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Fruit thinning using NAA shows potential for reducing
biennial bearing of ‘Barnea’ and ‘Picual’ oil olive trees
Arnon Dag
A,D
, Amnon Bustan
A
, Avishai Avni
A,B
, Shimon Lavee
C
, and Joseph Riov
B
A
Gilat Research Centre, Agricultural Research Organization, Ministry of Agriculture,
Mobile Post Negev 85280, Israel.
B
The Robert H. Smith Institute of Plant Sciences and Genetics in Agriculture, The Hebrew University of Jerusalem,
Faculty of Agriculture, Food and Environment, Rehovot 76100, Israel.
C
Institute of Plant Science, Faculty of Agriculture, Food and Environment, The Hebrew University of Jerusalem,
Rehovot, Israel.
D
Corresponding author. Email: arnondag@volcani.agri.gov.il
Abstract. Biennial bearing is a major horticultural and economic drawback of olive (Olea europaea L.) cultivation, which
particularly affects the olive oil industry under intensive production systems. The number of fruits per tree in an on-year is a
primary determinant of the biennial cycle. While fruit thinning using NAA shortly after full bloom is commonly practiced to
increase fruit size in table olives, the extent of its influence on biennial bearing is unknown. In the present study, the ability of
that common naphthaleneacetic acid (NAA) treatment (100 mg/L, 10 days after full bloom) to alleviate biennial bearing in
two oilolive cultivars, Picual and Barnea, was poor, although significant influence on the number of fruit was evident solely in
Barnea. Picual seemed less susceptible than Barnea to biennial bearing. Consequently, the effect of a broad range of NAA
concentrations (0–320 mg/L, 10 days after full bloom) on various yield parameters was investigated during a biennial cycle of
Barnea trees. There was a gradual proportional decline in the on-year number of fruits from ~50 000 to 10 000/tree in response
to increasing NAA concentrations. The number of return fruits in the off-year was reciprocal to the on-year fruit load,
but remained relatively small, below 15 000/tree. The dynamic relationship between fruit load and fruit size in both on- and
off-years was a significant compensation factor in fruit and oil yields. In both cultivars, an on-year fruit load smaller than
20 000/tree is likely to provide consistent yearly oil yields ranging from 10 to 12 kg/tree. The results demonstrate the
possibility of using NAA post-bloom spraying to balance biennial bearing in oil olives.
Additional keywords: alternate bearing, fruit load, fruit size, NAA (naphthalenacetic acid), oil content, Olea europaea.
Introduction
Biennial or alternate bearing is a widespread phenomenon in
many fruit tree species, which brings about severe instability in
management inputs (e.g. labour) and marketing. Although the
metabolic processes, their induction, and the signals involved are
only partially understood, they clearly differ among various fruit
tree species (Goldschmidt 2005). Different horticultural practices
are used to minimise alternate bearing in many species, but in
most cases, they are only partially effective (Monselise and
Goldschmidt 1982).
Olive (Olea europaea) has a very high tendency of alternate
fruit production. Being an industry-dependent commodity, the
economic problems arising from biennial bearing are especially
serious in the production of olive oil, where the consequences of
unstable yields on labour distribution, oil-mill capacity, storage
requirements, and produce quality are dramatic. For instance, an
intensively cultivated, fully alternate-bearing oil olive grove
(cv. Barnea) may fl uctuate between 22 and 3 t fruit or between
4.4 and 0.6 t oil/ha in successive on- and off-years, respectively
(Lavee 2006).
While environmental factors, such as temperature or water
and nutrient availability, directly affect vegetative growth and
the performance of most reproductive processes (induction,
evocation, differentiation, bloom, fruit set, and fruit growth
and ripening), endogenous determinants, namely the balances
of carbohydrate, mineral nutrients, and hormones, are
significantly involved in biennial bearing (Monselise and
Goldschmidt 1982; Troncoso et al. 2010). Lavee (2006) has
recently suggested a general scheme of alternate bearing in
olive, taking into account the above endogenous processes and
their interactions with environmental factors (e.g. temperature)
during subsequent on- and off-years. According to this scheme,
fruit production in olive is mainly dependent on the vegetative
growth of the previous growing season. On the other hand, the
degree of vegetative growth in any particular season is a function
of the amount of fruit present on the tree during that season.
Thus, the balance between the amount of developing fruit and
vegetative growth in any given growing season will affect and
control the potential fruit production for the following season.
Physiologically, biennial bearing in olive is thought to be
due to the inhibition of floral bud induction and differentiation
by growth substances that are produced and excreted by the
developing seeds (Lavee 1989, 2006; Baktir et al. 2004). In
addition, the metabolic effort required by a heavily yielding tree
CSIRO 2009 10.1071/CP09090 1836-0947/09/121124
CSIRO PUBLISHING
www.publish.csiro.au/journals/cp Crop & Pasture Science, 2009, 60, 1124–1130
that is furnishing oil production during the late season might
come at the expense of resources available for the subsequent
reproductive process (Monselise and Goldschmidt 1982; Cuevas
et al. 1994; Troncoso et al. 2010).
Attempts to interrupt the reproductive cycle of olive trees at
early stages of floral development (Lavee 1989; Fernandez-
Escobar et al. 1992) or during bloom (Lavee et al. 1999) have
been either too risky or inefficient in lessening alternate bearing.
On the other hand, chemical fruit thinning, which was first
examined decades ago (Hartmann 1952; Lavee and Spiegel
1958; Lavee and Spiegel-Roy 1967), is quite commonly
practiced, mainly to obtain large fruits in table olives (Martin
et al. 1980; Krueger et al. 2004). The fruit thinning is generally
carried out by post-bloom application of naphthalenacetic acid
(NAA), which is absorbed by the leaves and fruit and translocated
to the fruit pedicels and young developing embryos. Within
2 weeks of application, an abscission zone is formed, causing
some fruit to drop (Krueger et al. 2004). The common practice
in table olives is to apply 100 mg/L of NAA 10 days after
full bloom, and to add 10 mg/L per day until 20 days after full
bloom (Lavee and Spiegel 1958; Lavee and Spiegel-Roy 1967;
Krueger et al. 2004).
Despite the long-established use of chemical thinning in table
olives, there is little information on the potential use of NAA
to reduce biennial bearing, with respect to oil olive cultivars.
Solving the problem is of particular importance for the olive oil
sector, occupying a 10-fold larger area than that of table olives, to
guarantee an uninterrupted yearly supply of raw material for the
oil industry. The objectives of the present study were therefore:
(a) to test the ability of the common NAA fruit-thinning practice
in table olives to reduce alternate bearing in oil olive cultivars
(Picual and Barnea); (b) to examine the effects of NAA
application in a broad range of concentrations on parameters
of fruit and oil yield in oil olives in the year of application and in
the following year, and (c) to identify an NAA treatment that
would significantly reduce the tendency of Barnea to develop
alternate bearing under heavily producing, intensive growing
conditions.
Materials and methods
Two experiments were conducted between 2004 and 2007. The
first tested the effect of the table olive-thinning methodology on
the oil-olive cvv. Picual and Barnea, while the second focussed
on examining the effects of a much broader range of NAA
concentrations on the yield parameters of cv. Barnea.
Experimental site
The experiments were conducted in large commercial olive
orchards located in an arid area near Kibbutz Revivim
(34887
0
N; 15807
0
E; ~300 m a.s.l.) in the Negev highland desert
of Israel. The yearly mean precipitation is <100 mm between
November and February and it is unpredictable. The orchards are
drip-irrigated throughout the year to reach a total of 900 mm.
A detailed description of the environmental conditions and
the practices used in these orchards is provided by Dag et al.
(2008). Fertilisers are supplied continuously through the
irrigation water at 200, 30, and 300 kg/ha.year of N,P, and K,
respectively.
The first experiment was conducted in an orchard of Picual and
Barnea, planted in 1995 at spacings of 7 by 3.5 m (407 trees/ha,
Picual) and 7 by 5 m (286 trees/ha, Barnea), in which a uniform
plot (~1 ha) of each variety was selected. According to yield data
collected before the experiment, the first experimental year (2004)
was expected to be an on-year.
The orchard for the second experiment, based mostly on cv.
Barnea trees, was planted in 2000 at 7 by 4 m spacing. The
selected plot (2 ha) was characterised by strong fluctuations in
yield: in 2004, the average fruit yield was 22.8 t/ha, whereas in
2005, the plot was not harvested due to an absolute off-year.
Therefore, the following year (2006) was expected to be a highly
productive on-year.
NAA application
In all experiments, NAA was applied 10 days after full bloom
during the afternoon, using Alphatop (Milchan Bros., Ltd, Israel)
20% (w/v) NAA, at a spray volume of 4 L/tree. The adjuvant BB5
(alkyl phenoxy polyetheylene ethanol, produced by C.T.Z. Israel)
was added at 0.45 mL/L. In the first experiment, NAA was applied
at 100 and 120 mg/L on 29 April 2004, and again to the same
trees on 30 April (Barnea) and 8 May (Picual) 2006. In the
second experiment, NAA was applied on 30 April 2006, at
concentrations of 0, 40, 80, 120, 160, 240, and 320 mg/L.
Measurements
The fruit was harvested in the autumn, each treatment in
accordance with the appropriate average maturity level (3–4),
determined according to the international standard index for olive
ripeness (IOOC 1984). All trees of each treatment were harvested
on one day. All fruits were harvested from individual trees onto
nets using mechanical combs, gathered, and weighed. A sample
of 100 fruits was taken from each tree for a determination of
average fruit weight, and to calculate the number of fruits per tree.
The oil content of the fruit was determined by chemical extraction
according to Avidan et al. (1999).
Experimental design and statistical analyses
The experimental design was random blocks, one row each block,
with untreated neighbouring rows as a buffer. In each block
(4 in the first experiment and 6 in the second), uniform trees were
labelled (with at least 2 untreated buffer trees between them) and
were assigned randomly to the treatments. In the first experiment,
each block included 4 measured trees per treatment (16 trees in
total), and in the second, 1 tree per treatment (6 trees in total).
Data were analysed by 1-way ANOVA using JMP 5.0.1 software
(SAS Institute, Cary, NC, USA). Differences between treatments
were determined by Tukey-Kramer HSD test. Statistical analyses
were conducted mostly at a significance level of P < 0.05.
Results
Experiment 1
In the first set of experiments, NAA application at a concentration
similar to or slightly higher than that practiced with table olives
did not result in any significant reduction in the on-year yield
(2004) of either Picual (Table 1) or Barnea (data not shown).
Nevertheless, in the subsequent off-year (2005), the Picual trees
responded to the NAA treatment of the previous year by an almost
Fruit thinning and biennial bearing of oil olives Crop & Pasture Science 1125
2-fold increase in fruit yield compared to the control trees. Barnea,
on the other hand, produced extremely low yields in the off-year
(2005), although a slight increase was nevertheless observed in
the NAA-treated trees (data not shown).
In the second stage of this experiment (second treatment of
the same trees), cv. Picual displayed no apparent trend towards
biennial bearing. Fruit yield as well as oil yield were quite similar
in both years, and were not significantly affected by the NAA
treatments (Table 2). The number of fruits, however, did exhibit
indications of biennial bearing, but the reduction in fruit number
was almost completely compensated for by an increase in fruit
size. This phenomenon also occurred in NAA-treated trees in the
on-year. Barnea, in contrast, demonstrated unambiguous biennial
responses of all yield parameters. NAA applications significantly
reduced the number of fruit in the concurrent on-year (by ~26%),
and doubled the number of fruit in the subsequent off-year,
compared with control trees. Similar to Picual, the increase in
fruit size in NAA-treated trees in the on-year compensated for the
reduction in fruit number, so that fruit and oil yields did not differ
significantly from the controls. On the other hand, in the off-year,
the generally larger (2-fold) fruit size turned a small absolute
increase in fruit number into a notable relative rise in fruit and oil
yield, compared with the on-year (Table 2); the relative oil yield
increased from ~0.5 in 2006, to ~1 g/fruit in 2007.
Experiment 2
The broad NAA concentration range tested in the second
experiment in Barnea trees did not have noticeable effects on
the current vegetative growth up to 240 mg/L (data not shown).
Nonetheless, at the highest NAA concentration (320 mg/L), the
on-year spring vegetative growth was slightly impaired, leading
to some loss of apical dominance, which resulted in auxiliary
sprouting a few weeks later. On the other hand, the wide NAA
concentration range had an obvious effect on reproductive
processes: due to the different numbers of fruit in accordance
with the severity of the treatments, the time of harvest differed
considerably. In the on-year (2006), the fruit harvest period lasted
10 weeks, starting on 25 October in the trees that had received the
highest NAA concentration (320 mg/L), through 22 November
(240 and 160 mg/L), 19 December (80 and 120 mg/L), and ending
on 4 January 2007 with the control trees and trees treated with
40 mg/L NAA. In the subsequent off-year, however, all trees
were harvested earlier, and fruit harvest lasted only 2 weeks
(23 Oct.–5 Nov. 2007).
The most pronounced effect of NAA was on fruit load in the
on-year (2006): the reduction in fruit number was significant and
proportional to NAA concentration (Fig. 1). Thus, the number of
harvested fruits per tree declined from an average of 40 000 in
untreated trees to less than 15000 in trees treated with 320 mg/L
NAA. On the other hand, the number of fruits harvested in the
following off-year (2007) was fairly small in all treatments, only
partially compensating for the decrease in fruit number induced
by the NAA treatments in the previous on-year. While the control
trees carried ~2000 fruits per tree in the off-year, trees that
were treated with 240 mg/L NAA had 6600 fruits, and those
treated with the highest NAA concentration, 320 mg/L, diverged
slightly, producing more than 10 000 fruits per tree in the off-year.
Consequently, the calculated average biennial (2006–07) number
of fruits per tree steadily declined from 20 500 for fully alternate-
bearing control trees to 12 000 per tree for the highest NAA
concentration.
The decline in fruit numbers in the above experiment was
compensated for by a significant increase in fruit size in the on-
year. In the range 10 000–50 000 fruits/individual tree, a negative
logarithmic relationship was evident between fruit size and fruit
number (Fig. 2). In the off-year (2007), however, the fruit size was
unaffected by the number of fruits per tree, even when the latter
increased from 2000 to 7000. The fruit weight was somewhat
reduced in that year only when the fruit number exceeded 10 000
Table 1. Effect of 100 or 120 mg/L of NAA applied 10 days after full
bloom in the spring of the expected on-year (2004) on the fruit yield of oil
olive cv. Picual in that same year and in the following off-year
Values are means of 4 replicates s.e.
NAA application Fruit yield (kg/tree)
2004 2005
Control 67.9 ± 1.5 11.9 ± 3.6
100 mg/L 64.3 ± 3.6 19.3 ± 2.8
120 mg/L 57.3 ± 2.5 22.7 ± 6.6
P-value 0.1028 0.2942
Table 2. Effect of NAA application 10 days after full bloom in the spring of the expected on-year (2006) on yield parameters
of 2 oil olive cultivars, Picual and Barnea, in the same year and in the following off-year
Values are means of 4 replicates. Within a column, values followed by the same letter are not significantly different by Tukey’s HSD
NAA Fruit no./tree (1000) Fruit size (g) Fruit yield (kg/tree) Oil yield (kg/tree)
application 2006 2007 2006 2007 2006 2007 2006 2007
Picual
Control 20.7a 9.7 3.6c 6.8 70.2 59.3 13.1 9.8
100 mg/L 15.7b 8.8 4.2b 6.5 64.0 53.2 12.0 9.3
120 mg/L 12.9c 10.9 4.7a 6.0 58.1 61.8 10.6 10.7
P-value 0.019 0.071 0.038 0.066 0.068 0.113 0.062 0.093
Barnea
Control 40.1a 2.6c 2.3b 5.5 87.4 14.0b 19.0 2.5b
100 mg/L 28.5b 5.6b 3.0a 5.4 83.0 28.9a 17.3 5.2a
120 mg/L 29.5b 5.2a 2.9a 5.9 85.2 29.4a 18.2 5.2a
P-value 0.008 0.007 0.041 0.085 0.153 0.009 0.078 0.007
1126 Crop & Pasture Science A. Dag et al.
per tree (Fig. 2). Interestingly, the pit fresh weight was stable in
2006 at 0.55 g, regardless the effects of the NAA treatments on the
fruit size. Thus, the pulp/pit ratio increased in 2006 from 2.6 to 6.7
in accordance with the increase in fruit size and the decline in fruit
load from 50 000 to 10 000 fruit/tree. In the subsequent year, pit
fresh weight was higher, at the stable level of 0.81 g. The pulp/pit
ratio was 5.25 throughout NAA treatments.
The ‘trade-off’ between fruit load and fruit size had a
significant effect on fruit yield. Any level of reduction in fruit
number (% thinning) was accompanied by a much smaller
reduction in relative fruit yield. Even at the heaviest fruit
thinning—38% of the control fruit, a substantial relative fruit
yield was obtained—62% of the on-year control. A similar
analysis showed that the corresponding off-year yields
harmonised with those of the on-year. Thus, fruit thinning to
the range of 35–65% of fruit number in an on-year appeared to
slightly increase the average biennial yield (Table 3).
Nevertheless, in practice, NAA applications, excluding the
highest level tested, failed to break the alternate-bearing
character of the trees used in this experiment. Only the most
extreme NAA treatment was successful, bringing both on- and
off-year yields close enough to the quite stable average biennial
fruit yield (Table 3).
The oil yields correlated well with the fruit yields. The relative
oil content was quite uniform, ~19.3% in both years, when fruit
was harvested at the same maturation level, with no significant
effect of NAA treatment on relative oil contents. Thus, the
absolute oil content was strongly correlated with fruit size
(Fig. 3a) and, due to the relationship between fruit number and
size, the overall oil production was related to fruit number through
a saturation function (Fig. 3b). Below 15 000 fruits per tree, the
oil yield increased steeply; as the number of fruit increased, the
increase in oil yield gradually declined. The fluctuations in oil
yield between on- and off-years were very large, ranging from 18
to 2 kg/tree, respectively (Fig. 4). The NAA applications reduced
oil yield in the on-year, but this decrease was fully compensated
for by the corresponding oil yield in the subsequent year.
Interestingly, the average 2-year oil yield in all treatments was
around 10 kg/tree, close to the breaking point of the saturation
curve, which occurred at ~15 000 fruits per tree (Fig. 3b). The
application of 320 mg/L NAA, the highest concentration used in
this study, was far more effective than the other concentrations,
bringing the oil yield closer to the compensation point for
both years (Fig. 4).
Discussion
Post-bloom NAA application is commercially practiced in table
olive orchards to increase fruit size (Lavee and Spiegel 1958;
Martin et al. 1980; Krueger et al. 2004), an important quality
parameter in that industry. Although also intended to reduce
biennial bearing of table olives (Martin et al. 1980), information
about the efficiency of NAA application in this regard is scarce.
The problem of alternate bearing is fundamental to the olive-oil
R
2
= 0.827**
Fruit weight (g)
Log. (2006)
2006
2007
Number of fruit/tree (×1000)
7
6
5
4
3
2
1
0
0 10 20 30 40 50 60
Fig. 2. Effect of on-year thinning on the yield parameters of Barnea oil
olive trees (2006–07), demonstrating the ‘trade-off’ between fruit size and
number of fruits per individual tree in the on-year and following off-year.
** Significance of R
2
at P < 0.01.
R
2
= 0.863**
R
2
= 0.941*
R = 0.731
0
5
10
15
20
25
30
35
40
45
0 50 100 150 200 250 300 350
Number of fruit/tree (×1000)
NAA concentration (mg/L)
Ave. 2006–07
2
2006
2007
Fig. 1. Effect of NAA application 10 days after full bloom in an on-year
(2006) on the number of fruits on oil olive cv. Barnea trees in the same year
and in the following off-year (2007), and on the 2-year average. Values are
means of 6 replicates s.e.; *, **, significance of R
2
at P < 0.05, P < 0.01,
respectively.
Table 3. Effect of NAA concentration, applied 10 days after full bloom
in the spring of the expected on-year (2006), on fruit yields of cv. Barnea in
that same year and in the following off-year
Values are means of 6 replicates s.e. The calculated 2- year average is also
presented. Within a column, values followed by the same letter are not
significantly different at P < 0.05 by Tukey-Kramer multiple comparisons test
NAA Fruit yield (kg/tree)
conc. (mg/L) 2006 2007 2-year ave.
0 91.7 ± 4.74a 11.4 ± 1.21c 51.6
40 89.4 ± 4.12a 9.5 ± 2.97c 49.5
80 78.4 ± 2.57b 20.2 ± 5.33bc 49.3
120 79.0 ± 7.02ab 21.5 ± 3.87b 50.3
160 80.2 ± 4.57ab 32.3 ± 5.30ab 56.3
240 72.8 ± 5.02bc 35.2 ± 8.45ab 54.0
320 57.3 ± 5.88c 47.8 ± 3.92a 52.6
P-value 0.031 0.017 0.061
Fruit thinning and biennial bearing of oil olives Crop & Pasture Science 1127
industry, particularly in heavily producing intensive orchards
(Lavee 2006).
In the first biennial cycle (2004–05), the commonly used range
of NAA application (100 or 120 mg/L, 10 days after full bloom in
the on-year) was tested on two oil olive cultivars, Picual and
Barnea. NAA treatment resulted in an insignificant reduction in
on-year yields and a pronounced trend towards increasing yields
in the subsequent off-year, apparently having no effect on the
biennial pattern (Table 1). However, the figures for total fruit yield
in both years are likely to have masked any possible effects of
NAA treatment on fruit number and consequent fruit size in
each year. Since fruit number in an on-year is a determinant of
alternate bearing (Monselise and Goldschmidt 1982; Lavee
2006), it is likely that the common NAA practice in the
on-year not only affects fruit size due to thinning, but also
results in an increase in return fruit number in the subsequent
off-year. These assumptions were further investigated in the
2006–07 experiments.
The second cycle in this experiment (2006–07) confirmed
that any reduction in fruit number, within or between years,
was counterbalanced by an increase in fruit size (Table 2).
Nevertheless, the recommended range of NAA concentrations
for enlarging fruit size in table olives appeared to be inefficient at
reducing alternate bearing, at least for cv. Barnea. Interestingly,
one main difference between Picual and Barnea was fruit number
in the on-year (20 000 and 40 000, respectively). In both cultivars,
the fruit number dropped in the off-year; however, the greater the
fruit load in an on-year, the smaller it was in an off-year. Thus,
Picual experimental trees could produce more consistent fruit
and oil yields during a 2-year cycle. Barnea behaved differently: it
retained a clear biennial pattern even though the NAA treatments
had reduced the fruit load to ~30 000/tree (Table 2). Nevertheless,
the NAA treatments did induce some increase in the number of
return fruit in cv. Barnea in the off-year, suggesting that a further
reduction of fruit load in the on-year might generate even more
fruit in the subsequent year and alleviate alternate bearing.
That approach was examined by using a wider range of NAA
concentrations on cv. Barnea in 2006–07.
Increasing NAA concentration reduced fruit number
proportionally, down to less than 20 000 per tree with the
320 mg/L treatment (Fig. 1). Eventually, the time to harvest was
shortened, confirming the negative relationship between fruit
load and fruit maturation (Barone et al. 1994). The fruit number,
or more precisely, the number of developing seeds on a tree, is the
primary determinant of fruiting potential for the following year
(Monselise and Goldschmidt 1982; Lavee 2006). This is assumed
to be due to growth substances excreted by the developing fruit,
R
2
= 0.934**
Oil content (g/fruit)
2006
2007
Number of fruit/tree (×1000)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0 10 20 30 40 50 60
R
2
= 0.925**
24
20
16
12
8
4
0
Oil yield (kg/tree)
1 2 3 4 5 6 7
Fruit wei
g
ht (
g
)
(a)
(b)
Fig. 3. Relationship between oil production and yield parameters of individual Barnea trees as affected by thinning treatments in the on-
year (2006): ( a) oil content v. fruit size; (b) oil yield v. number of fruits per tree. ** Significance of R
2
at P < 0.01.
R
2
= 0.924*
2006
2007
20
18
16
14
12
10
8
6
4
2
0
Oil yield (kg/tree)
NAA concentration (m
g
/L)
Ave. 2006–07
R
2
= 0.92*
0 50 100 150 200 250 300 350
Fig. 4. Effect of NAA application 10 days after full bloom in an on-year
(2006) on the oil yield of oil olive Barnea trees in the same year and in the
following off-year (2007), and on the 2-year average oil yield. Values are
means of 6 replicates s.e.; * significance of R
2
at P < 0.05.
1128 Crop & Pasture Science A. Dag et al.
which regulate concurrent vegetative development and also act
as signals to initiate the metabolic activity that controls the
reproductive process for the following year (Stutte and Martin
1986; Cuevas et al.1994;Baktiret al. 2004; Lavee 2006). Thus,
the extent to which the reduction in fruit number or the earlier
harvest is responsible for the increase in fruit number in the
subsequent year can be argued. Nevertheless, the latter remained
relatively small, below 10 000 fruits per tree (Fig. 1), and the
expectation of a proportional increase that would reflect the
reduction in the number of fruit in the former year remained
unfulfilled. Phytotoxicity of high NAA concentrations has been
previously observed in table olives (Manzanillo) (Barranco and
Krueger 1990). In the present study, only the highest NAA
concentration (320 mg/L) impaired the normal spring vegetative
growth,whichwaslaterreplacedbynewauxiliarygrowth.It maybe
speculated that this secondary growth was responsible for the
exclusive return fruiting in this treatment. As mentioned by
Lavee (2006), summer vegetative growth may often mature in
the same year and be induced to bloom and carry fruit in the
following year.
The second yield parameter, fruit size, generally compensated
for the reduction in fruit load. Thus, the total fruit yield was
less affected by the fruit-thinning treatments in the on-year
(Table 2), even at a NAA concentration as high as 240 mg/L
(Table 3). This may be the explanation for the lack of response
by the vegetative growth to the reduction in the number of fruit:
the overall demand by the developing fruit did not change; hence,
the availability of supply for vegetative growth did not increase.
It is worth noting the tendency of cv. Barnea fruit size to converge
at around 2 g above a certain fruit load (Fig. 2). This is probably
due to the early maturation of the endocarp (pit hardening) and
the weaker sensitivity it has to fruit load, in contrast to the pulp.
While the endocarp size remained quite constant (0.55 g) at any
level of fruit load, the further growth demands of the mesocarp
(with the energetically expensive oil biosynthesis there) yielded
a remarkable increase in the pulp/pit ratio, from 2.6 to 6.7, in
respect to the reduction in fruit load. The delay in fruit maturation
and the low pulp/pit ratio both indicate that the high fruit loads
characterising on-years strongly challenge the yielding capacity
of Barnea trees. In contrast, in the off-year, there was no detectable
‘trade-off’ between fruit number and fruit size, indicating that
the fruit-bearing potential of the trees was far greater than the
actual yield, and that the maximum fruit size of cv. Barnea was, in
accordance with its genetic potential, up to 6 g (Fig. 2).
An indication of the desirable yearly yieldlevel may be found in
the calculated average fruit yield of the 2 years, which converged
to ~52 kg/tree, independent of the applied NAA concentration
(Table 3), possibly expressing the potential yearly yield of
consistently bearing olive trees under our conditions. Among all
NAA treatments, only the highest concentration, which caused the
most severe fruit thinning, came close to inducing relatively
constant yields in both years (Table 3).
Oil yield closely followed that of the fruit (Fig. 3b). This is in
agreement with previous findings that in olives, the relative oil
content in the mesocarp at full fruit maturity reaches a uniform
level, based on the genetic–environmental conditions, regardless
of fruit size and tree load (Lavee and Wodner 2004). Hence, the
absolute oil content was strongly correlated with fruit size
(Fig. 3a), as previously reported (Lavee and Wodner 2004).
The results of the present study indicate that the potential oil
yield of Barnea trees under similar conditions for two years is
close to 20 kg/tree (Figs 3b, 4). Thus, a consistent yearly yield of
8–12 kg oil/tree seems realistic, if fruit thinning is applied in an
on-year. In addition, a consistent intermediate yearly yield
provides an earlier harvest (compared with on-year), which is
an advantage where frosts or heavy rains might damage the fruit
or the oil quality.
In conclusion, cv. Barnea trees can be forced to bear consistent
fruit or oil yields if the fruit number is reduced to less than 20 000
per tree in an on-year. At this planting density, this level of fruit
load probably allows the optimum balance required by the olive
tree between development of the current crop, vegetative growth,
and processes that launch the subsequent reproductive cycle. It
remains unclear whether fruit thinning would be required every
2 years, or if once broken, the alternate yielding habit fades away,
until re-induced by external factors. Furthermore, the practical
application of severe fruit thinning requires serious economic
considerations due to the risks involved. These risks stem from the
fact that environmental factors are a strong determinant of fruit
yield in olives, which can often establish a new cycle of alternate
bearing (Lavee 2006) or damage the yield of a given year. Possible
differences in oil quality between on- and off-years, which are
currently under examination, may also be of concern.
Acknowledgments
We thank Yulia Subbotin, Moshe Aharon, and Isaac Tzipori for their technical
assistance. We also thank Izhar Tugendhaft and Nimrod Priel from ‘Negev
Oil’ for assisting in the field activities, and Shlomo Glidai and ‘Milchan Bros.,
Ltd’ for providing the chemicals and technical assistance.
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