Integrated organic-mineral nutrition improves growth and stress resilience in high density avocado cv. TKD-1

Introduction

Avocado growth is influenced by complex physiological, morphological and environmental interactions, particularly due to its monopodial growth habit, shallow rooting nature and rhythmic flush pattern. In avocado, vegetative growth typically occurs in distinct flushes, often coinciding or competing with reproductive development, which creates strong competition for assimilates between shoots, roots and developing fruits (Whiley et al., 2013) [22] . Shoot growth generally precedes root growth, but root growth is markedly suppressed during periods of high fruit load (Arpaia et al., 1995) [1] . Avocado exhibits both proleptic and sylleptic shoot types, forming characteristic growth modules which determine canopy structure, leaf area development and resource partitioning (Thorp et al., 1994) [21] . The shallow root system, concentrated at 30-40 cm depth, increases tree sensitivity to soil moisture imbalance, aeration stress and nutrient limitations (Scora et al., 2002) [19] . Therefore, balanced nutrient supply, favourable soil environment and optimized canopy growth regulation are critical to ensure continuous vegetative renewal and improved reproductive success in avocado orchards.

Materials and Methods

The field experiment was conducted at Pandrimalai, Dindigul district, Tamil Nadu (10°33' N, 77°73' E; 1251 m MSL), located in the Western Agro-Climatic Zone of Tamil Nadu. The region experiences a subtropical climate with an annual rainfall of ~800 mm, predominantly received during the North-East Monsoon (October-December), with mean temperatures ranging from 15-29 °C. Meteorological data recorded during the study period were obtained from the Office of the Assistant Director of Statistics, Dindigul. The experiment was carried out on 12-year-old avocado trees (var. TKD-1) planted at 5 × 5 m spacing under high-density planting. The study was laid out in a Randomized Block Design (RBD) consisting of 10 treatments replicated thrice, with 10 trees per replication, comprising a total of 300 plants.

Results and Discussion

The data pertaining to the effect of organic mineral nutrient treatments on tree height of avocado cv. TKD-1 are presented in Table 1. The results indicated that the application of organic mineral nutrients resulted in a marginal increase in tree height compared to the control; however, the differences among treatments were statistically non-significant (NS). Among the treatments, T 9 (25 kg compost + 50% K through polyhalite + 50% P through PROM) recorded the maximum tree height (7.98 m), which was statistically on par with T 8 (50% RDF + 50% K through polyhalite + 50% P through PROM) registering 7.93 m. The minimum tree height (7.38 m) was observed in T 10 (control). The increase in tree height under polyhalite nutrition appears to result from its balanced release of K, Ca, Mg and S, which collectively sustain metabolic activity and vegetative growth. Potassium acts as a major enzymatic activator for photosynthesis, assimilate transport and protein synthesis (Ng Kee Kwong, 2001) [12] , while sulphur supports oxidative processes and ferredoxin-mediated electron transfer during photosynthesis. This coordinated nutrient supply enhances carbohydrate accumulation and nitrogen metabolism, contributing to vigorous stem elongation, as similarly noted in sugarcane by Raghunath et al. (2021) [15] . Calcium-driven regulation of cell division and elongation further promotes vertical growth (Monshausen, 2012) [11] . Organic manures also increased plant height by improving nutrient mineralization, soil structure and releasing growthpromoting compounds. Their phosphorus content enhances meristematic activity and stem elongation (Noor et al., 2021) [13] and combined organic-inorganic applications have been shown to raise plant height in several crops (Zafar et al., 2011; Raiger, 2004; Khangarot, 2022) [23,16,7] . Trunk girth showed significant variation among the treatments (Table 1). Although slight variations were observed among treatments, the differences were statistically non-significant (NS). The highest trunk girth was recorded in T 9 (98.34 cm), which was statistically on par with T 8 (98.22 cm) and followed by T 5 (98.08 cm). The lowest trunk girth (97.05 cm) was observed in the control (T 10 ). Polyhalite improved trunk girth through complementary functions of its constituent nutrients. Calcium strengthened cell walls and supported xylem formation, while Mg and S maintained higher photosynthetic efficiency and protein synthesis, promoting sustained secondary thickening and greater deposition of structural carbohydrates. Similar benefits of polyhalite on stem thickening have been reported in the rice-wheat system (Kumar et al., 2025) [8] , tomato (Sacks et al., 2017) [17] , sugarcane (Bhatt et al., 2021) [3] and sunflower. PROM enhanced radial growth by supplying slow-release phosphorus and organic matter, supporting root expansion, nutrient uptake, ATP production and cambial activation. This steady P availability facilitates auxin-mediated secondary growth and increased translocation of assimilates to woody tissues. Vermicompost and PSB further strengthened trunk girth by improving P solubilization, nitrogen mineralization and microbial activity, collectively enhancing cambial activity. Similar responses in custard apple cv. Arka Sahan were documented by Bhatnagar and Singh (2015) [2] . Significant differences were observed in canopy spread among the treatments (Table 1). The maximum canopy spread was recorded in T 9 (5.37 m), which was statistically on par with T 8 (5.33 m) and T 6 (5.14 m). The minimum canopy spread (4.34 m) was observed in the control (T 10 ). The enhanced canopy spread under integrated nutrient treatments indicates improved vegetative growth due to balanced nutrient availability. The canopy volume of avocado trees was significantly influenced by organic mineral nutrient treatments (Table 1). Treatment T 9 recorded the highest canopy volume (178.51 m 3 ), which was statistically on par with T 8 (176.55 m 3 ) and T₅ (167.91 m 3 ). Ganeshamurthy, 2016) [18] . Leaf area differed significantly among the nutrient management treatments (Table 1). The highest leaf area was recorded in T 9 (25 kg compost + 50% K through polyhalite + 50% P through PROM), with a value of 254.99 cm 2 , which was statistically on par with T 8 (254.26 cm²). This was followed by T 5 (50% K through polyhalite + 50% P through PROM), which recorded a leaf area of 251.03 cm 2 .

In contrast, the control (T 10 ) exhibited the minimum leaf area (234.45 cm 2 ). Increased leaf area under integrated nutrient management may be attributed to improved nutrient uptake and enhanced photosynthetic efficiency. Enhanced leaf area under integrated nutrient management may be attributed to improved nutrient uptake and greater photosynthetic surface development. The synergistic interaction between nitrogen and sulphur plays a critical role in leaf expansion by promoting protein synthesis and chloroplast development, as reported by Bologna-Campbell et al. (2013) [5] . Adequate potassium supplied through polyhalite, owing to its high mobility, likely contributed to improved turgor regulation, stomatal functioning and assimilate translocation, thereby supporting vigorous leaf growth. The balanced supply of Ca, Mg, K and S may have further facilitated cell wall formation, laminar tissue strength and leaf enlargement, culminating in increased total leaf area and canopy spread.

Chlorophyll content (SPAD value) differed significantly among the treatments (Table 1). The highest chlorophyll content was recorded in T 9 (61.28 SPAD), followed closely by T 8 (61.06 SPAD) and followed by T 5 (60.10 SPAD). The lowest chlorophyll content (55. 16 [10,20,4] . Relative water content was significantly influenced by organic mineral nutrient treatments (Table 1). Treatment T 9 recorded the maximum RWC (96.39%), which was statistically on par with T 8 (96.01%) and followed by (94.31%). The minimum RWC (85.62%) was observed in the control (T 10 ). Higher RWC under integrated nutrient treatments suggests improved plant water status and stress tolerance. Polyhalite enhanced relative water content (RWC) by improving osmotic adjustment, stabilizing stomatal activity and maintaining cell turgor. Calcium strengthened membranes, reducing water loss and magnesium sustained photosynthetic efficiency, indirectly supporting hydration. These outcomes align with improvements reported in tomato (Sacks et al., 2017) [17] .

Compost further improved RWC by enhancing soil waterholding capacity, nutrient retention and root-zone aeration.

When combined with phosphorus, it improved ion balance, reduced sodium uptake and promoted antioxidant activity, lowering oxidative stress. Ding et al. (2021) [6] similarly found that PROM and compost enhanced stomatal conductance and RWC while reducing proline under stress; comparable outcomes were reported by Omara et al. (2022) [14] and Masjedi et al. (2025) [9] . The improved hydration and nutrient balance reduced PAL activity and proline accumulation, indicating lower stress signalling and more stable physiological functioning under integrated nutrient management. PAL activity showed significant variation among treatments (Table 1). The lowest PAL activity, which is physiologically desirable, was recorded in T 9 (18.13 units g -1 FW), which was statistically on par with T 8 (18.61 units g -1 FW) and followed by T 5 (20.74 units g -1 FW). In contrast, the highest PAL activity was observed in the control (31.67 units g -1 FW). Proline content also differed significantly among treatments (Table 1). The lowest proline content, which is considered favourable, was recorded in T 9 (3.25 µmol g -1 FW), followed by T 8 (3.51 µmol g -1 FW) and T₅ (4.63 µmol g -1 FW). The highest proline accumulation occurred in the control (10.41 µmol g -1 FW). Lower PAL activity indicates reduced stress-induced activation of the phenylpropanoid pathway and reflects improved metabolic stability. PAL is commonly upregulated under nutrient deficiency or environmental stress; therefore, its reduced activity under integrated organic-mineral nutrient treatments suggest enhanced nutrient availability and lower oxidative stress. The balanced supply of K, Ca, Mg and S from polyhalite, along with phosphorus from PROM, likely minimized stress signals and suppressed excessive PAL induction. Proline accumulation, a well-established indicator of physiological stress, was also lower under integrated nutrient treatments, indicating improved cellular hydration, membrane stability and osmotic balance. Adequate potassium-mediated stomatal regulation and sulfur-supported amino acid metabolism collectively contributed to reduced proline synthesis.

Conclusion

Integrated organic-mineral nutrient management significantly enhanced vegetative growth of avocado cv. TKD-1 compared to conventional fertilization. The combined application of compost with potassium through polyhalite and phosphorus through PROM (T 9 ) improved canopy development, leaf area, chlorophyll content and plant water status, while significantly reducing PAL activity and proline accumulation, indicating lower physiological stress. The balanced, slow-release supply of macro-and secondary nutrients from polyhalite, together with organic phosphorus sources, improved nutrient uptake efficiency and photosynthetic performance. Tree height and trunk girth showed minimal variation, suggesting that integrated nutrition primarily promoted canopy expansion and physiological efficiency rather than vertical growth. Overall, integrated organic-mineral nutrient application represents a sustainable and effective nutrient management strategy for avocado under high-density planting systems.

Authors Contribution

Work assessment, execution, lab work, data gathering, statistical analysis and final correction.

Declaration

There are no competing interests for the writers.

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