Effect of plant growth retardants on morpho-physiological, biochemical and yield parameters of sorghum (Sorghum bicolor L.)

Introduction

Sorghum (Sorghum bicolor L. Moench) is an important cereal crop cultivated predominantly by resource-poor, small, and marginal farmers in semi-arid regions. In India, sorghum grown during the rainy (kharif) season is utilized for both human consumption and livestock feed, whereas the post-rainy (rabi) season produce is primarily used for human consumption. Thus, sorghum plays a vital role in sustaining both human and livestock populations. During 2021-22, sorghum in India was cultivated over an area of 4.8 million hectares with a production of 4.6 million tonnes and an average productivity of 1000 kg ha⁻¹ (Anon., 2022) [2] . It is one of the most important cereal crops grown in semi-arid and arid regions due to its inherent tolerance to drought and high temperature (Reddy et al., 2010) [19] . It serves as a staple food, fodder, and industrial raw material, contributing significantly to food and nutritional security (Taylor, 2003) [23] . However, achieving higher productivity in sorghum remains a challenge due to excessive vegetative growth, lodging, and suboptimal partitioning of assimilates under varying environmental conditions. Plant growth retardants (PGRs) play a crucial role in modifying plant architecture by inhibiting gibberellin biosynthesis, thereby reducing plant height and improving canopy structure (Davies, 2010) [4] . Such modifications enhance light interception, photosynthetic efficiency, and assimilate partitioning towards reproductive organs, ultimately improving yield (Taiz et al., 2015) [22] . Among the commonly used growth retardants, mepiquat chloride, chlormequat chloride (CCC), and ethephon have been reported to influence morpho-physiological and biochemical processes in crops (Rademacher, 2000) [18] . Mepiquat chloride restricts excessive vegetative growth and promotes efficient utilization of assimilates, whereas chlormequat chloride regulates cell elongation and enhances stem strength, thereby reducing lodging. Ethephon, an ethylene-releasing compound, influences various physiological processes including senescence, source-sink relationships, and stress responses (Abeles et al., 1992) [1] . These growth regulators also affect key physiological parameters such as photosynthetic rate, transpiration rate, and chlorophyll content, which are directly associated with crop productivity (Ashraf et al., 2006) [3] . Despite the documented benefits of plant growth retardants in several crops, limited information is available regarding their comparative effectiveness on morpho-physiological, biochemical traits, and yield performance in sorghum under field conditions. Therefore, the present investigation was undertaken to evaluate the effects of different plant growth retardants on growth, physiological and biochemical parameters, and their consequent influence on yield in sorghum.

Materials and Methods

A field experiment was conducted during the Rabi season of 2022-23 at the Agricultural Research Station, Bheemarayanagudi, University of Agricultural Sciences, Raichur, Karnataka. The experimental site is located in the Northern Dry Zone of Karnataka and is characterized by a semi-arid climate. The soil of the experimental field was medium deep black with moderate fertility status. The experiment was laid out in a randomized complete block design (RCBD) with three replications, comprising eleven treatments. The treatments included control (no spray), water spray, mepiquat chloride at 1000, 1500, and 2000 ppm, chlormequat chloride at 1000, 1500, and 2000 ppm, and ethephon at 200, 400, and 600 ppm. The recommended package of practices for sorghum cultivation was followed uniformly for all treatments. The growth retardants were applied as foliar sprays at 35 days after sowing (DAS) using a hand sprayer with uniform spray volume. Observations on morphological parameters such as plant height (cm), total dry matter production (g plant⁻¹), total leaf area (cm² plant⁻¹), and leaf area index (LAI) were recorded at appropriate growth stages following standard procedures. Physiological parameters including photosynthetic rate (µmol CO₂ m⁻² s⁻¹) and transpiration rate (mmol H₂O m⁻² s⁻¹) were measured using a portable photosynthesis system. Total chlorophyll content (mg g⁻¹ fresh weight) was estimated using standard biochemical methods. At harvest, seed yield (kg ha⁻¹) was recorded from each treatment plot and computed on hectare basis. The data obtained from various observations were statistically analysed using analysis of variance (ANOVA) as described by Panse and Sukhatme (1967). Treatment means were compared using the appropriate critical difference (CD) at 5 percent level of significance.

Results and Discussion

The data on morphological parameters of sorghum are presented in Table 1. The control (T₁) recorded significantly higher plant height (258.60 cm) compared to all other treatments. Among the treatments, foliar application of mepiquat chloride @ 2000 ppm (T₅) recorded the lowest plant height (178.30 cm), followed by chlormequat chloride @ 1500 ppm (T₇) with 190.40 cm. The reduction in plant height due to growth retardants may be attributed to inhibition of gibberellin biosynthesis, which restricts cell elongation. Similar findings were reported by Kamran et al. (2018) [9] in maize and Kashid et al. (2010) [11] in sunflower. Growth retardants such as cycocel and mepiquat chloride act as anti-gibberellin agents by blocking the conversion of geranyl pyrophosphate to copalyl pyrophosphate, an early step in gibberellin synthesis (Moore, 1980) [14] . Total leaf area was significantly higher in T₅ (2325 cm² plant⁻¹) and the lowest was recorded in control (T₁) (1655 cm² plant⁻¹). Increased leaf area enhances photosynthetic capacity, thereby contributing to higher productivity. Leaf area increased up to 90 DAS and declined thereafter due to senescence and leaf shedding. Similar results were reported by Garg et al. (2006) [6] in cluster bean and Phulekar et al. (1998) [15] in groundnut. Total dry matter production was significantly higher in T₅ (195.64 g plant⁻¹), followed by T₇ (186.35 g plant⁻¹), while the lowest was recorded in control (122.84 g plant⁻¹).

Increased dry matter accumulation under growth retardant treatments may be due to enhanced photosynthetic efficiency and better translocation of assimilates towards reproductive organs. Reduced plant height and increased stem thickness also contribute to higher dry matter accumulation. These findings are in agreement with Gangadhara et al. (2022) [5] , Kashid et al. (2010) [11] , and Phulekar et al. (1998) [15] . Leaf area index (LAI) was significantly higher in T₅ (3.44) and lowest in control (2.45). Enhanced LAI may be attributed to increased branching and efficient assimilate translocation. Similar results were reported by Srivastava and Tiwari (1981) [20] and Islam and Jahan (2016) [7] . Seed yield was significantly higher in T₅ (1724.9 kg ha⁻¹), representing a 42.6 percent increase over control, followed by T₇ (1671.4 kg ha⁻¹), while the lowest yield was recorded in control (1354.5 kg ha⁻¹). The increase in yield may be due to improved morpho-physiological efficiency and better assimilate partitioning. Similar findings were reported by Kamran et al. (2018) [9] , Techapinyawat et al. (1995) [24] , Kashid et al. (2010) [11] , Kalyankar et al. (2007) [8] , and Pourmohammad et al. (2014) [16] . The data on physiological and biochemical parameters are presented in Table 2. Photosynthetic rate was significantly higher in T₅ (35.65 µmol CO₂ m⁻² s⁻¹), followed by T₇ (33.54 µmol CO₂ m⁻² s⁻¹), while the lowest was recorded in control (21.88 µmol CO₂ m⁻² s⁻¹). Increased photosynthetic rate may be attributed to higher chlorophyll content and LAI. Similar results were reported by Rajesh et al. (2014) [17] and Karuppusamy et al. (2021) [10] . Transpiration rate was significantly higher in T₅ (3.95 mmol H₂O m⁻² s⁻¹) followed by T₇ (3.82 mmol H₂O m⁻² s⁻¹), while the lowest was observed in control (3.18 mmol H₂O m⁻² s⁻¹). The increase in transpiration rate might be due to improved stomatal conductance. These results are in agreement with Liu et al. (2016) [13] . Total chlorophyll content was significantly higher in T₅ (2.77 mg g⁻¹ fresh weight), followed by T₇ (2.66 mg g⁻¹), whereas the lowest was recorded in control (1.93 mg g⁻¹). Increased chlorophyll content may be due to enhanced synthesis and delayed degradation of chlorophyll pigments. Growth retardants improve source-sink relationships and prolong chlorophyll synthesis (Stoddart, 1965) [21] . Similar findings were reported by Kashid et al. (2010) [11] , Wasnik and Bagga (1996) [26] , and Kulkarni et al. (1995) [12] . Overall, foliar application of mepiquat chloride @ 2000 ppm (T₅) was found to be the most effective treatment in improving morpho-physiological, biochemical parameters, and yield in sorghum, followed by chlormequat chloride @ 1500 ppm (T₇).

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