Proceedings
of The World Avocado Congress III, 1995 61-70
STRATEGIES
FOR MAXIMISING AVOCADO PRODUCTIVITY: AN OVERVIEW
B.N.
Wolstenholme
Department
of Horticultural Science University of Natal Pietermaritzburg,
South
Africa
A.W.
Whiley
Department
of Primary Industries Queensland
Maroochy
Horticultural Research Station
Nambour,
Queensland, Australia
Summary
The extent of the avocado productivity problem varies
regionally. In spite of greater tree vigour in the warm, humid subtropics,
sustained yields of 20 to 25 t ha-1 have been achieved. More typical
yields in the cooler, semi-arid winter rainfall areas are 8 to 12 t ha-1.
A target sustainable yield with current germplasm of 30 t ha-1 still
appears realistic. The majority of growers average less than half of these
yields.
Causes of low yield are both genetic and climatic,
with the latter recently quantified in Israel. A concerted breeding and
selection program is necessary to further modify residual evolutionary
adaptations which are counter-productive in orchards. Scion requirements should
emphasise yield, fruit quality, tree complexity and semi-dwarf growth;
rootstocks must have both disease tolerance and impart favourable horticultural
characteristics. A growing perception that phosphonate trunk injection
technology is less effective than when initially developed requires
investigation. ,
The expanded pheno-physiological growth model of
Whiley (1994) sheds new light on productivity constraints in the humid
subtropics. Fruit set can be limited not only by temperature and moisture
stress, but also by reduced photosynthetic efficiency of over- wintered leaves,
and attrition of feeder roots during flowering. Stored carbohydrates become
increasingly important as environmental stress increases. Opportunities for
managing shoot vigour at critical periods, optimising balanced root, shoot and
fruit relationships are discussed. Evidence of cross-pollination benefits must
be reconciled with orchard practicalities.
Orchard design and canopy management strategies
remain controversial. Initial high density plantings (200 - 800 tree ha-1)
are generally accepted, usually with one or two orchard thinnings at onset of
crowding. Staghoming, hedgerowing and tree containment by pruning or chemicals,
to optimise light interception and canopy bearing throughout the productive
life of the orchard, would benefit from ecophysiological research. Guidelines
from deciduous orchards are mentioned for possible relevance to avocado.
Additional Index Words: Persea americana Mill., breeding,
pheno-physiological model, canopy management.
1. Introduction
Globally, avocado production is dominated by Mexico
and tropical countries. Its coming-of-age as a world fruit crop has however,
been facilitated by research conducted in subtropical countries or areas,
notably California and Israel (relatively cool, semi-arid Mediterranean
climates), and South Africa and Australia (warm, humid, summer rainfall
subtropics), although the research base in many other countries is expanding
rapidly. This overview of productivity constraints and strategies to maximise
productivity will have a subtropical, southern hemisphere bias, but the main
principles should be globally applicable with modification.
2.
The avocado yield problem
The avocado yield problem was discussed in our paper
at the Second World Avocado Congress (Wolstenholme and Whiley,
1992). National
average yields are in the 4-8t ha-1
range, depending on the proportion of young trees. Good growers in the
cool, semi-arid, winter-rainfall subtropics can average 8-12 t ha-1,
or 12-16 t ha-1 in the warmer, humid summer-rainfall subtropics.
Respective averages for best growers are 12-15 and 20-25 t ha-1. Our
target yield of >30t ha-1 still appears realistic, but probably
unattainable on a large scale over a period of years with current germplasm and
technology.
2.1 Main reasons for low productivity
2. 1. 1 Evolutionary adaptive
strategies
The avocado tree (Guatemalan and Mexican subspecies
or ecological races, Bergh and Ellstrand, 1986) originated in subtropical to
highland tropical mesic rainforests of Mexico and Guatemala. It has a long
history of use and the fruit has undoubtedly been vastly improved by indigenous
selection. However, according to modem criteria cultivar selection is still in
its infancy and the tree has many residual features that are counter-productive
to the requirements of a modem orchard.
This potentially large canopy tree has
ecophysiological features suggesting a late successional to climax adaptive
strategy. It can be characterised as a small gap species, shade-tolerant in the
juvenile phase (Whiley, 1994). The vegetative bias, antagonistic to fruiting,
is still evident in vigorous cultivars on invigorating rootstocks under
favourable growing conditions. Rhythmic growth in two or three flushes is
associated with surprisingly short-lived leaves which are readily abscissed
under stress (including shade). Normally a wintergreen tree, it can become
deciduous or semi-deciduous during flowering under conditions of water, cold,
overbearing, salinity, or Phytophthora- induced stress (Whiley and Schaffer,
1994). The shallow, fibrous but desiccation- sensitive feeder root system, with
its high oxygen requirement and intolerance of saturated soils, is an
adaptation to "litter feeding" on the forest floor.
Associated with these factors is an energy-expensive,
oil-storing fruit with a large carbohydrate-rich seed, collectively imposing a
relatively low ceiling on yield (Wolstenholme, 1986, 1987). In its native
habitat, fruiting was much delayed and irregular. In response to moderate cold
and drought stress, accompanied by high (for an evergreen tree) starch storage
in the above-ground structural framework, heavy flowering led to strong crop
load adjustment through spring and summer fruit drop episodes, so that ultimate
fruit set is only about 0.001% of the flowers produced. A heavy crop however
severely depletes stored carbohydrates and sets the scene for alternate hearing
(Scholefield et al., 1985).
2.1.2 Tree architecture and
branching pattern
The main features of Rauh's growth model (Halle et al., 1978) affecting
avocado productivity relate to the potential for vigorous growth which may
compete with reproductive growth at critical periods. The model specifies that
although flowering is on the periphery of the tree from over-wintered shoots,
it is actually pseudoterminal, as a terminal bud can give rise to competitive
vegetative growth in the spring flush. Such indeterminate flowering panicles
certainly predominate in existing cultivars (Thorp et al., 1994; Whiley, 1994)
but determinate, entirely reproductive flowering panicles are also present. The
required partial separation of vegetative and reproductive growth peaks, to
reduce competition for resources therefore varies among shoots within the tree
(Verheij, 1986).
Spatial separation will be absent in indeterminate,
and partial in determinate inflorescences. Temporal separation will be minimal
in indeterminate, and substantial in determinate inflorescences. It is widely
accepted that fruit set and retention is lower in indeterminate inflorescences
(Blumenfeld et al., 1983; Whiley, 1994), especially in conditions promoting
spring flush vigour. Whiley (1990) established that the sink- source transition
in developing spring shoots takes ca 27 days during which most of the initially
set fruit drops. The fact that determinate flowering shoots set and hold more
fruit, and also produce larger fruit (Whiley, 1994), suggests that competition
for both current
and stored
carbon is substantial, and that determinate shoots are advantaged. The tree
architecture studies of Thorp and Sedgley (1993) and Thorp et al. (1994), while
contributing to our understanding of avocado branching patterns, need to be
related to the yield potential of different cultivars under varying
environmental conditions.
2.1.3 Environmental
constraints
The recent study of
Lomas and Zamet (1994) has highlighted climatic parameters limiting Israeli
national yields over a 27 year period, when the mean yield was 6.8 ± 2.5 t ha-1
with an annual C.V. of 36.7%, and a range from 1.4 to 12.9 t ha-1.
Seasonal (winter) rainfall reduced yield (r2 = 0.43) by an average 7
kg ha-1 mm-1 rainfall, probably through poor soil
aeration and root function, lower soil temperature, changed soil nitrogen
cycle, and alternative flowers (weeds etc) during pollination. They also noted that
frost (over 6 years), and especially the lowest minimum temperature being below
-1.5ºC, strongly reduced yield (± 2.4 t ha-1 ºC-1 below
-1.5ºC). Other factors reducing yield were low spring (flowering) temperatures (five night av.
min. <10ºC); and low soil temperature (low root activity?) in March
(September in SH) before peak flowering. A high spring heat stress index
(maximum temperature >33ºC) and high vapour pressure deficit during fruit
set were also detrimental.
2.1.4 Other factors
Historically, avocado trees in their native
habitat did not have to cope with Phytophthora cinnamomi, the root rot
fungus. The introduction of this pathogen into nurseries and orchards proved to
be devastating to tree health and productivity into the late 1970's. The
development of trunk injection technology of phosphonate fungicide has greatly
reduced the impact of Phytophthora root rot (Darvas and Bezuidenhout,1987; Pegg
et al., 1987; Whiley et al., 1995). However, more recently there are growing
perceptions that it is becoming less effective.
There is no doubt that other diseases (sunblotch
viroid, Verticillium wilt, fruit diseases) and a range of insect pests have the
potential to limit production, but control is relatively simple. Similarly,
poor technology and understanding of the needs of the crop along with extreme
climatic events including hail, wind and drought, contribute to low yields.
3.
Ecophysiology and pheno-physiology: widening the knowledge base'
In the past five years, our knowledge of avocado
ecophysiology has increased dramatically, as exemplified in the review by
Whiley and Schaffer (1994). The original phenological growth model of Whiley et
al. (1988), which assisted in guiding research, extension and orchard
management, has been expanded into a pheno-physiological model (Whiley, 1994;
Whiley et al., 1995), which has already led to innovative new manipulatory
strategies to increase productivity.
Modem portable instrumentation allows us to report
on carbon dioxide assimilation (A) for single, well-lit avocado leaves
with confidence (Whiley, 1994; Whiley and Schaffer, 1994). Full leaf expansion
occurs ca 30 days after bud-break, with sink-source transition at 80%
leaf expansion (24 days). Leaves reach Amax at ca 50 days. Orchard
trees in warm, mesic climates on good soils have a leaf Amax, of 17
μmol m-2
s-1,
which is in the high range compared with other fruit trees. Cold temperatures
(< 10ºC) result
in
photoinhibition and some loss of chlorophyll, with Amax dropping to
10 μmol m -2 s-1. Light
compensation point is 30 μmol m -2 s-1, indicative of shade tolerance, while light saturation
of A occurs at 1270 μmol quanta m-2 s-1 in (ca 60% of full sunlight).
However, these data are preliminary and there is a need to develop whole tree
models to provide insights into how best orchard canopies can be developed and
maintained for optimum production.
For a 'Hass' shoot in the spring flush, Wiley (1994)
found that the sink phase of leaves lasted 42 days, during which 86% loss of
initially set fruits occurred. The carbon budget of whole avocado trees under
orchard conditions has not been studied. The best integrator available is the
seasonal starch cycle, popularised by Scholefield et al. (1985) for a cold, stressful
climate where avocado trees are less vigorous, accumulate higher starch levels,
are semi-deciduous, and where fruit set is more dependent on stored
carbohydrate than current photosynthate than in the humid, warm subtropics
(Whiley, 1994). There is abundant evidence that excessive crop load, long
delayed harvest, climatic upsets and poor management can lead to severe
carbohydrate depletion and initiate alternate bearing, which is then difficult
to control unless drastic measures are taken (eg. pruning, fruit thinning)
(Whiley, 1994).
4.
Opportunities for orchard and tree manipulation
The
alternating (complementary) synchronous dichogamy (protogyny) of avocado
flowering appears to be an outcrossing mechanism, albeit with a fail-safe back-
up (Gazit, 1977; Davenport, 1986). Numerous studies have shown that
environmental factors modify flower opening. Low temperatures disrupt group
'13' cultivars ('Fuerte', 'Sharwil', 'Ettinger') more than group 'A' cultivars
('Hass', 'Gwen', 'Pinkerton', 'Reed') (Sedgley, 1977; Whiley and Winston,
1987), partly due to slower pollen tube growth (Sedgley and Grant, 1983). The
demise of 'Fuerte' in California was mainly due to low yield under cool
flowering conditions. Whiley and Winston (1987) were able to predict the
performance of 'A' and '13' group cultivars in different areas of Australia,
based on mean temperatures during flowering. It is now accepted in the
subtropics that group 'A' cultivars will yield better in cool climates, and
that 'Fuerte', 'Sharwil' etc only achieve high yields in warm (spring)
enviromnents. Lahav and Gazit (1994) point out the significant preference for
group 'A' cultivars in California, and that among the 12 new semi-commercial
cultivars selected in Israel in the last decade, only two (both late-flowering)
belong to the B group. The ability to delay flowering of group '13' cvs into a
warmer time slot (perhaps through a rootstock effect), may significantly
increase their yield potential.
The necessity for interplanting cultivars of
complementary groups and overlapping flowering times is still equivocal (Davenport and Lahav,
1992). The issue has been debated over several decades
(Bergh 1977; Gazit and Gafni, 1986; Davenport et al., 1994). The work of Degani
et al. (1989) with genetic markers shows that 'Ettinger' (B) is a good
cross-pollinator of 'Hass' (A), and that under environmental stress and
vegetative growth competition it is the hybrid progeny rather than the
self-pollinated fruitlets which develop the greater sink strength to survive
the massive abscission. By implication and common observation, solid blocks of
single cultivars (selfs) are more likely to yield adequately under low stress
conditions at fruit set. Lahav and Gazit (1994) summarise succinctly:
"Interplanting cultivars that belong to complementary flowering groups and
bloom at the same time tends to increase pollination rates and promote
cross-pollination and hence fruit set; yields are usually improved".
Widespread adoption of interplanting is prejudiced by practical issues such as
the need for closeness of pollinator and pollinated, their often incompatible
management and requirements; and also by insufficient research on the best
pollinators.
4.2 Ameliorating fruit set bottlenecks
Comprehensive pheno-physiological studies in the warm and cool humid
subtropics of S.E. Queensland by Whiley (1994) have highlighted a number of
resource limitations during the critical fruit set period. The first
comprehensive rhizotron studies identified major attrition of feeder roots from
flowering until spring flush maturity. This means reduced water and mineral
uptake, and probably also reduced root synthesis of cytokinins and gibberellins
at a time of increased demand. Over-wintered, partially photoinhibited leaves
have reduced carbon assimilation potential, while starch reserves decline
precipitously during early fruit ontogeny. Untimely and over-vigorous spring
flushing especially from indeterminate flowering shoots, aggravates the drain
on resources.
That such a situation is amenable to sophisticated
manipulation was shown by Whiley (1994). Pre-anthesis soil nitrogen application
successfully increased 'Hass' leaf N, chlorophyll concentration and A during
fruit set and early ontogeny. The subsequent and potentially competitive spring
shoot growth was controlled by a mid-anthesis foliar paclobutrazol application.
Preliminary results showed a substantial increase in yield over two seasons.
The importance of retaining over-wintered leaves until the sink:source
transition of the new cohort of spring flush leaves was also demonstrated.
4.3 Manipulation of vegetative-reproductive
competition
That
untimely and over-vigorous spring flushing can be detrimental to avocado fruit set
is overwhelmingly accepted (Blumenfeld et al., 1983; Whiley and Schaffer,
1994). Such vegetative competition has been reduced by decreased or delayed N
fertilisation and the strategic used of paclobutrazol (Köhne and Kremer-Köhne,
1987; Whiley et al., 1991). Excessive retardation of the spring flush is
counter-productive - these leaves are necessary to help meet the needs of
developing fruits (Cutting and Bower, 1990; Wolstenholme et al., 1990).
Similarly, encouragement of the summer shoot flush still seems beneficial, in
view of the high carbon demand of fruits, the short longevity of leaves
(especially stressed leaves), the need for starch accumulation during autumn
and winter; the importance of root growth during autumn and winter, and finally
the necessity for a new cohort of more efficient leaves to help cope with the
demands of flowering and fruit set the following spring.
Girdling (ringing, scoring, cincturing) is an ancient
manipulatory tool which may be coming back into fashion in the "clean and green"
modern world. It has been
researched
in Israel, Australia and recently in other countries, as a means of inducing
precocity in over-vigorous trees, increasing yield, inducing a "death
crop" in trees before tree thinning, and possibly increasing fruit size.
Effects vary according to the size of the girdle, the proportion of the tree affected, the timing of
the treatment, the health of the tree and duration of treatment (Lahav et al.,
1971; Trochoulias and O'Neill, 1976). It can be a drastic treatment affecting
not only carbohydrate apportionment but also plant growth substances, and some
degree of root starvation is inevitable. It needs to be used judiciously and
with a clear understanding of tree responses.
4.4 Canopy light interception, utilisation and
management
It
stands to reason that economic yield is a function not only of total canopy net
carbon assimilation, on a per ha basis, but also on the apportionment of that
fixed carbon among competing sinks, and particularly the relative share to the
economic end product, the fruits. Light and photosynthesis relationships of
single well-lit leaves differ dramatically from those of a full canopy, where
most leaves are shaded. Givnish (1988) notes the importance of the costs of
producing and maintaining leaves and other sinks, and of energetic trade-offs.
He notes that in Liriodendron trees the traditional leaf light
compensation point (CP) has little meaning for net carbon gain. The effective
14ecological compensation point" takes into account additional costs, e.g.
night leaf respiration, and construction of leaves and the associated support
and root tissue. Thus in a single leaf the light CP was 13 μmol quanta m-2 s-1; in a 1 m
tall tree 153 and in a30 m tall tree 1350 μmol quanta m-2 s-1 (±75% of full sunlight), i.e. the light
level at which total leaf benefits and costs just balance.
We have no comparable estimates for avocado, and the
questions of spacing, crowding, canopy surface and bearing volumes, light
requirements for seasonal carbon gain and flower initiation remain open to
innovative research. Jackson's (1985) injunction to quickly achieve a yield
ceiling is especially pertinent - the logic of discounted cash flow analysis
makes early yield more valuable economically than later yield with high real
interest rates, i.e. precocity of yield. South African researchers and growers
have perhaps been the most adventurous in translating this concept into high
density plantings, especially for 'Hass'. Köhne and Kremer-Köhne
(1990) clearly show the benefits of close initial spacing (400-800 trees
ha-1) and precocity-inducing rootstocks ('Duke 7' vs 'Martin
Grande') on cumulative yield and recovery of financial investment, at least
where land is expensive and nursery trees relatively cheap. Tree crowding is
delayed by growth retardants, and early fruiting, but initial tree thinning may
be necessary in the fifth year, and a second thinning a few years later, with
growth rates of I m p.a. possible in the humid subtropics. Further research on the
reasons for consistently high and low yielding trees (Smith and Köhne, 1992; Steyn et al., 1993)
will be instructive.
Even
with less intensive management, tree crowding is inevitable, with reduction in
light interception and loss of yield and fruit quality. Growers respond with a
variety of strategies, including chemical priming; staghorning (preferably of
orchard blocks rather than alternate rows - a form of orchard renewal or
"recycling"); hedgerowing with mechanical pruning devices; and pruning
for tree size containment (especially height reduction) and opening up the
canopy. Scientific and economic guidelines are sadly lacking, perhaps because
long-lived avocado orchards are a recent luxury (successful control of Phytophthora).
Rom (1994)
summarises light interception and utilisation guidelines for deciduous tree
orchards, discussing concepts of canopy surface area and volume, and their
optimum ratio for cumulative yield. The importance of relatively small tree
size, and of limiting tree height (also emphasised by Givnish, 1988) are
emphasised. The guidelines that tree height should not be more than 1.5 to 2
times the inter-row space (usually 2m) seems particularly important. However we
have a long way to go in evergreen fruit tree research.
5.
Conclusions
This limited overview has dealt only with certain
aspects of the avocado yield problem, and only with subtropical avocados. The
problem varies regionally, with higher yield potential (but greater management
inputs) in the humid warm subtropics. The current cultivar and rootstock
options are very restricted, and national breeding programmes for semi-dwarf
scions and rootstocks remain high priorities. Further exploration of the fast
disappearing indigenous gene pool in Central America is surely of international
concern.
Amongst the most significant advances since the
previous World Avocado
Congress
are the expansion in ecophysiological knowledge, and the development of a
pheno-physiological growth model. Sustained yields of above 20 t ha-1
have been shown to be achievable for both 'Hass' and ‘Fuerte'. However canopy
management research lags sadly. Environmental issues, including rising CO2 levels associated with
global warming, less predictable weather, and "clean and green"
resource sustainability, will test us into the next millennium. Improved
germplasm, also using the largely unexploited but potentially powerful tool of
a range of horticulturally and pathologically superior clonal rootstocks,
remain the greatest challenge.
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