MANAGING MACADAMIA TREE VIGOUR FOR HIGH PLANTING DENSITY
Excessive plant vigour in macadamia orchards presents several challenges to profitable and efficient operations. Failure to control excessive vigour can ultimately compromise the economic viability of macadamia orchards. The costs associated with managing vigorous growth, coupled with reduced yields and operational inefficiencies, can reduce profitability over time.
High plant vigour competes with fruit production by diverting resources away from reproductive growth. Efficient allocation of resources between vegetative and reproductive growth is crucial for maximising yield in tree crops like macadamia. On top of this vigorous growth leads to dense canopies, which shade lower branches and reduce light interception.
This limits photosynthesis and negatively impacts overall production as photosynthesis is crucial to producing energy and ultimately fruit yield. A shaded orchard floor not only reduces the growth potential of understory plants but also increases the risk of soil erosion. Loss of crops and nutrients due to soil erosion can further impact productivity and necessitate additional measures for soil conservation. Excessive growth can lead to crowding within the orchard, restricting machinery access for tasks such as harvesting and pesticide application. This can significantly reduce operational efficiency and increase labour costs.
Managing vigorous trees often requires additional inputs such as labour and expensive machinery. The need for low-density planting to prevent shading and crowding further increases production costs.
Methods of controlling tree vigour
Pruning is presently the primary management approach for controlling macadamia trees' excessive vigour. Pruning can lower tree size and open up the canopy to enable light and spray penetration, but pruning alone cannot sustainably keep trees small enough for high-density orchards without reducing yields. This is especially true for high-vigour varieties. Heavy pruning promotes vegetative growth at the expense of plant carbohydrate stores, reducing availability for flowering and fruit development.
Chemicals such as Paclobutrazol can efficiently manage plant vigour while promoting favourable fruit traits. Paclobutrazol has been shown to increase fruit size in stone fruit crops such as cherries. The use of growth regulators via drip irrigation systems provides for accurate dosage and timing control, allowing for short-term plant vigour management to regulate canopy development. This strategy reduces waste while also ensuring that the growth regulator is delivered to the tree's root zone. It's important to remember that growth retardants have various effects on different tree species and cultivars. Proper dose and application methods are crucial to minimise unwanted side effects.
Macadamia phenology schematic derived from (Smit, 2021) and (Stephenson & Gallagher, 1986b)
Pruning should be done as soon after harvest as possible. The competing central leader should be removed, and branches that overhang the lower branches should be clipped, as well as old, thick branches with minimal complicated growth on their stems.
Applying a plant growth retardant (PGR) to the soil after harvest can boost flowering and fruit production the next year. Additionally, fruit quality in other crops has been reported. Earlier nut maturity and drop may occur.
It is import that pruning and application of a PGR is done while Carbohydrate reserves are relatively high. RDI by itself will have very little impact on yield and fruit quality. Water deficits during flowering, nut set, and nut maturity phenological stages may significantly reduce but quality and total kernel recovery. It is advisable to apply RDI when the carbohydrate reserves are relatively stable (just after nuts begin to drop). With the introduction of dwarfing rootstocks which limit water flow, the efficacy of RDI will be improved.
Regulated Deficit Irrigation (RDI) is a water management strategy aimed at inducing controlled water stress in plants, optimising water use efficiency while minimising negative impacts on yield and quality. RDI can be timed to coincide with important growth periods in macadamia trees. Water stress, for example, can be used during periods of high shoot growth to limit vegetative growth and redirect resources towards reproductive growth and fruit development.
Progress in Breeding for Reduced Vigor
A rootstock's ability to manage tree vigour has been recognised as a crucial benefit for fruit-crop operators since it lowers care costs, enables high-density plantings, increases yield per unit area, and improves orchard efficiency and productivity. Dwarfing rootstocks can allocate more resources to reproductive growth rather than vegetative growth, perhaps leading to early fruit output and higher fruit quality. This might result in a faster return on investment for growers. Other advantages include a lesser demand for inputs such as water, fertilisers, and pesticides, resulting in a smaller environmental effect and increased sustainability of macadamia orchards. Breeding for rootstocks with lower vigour allows for the selection of additional traits such as enhanced tolerance to varied environmental conditions, such as soil type, water availability and wind.
Some of the present challenges for breeders include extensive selection processes due to the trees' protracted juvenile stages and modest growth rates. Rootstock vigour is impacted by numerous genetic variables, making it difficult to detect and choose desirable qualities using conventional breeding approaches. Understanding the genetic basis of vigour and using molecular markers for selection helps speed up the breeding process. Successful grafting and long-term orchard performance require compatibility between rootstocks and scion cultivars. Breeders must consider compatibility with existing scion cultivars to enhance utility. Macadamia cultivars have a relatively small genetic foundation, with the majority deriving from M. integrifolia. Expanding genetic diversity by adding wild species like M. jansenii and M. ternifolia gives both opportunities and problems in terms of possible characteristics of interest and overcoming barriers like cyanogenic glycosides. Incorporating genomic methods like marker-assisted selection (MAS) and genome-wide association studies (GWAS) into rootstock breeding projects necessitates investment in infrastructure and knowledge. However, these technologies have the potential to accelerate genetic gain and increase the efficiency of breeding efforts.
Possible mechanisms of rootstock-mediated vigour control in tree crops (Dhakal, et al., 2023) .
The diagram shows possible ways that rootstocks can regulate the growth of scions. The graft union plays a crucial role in regulating water flow, preventing water stress, and promoting plant growth in scions.
Reduced water flow via the graft union can be caused by either a lack of water being delivered from the root system or graft union anomalies that impede water transportation. Experiments conducted by Giulivo and Bergamini, have verified the notion by showing that trees grafted onto dwarfing rootstocks had lower stem water potential than trees grafted on strong rootstocks (Giulivo & Bergamini, 1982). This suggests that there is less water available in scions grafted onto dwarfing rootstocks. Dwarfing rootstocks have smaller xylem vessels, which reduces water flow to the scion. Hydraulic conductance, a fundamental metric characterising tree water status, is identified as a critical component of rootstock-scion water relations. The dwarfing effect may be caused by a decrease in hydraulic conductivity, which results in less water flow to the scion. The link between hydraulic conductivity and vigour management by rootstocks has been shown in a variety of fruit crops, including apple, pear, peach, olive, grape, and citrus. Low-vigour rootstocks often have poorer hydraulic conductivity than vigorous rootstocks.
Dwarfing rootstocks have smaller xylem vessels, which will reduce water flow to the scion. Hydraulic conductance, a fundamental metric characterising tree water status, is identified as a critical component of rootstock-scion water relations.
The dwarfing effect may be caused by a decrease in hydraulic activity, which results in less water flow to the scion. The link between hydraulic conductivity and vigour management by rootstocks has been shown in a variety of fruit crops, including apple, pear, peach, olive, grape and citrus. Low-vigour rootstocks often have poorer hydraulic conductivity than vigorous rootstocks.
Timing of vigour manipulation considering early vigour
Early vigour refers to plants' rapid and vigorous growth in the early stages of development, usually in the juvenile phase. It exhibits aggressive vegetative growth, which includes the formation of strong stems, vast root systems, and rapid canopy expansion. Genetic factors, climatic conditions, and management approaches all have an impact on early vigour, which is critical for orchard establishment, crop output, and stress tolerance. Early vigour is especially crucial in tree crops like macadamia for building a healthy and uniform orchard canopy, which increases early production and total productivity. Trees with high early vigour are frequently better able to compete for resources like sunlight, water, and nutrients, resulting in faster growth and greater crop output. Furthermore, vigorous root formation in the early stages improves the plant's ability to endure environmental challenges like drought or heat by enhancing water and nutrient absorption efficiency.
While early vigour is beneficial to productivity, excess vigour at maturity is negative. There may be genetic implications if major link between precocity and vigour at maturity exist. Thus, the goal is to create trees with high early vigour but moderate vigour at maturity. Understanding the correlation between EV and vigour at maturity is critical for cultivar selection, methods, and timing of vigour management. Further studies on the relationship between early and mature age vigour, as well as precocity, will provide crucial insight for breeding for scion and rootstock varieties, cultivar selection and vigour control strategy to maximise profitability and sustainability of high-density macadamia orchards.
Managing the vigour of macadamia trees in high-density orchards is critical for increasing production, resource utilisation, and long-term profit. Excessive vigour can cause problems such as decreased light interception, inefficient resource allocation, and difficulty in orchard management. Growers may, however, effectively regulate vigour by combining strategic techniques and novel approaches to maximise output and quality while minimising operational constraints.
References
Dhakal, P. et al., 2023. Macadamia Breeding for Reduced Plant Vigor: Progress and Prospects for Profitable and Sustainable Orchard Systems.. Sustainability, 15(19).
Giulivo, C. & Bergamini, A., 1982. Effect of rootstock-scion combination on water balance of apple tree, cv Golden Delicious.. Hamburg , F.R.G., Book of abstracts, 21st Int. Hon. Cong., .
Glisić, I. et al., 2016. Tree vigour and yield of plum grown under high density planting system.. Acta Hortic. , Volume 1139, pp. 131-136.
Jerie, P., van den Ende, B. & Dann, I., 1989. Managing tree vigour and fruitfulness in decidious orchards. s.l., ISHS Acta Horticulturae 240: Australian Temperate Fruits Review Conference.
Moutier, N., Ricard, J. & Le Verge, S., 2011. Vigour control of the olive tree in a high density planting system: Two experimental approaches. Acta Horticulturae , Volume 924, pp. 185-193.
Olien, W. & Lakso, A., 1986. Effect of rootstock on apple (Malus domestica) tree water relations. Physiologia Plantarum, 67(3), pp. 421-430.
Smit, A., 2021. The impact of water stress at different phenological stages on the, Pretoria : University of Pretoria.
Stephenson, R. & Gallagher, E., 1986b. Effects of temperature during latter stages of nut development on growth and quality of macadamia nuts. Scientia Horticulturae. Scientia Horticulturae, Volume 30, pp. 219-225.