South African
Avocado Growers’ Association Yearbook 1987. 10:58-61.
Proceedings of
the First World Avocado Congress
Theoretical and applied
aspects of avocado yield as affected by energy budgets and carbon partitioning
BN WOLSTENHOLME
Department of Horticultural Science,
University of Natal, Pietermaritzburg 3200, RSA
SYNOPSIS
Average avocado yields are low
compared to other fruit crops because of the high energy cost of producing
fruits with a high oil content and a large seed. Other major factors are the
evolutionary history and early stage of domestication of this crop, and the
ravages of Phytophthora root rot. Implications for breeding and certain aspects
of orchard management are discussed.
INTRODUCTION
Average yields of avocado orchards are low when compared with those of
other fleshy fruits, although higher than those of nut-bearing trees. Fruit
yield in tree crops is affected by many complex, interacting factors, but the
final analysis is determined by seasonal photosynthetic efficiency, and more
particularly by the 'harvest index'. The latter is a measure of the proportion
of photosynthate which is partitioned to the fruit. Cannell (1985) states that
it is now a truism that man has increased yield mainly by increasing the
harvest index of crops.
This paper is a limited review, in part theoretic of the presumed avocado yield problem, the reasons for low yields, and implications for yield improvement. The approach is to attempt a broad, conceptual perspective, with emphasis on the subtropical avocado industry, assuming good orchard management practices.
Avocado yields are obviously affected by cultivars, rootstock,
environmental (including edaphic) factors, cultural practices and tree age.
They can be expressed as national (or state) averages, which are especially low
as they include the full spectrum of grower capability, bearing and non-bearing
trees, and often unreliable obsolete statistics. It can be calculated, for
example, that the California average since 1981 was about 4,5 t ha-1
(Anon, 1985). The state average per bearing ha for a 10-year period to 1972/73
was 5,8t (Rock, 1977). Kotze (1986) referred to a South African average of
below 5 t ha-1, while average industry yields in Israel have been
given at around 10tha-1 (Ashkenazi, undated).
A more useful concept is that of a good commercial yield. This should be
qualified for cultivar and for tree age (eg mature trees), and must be a
realistic average ideally based on at least five years' data and a reasonably
large block of land (say > 10 ha). Gustafson (1979) stated that good growers
in southern California expect between 6,7 and 11,2 t ha-1 for
Fuerte, and 7,8 to 13,4 t ha-1 for Hass. Commercial production
varied between 5,6 and 16,8 t ha-1. Bergh (1986, pers comm) gave
good grower averages for the same area as 5,6 t ha-1 for Fuerte, 9 t
ha-1 for Hass, 11,2 t ha-1 for Pinkerton and 13,4 t ha-1
for Reed.
In South Africa, a conservative yield for Fuerte of 10 t ha-1
has been used for planning purposes. In SE Queensland, an orchard of mixed
cultivars could be expected to average 12 t ha-1 with reasonable
management (Whiley, 1987, pers comm). A recent economic analysis in the
climatically favourable Tzaneen district, South Africa, used a mature orchard
average of 18 t ha-1 for growers with 'intermediate' management
capabilities (Toerien et al, 1984).
Average yields are also available for the best growers, and constitute
target yields based on the best available technology in intensively managed
orchards in favourable localities. Toerien et
al (1984) used a figure of 21,6 t ha-1; Ashkenazi (undated)
referred to over 20t ha-1 for Hass in Israel, and Gustafson (1979)
stated that excellent orchards can produce over 16,8 t ha-1.
Occasional reported yields of 30 or more t ha-1 are possible in some
seasons, but are certainly not average yields as defined earlier.
Bergh (1977) discussed factors affecting avocado fruitfulness. The
writer's approach is to take a broader perspective, emphasizing evolutionary
history, Phytophthora and energy
costs of fruiting.
Centres of origin - Some disagreement exists over the centres of origin
and nomenclature of the cultivated avocados. Bergh (1975, 1977) retained the
'horticultural race' concept (Mexican, Guatemalan, West Indian) of earlier
workers, but also elevated them to three botanical varieties of Persea americana (var drymifolia, var guatemalensis and var ameri.cana
respectively).
However, Williams (1977) on the basis of field experience, recognised
two species, each with two botanical varieties, and only two centres of origin,
viz eastern and central Mexico
(Mexican and West Indian avocados) and central Guatemala (Guatemalan avocado).
Cultigens have subsequently arisen from crossing amongst the two species (eg
Fuerte). The unfortunately-named 'West Indian' avocado is believed to have
arisen from the Mexican avocado, although it has subsequently acquired genes
adapting it to the tropical lowlands.
The centres of origin of both of Williams' species were highland,
montane 'cloud' forests or rain-forests, essentially subtropical highland in
Mexico and tropical highland in Guatemala. The Guatemalan progenitors were
often gigantic trees.
Stage of domestication - Williams (1977) cites evidence that Mexican
avocados have been used as food for 9 000 to 10 000 years, the 'West Indian'
avocado for over 4000 years, and Guatemalan avocados for perhaps 2 000 years.
They were therefore undoubtedly improved by primitive man, probably for larger
fruit size in Mexican avocados, for smaller seed size, and for eating quality.
It must however be stressed that modern man has only exerted purposeful
selection on avocados for some 100 years, The avocado is therefore only a
partially domesticated tree according to the criteria for commercial orchard
culture.
Ecophysiological consequences of a rain-forest origin have been
summarised by Possingham (1986) and Kriedemann (1986). Those applicable to
avocado trees include a shallow, extensively suberized, relatively inefficient
root system with low hydraulic conductivity, a low frequency of root hairs, and
vesicular-arbuscular mycorrhizal associations. Leaves have a high stomatal
density but a limited vascular network; high quantum efficiency but limited
photosynthetic efficiency; light saturation at 20-25 per cent of full sunlight
(C3 pathway) and a low light compensation point. These features are typical of
shade-adapted plants. Leaves can also store large amounts of carbohydrates and
minerals. Episodic growth flushes result in leaves of varying age and
efficiency.
Flowering tends to be profuse but first set low, often
< 0,001 per cent. It is controlled by strong environmental cues leading to
synchronized growth cessation (Verheij, 1986), but there is no physiological
dormant period. Cropping is often biennial or irregular.
These evolutionary adaptations were necessary for competitiveness in
native rain-forests, but many are counterproductive in the orchard situation.
The same applies to the unique flower behaviour of avocados, apparently an
adaptation to unknown pollinators and to promote outcrossing, but fortunately
with a fail-proof system for self-pollination (Davenport, 1986).
Infection with Phytophthora cinnamomi
There is no doubt that Phytophthora
infection of avocado roots, even when seemingly mild and under control, has
been a major cause of poor yields. The fungus is not native to the Americas
(Zentmyer, 1985), so that avocado progenitors were not subjected to Phytophthora selection pressure.
Evidence from Australian rain-forests suggests that biological control is
possible (Broadbent & Baker, 1974). It is perhaps unfortunate, therefore,
that this highly susceptible plant did not have the opportunity to acquire
resistance during its evolutionary history.
The physiological effects of Phytophthora
infection are severe. Infected trees have lower leaf xylem water
potentials, reduced stomatal openings and therefore lower photosynthetic rates,
and disturbed mineral uptake patterns (Sterne et al, 1978; Whiley et al,
1986). Root:shoot balances are disturbed, loss of roots leading to compensatory
loss of foliage, severely aggravated by fruiting (Wolstenholme, 1981). The
dramatic effect on yield is hardly surprising.
Most of the dry mass of plants consists of carbon compounds, with over
90 per cent derived from photosynthetic carbon fixation. The different plant
parts ('sinks') compete for assimilates, which inter alia are energy sources for growth and respiration.
The order of priority amongst the carbon sinks is usually seeds >
fleshy fruit parts > shoot apices and leaves > cambium > roots >
storage (Cannell, 1985). Fruiting therefore, has strong priority, is energy
expensive, and reduces vegetative and especially root growth (Cannell, 1971;
Chalmers & Van den Ende, 1975; Heim et
al, 1979).
The energy cost of fruiting can be estimated from total annual carbon
budgets. Unfortunately, few carbon budgets have been constructed for trees, and
most of them suffer from serious limitations. Forestry models have however
shown a high annual dry matter (and therefore energy) allocation to fine root
turnover and root respiration (Cannell, 1985). The effect of Phytophthora on avocado would be to
greatly increase this allocation in largely futile attempts to replace rotted
roots.
Wolstenholme (1986) estimated the energy content of whole avocado fruits
at maturity from published data on total carbohydrate, protein and fat
contents. Heat of combustion values of 16,74 kJ g-1 of carbohydrate,
16,53 kJ g-1 of protein, and 38,91 kJ g-1 of fat are used
for extrapolations to specific yields per ha-1, ignoring respiratory
losses during fruit growth.
It was estimated that Fuerte fruits with 17 per cent oil content and a
flesh:seed ratio of 4:1 (fresh mass) contain 8 072 million kJ tonnes-1,
compared with 2 925 and 2 628 million kJ tonnes-1 for Valencia
oranges and apples respectively. Adopting a yield target of 100 t ha-1
as attainable for intensive apple orchards, the equivalent target (potential) yield
for avocados is about 32,5 t ha-1. The fact that the best avocado
growers are only achieving two-thirds of this is an indication of the lag in
avocado as compared with apple growing technology.
It is also clear that apparently low yields of avocado orchards are due
to two main factors, viz the high oil
content (oil is 2,3 times more energy-expensive than carbohydrate), and the
large seed with concentrated food reserves. In fact, within the assumptions
adopted, it was evident that only at oil contents above 17,8 per cent was the
edible flesh more energy expensive per unit fresh mass than the seed.
IMPLICATIONS FOR AVOCADO YIELD
IMPROVEMENT
Avocado selection and breeding Perhaps of the most immediate concern,
based on conclusions reached with other crops (Gifford & Evans, 1981;
Cannell, 1985), is altered assimilate partitioning away from wood production
towards earlier, heavier and more regular fruiting. This can be attained by
semi-dwarfed trees with greater branching and compactness. Cultivars such as
Gwen and Whitsell (Bergh, 1986, pers comm) may be capable of average yields of
16,8 and 22,4 t ha-1
respectively at appropriate close spacings in southern California,
without the benefit of dwarfing rootstocks. Further testing is needed to
confirm their early promise.
Traditional breeding methods (Bergh, 1975) may not be fast enough to
make the needed impact, for example incorporating dwarfing ability in Phytophthora-resistant rootstocks. Early
application of new breeding approaches to avocado problems made possible by
biotechnological advances, mutation breeding, etc, is desirable.
In energy terms, selection for small seed size and somewhat lower flesh oil content should increase yield potential. There is however no evidence that the low oil content of tropical West Indian type avocados results in higher yields, possibly due to large seed size, and to higher respiration rates in warm climates. Furthermore, there is evidence from other crops that potential yields are set during the early, intense cell division of fruit growth by the availability of energy reserves at the time.
Phytophthora control
With the advent of chemical control of Phytophthora (Darvas et al, 1978; Darvas et al, 1983; Coffey, 1985
a; Pegg et al, 1985; Whiley et al, 1987) avocado growers have entered
a new era of healthy, vigorous trees. More permanent control should arise with
new resistant rootstocks (Coffey, 1985b). Provided that the extra vigour of
trees can be satisfactorily controlled, average yields will show marked
increases.
The early onset of a vigorous, competitive spring growth flush is known
to be detrimental to fruit-set in vigorous cultivars such as Fuerte and
Sharwil. Emphasis is placed on keeping leaf nitrogen levels in Fuerte below 1,8
per cent as a practical means of controlling this vigour.
Spring growth flush vigour can also be manipulated by shoot tipping
(Blumenfeld et al, 1983), which
temporarily reduces the sink strength of the rapidly expanding vegetative shoot
tips, giving fruitlets a better opportunity to establish their mobilising
ability. In this regard, 'determinate' flower clusters appear to have a better
fruit-set than 'indeterminate' clusters. The growth retardant paclobutrazol
(Cultar(§) reduced the vegetative vigour of indeterminate flower clusters
(Bertling & Kohne, 1986). Significant increases in fruit-set of Fuerte and
Hass avocados have also been obtained by strategically timed foliar sprays of
paclobutrazol which target the emerging spring shoots (Wolstenholme &
Whiley, unpub data).
Avocado yields have been increased by girdling treatments. Israeli
(Lahav et al, 1971) and Australian (Trochoulias & O'Neill, 1976) workers
have reported yield increases of 2 to 5 t ha-1. Further research on
vigorous, healthy trees with good shoot systems appears justified.
It is now accepted that small trees in high density plantings are physiologically more efficient as fruit producers, and maximise early yields and yields per unit land area (Chalmers, 1986). However, the problems of translating these concepts to vigorously growing evergreen fruit trees are many, and were a recurring theme at a recent symposium (Cull & Page, 1986).
Accepting the need for close initial spacing, critical decisions on tree
thinning programmes arise as crowding sets in. Some aspects of avocado
photosynthesis have been researched (Bower et al, 1980; Ramasadan, 1980;
Scholefield et al, 1980). However,
further investigation of light relationships in the orchard situation is needed
for a more scientific basis for decision-making. We lack a sophisticated
understanding of whole-canopy architecture.
Very heavy flowering is characteristic of avocado trees, and is
undoubtedly a heavy drain on both energy and mineral resources at a critical
time. In a two-year study of mature Fuerte trees in California, abscissed
flowers constituted 8 per cent of annual dry matter production (Cameron et al, 1952). Trochoulias (1987) found
that flowers accounted for 2,7 per cent of the dry mass of a single 13-year-old
Fuerte tree sampled in spring. Heavy abscission of young fruitlets in spring
aggravates nutrient and energy losses. It appears that in the subtropics, where
heavy flowering is usually assured, selection for reduced flowering intensity
may have beneficial effects on yield.
Avocado fruits are often left on the tree after attaining minimum legal
maturity. The consequent increase in oil content from about 10 per cent,
sometimes to over 30 per cent in subtropical cultivars, adversely affects
energy partitioning in the rest of the tree. Wolstenholme (1986) estimated that
a 30 t ha-1 yield at 10 per cent oil content is energetically
equivalent to 12,54 t ha-1 at 35 per cent oil content. Late hanging,
although sometimes economically attractive, therefore severely depletes
carbohydrate reserves and correspondingly reduces the following season's crop.
CONCLUSIONS
Seemingly low average avocado yields are partly due to the highland
rainforest origin and the early stage of domestication of this crop by modern
man. The high energy costs of fruiting limit the potential target yield to
somewhat over 30 t ha-1. Best growers achieve two-thirds of this,
indicating considerable scope for research improvement.
In the longer term, breeding opportunities are believed to reside mainly
in altered dry matter partitioning, particularly from unproductive wood
production to fruit production. A range of Phytophthora-tolerant, dwarfing rootstocks are also needed.
For the average grower, the greatest short-term gains will come from
alleviation of environmental stress, including Phytophthora control. Provision for healthy root growth will also
improve water and nutrient supply, which ultimately limit yield.
In the medium term, research on growth retardant manipulation of
vegetative/ reproductive balances, girdling, and a better understanding of
canopy light relationships in high density orchards hold possibilities. The
main gaps in our knowledge are phenological matching of critical developmental
phases of root, shoot and fruit to relative source and sink strengths.
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