Proceedings
of The World Avocado Congress III, 1995 71-75
PHENO-PHYSIOLOGICAL
MODELLING IN AVOCADO - AN AID IN RESEARCH PLANNING
Anthony
W. Whiley and Jack B. Saranah
Maroochy
Horticultural
Research Station Department of Primary Industries
Queensland,
Nambour 4560 Australia
B.
Nigel Wolstenholme
Department
of Horticultural Science University of Natal
Pietermaritzburg
3209 South Africa
Abstract
A revised and expanded pheno-physiological model has
been developed for cv. Hass growing in a cool, humid subtropical climate in
south-east Queensland. The model quantifies the seasonal growth activities of
reproductive and vegetative components of the tree, details seasonal changes in
leaf nitrogen and chlorophyll concentrations, and records changes in the photoassimilation
efficiency of leaves from summer through to spring. The pheno-physiological
model has assisted in the development of disease prevention strategies and
limitations to production have been identified. Studies based on this
information have been implemented and successful outcomes achieved. These are
discussed in relation to a whole-tree approach to research and development of
avocado in subtropical regions.
1.
Introduction
Plant growth responses are largely precipitated by seasonal
environmental changes which either induce or release stress thereby promoting
associated changes in physiological processes. Ultimately this leads to a
progression of the cropping cycle or a sequence of phenological events which
normally occur in an orderly manner. In avocado (Persea americana Mill.)
tree phenology was first reported in the late 1950's (Chandler 1958; Venning
and Lincoln, 1958) with additional contributions to the knowledge base during
ensuing years (Kotz6, 1979; Wolstenholme, 1981; Davenport, 1982). However, the
first detailed conceptual model was not published until 1988 (Whiley et al.,
1988). This model has since been refined with the incorporation of
physiological data providing a more in-depth interpretation of tree growth and potential
yield limitations. We report on the use of this model as a tool for developing
new initiatives in research to enhance production in subtropical climates.
2.
Materials and Methods
A
commercial 'Hass' avocado orchard at Maleny in S.E. Queensland (latitude
26.50S, altitude 520 m) was chosen for the pheno-physiological study. The
climate is cool,
mesic subtropical with a high mean annual rainfall of 2000 mm in a summer/wet:
winter/dry pattern.
During 1988 'Velvick', a Guatemalan race seedling
selected in Queensland, was clonally propagated and grafted to 'Hass' scions.
In March 1989 five trees were planted in central positions 1 m from windows of
a rhizotron facility especially constructed for the study and described by
Whiley (1994). At the time of planting, under-tree mini- sprinklers (10 1 hr-1)
were installed at each site to supplement natural rainfall. Trees were
fertilized according to the schedule for tree age developed for avocados
growing in S.E. Queensland (Banks 1992).
Root measurements were made only of the white,
unsuberised "feeder" roots. Data were collected at monthly intervals
by tracing the outline of white roots visible at the soil-panel interface onto
transparent sheets of acetate with a black indelible pen. The information on
the acetate sheets was digitized by scanning to an electronic file using a
Hewlett Packard ScanJet Ilc. Root lengths were determined by computerized image
analysis (Sci-Scan Image Analysis System, Delta T, UK). This method gave a
total length (m) of visible white root at the soil-panel interface (0.58 m2 vertical window area).
Reproductive and vegetative phenology data were collected for four seasons at
monthly intervals, or more frequently when necessary, using a modified system
of Whiley et al. (1988) described by Whiley (1994).
The determination of nitrogen concentration was made
from 15 of the most recently matured summer-flush leaves selected at monthly
intervals from May until December 1992. After preparation nitrogen was measured
following a Kjeldahl digest.
Photoassimilation studies were carried out from
March to December 1992 on five fully expanded sun-exposed leaves on the
northern side of each tree. C02 assimilation (A) was measured at monthly intervals with a LICOR LI-6200
portable photosynthesis meter (LICOR, Nebraska, USA). All measurements of A
were made at or above light intensities of 1200 μmol quanta m-2
S–1 and between 0830 to 1030 h, a low stress time of day.
Chlorophyll concentrations were determined at monthly intervals from five
leaves on the northern side of the trees using the technique described by
Proctor (1981).
3.
Results and Discussion
The leaf concentration flux of nitrogen showed significant changes which could be related to critical stages of tree phenology (figure 1). Nitrogen concentration remained relatively stable from April until July, a period of extended quiescence in the canopy of the tree (figure 1a). However, there was a sharp decline during the growth of inflorescences probably due to remobilisation to support this adjacent sink, with recovery during anthesis but declining once more during fruit set and spring shoot growth. The decline in leaf N concomitant with fruit set and shoot growth has similarly been reported in citrus, where it was concluded that young vegetative flushes draw nitrogen from reserves in old leaves (Erner, 1988).
Net CO2 assimilation of sunlit summer-flush leaves reached its highest rate in
April (18.3 CO2 μmol m-2 S–1)
and then slowly declined through to May (figure 1 a). By June there was a rapid
decline in A which remained at about 10 μmol CO2 m-2 S–1 through to October. There
was a small recovery in A by November followed by a rapid decline as
leaves senesced. Except for a lag phase going into winter the pattern of
chlorophyll concentrations in leaves substantially mirrored A (figure 1 a).
The autumn/winter decline in A can be attributed to
at least four factors: increasing vapour pressure deficits (Schultze, 1986);
end product feedback-inhibition due to increasing leaf starch concentrations
(Schaffer et al., 1987); low temperature photoinhibition of photosystern II
(Smillie and Hetherington, 1983); and reduced leaf chlorophyll and N
concentrations (DeJong, 1982; Syvertsen, 1984). It is likely that all of these
factors contribute to the depression of A to a greater or lesser degree
during winter and spring.
The pheno-physiological model developed for 'Hass'
identified seasonal interactions between vegetative (roots and shoots) and
reproductive (flowers and fruit) components of the tree (figures 1 b and 1 c).
The offset bimodal cycle of root and shoot growth (sink phase) identified in
this study was shown to be critical with respect to the timing and distribution
of hunk injected phosphonate for Phytophthora root rot control in subtropical
climates (Whiley et al., 1995). Injection following the sink:source transition
of major flush periods when roots become strong sinks are the most effective
times for increasing phosphonate concentrations in roots and treatment is
application is based on key phenological events.
With respect to tree productivity one of the most
critical periods determining both yield and fruit size occurs immediately
following flowering when many fruits are dropped from the tree (figure 1 c).
Competition for resources between developing fruit and new shoot growth occurs
at a time when A (figure I a) and root growth (figure 1 b) are depressed.
Studies by Whiley (1994) have determined that in warm, humid subtropical
climates assimilates from current photosynthesis as opposed to stored sources
are critical for fruit retention and growth. From this identified deficiency of
low A during the critical spring period, research has demonstrated that by
increasing leaf N through soil applications of urea immediately prior to
flowering in conjunction with mid-bloom foliar applications of paclobutrazol
(Whiley et al., 1990), significant increases in 'Hass' yield are obtained. The
replacement of leaf nitrogen and the suppression of new shoot growth (by paclobutrazol)
results in higher A rates in the over- wintered canopy presumably increasing
the carbon supply to developing fruits.
4.
Conclusions
The
development of pheno-physiological models has assisted in interpreting tree growth
providing a greater understanding of temporal relationships between the
components of the tree with relation to growth and their physiological status.
This has been used to advantage in developing strategies to control
Phytophthora root rot and to improve fruit set, retention and yield. However,
despite this considerable improvement in our knowledge, sustainable yield
remains lower than the potential target of about 32 t ha-1 estimated
for avocados by Wolstenholme (1986). It is likely that we are reaching the
genetic limits of current cultivars and the next step forward will only occur
with improved rootstocks and varieties.
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Figure 1. Pheno-physiological model for cv. Hass on
cloned 'Velvick' rootstock growing at Maleny, S.E. Queensland where: (a) is the
seasonal change in leaf nitrogen (N) and chlorophyll concentrations and C02 assimilation (A); (b)
is the seasonal relationship between shoot and root growth; and (c) is the
relationship between floral development and fruiting. Data points are mean
values from five trees ± vertical SE bars which are obscured by symbols at some
points. Redrawn from Whiley
(1994).