Ecophysiological studies and tree manipulation for maximisation of yield potential in avocado (Persea americana Mill.)
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Date
1994
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Abstract
Tree fruit crops generally consist of scion and rootstock components, which through
interactive synergism affect tree performance. Coupled with tree architecture, sink/source
relationships (both spatial and temporal), genotypic responses to environments, and carry-over
seasonal effects present a high level of complexity which often confounds research results.
The development, description and use of pheno/physiological models as research and crop
management tools is a new holistic approach to reduce complexity and improve understanding
of the critical factors which influence crop productivity.
A pheno/physiological model is described for cv. Hass avocado growing in a cool, mesic
subtropical environment in S.E. Queensland, Australia. Seasonal shoot and root growth had
bimodal periodicity with root growth offset and delayed with respect to shoot growth. The
priority sink strength of developing shoots compared with roots was confirmed with 14(C)
studies. Root growth in summer extended through until late winter when there was a
substantial decline following anthesis - a critical time in fruit development with competition
between reproductive and vegetative sinks for limited resources. Delayed harvesting of fruit
over several seasons resulted in alternate bearing patterns, while removal of fruit at the
minimum legal maturity of 21 to 24% dry matter sustained successive high yields. With cv.
Hass, production was directly related to starch concentrations in trunks or shoots in July (midwinter)
immediately prior to anthesis. However, seasonal starch concentration fluxes in
trunks were much lower in coastal subtropical Australia compared with those previously
reported from interior areas in more southerly latitudes (7.5% vs. 18% maximum). Current
assimilate from over-wintered leaves was necessary to bridge the gap in early spring between
the depletion of starch reserves by new reproductive and vegetative shoot growth, and the
sink/source transition of the spring shoot growth.
Net CO2 assimilation of summer grown leaves reached ca. 17 µmol CO2 m(-2) s(-1),
approximately twice as high as previously reported rates on container-grown plants or trees in minimum temperatures were < 10⁰C for 50 days, this being the first report of this
phenomenon in field-grown avocado trees. Partial recovery occurred prior to senescence of
previous season's leaves in spring after minimum temperatures increased above 10⁰C. The
plasticity of the light response was high with the compensation point for net CO2 assimilation
at 30 µmol quanta m(-2) s(-1) and the light saturation point at 1270 µmol quanta m(-2)
s(-1). Net CO2
fixation from fruit photosynthesis was always less than losses through respiration but was
highest during the first few weeks of ontogeny, perhaps contributing to the fruit's own carbon
economy at a time when competition for assimilates was greatest. In general, CO2
assimilation studies with current technology applied to orchard trees in non-restrictive soils
have elucidated efficiencies more akin to deciduous than evergreen trees - thereby
compensating for short-lived leaves and energy expensive fruits.
Pheno/physiology models were used to substantiate the most effective timing for trunk
injection of ambimobile phosphonate fungicides for the control of Phytophthora root rot, a
serious disease of avocados, viz. at the completion of the leaf expansion phases when leaves
were strong net exporters. Preliminary studies demonstrated potential yield increases when
the assimilation efficiency of photoinhibited over-wintered leaves was improved through
increased nitrogen concentration, and spring shoot growth was partially suppressed with foliar
sprays of the growth retardant paclobutrazol.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1994.
Keywords
Avocado., Avocado--Physiology., Avocado--Growth., Theses--Horticultural science.