ORIGINAL PAPER

The influence of bearing cycles on olive oil productionresponse to irrigation

A. Ben-Gal • U. Yermiyahu • I. Zipori •

E. Presnov • E. Hanoch • A. Dag

Received: 21 June 2010 / Accepted: 22 September 2010

� Springer-Verlag 2010

Abstract Water requirements for olive oil production and

the effects of deficit irrigation were determined while

considering the relative fruit loads on trees occurring as a

result of biennial bearing cycles. Two Israeli olive (Olea

europaea) varieties (Barnea and Souri) were evaluated for

growth and yield parameters in a 4-year field study where

five relative irrigation rates were applied. Increasing irri-

gation increased stem water potential, vegetative growth,

and olive fruit yield with the increases tapering off at

application rates reaching 75–100% of potential crop

evapotranspiration. Tree water status, growth, and fruit

characteristic parameters were highly affected by both fruit

load and by irrigation level. Oil yield increases as a func-

tion of increased irrigation were initiated for each cultivar

only following an ‘off’ season when the treatments lead to

higher vegetative growth. The increased oil yields as a

function of increased irrigation were primarily explained

by higher tree-scale capacity for carrying fruit, especially

as irrigation alleviated measureable water stress. For the

Barnea cultivar in ‘on’ years, a secondary effect due to

increased oil per fruit as irrigation increased was evident,

particularly at the higher application rates.

Introduction

Olive oil production has historical importance throughout

the Mediterranean where there is evidence of human

cultivation and consumption from as far back as

5,000–6,000 years ago (Vossen 2007). Traditionally, olives

are not irrigated, but recently water application has been

recognized as constructive in order to (a) increase yields of

olives in regions with traditional rain-fed olive production

(Moriana et al. 2003), (b) allow high-density olive orchards

and (c) expand olive production into regions where there is

not enough rainfall to otherwise support the crop (Connor

2005). Under conditions like those in Israel, where growing

season rainfall is insufficient, irrigation has been found to

promote increased olive fruit and oil yields by as much as

fourfold (Grattan et al. 2006; Lavee et al. 1990; Moriana

et al. 2003; Samish and Speigel 1961; Spiegel 1955). This

impressive yield response to water application can be due

to a number of factors. First is that irrigation allows

avoidance of conditions of stress that would otherwise

cause reduced olive oil accumulation in fruits (Fernandez

and Moreno 1999). A second, probably more important,

reason that irrigation increases yields is an augmented

capacity for quantity of fruit per tree (d’Andria and Morelli

2002; d’Andria et al. 2004; Goldhamer et al. 1994; Grattan

et al. 2006; Gucci et al. 2007; Patumi et al. 1999, 2002).

For other than severely stressed trees, oil content on per

fruit basis is not usually found to be affected by water

application (d’Andria et al. 2004; Grattan et al. 2006;

Lavee et al. 2007; Moriana et al. 2003). In fact, oil per-

centage is commonly found to decrease as a function of

Communicated by E. Fereres.

A. Ben-Gal (&) � U. Yermiyahu � E. Presnov

Soil, Water and Environmental Sciences,

Agricultural Research Organization, Gilat Research Center,

85280 Mobile Post Negev II, Israel

e-mail: bengal@volcani.agri.gov.il

I. Zipori � A. Dag

Plant Sciences, Agricultural Research Organization,

Gilat Research Center, 85280 Mobile Post Negev II, Israel

E. Hanoch

The Extension Service, Ministry of Agriculture

and Rural Development, Bet-Dagan, Israel

123

Irrig Sci

DOI 10.1007/s00271-010-0237-1

increased amounts of applied water (Lavee et al. 1990;

Ramos and Santos 2010). Additionally, increased yields

may come at a price in terms of oil quality. Irrigation has

been found to decrease polyphenol content (Ben-Gal et al.

2008; Criado et al. 2007; Dag et al. 2008; Gomez-Rico

et al. 2006; Patumi et al. 1999) and sometimes increase free

fatty acids in oil (Dag et al. 2008).

Olives strongly follow a biennial cycle of growth and

production. Since flowering takes place on 1-year-old

wood and stem biomass growth becomes negligible when

fruit load is high, trees undergo alternated years of vege-

tative and reproductive growth. High yields are produced in

the ‘on’ years followed by ‘off’ years of low (sometimes

zero, often not commercial) yields. Although biennial

bearing is basically genetically determined, the degree to

which it occurs is greatly affected by environmental con-

ditions, especially the weather and by cultivation practices.

Alternate fruit bearing occurs under both extensive and

intensive growing conditions (Lavee 2006). Three principal

regulatory mechanisms that seem to interact strongly, thus

influencing or even inducing alternate bearing are as fol-

lows: hormonal control, nutritional (carbon and mineral

balance) control (Lavee 2006), and flowering site limitation

(Dag et al. 2010).

The objectives of this work were to evaluate water

requirements for olives in a modern Israeli orchard and to

determine effects of deficit irrigation on growth and pro-

duction while considering the relative fruit loads on trees

occurring as a result of biennial bearing cycles.

Materials and methods

Experimental site

The research was conducted in a two-hectare section of a

4-year-old (at onset of treatments) commercial olive

orchard with general upkeep and horticultural practices in

accordance with accepted local commercial practice. The

orchard was located (31�750N, 34�850E) adjacent to Kfar

Menachem and Revadim in the foothills of the Judean Hills

in Israel. The soil of the orchard was a clay vertisol

(sand 20%, clay 65%) with *20% gravel to stone-sized

rock fraction. The climate at the site is Mediterranean,

with highly variable seasonal rainfall averaging around

450 mm year-1 limited to the winter months November–

March. Summers are typically warm and dry with average

daily high temperatures in July and August of 30–35�C and

daytime relative humidity often dropping below 40%.

Winter rainfall and seasonal reference evapotranspiration

(ET0) for the 4 years of this study are found in Table 1.

The orchard section included both a local traditional cul-

tivar, ‘Souri’, and a newly bred local cultivar, ‘Barnea’,

interplanted such that between every four rows of Barnea

there were two rows of Souri. Orchard design and spacing

was to accommodate mechanized harvesting by trunk

shakers. Trees were planted every 3.75 m (Souri) and

4.25 m (Barnea) in rows spaced 7 m apart. The orchard

was drip-irrigated (UniRam 20 mm, 2.3 l h-1, emitter

every 0.5 m; Netafim, Hatzerim, Israel). The irrigation

application for each treatment was determined as a fraction

of the daylight-hour reference evapotranspiration (ET0)

calculated using Penman–Monteith equation (Monteith

1965). Meteorological data were collected from a station

located adjacent to the orchard.

Irrigation water application treatments

Five irrigation levels (30, 50, 75, 100, and 125%) were

given. Actual daily irrigation (I) was computed by:

I ¼ ETp� IrrLev

100; ETp ¼ ET0 � fc ð1Þ

where ETp is potential evapotranspiration, IrrLev is the

level of irrigation for the different treatments (%), and fc is

a cover factor estimated by midday shaded area. The IrrLev

fraction is equivalent to a crop factor (Kc) as found in FAO

crop water consumption methods (Allen et al. 1998). The

cover factor, fc, was estimated as 40% in 2006 and 50%

from 2007 to 2009. Frequency of irrigation events was

once every 3 days early and late in the season and once

every 2 days midseason (approximately June–September).

Statistical design was a randomized split-block with five

replicates per treatment. Each of the 25 experimental units

consisted of six adjacent tree rows (4 Barnea and 2 Souri)

with at least four olive trees per row. Two central trees of

each variety were monitored while those surrounding them

served as border-guard trees. The orchard was irrigated

with effluent that, originating as municipal wastewater in

the city of Jerusalem, underwent secondary, activated

sludge treatment and was stored in a nearby reservoir.

Nitrogen and potassium fertilizer was given with irrigation

water as recommended by the local extension service

equally to all treatments according to area-based calcula-

tions including consideration of nutrients originating in the

recycled wastewater. Seasonally, 200 kg nitrogen and

205 kg potassium were applied per hectare. The effluent

was low in salts and had no other characteristic expected to

influence plant performance.

Variable water application began in the spring of 2006

and continued during the four irrigation seasons (approxi-

mately April–November, depending on rainfall). The

beginning and end of each irrigation season (Table 1) were

dictated by the last and first significant rainfall events each

year. These dates, amount of winter rainfall, total accu-

mulated ET0 during the irrigation season, and the quantity

Irrig Sci

123

of irrigation given to the 100% return of ETp are provided

in Table 1 for each of the four experimental years. Daily

ET0 and rainfall for the four experimental seasons are

given in Fig. 1. Actual seasonal irrigation quantities for

each treatment level are shown in Fig. 2. Differential irri-

gation treatments ranged from 176 to 209 mm (25%

treatment) to 580 mm in the first year and between 730 and

820 mm for the highest (125%) treatment in the subsequent

years.

Alternate bearing

During the spring prior to the initiation of the experiment in

2006, the Souri cultivar flowered plentifully, while the

flowering of the Barnea trees was relatively meager. The

orchard’s two cultivars being in opposite biennial bearing

cycles continued throughout the reported experimental

years. In 2006 and 2008 Souri was ‘on’, with a relatively

high fruit load, and Barnea ‘off’, while in 2007 and 2009

Barnea was ‘on’ and Souri ‘off’. In the 2009 season, the

Souri trees were in a severe ‘off’ year, and no fruit was

harvested.

Measurements

Plant water status was assessed by midday stem water

potential using a Scholander-type pressure chamber

(MRC, Israel) according to Shackel et al. (1997), three

times a year, in May, July, and September. Vegetative

growth rate was measured by annual increased trunk

circumference at a marked point 40 cm above the ground

on each tree. Yield was determined for each monitored

tree, harvested at the appropriate ripeness level (50%

black fruit). Individual fruit weight was determined

from a sample of 100 fruits. Fruit number per tree was

calculated by dividing total fruit yield by average single

fruit weight. Oil percentage was measured by chemical

extraction with Soxhlet. Cold-pressed oil was obtained

with an ‘Abencor’ system (MC2 Ingenieria y Sistemas,

Spain).

Statistical analysis

Data were analyzed using the JMP 5.0 software (SAS

Institute, Cary, NC) and the curve-fitting and analysis tools

of SigmaPlot 10.0 (Systat Software, San Jose, CA). Rela-

tionships between irrigation levels and the measured

parameters were determined using regression analysis. All

statistical analyses were conducted at P B 0.05 unless

otherwise indicated.

Table 1 Seasonal reference climate and irrigation data

Year Rain prior (mm) Irrigation dates Seasonal ET0 (mm) Irrigation 100% (mm)

2006 502 aApril 23–Oct 27 1,119 484

2007 529 April 12–Nov 22 1,302 663

2008 419 March 27–Oct 26 1,292 612

2009 379 April 5–Oct 29 1,124 578

ET0 is reference evapotranspiration according to a modified Penmann Monteith equationa Differential irrigation treatments began May 28, 2006. From April 1–May 27 2006, 85 mm was applied to all treatments

ET

0 (m

m)

2

4

6

8

10

Jan

2006

Jul 2

006

Jan

2007

Jul 2

007

Jan

2008

Jul 2

008

Jan

2009

Jul 2

009

Jan

2010

Rai

n (m

m)

0

40

80

Fig. 1 Daily reference evapotranspiration (ET0) and rainfall during

the experiment

Irrigation treatment (% return ETp)

0 25 50 75 100 125S

easo

nal i

rrig

atio

n (m

m)

0

200

400

600

800

1000

2006200720082009

year

Fig. 2 Actual seasonal irrigation application for target irrigation

levels during the four experimental seasons. ETp is potential

evapotranspiration (reference ET X canopy cover factor)

Irrig Sci

123

Results

Tree water status

Midday stem water potential decreased with time over the

irrigation season as seen for the 2007 season in Fig. 3.

Well-watered trees had midday stem water potential of

approximately -1 MPa in May, -2 MPa in July, and -2.5

(Souri) to -3 MPa (Barnea) in September. Treatment-

related differences in stem water potential developed over

time (Fig. 3; Table 2). The Barnea trees, in an ‘on’ year

with higher fruit load, had consistently lower stem water

potential compared to the Souri trees that were in an ‘off’

year in 2007.

In each of the years, strong correlation of midday stem

water potential to irrigation level was found in September

measurements (Fig. 4; Table 3). In each year and for each

cultivar, extremely low values of stem water potential

ranging from -4 to -5 MPa were measured in the 30%

treatment. For both cultivars in 2006 and 2007 and for

Souri in 2008, stem water potential increased with

increased water application rate up to 100% return. For

Barnea in 2008 and both cultivars in 2009, stem water

potential was not differentiated for treatments of 50%

return and greater. The cultivar in its ‘off’ year consistently

had higher midday stem water potential in comparison with

that in its ‘on’ year as evidenced by higher absolute values

of September stem water potential across all the irrigation

treatments for the Barnea cultivar in 2006 and 2008 and for

the Souri cultivar in 2007 and 2009 (Fig. 4).

Growth

Trunk circumference was measured in March each year.

Initial measures of trunk circumference indicated high

uniformity in the orchard. Average circumference in March

2006 was 44.5 ± 3.6 cm and 44.6 ± 3.7 cm for Barnea

and Souri trees, respectively, with no differences between

treatment groups. Substantially higher measurements of

trunk growth were found in trees in ‘off’ years compared to

those in ‘on’ years (Fig. 5). Trunk expansion rates

increased with increased irrigation in ‘off’ years until

treatment levels 75–100%. In the first 2 years of the

experiment, the cultivars in ‘on’ years had relatively low

growth rates that were related linearly to water application

Irrigation treatment (% return of ETp)

0 25 50 75 100 125

Mid

day

stem

wat

er p

oten

tial (

MP

a)

-5

-4

-3

-2

-1

0

Souri May 17Barnea May 17 Souri July 02 Barnea July 02 Souri Sept 17Barnea Sept 17

Fig. 3 Stem water potential as a function of irrigation rate at three

dates during the 2007 irrigation season. Symbols are average of 10

replicate trees and lines are best-fit linear or 3-parameter exponential

rise to maximum curves. Regression equations and statistics are found

in Table 2

Table 2 Regression equations

and statistics for Fig. 2Cultivar Date Equation (Y = …) R2 P

Souri May 17 -1 ? 0.0019X 0.70 0.08

Barnea May 17 -1.4 ? 0.0032X 0.94 0.006

Souri July 2 -2.3 ? 0.84 9 (1 - exp(-0.013X)) 0.90 0.1

Barnea July 2 -3.1 ? 1.87 9 (1 - exp(-0.008X)) 0.96 0.04

Souri Sep 17 -13.4 ? 11.2 9 (1 - exp(-0.062X)) 0.98 0.02

Barnea Sep 17 -13.1 ? 10.4 9 (1 - exp(-0.055X)) 0.99 0.005

-5

-4

-3

-225 Sep 2008

Irrigation treatment (% return of ETp)

0 25 50 75 100 125 25 50 75 100 125

15 Sep 2009

Mid

day

stem

wat

er p

oten

tial (

MP

a)

-5

-4

-3

-2

-1

BarneaSouri

7002peS716002peS71

Fig. 4 Olive tree stem water potential measured in September of

each year. Heavy fruit loads (‘on’ years) were experienced in 2006

and 2008 in Souri trees and in 2007 and 2009 in Barnea. Symbols are

averages of 10 replicate trees. Lines are best-fit 3-parameter

exponential rise to maximum curves. Regression equations and

statistics are found in Table 3

Irrig Sci

123

rates throughout the range of treatments. In the third and

fourth years, the trunk growth of the cultivars in ‘on’ years

was not affected by irrigation rate (Fig. 5; Table 3).

Yields

Fruit yield

Fruit yield (kg/tree) increased slightly as a function of

increased irrigation for both varieties in 2006 and for Souri

in 2007 (Fig. 6; Table 3). Yield increased substantially as

irrigation increased to 100% treatment for Barnea in 2007,

but no significant difference was found between yields of

the 100 and 125% treatments. In 2008, fruit yield of both

varieties increased with increased irrigation rate up to the

100% treatment. In 2009, the Souri trees had essentially no

fruit and were not harvested. Barnea trees in 2009 strongly

responded to irrigation with the 50% treatment producing

double and the 75–125% levels nearly tripling the fruit

yield found for the 30% treatment. Throughout the four

experimental years, the maximum yields for trees in their

‘on’ year were measured in the 100% treatment.

Fruit number

Souri trees averaged only 2,000 fruits per tree in ‘off’ years

and some 12,500 fruits per tree in ‘on’ years. Barnea trees

had substantially greater numbers of fruit with an average

Table 3 Regression equations and statistics for Figs. 3, 4, 5, 6, 7, 8, and 9

Figure Cultivar Year Equation (Y = …) R2 P

3 Barnea 2006 15.7 ? 13.1(1 - exp(-0.074X)) 0.98 0.01

Souri 2006 9.6 ? 6.9(1 - exp(-0.041X)) 0.99 0.01

Barnea 2007 13.1 ? 10.4(1 - exp(-0.055X)) 0.99 0.005

Souri 2007 13.4 ? 11.2(1 - exp(-0.062X) 0.97 0.02

Barnea 2008 167 ? 164.2(1 - exp(-0.15X)) 0.99 0.001

Souri 2008 14.7 ? 11.5(1 - exp(-0.065X)) 0.99 0.003

Barnea 2009 -4482 ? 4479(1 - exp(-0.26X)) 0.98 0.003

Souri 2009 -108.9 ? 106.5(1 - exp(-0.14X)) 0.98 0.002

4 Barnea 2006-2007 -4.7 ? 15.5(1 - exp(0.045X)) 0.99 0.008

Souri 2006–2007 2.3 ? 0.04X 0.87 0.02

Barnea 2007–2008 3.5 ? 0.2X 0.24 0.01

Souri 2007–2008 -79.6 ? 88.5(1 - exp(0.10X)) 0.98 0.02

Barnea 2008–2009 -9.6 ? 14.1(1 - exp(0.06X)) 0.83 0.1

Souri 2009–2010 -11.1(1 - exp(0.04X)) 0.78 0.08

5 Barnea 2007 -36 ? 144.7(1 - exp(0.033X) 0.96 0.04

Souri 2007 7.1 ? 22.1(1 - exp(0.010X) 0.93 0.01

Barnea 2008 11.2 ? 50.6(1 - exp(0.012X)) 0.89 0.1

Souri 2008 -20.9 ? 76.7(1 - exp(0.040X)) 0.95 0.05

Barnea 2009 -127 ? 220.1(1 - exp(0.048X)) 0.96 0.04

6 Souri 2006 39011-20342 9 exp(-0.024X) 0.89 0.1

Barnea 2008 -22754 ? 41291 9 exp(-0.05X)) 0.93 0.07

Barnea 2009 -13071 ? 1322X - 7.7X2 0.98 0.02

7 Barnea 2006 14.2 9 exp(2.51/(X-25.5)) 0.98 0.02

Souri 2006 26.3 - 0.069X 0.96 0.003

Souri 2007 147.5 9 exp(11.6/(X - 11.1)) 0.99 0.002

Barnea 2008 18.4 9 exp(5.9/(X - 9.1)) 0.97 0.03

8 Barnea 2007 1.0 ? 0.82 9 exp(-0.61X) 0.99 0.02

Souri 2007 -0.91 ? 1.5(1 - exp(0.056X)) 0.99 0.01

Souri 2008 0.71 ? 0.91 9 exp(-0.91X) 0.98 0.02

Barnea 2009 0.29 ? 0.026 9 exp(0.018X) 0.97 0.03

9 Barnea 2007 0.57 ? 18.2(1 - exp(0.031X)) 0.99 0.05

Souri 2008 -12.8 ? 726.5(1 - exp(0.053X)) 0.97 0.03

Barnea 2009 -15.4 ? 32.5(1-exp(0.036X)) 0.96 0.04

Irrig Sci

123

of 9,500 and 34,000 fruits per tree in ‘off’ and ‘on’ years,

respectively. Starting in the second year for Barnea and

third year for Souri, the number of fruit per tree increased

with increasing irrigation rate up to the 75% treatment

level (Fig. 7; Table 3). In 2009, the amount of fruits in the

highest (125%) irrigation level was markedly reduced

compared to the 75 and 100% treatments as seen in the

statistically significant quadratic curve in Fig. 6-2009 and

defined in Table 3 (compared to the non-significant rise-to-

maximum regression curve). Maximum fruit per tree

reached over 20,000 for the 100% treatment in Souri trees

in 2008 and over 43,000 for the 100% treatment in Barnea

trees in 2009.

Oil in fruit

Percent oil in fruit was generally higher in ‘on’ than in ‘off’

years and decreased with increased irrigation level (Fig. 8;

Table 3). Percent oil in olives of both cultivars receiving

30% irrigation was consistently around 25%. In ‘on’ years,

2006-2007

3

6

9

12

2008-2009

Irrigation treatment (% return ETp)

Ann

ual t

runk

gro

wth

(cm

)

0

3

6

9

12 2009-2010

0 25 50 75 100 125 25 50 75 100 125

2007-2008

Barnea Souri

Fig. 5 Annual increases in trunk circumference measured in March

for the four experimental years. Heavy fruit loads (‘on’ years) were

experienced in 2006 and 2008 in Souri trees and in 2007 and 2009 in

Barnea. Symbols are average measured data (n = 10), lines are best-

fit regression of either linear or 2- or 3-parameter exponential rise to

maximum curves. Regression equations and statistics are found in

Table 3

2008

Irrigation treatment (% return ETp)

0 25 50 75 100 1250

30

60

90

2009

25 50 75 100 125

20072006

Fru

it yi

eld

(kg/

tree

)

0

30

60

90

120

BarneaSouri

Fig. 6 Olive fruit yield as a function of irrigation water application

rate. Symbols are average measured values (n = 10) and lines are

best-fit 3-parameter exponential rise to maximum curves. Dotted linesare not significant. Regression equations and statistics are found in

Table 3

Fru

its p

er tr

ee (

#)

0

10000

20000

30000

40000

50000

Barnea Souri

2006 2007

Irrigation treatment (% return ETp)

0

10000

20000

30000

40000

500002008

0 25 50 75 100 125 25 50 75 100 125

2009

Fig. 7 Average number of olive fruits per tree as a function of

irrigation water application rate. Symbols are average measured

values (n = 10), and lines are best-fit regression curves. Dotted linesare not significant. Regression equations and statistics are found in

Table 3

2008

Irrigation treatment (% return ETp)

10

20

302009

0 25 50 75 100 125 25 50 75 100 125

20072006

% o

il in

frui

t

10

20

30 Barnea Souri

Fig. 8 Percent oil by chemical extraction in olive fruit wet weight as

a function of irrigation water application rate. Heavy fruit loads (‘on’

years) were experienced in 2006 and 2008 in Souri trees and in 2007

and 2009 in Barnea. Symbols are average measured values (n = 10),

and lines are best-fit exponential reduction or linear regression curves.

Dotted lines are not significant. Regression equations and statistics are

found in Table 3

Irrig Sci

123

this decreased with irrigation amount fairly linearly to as

low as 18% (for Souri in 2006). In ‘off’ years, increasing

irrigation from 30 to 50% dropped oil percentages to as low

as 15%.

The total oil produced per fruit (calculated by multi-

plying fruit weight by % oil) was influenced inconsistently

by irrigation level (Fig. 9; Table 3). In the initial year of

the experiment, irrigation did not affect oil per fruit. In

2007, oil per fruit increased in Barnea (‘on’ year) and

decreased in Souri (‘off’ year) with the greatest effect

occurring between the 30 and 50% treatment levels. In

2008, both varieties showed trends of decreasing oil per

fruit with increased irrigation (Barnea significant, Souri

not), and in 2009 oil per fruit of Barnea trees again was

augmented by irrigation, but this time with greater relative

increases as irrigation level increased.

Oil yield

Oil yield was calculated as fruit yield multiplied by

chemically extracted oil percentage and is shown in

Fig. 10. Oil yield ranged from zero to almost 10 kg per tree

in ‘off’ years’ and from 6 to 19 kg per tree in ‘on’ years. In

the first year of the experiment, 2006, no effect of irrigation

was found on oil yield. In the second year, 2007, significant

increases in oil yield as a function of irrigation level were

found for Barnea, which was in an ‘on’ year, while no

effect was seen for Souri in an ‘off’ year. In the third year

of the experiment, 2008, significant increases in oil yield

from 30 to 75% return rates were found for Souri in an ‘on’

year, and a non-significant trend of increased oil with

increased water application was indicated for Barnea in an

‘off’ year. In the fourth year, 2009, the Souri trees pro-

duced no fruit and therefore no oil, and Barnea showed

strong increases in oil yield as a function of increased

irrigation until the 100% return treatment level.

Oil extraction efficiency determined as the ratio of % oil

from cold-press (Abencor) to % oil from chemical

extraction (Sohxlet) ranged from near 1 to 0.6. This ratio

was generally higher for Souri compared to Barnea and

consistently decreased for both varieties as a function of

increased irrigation treatment and increased water content

of fruit.

Discussion

In this study, increasing irrigation increased stem water

potential (Figs. 3, 4), vegetative growth (Fig. 5), and olive

fruit yield with the increases tapering off at application

rates above the 75–100% treatments (Fig. 6). Measured

tree status and yield parameters were highly affected by

fruit load. Trees in ‘on’ years generally had lower vege-

tative growth (Fig. 5), lower stem water potential (Fig. 4),

greater fruit yields (Fig. 6) but smaller individual fruits,

had higher oil percentages (Fig. 8), and had higher oil

yields (Fig. 10).

Oil yield increases as a function of increased irrigation

were initiated only in the second year of treatments, and

only for the cultivar Barnea for which the first year was an

‘off’ season when the treatments lead to higher vegetative

growth. Yields of fruit and oil for Souri trees were sig-

nificantly improved by irrigation just in the third year of the

experiment, which was this variety’s only ‘on’ year

Oil

per

frui

t (g)

0.0

0.3

0.6

0.9

1.2

Barnea Souri

2006 2007

Irrigation treatment (% return ETp)

0.0

0.3

0.6

0.9

1.2 2008

0 25 50 75 100 125 25 50 75 100 125

2009

Fig. 9 Average olive oil content of individual fruits as a function of

irrigation water application rate. Heavy fruit loads (‘‘on’’ years) were

experienced in 2006 and 2008 in Souri trees and in 2007 and 2009 in

Barnea. Symbols are average measured values (n = 10), and lines are

best-fit regression curves. Dotted lines are not significant. Regression

equations and statistics are found in Table 3

2009

25 50 75 100 125

2008

Irrigation treatment (% return ETp)

0 25 50 75 100 1250

5

10

15

20072006

Oil

yiel

d (k

g/tr

ee)

0

5

10

15

20

BarneaSouri

Fig. 10 Olive oil yield as a function of irrigation water application

rate. Symbols are average measured values (n = 10), and lines are

best-fit 3-parameter exponential rise to maximum or linear regression

curves. Dotted lines are not significant. Regression equations and

statistics are found in Table 3

Irrig Sci

123

subsequent to an ‘off’ year under irrigation treatments. We

suspect that fruit yield would continue to show an accu-

mulated positive response to irrigation in both ‘on’ and

‘off’ years, assuming continuation of treatment levels and

development of trees with greater fruit-bearing capacity.

The effects of deficit irrigation regimes on tree water

status developed each season gradually as moisture stored

in the soil from winter rains was depleted and climate

demand increased to maximum levels. The dynamic of

little to no stress in the first months of annual irrigation,

moderate stress in July and maximum stress in September,

measured as decreasing stem water potential in trees

receiving deficit irrigation applications, closely followed

those found in Tunisia (Guerfel et al. 2007) and are gen-

erally comparable with data from drought-stressed or def-

icit-irrigated orchards in Portugal (Ramos and Santos

2010), Italy (d’Andria et al. 2009; Giorio et al. 1999), and

Spain (Moriana et al. 2003, 2007; Perez-Lopez et al. 2007).

This pattern of dynamic decreasing of stem water potential

throughout the summer was found in additional studies to

be accompanied by reduced stomatal conductance (Ben-

Gal et al. 2009; Guerfel et al. 2007; Moriana et al. 2007)

and diminished photosynthetic capacity (Dıaz-Espejo et al.

2006; Guerfel et al. 2007). Of course, in many of the tra-

ditional Mediterranean olive-growing regions, there often

is significant rainfall occurring later into the spring, and

midsummer climate conditions at these locations will often

be less demanding regarding ET requirements than those at

our site in Israel. In such cases, tree water status would be

more slowly affected and reach less severe levels at

equivalent relative irrigation return rates. In all variations

of Mediterranean climates, little if any water stress would

be expected during flowering and fruit set stages, and

therefore management strategies for drought avoidance

during the spring are likely not effective or necessary. This

study supports others that indicate late season water stress,

following pit hardening and accompanying oil accumula-

tion in olive fruits, as particularly harmful to oil production

(Inglese et al. 1996; Moriana et al. 2007; Tognetti et al.

2006).

The results suggest that mechanisms for drought

avoidance in olives, involving stomatal closure, water

conservation, and maintenance of tree water potential and

turgor, are fruit load specific. Trees with a large number of

fruits will keep stomates open, continue transpiration, dry

their soil, and enter higher levels of water stress, compared

to trees with low fruit loads. This apparent favoring of

photosynthesis and supply of assimilates to the growing

fruits when fruit load is high, at the expense of water

budget and entering into water stress, has been well doc-

umented for apple (Fuji and Kennedy 1985; Giuliani et al.

1997; Gucci et al. 1994; Palmer 1992; Wibbe et al. 1993;

Wunsche and Palmer 1997; Wunsche et al. 2000) and

indicated for other deciduous fruit trees (DeJong 1986;

Marsal et al. 2005; Naor et al. 1997; Naor et al. 2008). This

theory, strongly supported by the current results, has been

suggested for olives (Moriana et al. 2002, 2003) but was

not found in a number of studies on ‘Leccino’ olives in

Italy. In separate 2-year studies, Proietti (2000) found no

influence of the presence or absence of fruit on leaf pho-

tosynthetic activity, and Gucci et al. (2007) reported no

effect of crop load on leaf water potential.

Deficit irrigation led to decreased biomass growth in this

study during all ‘off’ years and in the first years of the

experiment in ‘on’ years as well. While there is little data

concerning biomass or canopy growth reported in the lit-

erature on deficit irrigation of olives, it is generally

understood that shoot growth and canopy size in olives

increase as a function of sustained water supply (Connor

2005; Gomez-del-Campo et al. 2008; Tognetti et al. 2006).

Similar to us, d’Andria et al. (2004) reported significant

increased accumulated trunk growth as a function of

increased irrigation after 4 years of treatments, but they did

not supply annual data. Slightly different results compared

to ours were found by Perez-Lopez et al. (2007) who

reported decreased shoot growth rates for deficit regimes in

the second and third years of their study, but not in the first

year. Some enlightenment might be found in the Fernandez

and Cuevas (2010) review of irrigation scheduling using

stem diameter variations where they showed that trunk

growth rates were highly dependent on fruit load. Olive

trees in ‘on’ years grew very fast early in the spring and

until some 4 weeks after full bloom, but thereafter their

growth rate became very low to negligible. Conversely,

trees with light loads grew steadily throughout the season.

The stronger effect of water stress on growth of the trees in

‘off’ years in our current study could therefore be expected.

More intriguing is the fact that ‘on’ year trunk growth was

linearly increased by increased water application in the first

2 years of the experiment (for a single season for each

variety), but this did not continue in the next cycle of

alternative bearing (Fig. 4).

The literature suggests that fruit and oil yield response

to irrigation is highly cultivar specific (Lavee et al. 2007).

Ramos and Santos (2010) did not report differences in fruit

or oil yield between fully and deficit-irrigated mature

‘‘Cordovil’’ olives in their 2-year study. Moriana et al.

(2007) did find greater fruit and oil yields in well-irrigated

compared to deficit-irrigated ‘‘Cornicabra’’ trees in their

initial year when the trees were ‘on’ but no influence of

irrigation regime on yields in the subsequent ‘off’ year.

Lavee et al. (2007) found that increased yield due to

increased irrigation in the cultivar ‘‘Muhasan’’ was solely

due to increased fruit number. Oil per fruit was no greater

in well-irrigated compared to deficit-irrigated treatments.

Additionally, oil extraction in the well-irrigated fruits was

Irrig Sci

123

less efficient than in the less-irrigated fruits in their study.

Results of d’Andria et al.’s (2009) 4-year study were

similar to ours as total accumulated fruit and oil yield both

increased linearly as water application increased within

their four irrigation regime treatment levels from 0 to 100%

ETc. Patumi et al. (1999, 2002) and Tognetti et al. (2006)

also reported positive correlations between irrigation level

and fruit and oil yield. Regarding ‘on’ and ‘off’ relation-

ships between fruit and oil yield and irrigation rate, Ramos

and Santos (2010) reported irrigation treatments leading to

particularly high stress levels decreased relative yields

more in the ‘off’ than in the ‘on’ year of their 2-year study.

Perez-Lopez et al. (2007) showed a linear relationship

between canopy volume and yield in the 2 ‘on’ years and

no such relationship in the single ‘off’ year of their 3-year

study.

Studies of olives are expected to benefit from single

tree-level scrutiny, allowing sensitivity analysis of results

to variables expected to be naturally non-uniform as well as

affected by irrigation management. Therefore, analysis of

oil yields of individual trees was conducted in this study to

help determine how much of the yield effect was due to

trees with more fruits and how much was due to more oil

production per fruit (Figs. 11, 12). Tree-scale oil yield was

consistently correlated with fruit load (number of fruits per

tree) as seen in Figs. 11 and 12 for both Barnea and Souri,

and sometimes correlated with oil per fruit for Barnea only.

Fruit load was clearly both a function of natural variations

and 4 years of irrigation regime. Whole tree oil yield was

highly correlated with number of fruits on a tree but, as

seen in the lower treatment levels sitting below the

regression line in Fig. 11h and the higher treatments sitting

above it, it is not the only relevant factor. At least for

Barnea in ‘on’ years, it is evident that oil per fruit increased

by increased irrigation also plays a role (Fig. 11c, g). This

is additionally supported by the ‘‘separation’’ of the treat-

ments across the fruit load-oil yield regression line in

Fig. 10d and h. No such per fruit increases in oil were

evident for Souri where oil per fruit was negatively cor-

related with tree oil yield (Fig. 12e) and where the treat-

ments spread evenly along the regression line in Fig. 12f.

The results therefore suggest a dual mechanism for

increased tree-level oil yields as a function of increased

irrigation for the Barnea cultivar. The major effect, seen

distinguishing the most severe deficit regime from all the

other water application rates, was due to trees with higher

fruit-carrying capacity. Trees receiving greater amounts of

water in ‘off’ seasons had greater vegetative growth that

supported a larger number of fruits the following year. The

second, minor, effect was increased amount of oil per fruit

found as a function of increased irrigation rate, especially

at the highest treatment levels. Greater irrigation caused

larger fruits with more oil per fruit. This is in spite of the

fact that % oil in fruit was reduced as a function of

increased irrigation (Fig. 8). Similar results were presented

by d’Andria et al. (2009) who found linear increases of

fruit number per tree, fruit yield and oil yield (averages for

4 years) for both ‘‘Leccino’’ and ‘‘Frantoio’’ cultivars. But

fruit number could not explain all of the yield response in

that study as fruit yield decreased 41%, oil yield 37%, and

fruit number only 23% when irrigation level decreased

from 100% ETc. to 0% ETc.

The oil quality parameters from this study, which will be

presented in detail in a separate publication, reconfirm

previous findings that increasing irrigation leads to fruits

with greater water content (lower oil percentage) as well as

to relative decreases in total polyphenol content and

sometimes to increased free fatty acids (Dag et al. 2008).

They therefore support studies that have suggested conflicts

between olive oil yield and quality relating to water

application (Lavee et al. 2007; D’Andria et al. 2009;

Dabbou et al. 2010; Moriana et al. 2007) and that have

indicated that deficit irrigation regimes are likely to opti-

mize oil production. Water stress management for target oil

quality is becoming more popular and seems to become

a likely aspect of orchard management—especially in

g

Oil per fruit (g)

0

10

20

30

e

0

10

20

30

b

Y=-0.08+7.34X

R2=0.90, P<0.0001

c

Oil

yiel

d (k

g/tr

ee)

0

10

20

30

Y=1.66+32.6X

R2=0.55, P<0.0001

d

Y=7.20+0.0003X

R2=0.19, P=0.0025

h

Fruit load (# fruits/tree)

0.0 0.3 0.6 0.9 1.2 0 20000 40000 60000 80000

Y=4.28+0.0003X

R2=0.60, P<0.0001

f

Y=3.24+0.0004X

R2=0.88, P<0.0001

2007

a

0

10

20

30 305075100125

2006

2008

2009

Fig. 11 Correlation between tree-scale oil yield and oil per fruit (left)and fruit per tree (right) for individual Barnea trees. Markers are

measured data, and the lines are best-fit linear regression for each

entire data set for each season. Dotted lines are not significant

Irrig Sci

123

high-density orchards and orchards where relatively high

irrigation regimes are dictated (saline and wastewater

irrigation). It is likely that regulated, temporal, deficit

irrigation during certain olive growth stages will eventually

target desired water stress levels in attempts to secure

quality levels without harming yields. Additionally, the

fact that efficiency of cold-pressed techniques for oil

extraction decreased as irrigation level/water content of

fruit increased supports earlier work, suggesting that irri-

gation regimes should be properly matched to (and/or with)

extraction methods (Giovacchino et al. 1994; Dag et al.

2008).

Many of the parameters measured in this study are sus-

pected to be cultivar and environment specific (Fernandez

and Moreno 1999; Lavee et al. 2007). The unique case

where two varieties were in opposite alternate bearing

cycles allowed interpretation of the relative importance of

water availability across cultivars while considering irri-

gation and bearing levels. The absolute values of parame-

ters responding to irrigation/stress level were not cultivar

related, were very highly correlated to fruit load, and only

secondarily were a function of irrigation level.

The results suggest a number of likely methods to

improve irrigation management including irrigation

regimes that are cultivar specific and irrigation systems that

cater to each variety individually. Stress levels and water

requirements are highly dependent on fruit load and best

irrigation management must account for biannual bearing

effects. More work is necessary in order to confirm these

data over an even longer term, to determine optimum water

stress scheduling (stress levels and timing) for best yield–

quality combinations, and to further develop methods for

monitoring and maintaining water stress levels.

Acknowledgments The research was supported by The Chief Sci-

entist of Israel’s Ministry of Agriculture and Rural Development

(project #303-0300), the Netafim Company, and The USDA-ARS via

The Middle East Regional Irrigation Management Information Sys-

tem (MERIMIS). Thanks to TZ’’ABAR K’’AMA for providing the

orchard and orchard management support and to Professor Shimon

Lavee for consultation and advice.

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