L. and RAMOS, S.J., 2010. Interação entre Si e nitrogênio em arroz cultivado sob solução nutritiva. Ciência Agronômica, vol. 41, no. 2, pp. 184-190. http://dx.doi.org/10.1590/S1806-66902010000200003.
http://dx.doi.org/10.1590/S1806-66902010... ), supplementing the crop environment with Si did increase chlorophyll levels.

Levels of chlorophyll a were higher than those of chlorophyll b. This is usually the case in the majority of plants, since chlorophyll b is an integral part of antenna pigments only, whereas chlorophyll a is an integral part of antenna pigments and photosystem reaction centers (Taiz et al., 2017TAIZ, L., ZEIGER, E., MOLLER, I.M. and MURPH, A., 2017. Fisiologia e desenvolvimento vegetal. 6ª ed. Porto Alegre: Artmed Editora S.A., 120p.).

According to Hendry and Price (1993)HENDRY, G.A.F. and PRICE, A.H. 1993. Stress indicators: chlorophylls and carotenoids. In: G.A.F. HENDRY and J.P. GRIME, eds. Methods in comparative plant ecology. London: Chapman and Hall, pp. 148-152. http://dx.doi.org/10.1007/978-94-011-1494-3.
http://dx.doi.org/10.1007/978-94-011-149... , water deficiency is one of the environmental stresses responsible for leaf pigment reduction, altering the plant’s life cycle. According to Marques (2009)MARQUES, D.J., 2009. Estresse mineral induzido por fertilizantes potássicos em plantas de berinjela (Solanum melogena L.) e seu efeito sobre parâmetros agronômicos e metabólicos. Faculdade de Ciências Agronômicas da UNESP, Botucatu Dissertação de Mestrado em Agronomia/Horticultura., this kind of drought stress can reduce net photosynthesis, an effect generally associated with limited synthesis capacity and/or higher degradation of total chlorophylls (chlorophylls a and b), indicating that under stress plants seem to need alternative routes for dissipating energy in order to avoid photoinhibition and photooxidation problems.

According to Agarie et al. (1998)AGARIE, S., HANAOKA, N., UENO, O., MIYAZAKI, A., KUBOTA, F., AGATA, W. and KAUFMAN, P.B., 1998. Effects of silicon on tolerance to water deficit and heat stress in rice plants (Oryza sativa L.), monitored by electrolyte leakage. Plant Production Science, vol. 1, no. 1, pp. 96-103. http://dx.doi.org/10.1626/pps.1.96.
http://dx.doi.org/10.1626/pps.1.96... , this increase in chlorophyll levels induced by the application of Si is associated with the prevention of leaf senescence due to maintained photosynthesis and protection against chlorophyll destruction, especially under high temperature and low humidity conditions. Applying Si to rice under water deficiency conditions increases levels of glycolipids and phospholipids, related to the increase in levels of chlorophyll, and higher stability and improved functioning of the plasma membrane (Wang and Galletta, 1998WANG, S.Y. and GALLETTA, G.J., 1998. Foliar application of potassium silicate induces metabolic changes in strawberry plants. Journal of Plant Nutrition, vol. 21, no. 1, pp. 157-167. http://dx.doi.org/10.1080/01904169809365390.
http://dx.doi.org/10.1080/01904169809365... ).

With regard to shoot lipid peroxidation in the different cultivars under different water regimes (Table 4), the soil water regime induced differences when the plant was not supplemented with Si, with higher lipid peroxidation under water deficiency (50% WRC) and at 100% WRC. In contrast, when the plants were supplemented with Si from different sources and at different rates, there was no significant difference in the shoot lipid peroxidation figures under the three soil water regimes, i.e. the Si may have acted to induce a reduction in lipid peroxidation, regardless of the cultivar and rate applied. This result could be related to POD activity, since Si causes an increase in the activity of this enzyme under water deficiency conditions (50% WRC) or at 100% WRC.

With regard to the cultivars, there was higher lipid peroxidation in Guri INTA CL at 50% WRC when not supplemented with Si, but when supplemented, there was no significant difference between cultivars (Table 4), indicating that the Si had a very similar effect on both cultivars, possibly because of the fact that they were genetically similar, due to only a restricted group of genetically similar genotypes used extensively as genitors, despite the extensive genetic variability available in germplasm banks (Brondani et al. 2006BRONDANI, C., BORBA, T.C.O., RANGEL, P.H.N. and BRONDANI, R.P.V., 2006. Determination of genetic variability of traditional varieties of Brazilian rice using microsatellite markers. Genetics and Molecular Biology, vol. 29, no. 4, pp. 676-684. http://dx.doi.org/10.1590/S1415-47572006000400017.
http://dx.doi.org/10.1590/S1415-47572006... ).

It has been suggested that supplementing plants grown under water deficiency with Si mitigates the negative effects of water deficiency by improving photochemical efficiency and photosynthetic gaseous exchange, as well as activating antioxidant defense capabilities, attenuating lipid peroxidation (Rizwan et al., 2015RIZWAN, M., ALI, S., IBRAHIM, M., FARID, M., ADREES, M., BHARWANA, S.A., ZIA-UR-REHMAN, M., QAYYUM, M.F. and ABBAS, F., 2015. Mechanisms of silicon-mediated alleviation of drought and salt stress in plants: a review. Environmental Science and Pollution Research International, vol. 22, no. 20, pp. 15416-15431. http://dx.doi.org/10.1007/s11356-015-5305-x. PMid:26335528.
http://dx.doi.org/10.1007/s11356-015-530... ).

In the regression analysis between rates of Si and shoot lipid peroxidation, rice cultivars IRGA 424 RI and Guri INTA CL showed a reduction in lipid peroxidation only under soil water regimes of 50% WRC, fitting a quadratic polynomial model, with a reduction in lipid peroxidation as rates of sodium metasilicate increased, with respective minimum values at rates of 13 and 12 g L^-1 (supplementary data). Under soil water regimes of 100% WRC and the water blade, there were no significant fits to the polynomial models (supplementary data). In terms of sodium metasilicate rates, lipid peroxidation in the IRGA 424 RI cultivar at 50% WRC was reduced quadratically, with the trough at a rate of 11 g L^-1. At 100% WRC and under the water blade, there were no significant fits to the polynomial models. In contrast, the Guri INTA CL cultivar at 100% WRC, supplemented with Si (sodium metasilicate), exhibited negative linear behavior, with lipid peroxidation falling as rates of sodium metasilicate increased. At 50% WRC and under the water blade there were no significant fits to the polynomial models (supplementary data).

These results show that the presence of Si brought about a significant reduction in shoot lipid peroxidation in rice plants undergoing water deficiency. This could mean that the harm caused by ROS under water deficiency was attenuated by adding Si, in part due to increased POD and SOD activity. It is possible that applying Si might effectively increase the rice plant’s capacity to defend against oxidative stress induced by drought stress (supplementary data). Shi et al. (2005)SHI, Q.H., BAO, Z., ZHU, Z., HE, Y., QIAN, Q. and YU, J., 2005. Silicon mediated alleviation of Mn toxicity in Cucumis sativus in relation to activities of superoxide dismutase and ascorbate peroxidase. Phytochemistry, vol. 41, no. 66, pp. 1551-1559. http://dx.doi.org/10.1016/j.phytochem.2005.05.006. PMid:15963540.
http://dx.doi.org/10.1016/j.phytochem.20... reported that Si brought about a drop in membrane lipid peroxidation by stimulating enzymatic and non-enzymatic antioxidants.

Measurements of POD enzyme activity under the different soil water regimes indicated that enzyme activity was higher at 50% WRC. Higher rates of Si (8.0 and 16 g L^-1) increased POD activity under drought stress (50% WRC) and at 100% WRC compared to plants grown under the water blade (Table 5). This suggests that the increase in POD enzyme activity induced by adding Si could protect plant tissues under drought stress from oxidative damage, which could contribute significantly to increasing drought tolerance.

The regression analysis between rates of Si and POD activity showed that supplementing with Si at the highest rate (16 g L^-1) increased respective enzyme activity by 68, 57 and 22% under the three soil water regimes, compared to plants not supplemented with Si (Figure 3a). It is important to point out that Si has the potential to activate the antioxidant system, regardless of the presence of the stress factor. The results corroborate those found by Verma and Dubey (2003)VERMA, S. and DUBEY, R.S., 2003. Lead toxicity induces lipid peroxidation and alters the activities of antioxidant enzymes in growing rice plants. Plant Science, vol. 164, no. 4, pp. 645-655. http://dx.doi.org/10.1016/S0168-9452(03)00022-0.
http://dx.doi.org/10.1016/S0168-9452(03)... who observed an increase in POD activity in rice when subjected to stress conditions.

Figure 3
Relationship between guaiacol peroxidase (POD) enzymatic activity in the shoots of rice plants under different soil water regimes: 50 and 100% WRC and water blade at different Si rates (means for sodium metasilicate and potassium metasilicate sources and IRGA 424 RI and Guri INTA CL cultivars) (a). Superoxide dismutase (SOD) enzymatic activity at different Si rates (means for sodium metasilicate and potassium metasilicate sources IRGA 424 RI and Guri INTA CL cultivars and different soil water regimes: 50 and 100% WRC and water blade) (b). Santa Maria, RS, 2019.

Taking into account the mean enzymatic activity of SOD, the regression analysis shows a linear increase in enzyme activity as rates of Si increase, with an increment of 15% at the highest rate (16 g L^-1) compared to plants not supplemented with Si (0 g L^-1). This can be positive if the objective is to mitigate the effect of stress that involves the production of ROS in the rice crop (Figure 3b). The increase in antioxidant enzyme activity in the presence of Si indicated that Si could be involved in increasing the expression of genes related to the production and activation of antioxidant enzymes (Biju et al., 2018BIJU, S., FUENTES, S. and GUPTA, D., 2018. The use of infrared thermal imaging as a non-destructive screening tool for identifying drought-tolerant lentil genotypes. Plant Physiology and Biochemistry, vol. 127, pp. 11-24. http://dx.doi.org/10.1016/j.plaphy.2018.03.005. PMid:29544209.
http://dx.doi.org/10.1016/j.plaphy.2018.... ).

The increase in SOD activity under stress conditions could result in higher efficiency in the elimination of ROS (Wang et al., 2005WANG, F.-Z., WANG, Q.-B., KWON, S.-Y., KWAK, S.-S. and SU, W.-A., 2005. Enhanced drought tolerance of transgenic rice plant s expressing a pea manganese superoxide dismutase. Journal of Plant Physiology, Stuttgart, vol. 162, no. 4, pp. 465-472. http://dx.doi.org/10.1016/j.jplph.2004.09.009.
http://dx.doi.org/10.1016/j.jplph.2004.0... ). These ROS cause oxidative stress, which in the final analysis results in cell damage that could kill the plant (Sfalcin, 2009SFALCIN, R.A., 2009. Avaliação de parâmetros fisiológicos e bioquímicos em berinjela (Solanum melongena L.) cultivada sob diferentes potenciais de água no solo. Botucatu: Universidade Estadual Paulista. Dissertação de Mestrado em Agronomia/Energia na Agricultura).). Schmidt et al. (1999)SCHMIDT, R.E., ZHANG, X. and CHALMERS, D.R., 1999. Response of photosynthesis and superoxide dimutase to silica applied to creeping bentgrass grown under two fertility levels. Journal of Plant Nutrition, vol. 22, no. 11, pp. 1763-1773. http://dx.doi.org/10.1080/01904169909365752.
http://dx.doi.org/10.1080/01904169909365... reported that Si increases antioxidant enzyme levels under low humidity conditions.

4. Conclusion

Chlorophyll a and b and shoot and root dry weight increased at higher rates of Si under the three soil water regimes, in both sources.

Shoot lipid peroxidation was greater when the plants was not supplemented with Si, under water deficiency (50% WRC) and at 100% WRC. In contrast, when the plants were supplemented with Si from different sources and at different rates, there was no significant difference in the shoot lipid peroxidation figures under the three soil water regimes, i.e. the Si have acted to induce a reduction in lipid peroxidation, regardless of the cultivar and rate applied. This result related to POD activity, since Si caused an increase in the activity of this enzyme under drought stress conditions (50% WRC) or at 100% WRC. Furthermore, the SOD enzymatic activity increased at higher rates of Si (16 gL^-1).

The leaf area increased in the three soil water regimes associated with a higher dose (16 g L^-1) of Si, demonstrating its beneficial action, even in regimes of drought stress (50% CRA).

Therefore, Si did indeed attenuate water deficiency stress in rice plants emerging from seeds treated with dietholate.

Supplementary Material

Supplementary material accompanies this paper.

Suppl. data. Relationship between concentration of shoot and root dry weight and shoot lipid peroxidation for the IRGA 424 RI and Guri INTA CL cultivars under different soil water regimes, Si rates and Si sources. Santa Maria, RS, 2019.

This material is available as part of the online article from http://www.scielo.br/bjb

Acknowledgements

This research was financially supported by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES).

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