Effect of Bio-regulators on Grafting Success of Macadamia nut (Macadamia integrifolia Maiden and Betche)

Introduction

Macadamia (Macadamia integrifolia Maiden and Betche) belongs to the family Proteaceae and is an important nut crop of the world. It is indigenous to the rainforests of north-eastern Australia and has subsequently been introduced into several tropical and subtropical regions. Major commercial development of macadamia has taken place in Hawaii, particularly during the last six decades (Sharma et al., 2015) [1] . At present, it is commercially cultivated in the countries such as Australia, Brazil, Indonesia, Kenya, New Zealand and South Africa. Macadamia nuts are highly valued for their distinctive buttery flavour, crisp texture and nutritional properties and are consumed raw, roasted or processed into cakes, confectionery, ice cream, salads and other food products. Macadamia is represented by several species in Australia, but M. integrifolia, M. tetraphylla and their hybrids are predominantly cultivated commercially. Both species possess a somatic chromosome number of 2𝑛 = 28. M. integrifolia is commonly known as the smooth-shelled type, producing highquality kernels, whereas M. tetraphylla is characterised by a rough, pebbled shell and slightly elongated nuts. Macadamia is primarily propagated through seeds for raising rootstocks, whereas vegetative propagation through grafting is preferred for the multiplication of superior cultivars and early bearing. Different grafting techniques, including simple, whip, side, wedge and cleft grafting are used for macadamia propagation. Well-matured, healthy nuts are commonly used for raising rootstock seedlings (Tripathi et al., 2023) [2] . Suitable rootstock seedlings are generally selected for grafting, based on their age and stem thickness. Grafted plants have the advantage of maintaining the desirable characteristics of the scion and generally attain bearing earlier than seedling plants. Therefore, efficient grafting techniques are essential for rapid multiplication and establishment of elite macadamia planting material. Graft union formation is a complex physiological process involving wound healing, callus formation, cambial reconnection and subsequent development of vascular tissues. Plant growth regulators, particularly auxins and cytokinin, play important roles in cell division, differentiation and vascular development during graft union formation. Following grafting, auxin accumulation in the scion and cytokinin activity in the rootstock contribute to wound responses and vascular tissue differentiation at the graft interface (Melnyk et al., 2015) and increase grafting success. Hence, the present study has been conducted with the objectives to study the effect of bio-regulators on grafting success and graft survivability of Macadamia nut.

Material and Methods

The study was conducted from November 2025 to June 2026 at Varashree Nursery, Gajanuru, Shivamogga district, Karnataka. The site lies in the Southern Transitional Zone (Zone 7) of Karnataka at an altitude of 570-588 m above mean sea level. The mean maximum temperature during the experimental period was 32.32 °C, the mean minimum temperature was 19.28 °C and relative humidity ranged from 32.50 to 91.50 per cent. The experiment was laid out in a Randomized Complete Block Design (RCBD) with three replications. There were 14 treatments, 20 grafts per treatment and 840 grafts in total were taken for the experiment. The treatments comprised of viz., T 0 -Control (No dip), T 1 -Water dip, T 2 -NAA @ 10 ppm, T 3 -NAA @ 20 ppm, T 4 -NAA @ 30 ppm, T 5 -BAP @ 10 ppm, T 6 -BAP @ 20 ppm, T 7 -BAP @ 30 ppm, T 8 -GA 3 @ 5 ppm, T 9 -GA 3 @ 10 ppm, T 10 -GA 3 @ 15 ppm, T 11 -Biovita @ 2500 ppm, T 12 -Biovita @ 5000 ppm and T 13 -Biovita @ 7500 ppm. Grafting was carried out during November-December using 3-4 months old M. integrifolia scions and 5-6 months old M. tetraphylla rootstocks were used for grafting with the following treatment details. The one-season-old, pencil-thick, healthy scion shoots were selected from regular-bearing mother plants. Leaves were removed while retaining the petiole and curing was carried out ten days before grafting. Scions were collected during the morning, wrapped in moist cloth and grafted on the same day after treatment with 0.1 per cent Bavistin. Wedge grafting was followed, the scion was dipped in the respective bio-regulator solution and inserted into the V-shaped cleft of the rootstock. The graft union was secured with polythene strip and grafts were maintained under polyhouse conditions. Observations were recorded at monthly intervals at 60, 90, 120 and 150 days after grafting (DAG), with sprouting percentage additionally recorded at 30 and 45 DAG. Days to sprouting, sprouting percentage, graft success, scion girth, scion length and graft survivability were considered. Fisher's analysis of variance was used following Panse et al. (1967) [4] and critical difference (CD) values were worked out at P = 0.05 wherever the F-test was significant.

Results and Discussion

The bio-regulator treatments significantly influenced the selected grafting and growth parameters. Results and their interpretation are presented together for each parameter. Among all the different bio-regulator treatments evaluated, the scion treated with (T 9 ) GA 3 @ 10 ppm recorded the earliest sprouting (22.86 days), followed by T 8 (24.13 days) and T 10 (24.14 days), whereas T 0 required the longest period (36.47 days) (Table 1.). Sprouting percentage was highest in T 4 (NAA @ 30 ppm) at 30, 45 and 60 DAG (60.84, 63.46 and 65.84%, respectively), while T 0 recorded 0.00, 27.15 and 31.82 per cent. The earlier sprouting under GA 3 may be associated with its role in promoting shoot elongation and active growth. Castro et al. (1991) [5] reported growth-promoting effects of GA 3 in macadamia rootstocks. The higher sprouting percentage under NAA may be related to auxin-mediated callus development and vascular reconnection at the graft interface processes, important for graft establishment (Pina et al., 2005) [6] . ). At 60 days after grafting (DAG), T 4 (NAA @ 30 ppm) recorded the highest graft success (61.84%), followed by T 3 (NAA @ 20 ppm) (58.42%), whereas the control (T 0 ) recorded the lowest (30.48%). At 90 DAG, T 4 recorded the maximum success rate (61.84%), followed by T 3 (58.42%), while T 0 recorded the minimum (32.64%). At 120 DAG, the highest graft success was recorded in T 4 (64.22%), followed by T 3 (63.16%), whereas T 0 recorded the lowest (34.22%). At 150 DAG, T 4 maintained the highest graft success (64.04%), followed by T 3 (62.98%), while T 0 recorded the minimum (34.06%). The superior performance of NAA may be attributed to enhanced cell division, callus proliferation, cambial activity and vascular tissue differentiation at the graft interface. Similar findings were reported by Sharma et al. (2019) [7] , Castro et al. (1991) [5] , Choudhury et al. (2021) [8] , Aminzadeh et al. (2013) [9] , Ahmed et al. (2012) [10] and Pina et al. (2005) [6] . 3.). Before grafting, scion girth ranged from 4.08 to 4.31 mm. At 60 days after grafting (DAG), T 4 (NAA @ 30 ppm) recorded the maximum scion girth (8.18 mm), followed by T 3 (NAA @ 20 ppm) (6.10 mm), whereas the minimum was recorded in T 0 (Control) (4.89 mm). At 90 DAG, T 4 recorded the highest value (8.31 mm), followed by T 3 (7.62 mm), while T 0 recorded the lowest (5.06 mm). At 120 DAG, T 4 recorded the maximum scion girth (8.96 mm), followed by T 3 (7.52 mm), whereas T 0 recorded the minimum (5.19 mm). At 150 DAG, T 4 recorded the highest scion girth (9.10 mm), followed by T 3 (8.84 mm), while T 0 recorded the lowest (5.51 mm). The enhanced girth of scion under NAA may be attributed to increased cambial activity, callus proliferation and vascular differentiation. Similar findings were reported by Choudhury et al. (2021) [8] , Castro et al. (1991) [5] , Hartmann et al. (2018) [11] and Ahmed et al. (2012) [10] . 4.). At 60, 90, 120 and 150 days after grafting (DAG), T 9 (GA 3 @ 10 ppm) recorded the maximum scion length of 19.58, 28.30, 34.50 and 37.68 cm, respectively, whereas the control (T 0 ) recorded the minimum values of 9.47, 12.51, 18.23 and 20.28 cm, respectively.

The enhanced scion elongation under GA 3 may be attributed to stimulation of cell division and elongation, rapid extension of internodal tissues and improved translocation of assimilates towards actively growing tissues. The present findings are in accordance with Castro et al. (1991) [5] , Choudhury et al. (2021) [8] , Aminzadeh et al. (2013) [9] and Ahmed et al. (2012) [10] , who reported improved shoot growth following growth-regulator application. [7] , Pina et al. (2005) [6] and Ahmed et al. (2012) [10] , who highlighted the importance of auxins, callus formation and vascular reconnection in successful graft establishment.

Conclusion

The present investigation demonstrated that bio-regulators influenced graft establishment, scion growth and survivability in macadamia nut. NAA @ 30 ppm (T 4 ) was the most effective treatment for sprouting percentage, graft success, scion girth and graft survivability, whereas GA 3 @ 10 ppm (T 9 ) was most effective for reducing days taken for sprouting and increasing scion length. Therefore, NAA @ 30 ppm is promising for graft establishment and survivability, while GA 3 @ 10 ppm is promising for early sprouting and vigorous scion growth under the experimental conditions.

References

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