Watermelon
Crop Information
Horticultural Traits
- by Todd C. Wehner
- Department of Horticultural Science
- North Carolina State University
- Raleigh, NC 27695-7609
Vines
Vine length of watermelon varies from dwarf to long. For
example, 'Charleston Gray' and 'Jubilee', large-fruited
varieties, have vines up to 30 feet long. Short or medium
length vines are well suited to varieties with small or
medium sized fruit. For example, 'Sugar Baby', 'New Hampshire
Midget', and 'Petite Sweet' are short vined, and 'Crimson
Sweet' has intermediate vine length.
Dwarf mutants have been discovered in watermelon. Two genes
cause dwarfing when they are in homozygous recessive condition:
dw-1 and dw-2. 'Kengarden' has the genotype dw-1 dw-1. Another
gene mutant (Japanese Dwarf, dw-2 dw-2) has increased branching
from the crown. Dwarf plants having both sets of genes (dw-1
dw-1 and dw-2 dw-2) have hypocotyls 50% the length of normal
vining plants, so can be selected in the seedling stage
(Table 3.2).
Sex Expression
Most modern varieties are monoecious, and that appears
to be the prefered type of sex expression for commercial
seed production of inbred lines and hybrid varieties. Andromonecy
(aa) is recessive to monoecy.
Most varieties have a ratio of 7 staminate to 1 perfect
or pistillate flower. There are some varieties with a ratio
of 4 staminate to 1 pistillate flower. It may be possible
to breed for gynoecious sex expression by selecting for
increased proportion of pistillate nodes in a segregating
population. There is no advantage to andromonoecious sex
expression, since the perfect flowers must be pollinated
by bees in order to set fruit. Thus, they are no more likely
to set without bees or to be self-pollinated, than monoecious
varieties.
Male sterility is useful for the production of hybrid seeds
without the requirement for expensive hand pollination.
The glabrous male sterile (gms) mutant provides male sterility,
but the plants are less vigorous, have poor seed set, and
are susceptible to cucumber beetles because they lack hairs.
A second male sterile mutant, the Chinese male sterile (cms),
has been more useful for hybrid production (Fig. 3.3).
Fruit can be set parthenocarpically. Although there are
no gene mutants that make plants parthenocarpic, fruit set
may be achieved without pollination by applying growth regulators
to the plants. Thus, commercial production of seedless watermelon
may be possible in areas where bees have been excluded by
applying growth regulators at a particular growth stage
to diploid pistillate flowers that would otherwise produce
seeded fruit.
Yield
Yield varies among watermelon accessions and current varieties.
Growers want high weight per acre of marketable size fruit,
with a low percentage of culls. The yield goal expressed
by many growers is at least one load (45,000 lb.) per acre.
Most watermelon breeders are selecting for yield in their
programs, but it is not clear whether significant progress
has been achieved.
In the production of triploid hybrids, up to one third
of the field must be planted to a diploid seeded variety.
Therefore, higher yield of seedless watermelon per acre
could be obtained by using a more efficient pollenizer that
would allow more than two thirds of the field to be planted
to the triploid variety. Alternatively, parthenocarpic fruit
set (genetic or hormone-induced) to stimulate fruit set
would permit the entire field to be planted to the triploid
variety.
Earliness
Early maturity is desirable because prices for watermelon
usually are best at the beginning of the local season. However,
late maturity is associated with varieties that have large
fruit size and high yield. Thus, it may be necessary to
sacrifice some earliness to obtain high yield or large fruit.
Time from pollination to fruit harvest ranges from 26 days
for early maturing, small-fruited varieties such as ‘Petite
Sweet’ to 45 days for large-fruited varieties such
as ‘Super Sweet’.
The selection process for early maturity should involve
both days from seeding or transplanting to first fruit set,
and days from first fruit set to fruit harvest. Days to
fruit harvest should be based on fruit having fully developed
sugars as verified by a hand-held refractometer or by taste
evaluation.
Fruit Size, Shape, and Rind Pattern
Fruit size. Fruit size is an important consideration in a breeding
program since there are different market requirements for
particular groups of shippers and consumers. The general
categories are: micro (<3 lb.), mini (3-8 lb.), icebox (9-13 lb.), small, sometimes called
pee-wee (14-18 lb.), medium (19-24 lb.), large (25-32 lb.), and giant (>32
lb.). Fruit size is inherited in polygenic fashion, with
an estimated 25 genes involved. Shippers in the United States
work with particular weight categories, such as 19-24 lb.
for seeded and 14-18 lb. for seedless.
Old varieties tend to have larger fruit size than current
varieties, because one of the things growers were interested
in was winning competitions for fruit weight. Competitions
are still being held to grow the largest fruit, but commercial
production concentrates on high quality. Another reason
for larger fruit in the past is that they are more efficient
for hand harvest and shipping; large fruit handled individually
permit more weight to be moved per unit. Also, there was
demand for large fruit to be sold or served by the slice
for restaurants and cafeterias. Today, most supermarkets
request fruit of seeded watermelons that weigh 19-24 lb.
Small- or medium-fruited types were the result of adapting
watermelon to the northern areas of the United States. Varieties
developed for the northern United States were bred from
early maturing Asian varieties brought from Japan and Russia.
A.F.Yeager produced the early varieties ‘White Mountain’
and ‘New Hampshire Midget’ from sources, which
have 2 to 4 lb. fruit with a 65-day maturity. The early
variety ‘Petite Sweet’ has 5 to10 lb. fruit.
‘Sugar Baby’, a small-fruited variety popular in some parts of the world, was selected in Oklahoma by M. Hardin in 1956. Even though mini and icebox varieties with 4 to 11 lb. fruit have
been developed to fit easily in a small refrigerator, most
of the demand in the marketplace for small fruit has been
met using sections cut from a large fruit. A large watermelon
fruit cut into quarters has the same weight as an icebox
melon, but it has a different shape, and consumers can see
what they are buying.
Fruit shape. Fruit shape is also an important part of market type. The
general categories are round, oval, blocky, or elongate.
There is one gene involved in round vs. elongate, with the
F1 being intermediate (blocky). In some cases, fruit shape
is related to cotyledon shape at the seedling stage. Plants
with elongate fruit have elongate cotyledons, and plants
with round fruit have round cotyledons. However, others
have concluded that selection for fruit shape at the seedling
stage is ineffective. Among old varieties with elongate-shaped
fruit, there was greater susceptibility to production of
gourd-neck or bottle-neck fruit, which are culls. Old varieties
with round fruit were more susceptible to hollowheart. Thus,
some of the first hybrids were made between elongate and
round inbreds to reduce the incidence of these defects.
Recently, genetic resistance to those defects has been incorporated
into new varieties, and has made fruit shape less important
to consider.
Rind pattern. The third area of importance in market type is rind pattern,
which can be gray, striped, or solid. Stripes on the rind
can be narrow, medium, or wide where the stripes are the
dark green areas. The striped pattern can be on light green
or medium green background. Solid rind color can be light
or dark green. Solid dark green is dominant to gray rind
pattern. Solid dark green is dominant to striped, and striped
is dominant to solid light green rind pattern. However,
the striped pattern can be seen on a solid dark green fruit
after the color has been bleached by the sun.
In addition to the common rind patterns, there is furrowed
vs. smooth rind, controlled by the recessive gene, f (Table
3.2). Most current varieties have smooth rind. Another interesting
mutant is golden rind, which is controlled by the recessive
gene, go. Its usefulness as an indicator of fruit ripeness
is limited because the change in fruit color at fruit maturity
is accompanied by chlorosis of the leaves. Furthermore,
it does not appear to be a reliable indicator of ripeness,
and may be disadvantageous for yield, especially if the
grower is using a multiple harvest system.
Summary. We propose that watermelon varieties be categorized by
fruit size, shape, and rind pattern as follows: Fruit size
- micro, mini, icebox, small, medium, or giant. Fruit shape - round, oval,
blocky, or elongate. Rind pattern - gray, solid light, solid
medium, solid dark, or narrow, medium, or wide striped on
a light green or medium green background (Table 3.1, Fig.
3.2). Using these categories, we would classify 'Allsweet'
as large, elongate, with wide stripes on a light green background.
'Crimson Sweet' would be classified as medium size, round,
with medium stripes on a light green background. 'Charleston
Gray' would be large, elongate, with gray rind.
External Fruit Quality
Rind durability is important on varieties that are to be
shipped to market. On large-fruited varieties, the rind
should be thick and tough; whereas on small-fruited varieties,
the rind should be thin and tough. Rind thickness should
be a small percentage of flesh diameter to keep it in a
balanced proportion for best appearance. Large-fruited varieties
look better with a thicker rind, and need the extra protection
for postharvest handling and shipping. The rind can be tough
and hard as in 'Peacock' or tough and soft as in 'Calhoun
Gray'. Brittle rind as in 'New Hampshire Midget' is not
useful for varieties that are to be shipped to market.
Rind flexibility can be tested by cutting a 1/16 to 1/8
inch x 3 inch piece of rind from a fruit and bending the
rind into an arc. If the rind bends into a tight arc, it
is flexible and tough. If it breaks early in the attempt,
it is tender and explosive.
Rind toughness can be measured by driving a spring-loaded
punch into the rind. A tough rind would require more force
to punch through, whereas a tender or brittle rind requires
less force. Watermelon breeders often use faster methods
to test for rind toughness, however. One method is to drop
the fruit onto the ground from a particular height (for
example, knee height) to see whether it breaks open or not.
The drop height would depend on the soil type of the field
being used. Another method is the "thumb" test,
where the breeder presses on the rind at a particular location
on each fruit. If the rind breaks when only a small amount
of force is applied, then it has a tender rind; otherwise
it should be resistant to shipping damage.
Internal Fruit Quality
Flesh color is one of the primary traits consumers look
for in a watermelon fruit. Color can be scarlet red, coral
red, orange, canary yellow, salmon yellow (golden), or white.
Coral red (YY) is dominant to orange (y-o y-o), which is dominant
to salmon yellow (yy). Canary yellow (CC) is dominant to
non-canary yellow (cc), and epistatic to (overcomes) the
y locus for red-orange-salmon yellow. Coral red is recessive
to the white flesh color, which is found in citron (Table
3.2). Scarlet red color from 'Peacock' has been used to develop
many new varieties because of its attractive color. The inheritance of scarlet red flesh color is due to the single dominant gene, Scr.
Varieties with scarlet red flesh include ‘Dixielee’,
‘AU-Sweet Scarlet’, ‘Red-N-Sweet’,
and ‘Sangria’.
Sugar content is a major component of flavor. Breeders
select for high sugar content as indicated by taste and
refractometer readings. Refractometer readings are easily
made in the field using a handheld unit, and provide data
on percentage of soluble solids (°Brix). These translate
to sugar content, which should be a minimum of 10%. Newer
varieties have Brix as high as 14%. Some varieties have
higher levels of fructose, which tastes sweeter than sucrose.
The difference in taste is not measured by a refractometer.
Selection should be made for good watermelon flavor, independent
of sweetness (sugar content). Flavor should include freedom
from bitterness, which is controlled by a single dominant
gene, and may be introduced in crosses with C. colocynthis
accessions. Another component is caramel flavor as in 'Sugar
Baby' fruit, which some taste testers find unpleasant. The
flavor is sometimes associated with dark red flesh color.
Its inheritance is not known, but caramel flavor does respond
to selection. Thus, breeders should select lines with mild
(not bitter) taste, high sugar content (°Brix), freedom
from caramel flavor, and excellent "watermelon"
taste. It is important that varieties with excellent taste
be included as checks in all selection blocks to provide
a comparison for the plant breeder. Examples of varieties
with good quality that are commonly used include ‘Allsweet’,
‘Crimson Sweet’, and ‘Sweet Princess’.
Flesh texture is an important part of internal quality.
Watermelon fruit can have flesh that is soft or firm, and
fibrous or non-fibrous. The objectives for plant breeders
should be to develop varieties with flesh that is firm and
non-fibrous. The genes controlling those traits are not
known, but they are heritable.
Seeds and Seedlessness
Seed color can be white, tan, brown, black, red, green,
or mottled. White seed color usually is not preferred since
it suggests that the fruit is immature, and can make it
difficult to distinguish mature from immature seeds. On
the other hand, white seeds may be a useful objective for
the development of near-seedless varieties that have few,
small, and inconspicuous seeds. Black seed color is attractive
with red or canary yellow flesh color. Black, brown, or
tan seeds look good with orange flesh color.
Seed size should be large for confectionery (edible seeded)
type, and small or medium sized for the standard (edible
flesh) type (Fig. 3.4). A new seed size mutant discovered
recently is called tomato seed. The seed size is about half
that of the small watermelon seed size, and is controlled
by a single recessive gene, tss.
Seed number should be high for the confectionery type,
but should be low or medium for the edible flesh type. Seed
number should be lower in small-fruited varieties so that
the seeds will not appear to include more than the usual
percentage of the fruit volume. Seed number should be high
enough to make seed production economical, but low enough
to make the flesh easy to eat.
In theory, seedless triploid hybrids should provide higher
yield than diploid hybrids because no energy is used in
seed production. However, in practice this may not be the
case. Fruit production in triploids is limited by the availability
of viable pollen to induce fruit set.
During the development of tetraploid inbreds, seed yield
is often low in early generations, so selection for fertility
is essential. Some tetraploids are more fertile than others,
and should be selected to keep seed costs low for triploid
hybrid production, since the hybrid seeds are produced on
the tetraploid parent line.
Triploid hybrids are generally seedless, but occasionally
hard seed coats form in the fruit The presence of objectionable
seed coats is affected by environment, but can also be selected
against in the development of the inbred parents of the
hybrid. Inbred parents that do not develop objectionable
seed coats in the fruit in different production environments
should be selected for triploid hybrids.
