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www.publish.csiro.au/journals/ajar Australian Journal of Agricultural Research, 2004, 55, 609–624
Macadamia physiology review: a canopy light
response study and literature review
D. O. Huett
NSW Agriculture, Centre for Tropical Horticulture, PO Box 72, Alstonville, NSW 2477,
Australia; email: david.huett@agric.nsw.gov.au
Abstract. The management of mature macadamia orchards has evolved largely through the need to control inter-row
crowding to maintain machinery access. The current study was undertaken to identify physiological constraints to
production and priorities for future research and development. The two components of the study were a preliminary
field study to determine the impact of current pruning and hedging management strategies on canopy photosynthetic
performance and a literature review to identify physiological issues affecting orchard productivity.
The field photosynthesis study demonstrated that emerging flush leaves have a negative light saturation net
assimilation rate (A
) that increases to that of mature leaves (A
8–10 µmol CO
.s) over 28 days. Leaf age
has no effect on A
of light-adapted leaves. Shade-adapted macadamia leaves cannot attain the photosynthetic
capacity of light-adapted leaves. This means that a late hedging strategy to remove around1mofcanopy from the
side of trees to improve orchard access reduces the photosynthetic capacity of the orchard until sufficient flushing
occurs to restore the canopy.
The literature review focussed on light interception and distribution, photosynthesis, carbohydrate and nitrogen
cycling, environmental response, flowering and fruit set, nut abscission, canopy management, and nutrition and
disease control. Light interception modelling work, which has been widely adopted for deciduous temperate fruit
crops, has immediate application to macadamia production and can explain many of the yield responses recorded
by macadamias to canopy management. Macadamia yields appear to increase up to about 96% light interception.
Variation in light distribution within canopies that affects the yield and quality of temperate crops is also a present
problem with macadamias. It leads to uneven distribution of leaf and fruit throughout the canopy and to a heavily
shaded void developing in the middle of trees. The relationship between irradiance, leaf photosynthetic capacity
and longevity, flushing, flowering, and fruit set is poorly understood. No information is available on the training of
young trees to improve light distribution and canopy photosynthetic efficiency.
The current practice of light annual hedging of mature macadamias appears to restrict the production of fruiting
wood. Earlier nut abscission can be achieved by the use of ethephon, which offers greater flexibility in the timing
of hedging. Research work is required on the effects of timing, frequency and severity of hedging on fruiting wood
production, flowering, and fruit set of macadamias. Information on the need to supplement mechanical hedging
with manual pruning to improve light distribution throughout canopies is also required.
The cyclical and highly variable nature of macadamia yields needs to be further analysed to determine whether a
consistent patter n exists and whether environmental factors are an influence. A simple measure of the storage levels
of the major assimilates, carbohydrates and nitrogen compounds, is unlikely to predict cyclical yield patterns because
both are continually cycled within the tree and it appears that, from comprehensive studies on another evergreen
fruit crop, avocado, the major supply of carbohydrate during the fr uit-filling stage is from current photosynthesis.
Several studies have failed to demonstrate irrigation responses by field-grown macadamias. The cyclical and
variable yield of macadamias demonstrates that yield responses from field experiments cannot be expected in less
than 5 years, even assuming adequate plot size, buffering, and replication. On the drier sites with light-textured soils
in south-eastern Queensland, yield responses could be expected.
Following the success of temperate fruit tree crop breeding programs and trends with other evergreen tree crops,
macadamia breeding needs to focus on dwarfing clonal rootstocks that provide uniformity, and vigour control to
improve cropping efficiency, to reduce canopy management costs and to minimise the reduction in yield following
tree topping.
Additional keywords: carbohydrate, flowering, light interception, net assimilation rate, nitrogen cycling,
photosynthesis, photosynthetic photon flux density, pruning, nut abscission, rootstocks, water requirements.
© CSIRO 2004 10.1071/AR03180 0004-9409/04/060609
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