The pistachio tree; botany and physiology and factors that affect yield

the tree

THE PISTACHIO TREE; BOTANY AND PHYSIOLOGY

AND FACTORS THAT AFFECT YIELD

Louise Ferguson, Vito Polito and Craig Kallsen

he pistachio is the single most successful

plant introduction to the United States in

the twentieth century.

ORIGIN AND HISTORY

The pistachio is native to western Asia and

Asia Minor, where it is still found growing

wild in numerous hot, dry locations in

Lebanon, Palestine, Syria, Iran, Iraq, India,

Southern Europe and the desert countries of

Asia and Africa. It was introduced to Europe at

the beginning of the Christian era. The first

pistachio introductions to the United States

were by the USDA plant exploration service in

1890. The first California introductions were

planted at the Plant Introduction Station in

Chico, California in the Northern Sacramento

Valley in 1904.

BOTANICAL CLASSIFICATION

The pistachio of commerce is the only edible

species among the 11 species in the genus

Pistacia; all are characterized by their ability to

exude turpentine or mastic. Several are referred

to as pistachios, but the name is generally

reserved for the edible nut of commerce. Its

Latin name is Pistacia vera L. A member of

the family Anacardiaceae, it is related to the

cashew, mango, poison ivy and oak, pepper

tree and sumac.

The tree has a pinnately compound leaf.

Each leaf subtends a single axillary bud. Most

of these lateral axillary buds differentiate into

inflorescence primordia and produce a nut-

bearing rachis the following year; thus,

pistachios bear laterally on one-year-old wood.

Botanically, pistachio nuts are drupes, the same

classification for almonds, peaches, apricots,

cherries and plums. All drupes consist of three

parts; an exocarp, a fleshy mesocarp and an

endocarp that encloses a seed. The difference

lies in the edible portion. In pistachios and

almonds the seed is consumed, rather than the

mesocarp as in stone fruit.

The pistachio tree is dioecious (i.e. two

houses”), meaning the male flowers are borne

on one tree and the female flowers on another.

Therefore, both male and female trees are

required to produce nuts. The female flower is

apetalous (no petals) and has no nectarines,

thus does not attract bees. The pollen is spread

by wind. The pistachio tree is deciduous, so it

loses its leaves in the fall and remains dormant

through the winter.

The rooting habit of the tree is classified as

a phreatophyte. Phreatophytes have extensive

root systems allowing them to mine the soil

deeply. Thus, pistachios are adapted to survive

long periods of drought.

Pistachios are characterized by a long

juvenile period, typically bearing few nuts

before five years of age. They achieve full

bearing between 10 to 12 years of age. The tree

has an upright growth habit characterized by a

strong apical dominance and a lack of lateral

vegetative buds in older trees. These

characteristics have strong implications for

young tree training, mature tree pruning and

rejuvenation of fruiting wood in older trees.

CLIMATIC REQUIREMENTS

Areas suitable for pistachio production have

long, hot, dry summers and moderate winters.

Pistachios grow best in areas with 2200-2800

heat units.

Heat Units = X Number of days

In month (April-Oct)

Mean Monthly Mean Monthly

Tmax

Tmin

Generally, pistachios should not be planted

above 2500 feet where summer heat is usually

insufficient for complete kernel development.

Elevations of 200 to 800 feet have proven ideal

in the central California valleys. Late spring

rains, frosts, and strong desiccating winds

interfere with pollination. High humidity

through the growing season promotes fungal

diseases that subsequently overwinter on both

male and female trees and reinoculate the tree

the following season. Strong winds are

generally detrimental to young tree training.

Historically, for both the female Kerman

and male Peters to produce good, even, timely

budbreak, normal inflorescences, viable pollen,

good fruit set, and normal vegetative growth,

pistachios in California have required at least

900 winter chilling hours below 45

F (7.2

C).

When cumulative hours below 45

F (7.2

have fallen to 670, as they did in 1977-78, the

bloom and foliation have been irregular and

delayed, leaves deformed and yield reduced.

Pistachios can be successfully grown on a

number of soil types. In California, the Pacheco

sandy loams of the southwest San Joaquin

Valley produce the best yields. In areas with

shallow hardpan soils, tree size and

productivity are limited. The tree grows best

on well-drained soils and is intolerant of

saturated conditions. It appears to tolerate

alkalinity and salinity well. The topic of soils

and their modification for pistachio production

is discussed later, in the chapter 4, “Site

Evaluation and Soil Modification” in this

manual.

As stated earlier, pistachios are

phreatophytes and as such can survive harsh

climates without irrigation. Also, the stomata

on their leaves are somewhat less sensitive to

desiccating conditions than stomata on many

other trees. Therefore, pistachios can transpire

a great deal of water under San Joaquin Valley

conditions. The result is a tree that is adapted

for survival, but for economic production,

adequate irrigation is necessary. Irrigation of

pistachios, how much, when, and the method of

application, have important implications for

production. Pistachio irrigation has a

significant impact on young tree development,

soil-borne and aerial diseases, crop yield and

quality (both current and subsequent years) and

tree growth. Irrigation and its impact on these

processes is discussed in chapter 13, “Tree

Water Requirements and Regulated Deficit

Irrigation” in this manual.

SEASONAL PHENOLOGY

The lateral axillary inflorescence buds on one-

year-old wood begin to swell in late March.

Within the first two weeks of April, the 100 to

300 flowers per paniculate rachis are pollinated

and set. Throughout the balance of April and

May the nut shell (endocarp), but not the

nutmeat (seed), enlarges. Through this period

the nut shell is soft and vulnerable to insect

attack and the splitting that appears to be a

result of rain. In June the nut shell hardens, and

from late June through early August the

nutmeat enlarges until it fills the shell. Through

late August and September the nut ripens, the

radial suture around the shell’s long

circumference splits, the hull degrades, and

abscission of the individual nut from the rachis

commences.

Shoot growth is simultaneous with shell

growth. Growth begins in late April and

concludes in late May. The new extension

growth produces pinnately compound leaves

with lateral inflorescence buds in the axils and,

generally, a single apical vegetative bud. The

buds differentiate throughout April, May and

June, become quiescent in July, August and

September, and resume differentiation in

October. Sometimes there is an additional flush

of shoot growth in late June. This growth

produces primarily vegetative lateral buds as

opposed to the inflorescence buds produced by

the spring flush. In August, leaves distal to

heavy fruit clusters often display a marked

depletion and senescence. Most leaves drop by

the end of November, and the tree remains

dormant through the following March. As the

trees mature, their strong apical dominance

becomes more marked.

PHYSIOLOGICAL PROBLEMS

Pistachios display three physiological

conditions. The first is alternate bearing; an

annual fluctuation of large crops with poor

crops. The second is the production of blank, or

unfilled nuts. The third is nonsplit nuts, nuts

that fail to split along the lateral nut suture. All

three phenomena appear to be ultimately

related to crop load and are therefore probably

related to carbohydrate competition. Thus far,

little is known about the specific mechanism of

each, though correlation with crop load is

apparent in each case.

Alternate bearing

As stated earlier, pistachios bear laterally on

one-year-old wood. As the trees age, they

develop an alternate bearing pattern with

increasingly large and small crops. Though the

specific mechanism of this phenomenon has

not been defined, evidence suggests that it is a

problem of carbohydrate competition, perhaps

mediated by growth regulator signals. During

the period of nut fill in July, the fruit buds

distal to fruit clusters die and abscise. The

heavier the currently borne crop, the greater the

subtending bud abscission. Thus, following a

heavy crop year, an individual branch may bear

no fruit. Attempts to alleviate the cycle by

nutritional and growth regulator sprays have

not been successful. However, some success in

damping the swing has been achieved with

rejuvenation pruning of older trees. Currently,

pruning appears to be the only method

available to mitigate alternate bearing.

Alternate bearing has not been demonstrated to

harmful to the tree and may therefore only be a

marketing problem. Alternate bearing is not

unique to pistachio trees. Several types of fruit

trees alternate bear. However, only pistachios

appear to possess the phenomenon of

premature bud abscission as the mechanism

that produces alternate bearing.

INTRODUCTION TO BLANK AND

NONSPLIT NUTS

Pistachio fruits consist of a nutmeat (kernel)

enclosed in a thin, hard shell (endocarp)

surrounded by a fleshy hull (mesocarp and

exocarp). The fruit grows from the pistil of the

female flower. The pistil has a single ovary at

its base. The ovary forms the fruit wall that

includes the shell and the hull. Within the

ovary is a single ovule. The ovule, which

contains the female germ cell (egg), will

develop into the edible kernel of the nut.

Extending from the ovary is a three-part style.

Each of the three parts of the style terminates in

a stigma. As the flower opens, these stigma

surfaces become receptive to pollen.

Fruit set typically follows successful

pollination. Pollen is released from staminate

flowers on male trees and is carried by air

currents to stigmas of the pistillate flowers.

When a pollen grain lands on a receptive

stigma surface, it germinates to form a pollen

tube. The pollen tube is an elongate cell that

grows through sigma and style tissue to the

basal ovary and into the ovule. As it does so, it

carries male germ cells to the egg cell. Many

pollen tubes germinate and grow through the

style, but only one successfully reaches the

ovule. That pollen tube enters the ovule and

releases its contents. Fertilization involves the

fusion of a male germ cell with the egg cell.

Thus, the reproductive process leading to fruit

set in pistachio can be seen in three parts:

pollination, involving the transfer of pollen to

the stigma; pollen tube growth, where the male

germ cells are transferred through the stigma

and style to the ovule, and fertilization, the

fusion of the male and female germ cells in the

ovule.

The fusion of the male and female germ

cells produces a single-celled zygote that

eventually grows to form an embryonic plant.

This embryonic plant comprises the kernel.

This process begins slowly, however. The first

division of the zygote does not occur for

several weeks after flowering during which

time the ovary grows to its final size. After

ovary growth is complete, the kernel grows to

fill it. This is an unusual pattern of growth, and

the differential timing of pistachio ovary and

kernel development has implications for both

blank nut production and shell splitting.

BLANK NUTS

Blank nuts result when there is fruit set and

ovary growth, but the embryo fails to grow,

leaving the nut shell empty or blank. Blanking

can occur during two different phases of

pistachio nut development, nut setting and nut

filling. It can be affected by crop load and

production practices.

Blanking during nut set

Chronologically, the first empty shells (blanks)

are produced as a result of events that occur at

the time of fruit set. This can occur under a set

of circumstances where pollination occurs but

fertilization fails either because pollen tubes do

not complete growth to the ovule, or the ovule

is not viable when the pollen tubes do arrive.

Under this scenario, the stimulus of pollination

and/or pollen tube growth is sufficient to

induce fruit set, but the failure of successful

fertilization means there is no embryo formed,

so there is no kernel to fill the shell. This

phenomenon of fruit set without fertilization is

called parthenocarpy and is the basis for the

production of several types of seedless fruits,

including some seedless citrus varieties.

Parthenocarpy is fairly common among plants,

however, it normally is found in fruits that have

many seeds rather than in single-seeded fruits

such as the pistachio.

There is some experimental evidence that

pollination-induced parthenocarpy is a

potential mechanism leading to blank pistachio

nuts. In one study, flowers were pollinated with

pollen that had been exposed to a high dose of

gamma radiation. The radiation treatment was

at a threshold level that permitted pollen

germination but inhibited full pollen tube

growth. These experiments resulted in a high

percentage of blank nuts. One explanation is

that the pollination stimulus, which was not

eliminated in the irradiated pollen, is sufficient

to set the fruit, most likely by triggering a

hormonally mediated signal that leads to fruit

set. Research is currently underway to

determine if this finding has implications in the

field.

This form of blanking may be associated

with poor boron nutrition. Boron is known to

be involved in several important aspects of

plant reproductive biology, including pollen

tube growth and ovule longevity, both of which

may have a role in pollination-induced

parthenocarpy. Research has demonstrated

boron leaf levels below 120 ppm dry weight

(August leaf sample) are associated with an

increased percentage of blank nuts at harvest.

Blanking during nut fill

Blanks may also develop in July during kernel

enlargement, when a certain percentage of the

fertilized embryos fail to enlarge to fill the

shell. It is not known what determines the

percentage of nuts filled, but it is suspected the

tree’s stored carbohydrate capacity initially

determines the percentage of filled nuts. This

theory is corroborated by demonstrations that

thinning a cluster prior to nut growth will result

in a higher percentage of filled nuts on the

thinned cluster. However, the thinned and

unthinned clusters have virtually the same

absolute number of filled nuts, though the

thinned clusters may have slightly larger nuts.

This also has been demonstrated on a whole

tree scale with pruning experiments.

Blanking is more sensitive to insufficient

irrigation than is splitting.

Effect of alternate bearing on blanking

Production of blank nuts is strongly affected by

alternate bearing. As can be seen in Table 1,

the percentage of blank nuts is always higher in

the ‘off’ crop year. This is a further, though not

proven, corroboration of the idea that the

carbohydrate status of the tree entering a crop

year sets the limits on the crop load a tree is

able to set and carry through maturation to

splitting. Markedly different crop loads within

a given crop year (as shown in Table 1) had

virtually the same percentage of blank nuts. For

example, in 1989 the control trees with 0.2

pounds of crop per tree had 19.1% blanks,

while the topped and hedged trees with 11.4

pounds of crop had 16.0% blanks. There was

no statistically significant difference in these

percentages despite the difference in crop

loads. Plate 3A shows “on” and “off” current

year pistachio shoots in mid July after axillary

Pistachio nuts may split along the

longitudinal ridges of the shell and at the tip of

the shell. Splitting can occur in any

combination of one or both of the longitudinal

ridges, with or without the tip splitting, or at

the tip alone. Investigation of the anatomical

structure of the longitudinal and tip split

regions indicates that these parts of the shell

differ from one another structurally suggesting

that different mechanisms may be involved in

shell separation at each site.

flower bud abscission. The “on” year shoot is

the one with nuts.

Conclusion

At present, there appears to be very little that

can be done to affect the percentage of

blanking. However, maintaining boron leaf

levels above 120 ppm and providing sufficient

water to avoid water stress during the season

will at least avoid exacerbation of blank nut

production.

Shell splitting is dependent upon nutmeat

growth and development within the shell.

Figure 3a shows the relationship between shell

split and kernel growth. Note that kernel

growth begins after the shell has reached its

full size. The first split shells are seen about the

time the kernel has grown to fill the shell and

would be exerting physical pressure on it. At

this time, the shell is fully lignified (hardened),

and the cells that form the regions where

longitudinal splitting will occur are dead. This

fact would seem to rule out the possibility that

biochemical factors are involved as a

controlling mechanism in shell split making it

unlikely that a chemical agent to enhance

splitting will be discovered.

NON SPLIT NUTS

The edible pistachio, unlike the species used

for rootstocks, is characterized by splitting of

the nut shell at maturity. Splitting begins about

the end of July, at least one month before fruit

maturity, and continues through mid-

September, progressing simultaneously with

nutmeat maturation. Final nutmeat maturity is

indicated by separation of the hull from the

shell. This is accompanied by a breakdown of

chlorophyll pigments in the hull, allowing the

red pigments to become visible. Thus, the most

obvious indicator of shell splitting is the

appearance of red color in the hull.

Figure 3a. This graph illustrates kernel and shell growth. The major (in the plane of the longitudinal

split lines) and minor (perpendicular to the plane of the longitudinal split lines) diameters are shown.

Data points represent means of 20 samples. The arrow indicates the time split shells were first seen in

any nut (from Polito, V. S. and K. Pinney. 1999. Endocarp dehiscence in pistachio [Pistacia vera L.]

Internat. J. Plant Sci. 160:827-835).

170 175 180 185 190 195 200 205 210 215 220 225 230

Julian Date

Size (mm)

First

Split

Major Diameter (S)

Minor Diameter (S)

Major Diameter (K)

Minor Diameter (K)

July

June

ugust

Table 1. Effect of 'on' and 'off' year versus individual tree crop load on the percentages of blank,

split and non-split nuts (by weight).

Year & Non-

Treatment Tree yield Blank Split Split

lbs/tree % % %

1985 'Off'

Control 6.4 a 8.1 a 86.4 a 3.2 a

Hedge 4.6 b 7.2 b 86.1 a 3.0 a

Top 3.7 b 8.4 a 84.9 a 2.9 a

Hedge & Top 1.8 c 8.1 a 85.8 a 2.7 a

Average 8.0 85.8 3.0

1986

'On'

Control 49.2 a 2.1 a 63.4 a 30.4 a

Hedge 44.6 a 2.5 a 65.5 a 28.1 a

Top 39.9 b 2.9 a 64.2 a 30.1 a

Hedge & Top 28.1 c 2.6 a 62.5 a 29.4 a

Average 2.5 63.9 29.5

1987

'Off'

Control 3.5 c 10.1 ab 83.3 ab 3.7 a

Hedge 7.6 b 8.5 b 83.8 a 3.1 a

Top 11.7 a 11.5 a 80.3 b 3.7 a

Hedge & Top 14.2 a 10.2 ab 83.1 ab 3.6 a

Average 10.1 82.6 3.5

1988

'On'

Control 34.1 a 2.7 a 72.9 a 20.3 a

Hedge 26.3 b 2.8 a 72.6 a 21.3 a

Top 24.5 b 2.2 a 72.3 a 22.4 a

Hedge & Top 26.2 b 2.2 a 72.6 a 22.7 a

Average 2.5 72.6 21.6

1989

'Off'

Control 0.2 c 19.1 a 47.6 c 19.1 a

Hedge 0.6 c 17.7 b 61.2 b 13.5 c

Top 6.0 b 14.1 c 63.6 a 15.1 b

Hedge & Top 11.4 a 16.0 a 64.4 a 12.8 c

Average 16.7 59.2 15.1

1990

'On'

Control 37.1 a 11.2 a 67.5 a 21.1 a

Hedge 35.3 a 9.3 a 68.5 a 22.1 a

Top 30.4 b 11.7 a 63.5 a 22.8 a

Hedge & Top 27.2 b 13.8 a 67.4 a 18.7 a

Average 11.5 66.7 21.2

1991

'Off'

Control 3.0 c 19.2 a 59.0 a 21.1 b

Hedge 4.2 c 16.0 a 57.1 ab 26.0 b

Top 17.2 b 15.1 a 50.1 bc 34.0 a

Hedge & Top 25.8 a 17.0 a 47.1 c 35.1 a

Average 16.8 53.3 29.1

* Values within a crop year column followed by the same letter are statistically equal.

There is a correlation between kernel-to-

shell size ratios and longitudinal splitting.

Table 2 shows the relationship of kernel

diameters to inner shell diameters for six

samples of undried, filled nuts. In each case,

the ratio of kernel to shell size is greater for

fully split nuts than it is for tip-split or unsplit

nuts. Interestingly, there is no statistical

difference between tip-split and unsplit nuts, a

finding that is consistent with anatomical

indications that tip split and longitudinal split

involve different mechanisms. Furthermore, for

fully split nuts, the kernel-to-shell diameter

ratios in the minor axis, i.e. perpendicular to

the plane of the longitudinal ridges, is

consistently greater than one. This means that

for fully split nuts, kernel size is greater than

shell size in the direction where force against

the inside of the shell would tend to drive the

shell halves apart. These correlations would

seem to implicate mechanical force generated

by the growing kernel against the shell as the

mechanism for shell splitting. It should be

noted, however, that experimental evidence to

verify this inference is lacking.

Table 2. Ratio of kernel to shell (K:S) diameters for undried, filled nuts. Nuts were considered in

three categories: nuts that had split fully along both longitudinal ridges, nuts that had split at the tip of

the shell only, and nuts with unsplit shells. Ratios are for kernel diameter and shell inner diameter at

their widest point. The major diameter is in the plane parallel to the longitudinal ridges; the minor

diameter is in the plane perpendicular to the longitudinal ridges. For each sample n = 20; samples 4

through 6 had fewer than 20 tip-split nuts. For each diameter ratio for each sample, values with

different letters differ significantly (P <0.05).

or Diameter (K:S Ratio) Minor Diameter (K:S Ratio)

Sam

le Full S

lit Ti

lit Uns

lit Full S

lit Ti

lit Uns

lit

1 0.97 a 0.90 b 0.89 b 1.06 a 0.90 b 0.96 b

2 0.98 a 0.85 b 0.85 b 1.07 a 0.90 b 0.90 b

3 0.95 a 0.88 b 0.83 b 1.06 a 0.94 b 0.92 b

4 1.02 a -- 0.90 b 1.10 a -- 0.97 b

5 0.94 a -- 0.83 b 1.02 a -- 0.88 b

6 0.97 a -- 0.81 b 1.02 a -- 0.90 b

If it is the growth of the kernel that drives

shell split, then factors that enhance kernel size

relative to shell size would lead to more split

nuts. Crop load is one such factor; irrigation

management is another. Both can affect the

percentage of split nuts.

There is an inverse relationship between

tree crop load, the percentage of nuts with split

shells, and the percentage of blank nuts (Table

1). As crop load increases, the percentage of

nuts with split shells decreases, and the

percentage of blank nuts decreases. Thus, in

'heavy' crop years the marketable crop is

decreased by non-splits, and in 'light' crop

years it is decreased by blanks. Further, it is

interesting to note that the percentage of non-

split nuts and blanks is much more strongly

correlated with the 'heavy' and 'light' crop year

than with individual tree crop load. A review of

the seven years of crop production in trees with

altered crop loads in Table 1, shows that only

when the crop load is as little as 0.2 pounds per

tree versus 11.4 pounds per tree (1989), or 3.0

versus 25.8 pounds per tree (1991), are

significant differences produced in the

percentage of split shells. This suggests that the

ability of the tree to support and mature a crop

is a stronger factor in determining the

percentage of splitting than the actual pounds

of crop on the tree in any given season.

Effects of female cultivar, pollen

source and rootstock on shell splitting

Research within the past 20 years has

demonstrated the effects of scion cultivar and

pollen source, but not thus far of rootstock, on

pistachio nut shell splitting. First, it is well

established that variability in shell splitting

exists among edible pistachio cultivars. This is

one of the primary selection criteria for new

cultivars and was among those for the selection

of 'Kerman'. Second, research demonstrated

pollen from 'Peters' and 'Ask' male trees

produced a higher percentage of split nut shells

than pollen from 'Atlantica' males. There is no

evidence that different rootstocks producing

differences in shell splitting. In rootstock trials

currently being conducted in California, no

significant differences in shell splitting

percentages have been detected among P.

atlantica, P. integerrima and the hybrids of

these two rootstocks.

Effects of preharvest production

practices on shell splitting

Certain field production practices do affect

shell splitting. In decreasing order of degree of

impact, these factors are: harvest time,

irrigation management, boron nutrition and

dormant pruning.

There are research results that show the

highest shell split percentages are achieved

when harvest is delayed until the maximum

number of nuts display hull dehiscence, or

separation, from the nut shell. Dehiscence is

signaled primarily by a hull color change to

red. In practice, progress of hull dehiscence is

evaluated by observing the early color change

in the hulls and, at that time, randomly

sampling trees for split nuts. Specifically,

collect a 100-nut sample from around the tree,

remembering those in the upper southwest

quadrant will mature first, and determine the

percentage of nuts on which the hull is easily

removed and the shell is split. Do this daily

until the increase in the percentage of split nuts

appears to be slowing. However, this period of

maximum nut shell splitting must also be

balanced against the threat of navel

orangeworm (NOW) infestation, (the

possibility of a spray with a preharvest

interval), as well as the availability of

harvesting machinery. Delaying harvest until

maximum nut shell splitting percentages are

achieved may result in NOW infestation and

shell staining.

Additional research has demonstrated

insufficient irrigation from mid-August through

early September will significantly decrease the

percentage of split nuts. Further, preliminary

data currently being generated suggests that

regulated deficit irrigation from mid-May

through the end of June may increase the

percentage of shell splitting.

Studies showed that a late dormant spray of

2-5 pounds of Solubor per acre, applied at

budswell, will significantly increase the

percentage of split nut shells. This can be tank

mixed with the late dormant zinc spray.

Dormant pruning has a negligible effect on

the percentage of splits. The alterations in yield

presented in Table 1 were produced by dormant

pruning. From this data, it can be seen that only

when dormant pruning produced differences in

crop load per tree that varied significantly were

there significant differences in the percentage

of splits, as in 1989 and 1991. Further, these

differences were produced by pruning

treatments done four and seven years earlier.

Later research demonstrated approximately

half a pistachio tree's fruit buds can be

removed, and the tree will compensate by

setting more nuts per cluster with the same

percentage of split nuts as the unpruned

controls. Thus, dormant pruning has a limited

effect on shell splitting; unless the pruning is

quite severe, it does not impact tree crop load,

and therefore will not impact shell splitting.

Further, as discussed previously, shell splitting

appears to be more responsive to 'on' and 'off '

crop year than individual tree crop load.

Thus far, no information has been

generated in California demonstrating the

effects of irrigation water quality on shell

splitting. Thus, it is not ranked in the

production practices discussed here. However,

preliminary information is available from Israel

that irrigation water salinities of 4,000 mg/liter

of total soluble solids (TSS), primarily sodium

(Na) and chlorine (Cl), have decreased shell

splitting. Currently, rootstock trials are in

progress in California to determine the relative

salinity tolerance of P. atlantica and P.

integerrima and the two hybrids of these two

rootstock species, to salinities ranging from

ECws of 0.75 through 8.0.

Effects of postharvest factors on shell

splitting

No postharvest practice has yet been

demonstrated to significantly impact the

percentage of split pistachio nut shells.

However, pistachio shells have a high

percentage of moisture. Anytime the harvested

pistachio nut is subjected to heat, during

postharvest transport, preprocessing waits and

drying, this heat will decrease the moisture

content of the shells, literally shrinking the

shell about the nut and increasing the width of

the split. Thus, pistachios can leave the field

with a lower percentage of wide splits than

they have when they arrive at the processor.

During processing, this increase in split width

occurs very early in drying and increases as

dryer temperatures increase from 125

F to 190

F. This increase can result in nut kernels

dropping out of the shell.

Conclusion

Controllable factors that affect shell split

include: harvest timing, irrigation and boron

nutrition. To obtain the best percentage of split

shells with the 'Kerman' female and 'Peters'

male cultivars: harvest timing should be based

upon the appearance of hull color and a

sampling for the percentage of splits. Trees

should not be water stressed from mid-August

through September; and boron levels should be

maintained above 120 ppm by dry weight of

July leaf sample. Hopefully, through the long-

term rootstock evaluation, and cultivar and

rootstock breeding program in progress,

information concerning the effects of rootstock

and salinity on shell splitting, as well as new

female cultivars and male pollinizers, will be

available in the next few years.