Effect of plant growth substances on growth and yield of Nagpur mandarin

Introduction

In India, the Nagpur mandarin (Citrus reticulata Blanco) occupies a premier position among cultivated loose-skinned oranges, prominently grown in the Vidarbha region of Maharashtra (Anavrat, 2017; Shraddha et al., 2024) [3] . Despite an estimated cultivation area of 1.2 to 1.4 lakh hectares in Vidarbha, the average productivity remains relatively low at 8 to 10 tonnes per hectare, which is significantly below the crop's genetic and regional potential. Maximizing the yield efficiency of mature orchards is a highly complex horticultural challenge. The transition from flowering and initial fruit set to final yield is heavily dictated by physiological factors, endogenous hormonal balances, environmental stressors, and the efficient mobilization of nutrients toward developing reproductive sinks. Under natural conditions, disturbances in the balance of endogenous growth regulators lead to reproductive losses, severely capping the final harvestable yield. To maximize yield, the exogenous application of plant growth regulators (PGRs) and phytohormones has become an important component of modern agrotechnical practices (Monselise, 1979; Almeida et al., 2004) [2,15] . Historically, compounds such as auxins (NAA, 2,4-D) and gibberellins (GA3) have been extensively utilized in the global citrus industry to improve fruit set and enhance overall production efficiency (Anthony et al., 1999; Nawaz et al., 2008) [5,16] . In recent years, new-generation growth promoters like CPPU, N-ACTA, Brassinolides, and Folic Acid have demonstrated exceptional efficacy in managing plant stress, enhancing cellular division, and optimizing the source-sink relationship. CPPU functions synergistically with endogenous auxins to profoundly enhance early cell division, plant growth, and sink strength in developing fruitlets (Zhang & Whiting, 2011; Xin Xie et al., 2024) [23,25] . Brassinolides act as potent steroidal hormones that maintain plant metabolism and developmental processes under environmental stress conditions, effectively acting to optimize yields (Roy, 1973; Peng et al., 2004) [17,19] . N-ACTA has been proven to channelize assimilate translocation directly to developing reproductive organs (Hota et al., 2017a) [10] . Furthermore, Folic acid (vitamin B9) aids in crucial nucleic acid synthesis and one-carbon metabolism necessary for vigorous early reproductive cell division (Hanson & Gregory, 2011; Abd El-Rahman et al., 2018) [1,9] .

Materials and Methods

Experimental site and weather conditions

The field experiment was conducted during year 2021 to 2023 at the experimental farm of the All India Coordinated Research Project on Fruits (Citrus), Dr. Panjabrao Deshmukh Krishi Vidyapeeth, Akola, Maharashtra. The site lies at 20°42' N latitude and 77°02' E longitude, with an elevation of 307.42 m above mean sea level (MSL). The experimental trial was executed on uniform, eighteen-yearold Nagpur mandarin trees that were planted at a standard spacing of 6 m × 6 m. The orchard soil consisted of medium to heavy black cotton soil, which was moderately deep, porous and possessed good drainage capabilities.

Experimental design and treatment details

The field trial was laid out using a Randomized Block Design (RBD) with 10 different treatments. These treatments were replicated three times, with two uniform trees forming a single experimental unit, resulting in 60 total trees under observation. The treatments comprised foliar applications of various growth regulators, applied initially at the pea-size stage of fruit development and subsequently at one-month intervals corresponding to July, August and September.

Treatment details

Results and Discussion

Effect on vegetative growth parameters

The pooled initial plant height across the orchard ranged (2020) [7,18] in Kinnow mandarin, who also reported limited height variations post-PGR application.

The pooled data demonstrated that the significantly highest leaf area was recorded in T 7 (45.39 cm²), closely followed by T 8 (45.24 cm²) and T 6 (45.19 cm²), whereas the untreated control recorded the lowest (39.25 cm²). The sequential application of CPPU followed by N-ACTA and Brassinolides in T 7 actively stimulated cellular division and expansion, resulting in highest leaf area increase the overall photosynthetic capability of the canopy (Atheer M.I. Al Janabi et al., 2017; Jyoti et al., 2024) [6] . [4] .

Graded fruit yield analysis:

The fruits were graded by diameter into premium "A" Grade (>7.5 cm), "B" Grade (>6.5 cm), and "C" Grade (<6.5 cm). The maximum number of premium "A" grade fruits per plant was significantly achieved in T

Leaf and soil nutrient status

Based on the pooled analysis, leaf Nitrogen ranged from 1.75% (T 10 ) to 2.20% (T 7 ), Phosphorus from 0.14% to 0.23%, and Potassium from 1.45% to 1.89%. While treatment T 7 showed higher leaf retention of all three elements, the analysis of variance confirmed that these differences were statistically non-significant.

Similarly, the final pooled status of soil available Nitrogen ranged from 205.04 to 213.01 kg/ha, available Phosphorus from 14.95 to 17.55 kg/ha, and available Potassium from 304.00 to 343.65 kg/ha. The available calcium carbonate in the soil also remained completely stable between 7.34% and 7.49%. these variations between treatments proved to be statistically non-significant.

Economic viability

Treatment T 7 yielded the highest returns by a substantial margin, achieving a highly BCR of 5.89. Despite the supplementary capital costs associated with procuring and applying these advanced new-generation growth substances, the improvements observed in total yield and the shift toward premium marketable "A" and "B" grade fruits directly resulted into larger revenues. Other treatments such as T 6 (5.67) and T 9 (5.62) also demonstrated high profitability. In contrast, the control group (T 10 ) recorded an economically inferior BCR of 2.46.

Conclusion

Treatment T 7: a targeted sequential foliar spray of CPPU (10 ppm) + Urea (1%) at the pea-size stage, followed by monthly applications of N-ACTA (10 ppm) + Brassinolides (4 ppm) + Folic acid (40 ppm) from July through September. This specific chemical formulation optimized the plant's physiological source-sink dynamics, maximizing the total number of harvested fruits per plant (816.33), the highest fruit yield per plant (118.63 kg). Treatment T 7 is economically viable benefit-cost ratio of 5.89. The application of CPPU, N-ACTA, Brassinolides, and Folic acid coordinates internal carbohydrate partitioning and provides a potent, to secure higher yields.

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