Foliar application of silicon improves growth of soybean by enhancing carbon metabolism under shading conditions

Highlights

• Si potentially improves root architecture in intercropping.

• Si increases photosynthesis and stomatal conductance under shading.

• Foliar application of Si enhances soluble sugars and sucrose in leaves.

• Si plays a role in alleviating drastic effects of shading stress.

Abstract

An experiment was set up to investigate physiological responses of soybeans to silicon (Si) under normal light and shade conditions. Two soybean varieties, Nandou 12 (shade resistant), and Nan 032–4 (shade susceptible), were tested. Our results revealed that under shading, the net assimilation rate and the plant growth were significantly reduced. However, foliar application of Si under normal light and shading significantly improved the net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and decreased intercellular carbon dioxide concentration (Ci). The net photosynthetic rate of Nandou 12 under normal light and shading increased by 46.4% and 33.3% respectively with Si treatment (200 mg/kg) compared to controls. Si application also enhanced chlorophyll content, soluble sugars, fresh weight, root length, root surface area, root volume, root-shoot ratio, and root dry weight under both conditions. Si application significantly increased the accumulation of some carbohydrates such as soluble sugar and sucrose in stems and leaves ensuring better stem strength under both conditions. Si application significantly increased the yield by increasing the number of effective pods per plant, the number of beans per plant and the weight of beans per plant. After Si treatment, the yield increased 24.5% under mono-cropping, and 17.41% under intercropping. Thus, Si is very effective in alleviating the stress effects of shading in intercropped soybeans by increasing the photosynthetic efficiency and lodging resistance.

Introduction

The potential of silicon (Si) for alleviating the effects of biotic and abiotic stress in plants is well known (Frew et al., 2018). Silicon plays a vital role in counteracting abiotic stresses such as nutrient imbalance, water deficit, temperature, shade and physical stresses (Helaly et al., 2017; Tripathi et al., 2016b). Silicon belongs to the IVA group in the periodic table and is the second most abundant element in soils (Sommer et al., 2006), but its availability to plants as silicic acid could be limiting, hence the application of silicate fertilizers is required in crop production (Ma and Takahashi, 2002). In soil solution, it is present as a silicic acid with a concentration of 0.1–0.6 mM. However, most Si exists in soil combined with oxygen to form oxides and silicates, which are not easily taken up by plants. On the agronomic basis, monosilicic acid phase of Si is essential. Plants take up silicon in the form of silicic acid and farmers use monosilicic adic, Si(OH)₄, to provide crops with Si. Several studies have shown the beneficial effects of Si application in terms of faster growth and increased yield and improved uptake of macro- and micro-nutrients (Ma and Takahashi, 1990; Tripathi et al., 2016b).

Si is not considered as an essential element for plants, although recent studies have been debating whether that is still true. It has been the subject of global attention as it can enhance a plant's resistance to disease and herbivorous insects, thus reducing the use of fungicides and pesticides. Si is also now considered as an environment-friendly element (Jian and Eiichi, 2002). The effects of Si vary among plant species, but in general it enhances instantaneous water-use efficiency and photosynthesis, as well as remedying nutrient imbalances (Farooq et al., 2013; Feng et al., 2010). Plants have a well-developed defense system against oxidative damage, including antioxidant enzymes and compounds (Hussain et al., 2020b). The enzymatic antioxidant activities decreased under stress from metal contamination and silicon enhanced the activity of superoxide dismutase, peroxidases, catalase, and ascorbate peroxidase, which help plants cope with metal stress (Adrees et al., 2015). Si application increased the activities of non-enzymatic antioxidants like glutathione, non-protein thiols and ascorbic acid in rice, cucumber and pak choi under Mn and Cd stress (Song et al., 2009; Wang et al., 2015). The addition of Si had minimal effects on photosynthesis and chlorophyll fluorescence in tomato and maize plants under normal conditions, but under stress, Si improved photosynthesis and chlorophyll fluorescence yields (Al-aghabary et al., 2005). Si application on agricultural crops increased photosynthesis and enzymatic activities resulting in higher yield and increased biomass. Si increased the chlorophyll pigments in maize, wheat, soybean and other plant species and foliar application of Si significantly enhanced PSII efficiency (Tripathi et al., 2003). The uptake of N, P, Ca and Mg by plant were observed to be also increased after Si application.

Silicon application was reported to have beneficial effects on plant growth as it enhanced resistance to lodging, biotic and abiotic stress, and had indirect effects such as pH adjustment and assimilation of macro- and micronutrients contained in silicate fertilizers. Si application improved biomass production (Mehrabanjoubani et al., 2015; Vaculík et al., 2009), yield and quality of a wide range of crops including monocots like rice, wheat, maize, barley, millet, sorghum and sugarcane that actively take up and accumulate large amounts of Si in their organs, and some dicots like cotton, vegetable and fruit crops (Liang et al., 2015a). Earlier findings suggested a positive role of Si in enhancing stem length, leaf area, photosynthesis, stomatal conductance, transpiration rate, stomatal number and size, pigment concentration and chlorophyll fluorescence under stress conditions (Abbas et al., 2015; Mateos-Naranjo et al., 2013; Shen et al., 2010a, 2010b, 2014; Xiaoqin et al., 2011).

Shade is a pervasive abiotic stress in agricultural crop production especially in intercropping (Hussain et al., 2020b; Yang et al., 2018). When soybeans are intercropped with maize, they suffer shading from the adjacent maize canopy. Shade leads to stem and petiole elongation and increased internodal distance, and reduces root diameter and stem strength (Hussain et al., 2019d; Wei-Guo et al., 2018; Wen et al., 2020). Shading makes soybean plants susceptible to lodging thereby inhibiting the transport of photo-assimilates, nutrients, and water (Li et al., 2014), which reduces mechanized yield (mechanical harvesting efficiency) (Hussain et al., 2019b; Wei-Guo et al., 2018). It affects the rate of photosynthesis by blocking electron transport from photosystem II to photosystem I, thus reducing the electron transport rate, the amount of ATP produced, and rubisco activity (Huang et al., 2018; Hussain et al., 2019a; Valladares and Niinemets, 2008; Yao et al., 2017). In addition, shading decreases quantum yield, effective quantum yield of photosystem, photochemical quenching (Hussain et al., 2019c), and electron transport rate (Hussain et al., 2019a). Shade is also known to reduce nonstructural carbohydrates such as soluble sugars, starch and sucrose, which not only provide energy but also contribute to the accumulation of structural carbohydrates (Wu et al., 2017).

In the literature, there have been many reports about the positive role of Si in plant growth under conditions of salt stress, metal toxicity, drought stress, radiation damage, nutrient imbalances, high temperature, and freezing (Ma et al., 2002). However, not much is known about the effect of Si on soybean plants under shade conditions. Therefore, the current study was aimed to investigate the role of Si in the improvement of morphological characteristics, photosynthetic parameters, and non-structural carbohydrates in soybeans grown under shade. The effect of Si on various yield parameters of soybean was also studied.