Growth regulators promote soybean productivity: a review

Root

Botanically, the otanically, the root is an underground organ and functions primarily in absorbing soil minerals and moisture and as well offers firm anchorage. Roots, as the central boundary between plants and the surrounding environment, exert key functions in the growth and development of plants (Marschner, 2011). Root growth and development directly affect the growth of the soybean plant. With PGRs affecting growth, understanding their influence on the roots is very important.

In soybean growth, most root traits arise from the features of the soybean plant rather than the soil unlike in most crops where the root length and weight are dependent on the oxygen content of the soil which is ascertained by the soil porosity (Yang, Wu & Zhang, 2001). Soybean roots under various soil parameters such as moisture, temperature, voids can cause variations in root morphology in addition to other environmental factors (Jian, Xiaobing & Guanghua, 2002). Conditions such as waterlogging stress and drought highly influence root growth. Waterlogging stress inhibits root growth by decreasing the size of the root whiles drought influences the root architecture through longer lateral root and root hairs development for better water absorption (Kim et al., 2018). Waterlogging fills soil pores which limits gaseous exchange, thus triggering adventitious root formation (Steffens, Wang & Sauter, 2006). PGRs are positive promoters of growth, with ET regulating adventitious root formation. The availability of ET and GA₃ triggered a substantial increase in the development and penetration of adventitious roots of several plants (Steffens, Wang & Sauter, 2006).

In soybean, endogenous GA increased with the application of ET coupled with increased plant height and adventitious root formation, proposing the ability of ET to improve GA accumulation and hence, improve root growth (Kim et al., 2018). The application of 2-chloroethylphosphonic acid (ethephon, ETP) increased the RSA of the soybean plant, providing an increased surface area for water absorption (Kim et al., 2018). ET and GA control the formation, number, and length of adventitious roots synergistically, hence, exogenous ET application boosts GA accumulation in soybean plants which enhance the RSA and improves root growth. Exogenous application of IBA to soybean hypocotyl induced root development by significantly increasing the number of lateral roots (Chao et al., 2001).

Shoot

The shoot of plants encompasses its stem and leaves. The leaves function to effectively capture photosynthetically active radiation (PAR). Leaf development is thereby well-thought-out as the principal phenomena of growth and shoot morphogenesis during canopy developmental stages in plants. The application of PGRs is often used as a measure to ensure the reduction of lodging incidence by mimicking or altering the production of hormones essential in improving stem structure and increasing yields (Berry & Spink, 2012).

In soybean, the application of ETP showed greater shoot growth resulting in increased plant height (Kim et al., 2018). The increase may be related to the elongation of internodes and heightened cell division and expansion. Growth regulator treatments increased the leaf area per plant and leaf area index (Manu, Halagalimath & Chandranath, 2020). For instance, cycocel treatments enhanced a prolonged assimilation surface area which delayed leaf senescence, hence, retaining more leaves per plant. Also, cycocel reduced chlorophyll degradation, protease activity, and stimulated the synthesis of soluble proteins and enzymes (Khalilzadeh, Seyed Sharifi & Jalilian, 2018).

Cytokinin is known to induce shoot regeneration and elongation. Its application was observed to control shoot branching and stimulate the growth of axillary bud (Shimizu-Sato, Tanaka & Mori, 2009). Exogenous application of cytokinin alleviated lateral branch inhibition in soybean plants subjected to high-aluminum toxicity, thus, promoting shoot growth (Pan, Hopkins & Jackson, 1988). Although cytokinin has been observed to increase shoot development in several plants, very little information is available on soybean shoot development, hence, presenting a gap for future research. Soybean cv. BS-3 sprayed with 100 ppm of IAA at three different times increased plant height, number of flowers, pod number, percent of fruit set, seed number per plant, seed yield per plant, and total seed yield (t/ha) (Sarkar, Haque & Abdul Karim, 2002). Application of 100 ppm NAA at the flowering stage increased the number of branches per plant, plant height, leaf number per plant, average pod weight, leaf area, dry matter, and seed yield (Deotale et al., 1998).

Root nodules

Root nodules are the nitrogen-fixing cell protrusions that contain Gram-negative bacteria found on the roots of leguminous plants. These protuberances are formed through a process known as nodulation. Nodulation provides atmospheric nitrogen (N₂) needed for the synthesis of nitrogen-containing compounds like nucleic acids and proteins necessary for enhancing plant growth physiognomies, crop yield, and conserve soil fertility (Mangena, 2018). According to Ohyama et al. (2011), one element positively influencing soybean productivity is biological nitrogen fixation. However, it is established through research that the root nodules of legumes including soybean improve soil fertility through atmospheric nitrogen fixation and subsequently, increasing yield. Several studies have revealed that exogenous PGRs promote nodule formation (Mishra et al., 1999). By import, plant growth regulators could improve soybean yield as it promotes nodulation (Sudadi & Suryono, 2015).

Nodule formation in soybean is extremely sensitive to exogenous plant growth regulators such that they intricately regulate nodule development and affect nitrogen fixation. For instance, the addition of L-tryptophan and indole acetic acid to sterile sand media of neutral pH increased the number of nodules (Sudadi, 2012). Likewise, the addition of epi-brassinolide to growth media improved nodulation in soybean by increasing the number of nodules per plant. Sudadi & Suryono (2015) reported that L-tryptophan (0.001 mg L⁻¹, 0.1 mg L⁻¹, 1.0 mg L⁻¹) increased the number of root nodules per plant. Similarly, Basuchaudhuri (2016) reported that applying tryptophan and IAA induced root nodulation in soybean. In another study, the exogenous application of 6-Benzylaminopurine (BAP), indole acetic acid, salicylic acid, gibberellic acid, and jasmonic acid increased nodule number per plant compared to untreated plants, though high concentrations of GA₃ (1 µM) and JA (100 µM) significantly decreased nodule number by about 70% and 95% respectively. The results revealed that the effect of these growth regulators is concentration-dependent, however, 10 nM IAA, 50 nM BAP, 10 µM JA, 10 nM, and 100 nM GA₃ and 100 µM to 1 mM SA had the best effect and is considered ideal for high nodule numbers in soybean (Roy Choudhury, Johns & Pandey 2019). According to Mens et al. (2018), the exogenous application of cytokinins (BAP, N⁶-(Δ²-isopentenyl)-adenine and trans-zeatin) at low concentrations promoted nodule development while high concentrations decreased nodulation in soybean. Seed priming with low cytokinin concentrations (10⁻⁹ mol/L) promoted nodulation by increasing the total nodule area, thereby improving biological nitrogen fixation under controlled environments (Kempster et al., 2021). By this, it can be suggested that plant growth regulators intricately regulate nodulation and thus, effective nodulation requires strict regulation of the concentration of growth regulators applied.

Hormone

Plants produce chemicals that regulate processes of growth and development like reproduction, continuous survival, and senescence (Mustafa et al., 2016). These endogenous chemical substances known as phytohormones (plant hormones) are a product of plant secondary metabolism, acting as chemical messengers that coordinate a host of signaling pathways and regulate varied cellular responses that facilitate plants’ adaptation to abiotic stress (Kazan, 2015; Fahad et al., 2015). Application before/during stress exposure may modulate the endogenous hormone levels to activate plants’ response to stress (Kaur, Gupta & Kaur, 1998b). Fluctuations in levels of the major plant hormones (auxin, abscisic acid, cytokinin, ethylene, and gibberellins) influence all aspects of plant growth (Zhao et al., 2012). These hormones intricately regulate plant growth by either promoting or inhibiting growth from germination to reproductive growth and flowering, hence classified as growth promoters (gibberellins, cytokinin, and auxin) and growth retardants (abscisic acid and ethylene; Nadeem et al., 2019). Under normal growth and environmental conditions, these hormones are naturally synthesized in certain plant parts but their production minimizes when conditions are unfavorable to promote optimal growth and productivity (Zahir, Asghar & Arshad, 2001). Synthetic compounds that mimic the activities of plant hormones otherwise known as PGRs are exogenously applied to alter the endogenous hormone levels in plants and to augment growth and yield (Mustafa et al., 2016).

Plant growth regulators increase endogenous hormones in plants. In soybean, exogenously applied methyl jasmonate (MeJA) increased the abscisic acid content in plants subjected to salt stress, thus, accelerating tolerance to salt stress (Yoon et al., 2009). A report by Hamayun et al. (2015) revealed that exogenous application of kinetin (Kn) significantly increased the endogenous bioactive gibberellins (GA₁ and GA₄), jasmonic acid, and free salicylic acid contents in soybean plants subjected to salt stress. This was evident as Kn up-regulated the biosynthesis pathways of gibberellins, jasmonic acid, and free salicylic acid unlike the control. Qi et al. (2013) reported that the foliar application of DTA-6 on soybean leaves increased the endogenous hormone (Zeatin Riboside, gibberellins, and indole acetic acid) levels. This increased PEPCase and Rubisco (key photosynthetic enzymes) activity, photosynthetic rate, high biomass accumulation, and consequently increased productivity. In conclusion, exogenous PGRs regulate the endogenous hormone levels to increase plant metabolic processes associated with growth and productivity.

Enzyme

Enzymes are mostly utilized in both plants and animals as an effective defense mechanism for eliminating reactive oxygen species (ROS). ROS induces cellular oxidative damage which impairs cell components with a concomitant effect on cell function leading to death (Foyer & Noctor, 2011). As reviewed by Hasanuzzaman et al. (2020), various enzymes like peroxidase (POD), catalase (CAT), superoxide dismutase (SOD), and glutathione-s-transferase (GST) in addition to non-enzymatic antioxidants such as reduced ascorbate (AsA) and glutathione (GSH) cooperatively reduce oxidative damage under stress conditions (Fig. 2).

SOD degrades excess ROS by transforming O₂^•− to O₂ and H₂O₂ (Bresciani, Da Cruz & González-Gallego, 2015) whiles CAT catalyzes the conversion of H₂O₂ to H₂O (Furukawa et al., 2017). Both antioxidant enzymes are essential and considered as a preliminary defense mechanism against oxygen toxicity. Ascorbate peroxidase (APX) is a widely distributed antioxidant in plant cells with its isoforms having a higher affinity for H₂O₂ than CAT, rendering APXs as an efficient scavenger of H₂O₂ under stressful conditions (Sharma, Jha & Dubey, 2019). GSH is an essential redox molecule found in all living cells and forthrightly associated with antioxidant defense and maintenance of intracellular redox homeostasis by stimulating ascorbic acid synthesis, through the Ascorbate-Glutathione cycle (Wang, Yang & Yi, 2012). GSH also plays a direct role as a free radical scavenger due to its ability to react with H₂O₂, OH^•, and O₂^•−. In soybean, POD, APX, polyphenol oxidase (PPO), and CAT activities were up-regulated under salinity stress with a significant increase in MDA and hydrogen peroxide. Also, applying PGRs stimulated the expression of these enzymes hence, influencing stress response. PGRs (salicylic acid and nitric oxide) inhibited the production and accumulation of ROS (H₂O₂, O₂^•−, OH^•), decreased lipid peroxidation and increased proline accumulation under salt stress (Tan et al., 2008). This significantly decreased MDA and increased the total antioxidant activity (by scavenging ROS which impeded its harmful effects), thereby increasing tolerance to salt stress. In soybean, exogenously applying SA, sodium nitroprusside (SNP), and their combination (SA+SNP) regulated the proline levels and reduced salt-induced oxidative damage marked by a reduction in the MDA content and subsequently, decreased lipid peroxidation and enhanced catalase activity (Simaei et al., 2011). Applying ETP alleviated water stress in soybean plants by up-regulating GST3 and GST8 (Kim et al., 2018). ETP significantly enhanced the expression of GST3 and GST8 at the biochemical and transcriptional level. Hence, an increase in GST3 and GST8 levels heightened GSH activity, reduced reactive oxygen species, alleviated cell damage in photosynthetic apparatus, and enhanced phenotype (Kim et al., 2018).

Foliar application of NAA improved nitrate reductase activity in soybean (Senthil, Pathmanaban & Srinivasan, 2003). Also, the application of SOD simulation material (SOD_M) and DTA-6 increased SOD, CAT, and POD activities in soybean leaves and lowered MDA, resulting in enhanced seed yield and delayed leaf senescence (Zheng et al., 2008). DTA-6 and uniconazole (S3307) increased the POD activity in pods, reduced vital abscission enzyme activity, MDA activities, and lipid peroxidation, thereby enhancing the accumulation and transportation of assimilates as well as antioxidant capacity (Sun et al., 2016). In addition, exogenously applied SNP induced the antioxidant machinery in soybean plants by enhancing various antioxidant enzyme (CAT, SOD, POD, and APX) activities (Jabeen et al., 2021).

Brassinosteroids and JA are well known for their antioxidant capacity and their major involvement in regulating the AsA-GSH cycle in plants. These PGRs boost the plant defense system by increasing antioxidant system capacity, enzymatic activity, and upregulating the expression of antioxidant genes (APX, SOD, POD, CAT, and glutathione reductase (GR); Bali et al., 2018). In soybean, seed priming with EBL increased the antioxidant enzyme (CAT, APX, SOD, and GR) activity and the contents of non-enzymatic antioxidants such as AsA, GSH, and glutathione disulfide (GSSG) under salt stress, which improved salt tolerance, biomass accumulation, and subsequently yield (Soliman et al., 2020). Similar results were obtained with exogenous EBL application in salt-stressed soybean plants as reported by Alam et al. (2019). Furthermore, exogenous 28-homobrassinolide enhanced the AsA-GSH cycle by activating the antioxidant system, marked by improved activities of APX, SOD, CAT, monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and GR), leading to efficient ROS scavenging in soybean seedlings (Hasan et al., 2020). According to Mir et al. (2018), the exogenous application of JA enhanced the contents of non-enzymatic antioxidants and antioxidant enzyme activity, thereby boosting redox homeostasis and antioxidant capacity in Nickel-stressed soybean plants. Similar results were also reported by Sirhindi et al. (2016). In another study, MeJA treatment increased CAT, SOD, and APX activity and the level of dehydroascorbic acid (DHA) in soybean leaves under cadmium stress, thereby ameliorating cadmium toxicity (Keramat, Kalantari & Arvin, 2009). Eventually, all these resulted in heightened growth and productivity of the soybean plant.

Photosynthesis

Photosynthesis is a vital phenomenon in crop production as it provides the energy needed for plant growth and development, thus, regarded as the basis of growth in plants. Murata (1981) reported a relatively high positive correlation between the optimal growth rate and leaf photosynthesis of many crops. However, the growth rate of plants may not directly reflect their photosynthetic rate. Leaf photosynthesis is a vital component of crop biomass production, thus, an essential determining factor of grain yield through canopy photosynthesis. For instance, Connor, Loomis & Cassman (2011) revealed that photosynthesis produces simple carbohydrates from which more complex carbohydrates (proteins and lipids) are obtained to form the dry matter of plants. In soybean, high seed yield results from enhanced canopy photosynthesis after seed germination. It is hypothesized that a decline in canopy photosynthesis significantly reduced soybean productivity, attributed to decreased pod setting (Jiang & Egli, 1993). Furthermore, the improved production of assimilates at the fruit development stage in soybean may increase yield. Based on the above results, it can be deduced that enhanced photosynthesis may improve growth, biomass production, and subsequently increase grain yield in soybean. Therefore, improving photosynthesis in soybean is essential as it may increase productivity without altering other genetic factors (Long et al., 2006).

Plant growth regulators enhance chlorophyll content, improve the photosynthetic ability, and increase translocation of assimilates from source to sink , thereby increasing biomass production and ultimately yield. Shao et al. (2014) reported a positive correlation between the rate of photosynthesis and chlorophyll content, indicating that chlorophyll content highly influences photosynthesis in plants. However, this relationship is limited in chlorophyll range, in that increased chlorophyll production may not necessarily increase photosynthetic rate. Generally, cycocel is reported to increase the translocation of photosynthates (Grewal et al., 1993). Foliar spray of cycocel (500 ppm) on soybean leaves at flower initiation (R1), pod initiation (R3), and flower + pod initiation (R1+R3) growth stages increased the chlorophyll content (Devi et al., 2011). The marked increment in chlorophyll content improved photosynthesis and greatly enhanced the translocation of photoassimilates to sink organs (seeds) leading to increased biomass accumulation. Similarly, the foliar spray of NAA (40 ppm), SOD_M, DTA-6, and ABA (300 mg/l) on soybean plants increased the rate of photosynthesis and chlorophyll content of soybean plants (Qi et al., 2013; Travaglia, Reinoso & Bottini, 2009; Zhao et al., 2008). The main factors for the enhanced photosynthesis in the DTA-6 treated plants were the increased activities of key photosynthetic enzymes (PEPCase and Rubisco), improved electron transport activity which elevated the rate of CO₂ assimilation, eventually improving the photosynthetic apparatus.

Also, DTA-6 and uniconazole significantly increased leaf photosynthetic ability. These growth regulators maintained high photosynthetic activity, increased photosynthetic rate, and subsequently biomass accumulation due to enhanced translocation of photosynthates (sucrose and starch) to different organs resulting in improved yield (Liu et al., 2019). Exogenously applied SA maintained the stability of PSII, increased chlorophyll a/b binding proteins (Chla/bBP), Rubisco activase, Rubisco subunits, oxygen-evolving enhancer protein 1 and 2 (OEE1 and OEE2, respectively), ferredoxin NADP reductase (FNR), and photosynthesis-related proteins in soybean. This improved stomatal conductance, photosynthetic rate, and water use efficiency (WUE), hence, increasing photosynthesis. Also, antioxidant enzyme activity increased with SA treatment (Sharma et al., 2018). These findings suggest that plant growth regulators enhance photosynthesis in soybean by increasing the chlorophyll content, distribution of photosynthates, and biomass accumulation which eventually improves growth and yield.

As recently reviewed by Müller & Munné-Bosch (2021), hormonal crosstalk positively regulates photosynthesis in plants. For instance, the exogenous application of IAA caused a marked increase in endogenous JA and ABA levels leading to significantly higher chlorophyll contents, thus, elevating the rate of photosynthesis in clover (Zhang et al., 2020). Furthermore, exogenous BAP increased endogenous zeatin levels which enhanced the F_v/F_m, ΦPSII (relative quantum yield of PSII), and electron transport activity in wheat (Yang et al., 2018). In soybean, MeJA treatments resulted in elevated levels of ABA which increased the photosynthetic rate (Yoon et al., 2009). In addition, the foliar spray of DTA-6 increased endogenous Zeatin Riboside, gibberellins, and IAA levels which increased PEPCase & Rubisco levels and the rate of photosynthesis in soybean. This enhanced biomass accumulation, thereby improving productivity (Qi et al., 2013). Though there is extensive research to support the positive regulation of hormonal crosstalk on photosynthesis in several plants, that of soybean is limited, thus, presenting a gap for future research.

Water utilization

Water utilization is very essential in the growth of plants. Every level of development requires a certain amount of water which is mostly dependent on the season, place and time of planting, and cultivars involved. The sufficient quantity of water available at these times of growth and development is essential for the diverse parameters of growth such as seed weight and size among others.

Generally, water use efficiency (WUE) defines the quantity of carbon assimilated as biomass produced per unit of water used by the plant. Hence, WUE is expressed as the relationship between yield/biomass to the soil water evaporation component and transpiration component which together denotes Evapotranspiration (ET) or the overall water available to the crop, comprising precipitation and the quantity of water available through irrigation i.e., Yield = ET*WUE where ET = T_soil + T_crop (Kuglitsch et al., 2008). Understanding plant water use is further detailed down by analyzing the water use at the leaf and canopy levels. The leaf level WUE is expressed as the net photosynthetic rate (A_n) divided by the transpiration rate (E) (Hatfield & Dold, 2019). In soybean, analysis of WUE at leaf and field scales revealed that a 1% increase in leaf-scale WUE showed ∼10% increment in field-scale explaining the 90% yearly variability in field-scale (Gorthi, 2017). WUE can hence be utilized as secondary criteria for selecting seed yield genotypes.

PGRs, being popular for their influence on various morphological and physiological parameters of plants, have limited information regarding water utilization in soybeans. According to Kamal et al. (1998), the foliar application of 2-ppm S-abscisic acid on soybean at the reproductive stage increased seed yield, pod number, and seed number under conditions of optimal or moderate water stress whereas severe conditions reduced seed yield. The WUE of soybean was enhanced after the application. GA₃ application during flowering and pod development, significantly increased rainwater use efficiency and productivity (Giri et al., 2018). This then proposes that GA₃ application enhanced the effective use of water by the soybean plant which was reflected in the increment in its yield.

Fertilizer utilization and nitrogen fixation

Nitrogen is a key nutrient for plant growth. Generally, legumes can fix nitrogen in soils which are further utilized by the plant for growth (Wang et al., 2020b). Di-nitrogen (N₂) fixation by leguminous crops has attracted much interest in recent times due to the nitrogen nutrition it provides in a highly sustainable and economically competitive way hence, participating in environmentally sound agricultural production as well as high-quality crop products (Vollmann et al., 2011).

In soybean, an increase in growth and yield have been reported to be dependent on large inputs of nitrogen with the pod initiation (R3) to full seed (R6) growth periods, requiring higher N contents (Oplinger, 1991). Despite this high requirement of nitrogen, only 25–60% of this nitrogen is naturally available with the rest compensated for by the application of nitrogenous fertilizers to the soil. Although there have been several conflicting reports on the benefits of applying nitrogenous fertilizers for soybean yield, it has been firmly agreed that soybean plants act as sinks for soil nitrogen and utilize it effectively (Varvel & Peterson, 1992). The effective utilization of this soil nitrogen during the early pod filling stage increased yield (Oplinger, 1991). However, increasing soil nitrogen at planting reduces soybean response to N fertilizer (Stone, Whitney & Anderson, 1985).

Despite all the benefits stated by several researchers about exogenous nitrogen application, it can be observed that the benefits were directly related to the time of application. For instance, the application of N fertilizer at planting reduced nodule formation, increased grain yield at flowering and early pod fill growth stages (Oplinger, 1991). Also, starter N application at the beginning flowering/flower initiation (R1) stage did not influence protein/oil concentrations or increase yield but increased dry matter content and plant N content (Wood, Torbert & Weaver, 1993). At the beginning seed (R5) stage, N increased grain yield, dry matter accumulation, seed protein concentration but reduced seed oil concentration. In all, with N fertilizer application improving soybean growth during the early season, it can hence be proposed that application at this stage is beneficial to the soybean plant and its nitrogen content, however, for effective utilization of the applied fertilizer, the R5 stage may be the most reliable. The above results, however, may not be independent of other factors. Per speculations, environmental factors may limit soybean growth by restricting N fixation, which may result in a positive response to N fertilizer. Hence, factors such as nitrogen concentration, cultivar, and environmental parameters may be responsible for several conflicting reports in terms of fertilizer utilization in soybean plants.

Despite all the above reports, one major limitation is the fact that the above researches were conducted one to three decades ago. Current information on fertilizer utilization is however very limited with none on how PGRs regulate fertilizer utilization. However, from knowing the various impacts and limitations of PGRs and fertilizers, the following hypothesis can be made. With the majority of PGRs involved in improving growth and yield, their application together with N fertilizers might enhance growth and yield in double folds than the individual increases and probably reduce the negative effects such as low nodule formation. A study by Devi et al. (2011) indicated that applying the required amount of NPK fertilizer at planting before applying various PGRs (50 ppm Salicylic acid, 200 ppm Ethrel, and Cycocel) at three different stages of development (R1, R2, and R1+R2) increased growth, yield and yield components, photosynthetic pigments, and protein and oil contents. Although it was not categorically stated that the result was due to the fertilizer application, there is a possibility of the fertilizer playing a role and revealing the chance of our hypothesis being true. This hypothesis, however, needs to be thoroughly investigated to reach a conclusive finding.

Yield and yield components

Major evidence of the extent of growth in plants is their yield. The yield is hence known as the productivity or output of the plant. Several traits of the plant known as the yield components cumulatively determine the overall yield. In soybean, components such as pod number, seed weight, seed number, and population density among others contribute to the overall yield and an increase in these components may significantly influence the yield. Enhancing these components and subsequently, the overall yield requires the adoption of various agronomic practices such as the application of plant growth regulators.

Numerous researches have studied the influence of PGRs on these components and their influence on the overall yield of plants. Applying NAA revealed an increment in the yield of soybean (Basuchaudhuri, 2016). Seed yield, pod number per plant, and overall yield of soybean increased after NAA application (between 10–100 ppm; Dhakne et al., 2015). A higher pod and seed number per plant were also recorded in soybean treated with 250 ppm of CCC resulting in higher seed yield (Kumar et al., 2002). Soaking seeds with 10 ppm gibberellic acid before sowing, then spraying again at the vegetative and flowering stage recorded a significant increase in yield (2.62 tons per hectare; Domingo, 1981). Soybean plants treated with 300 mg/l of BA decreased pod abortion in the lower, middle, and upper third of the canopy resulting in high productivity (Borges, Junior & De Freitas Santos, 2014). BAP application to racemes before efflorescence, reduced flower and pod number whereas application around 7 d after efflorescence considerably improved the rate of pod set (Nonokawa et al., 2007). The time of application can affect the productivity of the soybean plant, thus application during the end of flowering is more advantageous for soybean cultivation.

A study involving the foliar application of Tri-iodo benzoic acid (TIBA) to soybean plants revealed an enhanced pod number per node (Noodén & Nooden, 1985). Also, Kumar et al. (2002) detected a substantial increase in the pod and seed number per plant when 50 ppm of TIBA was applied and consequently increased the grain yield in soybean. Treatment with 15 to 120 ppm TIBA facilitated fruit development and better conditions for podding and also increased seed yield. However, higher levels were injurious to the plant (Buzzello et al., 2013). Again, seed yield, weight, and harvest index were observed to increase in soybean cv. Harit Soya when treated with 20 ppm GA₃ at bud initiation and 50% flowering stage (Upadhyay & Rajeev, 2015). Higher concentrations of GA₃ at 100 ppm efficiently augmented the number of pods and seeds per plant, seed weight per plant, and overall seed yield of soybean (Sarkar, Haque & Abdul Karim, 2002). Soybean cv. JS-9305 saw a significant increase in the seed and pod number per plant, seed weight, and total yield when seeds were primed (Agawane & Parhe, 2015). Different concentrations of GA₃ significantly influenced the pod number per plant, seeds per pod, 1,000 seed weight, and economic yield of soybean (Dhakne et al., 2015).

Nagel et al. (2001) revealed that the exogenous application of cytokinin improved the total yield of soybean by increasing the pod and seed number per plant. Total seed production increased with increasing cytokinin concentration, suggesting that the total yield of soybean may depend on cytokinin levels. Foliar application of plant growth regulators at both R1 and R3 stages decreased flower drop and efficiently influenced the transport of assimilates from source to sink. This, however, increased the pod number per plant significantly in many crops (Copur, Demirel & Karakuş, 2010). In soybean, applying GA₃ at the R1 and R3 stages significantly increased the pod number per plant.

Also, PGRs increased pod length which determines the seed per pod of soybean plants, depending on the time of application. For instance, the application of salicylic acid at the R1 stage and GA₃(foliar) at the R1+R3 stages recorded the highest pod length in soybean compared to other treatments applied at different stages of growth (Khatun et al., 2016). According to Dhankhar & Singh (2009), applying GA₃ increased the pod length of the soybean plants. The seed number per pod increased with the application of salicylic acid and Kn spray at vegetative, R3, and R1+R3 stages which were the same for the foliar application of GA₃ at the R1+R3 stage. The application of salicylic acid and 100 ppm GA₃ had the best effect. Similarly, Devi et al. (2011) recorded an increase in the seed number of soybean with the application of 50 ppm salicylic acid at the R1+R3 stage. Foliar spray with 50 ppm salicylic acid and 200 ppm of ethrel applied at R1, R2, and R1+R3 growth stages in soybean cv. JS 335 increased seed yield, attributed to better vegetative growth (Devi et al., 2011).

Further researches have reported that the application of PGRs increases the seed weight of soybean. Zholobak (1986) recorded a 20% increase in 100-seed weight when Kn was applied compared to the control. Similarly, Khatun et al. (2016) recorded the highest 100-seed weight in soybean with the application of salicylic acid and Kn spray at the R1+R3 and R3 stages respectively. Similar results were recorded for the foliar application of Kn at the vegetative stage. Devi et al. (2011) also reported an increase in the 100-seed weight of soybean with the application of Kn (500 ppm) at the R1+R3 stage. Kn application (40ppm) also increased the total pod set per plant, net return, and yield of soybean respectively with high concentrations recording the highest yield (Passos et al., 2011). This increase is attributed to the ability of plant growth regulators to improve photosynthesis and enhance the translocation of photosynthates in reproductive sinks of soybean plants which increased the cumulative effect of yield components and subsequently, increased the overall yield of soybean. However, the time of application of the growth regulators significantly influenced the 100-seed weight of soybean.

These results project that flower and pod dropping of soybean can be minimized by using exogenous BA, GA₃, Kn, and SA. Also, these PGRs can influence the diverse yield components which intend to affect the overall yield by restricting stress impacts. Under various environmental stresses, the application of PGRs may reduce the effects of these stresses on growth and overall yield. For instance, under salt stress, foliar application of n-triacontanol increased the specific leaf area (SLA) by increasing the leaf osmotic potential which may be due to increased organic solutes (Na⁺ and Cl⁻) in the leaves (Dhakne et al., 2015). One detrimental effect of salt stress in plants is to induce a reduction in relative water content of leaves, leading to loss of turgor which makes available little water for cell extension processes (Krishnan & Kumari, 2008). However, the foliar application of n-triacontanol alleviated this effect and increased the SLA. Also, exogenous ABA increased soybean yield under water deficit conditions (Zhang et al., 2004).

Quality

Mostly, the nutritional constituents of plants describe their quality. Research evidence suggests that plant growth regulators significantly increase the nutritional value/quality of soybean seeds. Seed quality parameters like protein content are of special concern to food-grade soybean production. Low protein content in soybean seeds is undesirable for soymilk as well as tofu yield. Soluble protein has essential functions in plant growth and is a key constituent of several plant enzymes reflecting the plant’s metabolism in totality. Foliar applications of 100 mg/l salicylic acid, 10 mg/l paclobutrazol (PP₃₃₃), or 5 g/l humic acid to soybean plants had a positive influence by maximizing its nutritional value (El-Aal & Eid, 2017). Salicylic acid at 50 ppm increased the quality of soybean and yield components compared to the control (Devi et al., 2011). Treating plants with GA₃ reduced seed protein whiles GA₃ spray increased oil content (Travaglia, Reinoso & Bottini, 2009). Higher doses of NAA also resulted in larger seed size and high protein contents but lower oil contents in soybean (Basuchaudhuri, 2016). Exogenous cycocel application increased protein and oil contents in soybean seeds (Devi et al., 2011). Although the application of PGRs influenced the nutritional value of the soybean plant, the time of application played an essential role. Application of GA₃ and salicylic acid (spray) significantly improved the protein percentage of soybean with the highest protein percentage recorded for GA₃ application at R1+R3 stage and salicylic acid (spray) at the vegetative, R1, and R3 stages (Khatun et al., 2016). Although GA₃ did not greatly influence the oil content, application during flowering and pod development showed a significant increase. The increase in oil yield correlated directly with grain yield of soybean (Giri et al., 2018). Furthermore, spraying salicylic acid on leaves and stems of soybean cv. BARI at the vegetative, R1, R3, and R1+R3 stages also increased the protein and moisture content of seeds (Khatun et al., 2016). In a recent study, foliar application of JA (0.5 mM) and SA (1 mM) enhanced the oil quality and yield as well as the fatty acids of soybean seeds by increasing the unsaturation index (UI), linolenic acid, and linoleic acid contents while reducing the oleic acid content. SA had the best effect on oil quality and yield both under salt toxicity and normal conditions (Ghassemi-Golezani & Farhangi-Abriz, 2018). Moreover, floral application of exogenous MeJA (0.1 mM) and SA (0.1 mM) increased the isoflavones content in soybean seeds (Saini et al., 2013b). Similarly, Hamayun et al. (2015) reported a significant increase in the isoflavones content of soybean with the exogenous application of Kn which enhanced their biosynthesis directly by upregulating the expression of key pathway genes. Taken together, PGRs have the potency of enhancing the quality of soybean seeds both under stressed and normal conditions.

Low temperature

Low temperature, one of the major limiting factors of plant growth and productivity, is defined as a drop in temperature to a level low enough but not freezing (>0 °C) to influence cellular function, abnormalities at various levels of cell organization, and inhibit growth. Membrane damage, reduced cellular respiration, and enhanced ROS production are the characteristic effects of low temperature stress in plants (Xing & Rajashekar, 2001). In soybean, all growth and developmental stages (from sprouting to seed filling and maturity) are negatively affected by low temperature, making soybean a cold-sensitive plant (Holmberg, 1973).

Low temperature reduces germination rate in soybean by delaying germination, that is, the lower the temperature, the lesser the rate of germination. Several studies have reported the adverse effects of low temperature on pod set and seed yield in soybean due to major factors such as poor growth, abscission of flowers and pods, and inadequate seed filling (Matsukawa, 1994). For instance, low-temperature treatment before and during flowering resulted in ruinous pod abscission in soybean (Michailov, 1989). Funatsuki et al. (2004) recorded a high reduction in soybean yield resulting from malformed pods and reduced seed filling upon exposure to low temperature before and during flowering. Furthermore, exposing soybean plants to low-temperature stress before flowering decreased the rate of fertilization and pod set marked by a decline in male fertility (Goto, 1972), attributed to the abnormal development of pollen grains, decreased pollen dispersal, and reduced number of pollen grains on the stigma (Ohnishi, Miyoshi & Shirai, 2010). According to Kurosaki & Yumoto (2003), long-term exposure of soybean to low temperature diminished pod set caused by a decline in the number of fertilized flowers which severely influenced grain yield. Similarly, Ohnishi, Miyoshi & Shirai (2010) revealed that low-temperature stress significantly decreased pod set at the flowering stage (12.5 d and 3-4 d before anthesis) by disrupting normal development of pollen grains, pollen dispersion, reduced number of pollen grains deposited on stigma, thus reducing pollination. However, the response to low temperature at the flowering stage and pod setting ability upon exposure to low-temperature stress is cultivar dependent, mainly based on the tolerance level (Kurosaki & Yumoto, 2003).

To eliminate the detrimental effects of low temperature on the growth and productivity of soybean and further improve tolerance to low temperature, plant growth regulators have been demonstrated through several studies as an effective agronomic technique. In soybean, tolerance to low temperatures has been achieved through the exogenous application of PGRs, and this promoted growth and seed yield. For instance, low concentrations of 5-aminolevulinic acid (ALA) induced low-temperature tolerance in soybean plants by improving heme-catabolism and antioxidant enzyme (heme-catalase (CAT) and heme- oxygenase-1 (HO-1)) activity. These enzymes inhibited oxidative damage by scavenging ROS, thereby protecting soybean plants from the ravaging influence of low-temperature stress. Pre-treatment with 5 µM of 5-aminolevulinic acid before cold treatment had the best effect (Balestrasse et al., 2010). Based on the results outlined above, it can then be proposed that the exogenous application of PGRs enhance soybean tolerance to low temperature and may be useful in preventing yield losses due to low temperatures.

Salt stress

Salt stress as an abiotic stress factor embraces all salt-related problems emanating from the excessive supply of NaCl (sodium chloride) either by natural accrual or artificial supply through irrigation (Flowers & Flowers, 2005). Salt stress has overwhelming effects on the overall growth and development of plants by affecting numerous vital physiological processes in plants (El Sayed & El Sayed, 2011) like protein synthesis, photosynthesis, and energy metabolism (Ahmad et al., 2019). Generally, salt stress interferes with complex hormone interaction, ion toxicity, osmotic effects, and nutritional balances in plants (Ahmad et al., 2019).

In soybean, salt stress disrupts overall growth and development (Phang, Shao & Lam, 2008) by influencing photosynthesis, seed germination, nutritional imbalance, nutrient uptake, and productivity (Zhu, 2016). For instance, Shu et al. (2017) indicated that salt stress impedes germination by dwindling the GA/ABA ratio in soybean. Similarly, Kumar (2017) indicated a decrease in germination and seedling growth in soybean under salt stress. Khan et al. (2016) and Ghassemi-Golezani et al. (2010) reported a significant reduction in all yield-related traits under salt stress, eventually leading to significant decrease in final yield of soybean. Ruzhen & Yiwu (1994) recorded nearly 40% yield loss in soybean as a result of high soil salinity. A recent study reported a decline in the amount of grain oil and seed protein of soybean under salt stress. In contrast to the control, the oil and protein contents diminished as the salt concentration escalated (Ghassemi-Golezani et al., 2010). In another study, Egbichi et al. (2013) reported that exposing soybean plants to salt stress-induced oxidative damage of membrane lipids in root nodules.

Several techniques such as conventional breeding and exogenous application of PGRs have been reported to enhance plants’ tolerance to salt stress, promote plant growth, and consequently increase productivity (Javid et al., 2011). Though soybean plants are adapted to several mechanisms that induce resistance to salt stress, agronomic techniques to eliminate salt stress in soybean are minimal. Plant growth regulators offer a lasting solution to the many growth challenges faced by soybean plants under salt stress. The exogenous application of fluridone (FLUN) promoted seed germination of soybean under salt stress by inhibiting ABA biosynthesis and promoting GA biosynthesis (Shu et al., 2017). Exogenously applied Kn ameliorates the adverse effects of salt stress by declining ABA levels (growth inhibitory hormone) and elevating GAs and SA (growth and defense hormones) levels. These endogenous hormone fluctuations caused by the influence of exogenous Kn, significantly increased fresh weight, plant height, and dry weight of soybean plants thereby promoting growth through enhanced chlorophyll content and leaf area (Hamayun et al., 2015). In another study, the combined effect of exogenous SA and SNP, a nitric oxide (NO) donor significantly improved shoot biomass (both fresh and dry), and the leaf area of soybean plants under salt stress by increasing the chlorophyll content. This stimulated the photosynthetic ability of the soybean plants and subsequently improved growth. However, 100 mM salicylic acid and/or 100 µM sodium nitroprusside had the best effect (Simaei et al., 2011).

Plant growth regulators, either single or combined can alleviate salt-induced oxidative damage and confer resistance to salt stress (Dong et al., 2017). According to Krishnan & Kumari (2008), foliar spray of n-triacontanol (TRIA, a saturated primary alcohol) reduced the harmful effects of salt stress on soybean plants and promoted growth. This was evident with the observed increase in all growth parameters (fresh weight, dry weight, root and shoot length), physiological parameters (relative water content, SLA, and leaf weight ratio), and biochemical parameters (total soluble sugars, soluble proteins, nucleic acids (DNA and RNA) and chlorophyll content) tested. The accelerated growth resulted from enhanced cell division and expansion (Sun et al., 2016), increased nutrient content (K⁺ and Ca²⁺) and its metabolism which enhanced the absorption of Na⁺ ions (Song et al., 2016), increased nitrogen availability (Parashar & Verma, 1993) and decreased chlorophyllase and protease activity (Dong et al., 2017). Thus, suggesting the ability of n-triacontanol to restore normal metabolism in salt-stressed soybean plants. Although growth was accelerated, there was a decline in proline accumulation and leaf osmotic potential. From the above report, it is speculated that plant growth regulators can ameliorate the adverse effects of salt stress on soybean plants and accelerate growth, thereby increasing productivity.

Drought

Drought stress is characterized by low atmospheric and soil humidity as well as high ambient temperature influenced by an imbalance between water intake from the soil and evapotranspiration oscillations (Lipiec et al., 2013). Drought stress is detrimental to the growth and development of plants by influencing their biochemical, morphological, physiological, and genetic resources which diminishes productivity (Khan & Mazid, 2018). Thus, drought hinders the proper functioning of plants by interrupting the water use potential and turgidity. Moreover, drought influences oxidative damage which may degrade proteins, lipids, nucleic acids, and cause lipid peroxidation in plants (Lipiec et al., 2013).

Soybean is a drought-sensitive crop and its exposure to drought stress particularly during seed filling (Desclaux, Huynh & Roumet, 2000), flowering, and pod-filling stages (Kobraee, Shamsi & Rasekhi, 2011) reduces photosynthesis, thus, decreasing yield (Liu, Jensen & Andersen, 2004). During drought stress, the rate of abortion at the early pod-filling stage increases in soybean and thus affect productivity (Lie, Anderson & Jensen, 2003). In soybean, water stress including drought stress approximately caused a 40–60% yield loss (Ahmed et al., 2013). Shou et al. (1991) recorded a 46% and 20% decrease in soybean yield at the flowering and seedling stages respectively, due to decreased photosynthesis under water stress.

In an attempt to increase soybean growth and productivity under drought conditions, many drought-tolerant soybean varieties have been developed through conventional and molecular breeding techniques. Another optional approach by researchers to effectively promote plant growth and increase drought tolerance is the application of growth regulators. Exogenous application of PGRs such as abscisic acid, brassinolide, and uniconazole under drought conditions increases soybean tolerance to drought stress and improves productivity. For instance, uniconazole application under drought stress improved soybean tolerance to drought which increased biomass accumulation and seed yield (Zhang et al., 2007). The higher yields recorded in uniconazole-treated soybean plants may be as a result of the enhanced transport of ¹⁴C assimilates from leaves to pods. Zhang et al. (2008) indicated that treatment with BL before drought stress minimized water-deficit yield loss in soybean. In another study, foliar application of MeJA eased drought stress in soybean plants thereby improving drought tolerance and yield in comparison to unstressed plants (Mohamed & Latif, 2017). The drought tolerance exhibited by soybean plants treated with MeJA (foliar spray) resulted from the elevated levels of soluble sugars and secondary metabolites including flavonoids and phenolic compounds associated with plant defense response against several biotic and abiotic stresses (Kim et al., 2007).

This implies that exogenous PGRs mitigate drought stress and improve drought tolerance in soybean through increased water use potential, leaf water potential, nitrate reductase activity, chlorophyll contents, and photosynthesis (Zhang et al., 2007), contributing to increased translocation of ¹⁴C assimilates, dry weight accumulation, and high leaf area (Mohamed & Latif, 2017). This significantly increased the yield-related parameters and subsequently improved the overall seed yield of soybean, indicating the ability of PGRs to stimulate soybean growth and improve yield even under drought stress.

Waterlogging

Waterlogging occurs when plant roots are surrounded by water or the shoots are partially or entirely submerged (Sullivan et al., 2001). Waterlogging is the most frequent flooding condition, characterized by excessive rainfall and/or the overspill of rivers (Kim et al., 2015) and it is one of the limiting factors in global soybean (a flood-sensitive crop) cultivation. Waterlogging negatively influences many physiological processes in plants by reducing photosynthesis, nutrient uptake, and causes hormonal imbalance that negatively affects the growth and yield of soybean (Kim et al., 2015; Valliyodan et al., 2017). For example, substantial yield loss was observed in soybean under severe waterlogging stress during the vegetative (17 to 43%; Reyna et al., 2003) and reproductive (50 to 56%; Scott et al., 1990) growth stages. Similarly, soybean plants exposed to waterlogging stress recorded a 17% and 57% yield loss at the vegetative and reproductive stages respectively, compared to non-stressed plants (Nguyen et al., 2012). A recent report by Rhine et al. (2010) suggested a 39% yield loss in waterlogging-tolerant soybean varieties as against the 77% yield loss in susceptible varieties under severe waterlogging conditions.

Plants develop many strategies to mitigate the adverse effects of waterlogging stress which improves their tolerance. Among them, is the escape strategy which allows gaseous exchange between cells and their environment due to the changes that occur in both the anatomical and morphological structures of the plant including adventitious root formation, shoot elongation, and aerenchyma cell development (Bailey-Serres & Voesenek, 2008). Recently, several studies have reported the use of exogenous plant growth regulators as an effective alternative to mitigate the deleterious effects of waterlogging stress in plants. Kim et al. (2015) indicated that the application of GA, and ETP improved tolerance to waterlogging stress than in control plants. Treatments 50 µM and 100 µM ETP had the best effect, recording the highest GA concentrations even after short-term stress. The results revealed that GA- and ETP-treated soybean plants developed an escape strategy under waterlogging by the hyper-elongation of the shoot which increased the overall plant height. This then confirms that the accumulation and fluctuations in the endogenous GA levels induce the escape strategy. In another study, Kim et al. (2018) reported the ability of ETP to ameliorate waterlogging stress in soybean plants. This ameliorative effect of ETP was due to its ability to induce the overexpression of antioxidant enzymes. Overall, this could promote growth and improve soybean productivity under waterlogging stress.

Root length is among the key response indicators to waterlogging stress in plants. Tolerance of soybean plants to waterlogging stress correlates sturdily to the overall root growth (length, surface area, and dry weight; Sallam & Scott, 1987). Recently, Kim et al. (2018) documented that waterlogging stress can impede root growth in soybean by reducing the root size and RSA. However, the exogenous application of ETP to soybean plants subjected to waterlogging stress increased root size and RSA which promoted root growth and nutrient (K and P) uptake.

ABA is a key hormone in water stress response. According to recent research, endogenous ABA contributes to the development of aerenchyma cells in root tips during waterlogging, thus facilitating the absorption and transport of oxygen (Shimamura et al., 2014). Endogenous ABA decreases in response to waterlogging stress to keep the stomata open, providing greater surface area for oxygen entry and removal of excess water (Shimamura et al., 2014). Increasing research suggests that the downregulation of endogenous ABA suppresses suberin biosynthesis which keeps the root cell unsuberized for aerenchyma cell development in soybean. Similarly, Kim et al. (2015) observed a significant decrease in ABA contents and well-developed aerenchyma cells in waterlogging tolerant soybean variety compared to non-tolerant variety and control. This suggests the crucial role of ABA in regulating soybean tolerance to waterlogging stress, thus promoting growth and productivity. In conclusion, the exogenous application of plant growth regulators can mitigate waterlogging stress in soybean by improving waterlogging tolerance through the development of the escape strategy, increased accumulation of endogenous GAs, induced antioxidant enzyme activity, and increased root growth which promoted nutrient uptake. This in effect promoted the overall growth and productivity of soybean under waterlogging stress. However, the significant internode growth observed in the GA- and ETP-treated soybean plants subjected the plants to lodging (Seo et al., 2017). We can therefore propose that the use of exogenous GA and ETP may be ineffective for commercial soybean production.