A Breeding Program for Petunia hybrida
Jessica Gaus
November 18, 2002
Abstract
Petunia (Petunia hybrida) is the third most important bedding plant in the United States
with sales expected to reach $138 million in 2002. As a general rule, cultivars of petunia can be
divided into five distinct groups - grandiflora, multiflora, floribunda, milliflora, and the
spreading types. These five types are physiologically distinct and are developed for different
uses in the garden including container planting, hanging baskets, and mass planting in the
landscape. These five groups are propagated asexually by cuttings or sexually by seed. Within
the last ten years, the genus of Petunia has undergone tremendous restructuring since the genus
of Calibrachoa was separated out in 1985. Currently there are 14 species of Petunia that are
closely related and fully crossable with Petunia hybrida. These 14 different species are all
native to South America and offer a number of interesting traits like better branching, prolific
rooting ability, and glabrous leaves that may be useful to a breeding program. Although there is
still plenty of debate concerning the true origin of Petunia hybrida most agree that it was an
interspecific hybrid made between P. integrifolia and P. axillaris.
Although, all petunia taxa have a gametophytic system of self-incompatibility it is not
really a problem for breeding programs because due to the existence of a single self fertility
allele. From the beginning, breeders have selected for this allele so extensively that almost all
breeding stock is now totally self-compatible. There are three different types of male-sterility in
petunia. Breeders recognized the importance of this male-sterility and developed a system for
hybrid seed production that eliminated the need for emasculation of the female parents. In this
system a cytoplasmic male sterile inbred line (S) is used as the female parent, which is crossed to
a complementative male fertile, inbred line with normal cytoplasm. Since the F1 hybrid does not
need to be male fertile, restorer genes are not necessary. The pollination procedure for petunia is
not elaborate, but it is highly labor-intensive. Emasculation of the female flower prior to
anthesis is necessary to prevent self pollination when cross pollinations are being made. Under
controlled conditions in a greenhouse, pollen is then transferred from the male plant to the
female plant using tweezers or a pipe cleaner.
Although, petunia breeding programs have existed for over 100 years, little information
has been recorded regarding the qualitative and quantitative inheritance of most traits. Today,
there is only information available regarding traits like floral longevity, plant height, flowering
time, flower size, and leaf variegation. The genetics and the biochemistry of flower color are
astoundingly complicated. The entire system is broken down into ten different groups of genes
that control different aspects of flower color in the flower like anthocyanins, hydroxylation,
glycosilation, methylation, acylation, flavonols, venation patterns, flower color intensity, pH, and
pollen colors. Within these 10 groups there a least 40 different genes that all have different
patterns of inheritance. Petunia cultivars are usually fairly insect resistant, however they often
have problems with disease caused by Pythium, Rhizoctonia, powdery mildew, Sclerotinia, and
Botrytis either during greenhouse production or in the landscape. Additionally, virus infection
has recently become a serious problem now that so many petunia cultivars are being vegetatively
propagated.
Most breeding systems for petunia closely resemble a pedigree system with selection
occurring both in the greenhouse in the pack stage and in the field. Due to the recent explosion
in the number of different types of petunia, selection for different environments is becoming
increasingly important. Breeders must now select for superior performance in several different
types of environments including the pack, containers, hanging baskets, and in a landscape
planting. Inbreeding depression is a problem is a problem with petunia since it is a natural
outcrosser. To avoid the loss of vigor associated with inbreeding depression, mass and full-sib
selection is often used within a family late in the program (F6, F7).
Introduction
Petunia hybrida is divided into five distinct groups - grandiflora, multiflora, floribunda,
milliflora, and the spreading types (Corr, 1998; Dole and Wilkins, 1999; Ewart, 1984; Kessler,
1998). Typically, single grandiflora petunias have large flowers (3.5-5 inches across) which are
often ruffled or fringed, a coarse plant habit, broad leaves, thick peduncles and sepals, and thick
anther filaments. Multiflora petunias have a more delicate overall phenotype with smaller
flowers (1.5-2 inches across), narrower sepals, finer anther filaments, a more compact growth
habit, and better disease resistance. The floribunda type is the result of a cross between
grandiflora and multiflora types that resulted in an intermediate phenotype, which shares
characteristics associated with both parents. In general, floribundas offer the disease tolerance of
multifloras with larger flowers (3 inches across). Milliflora petunias are a miniature version of a
standard petunia that was introduced in 1996. Millifloras are usually quite compact with plant
heights and widths of only 6 to 8 inches across with 1 inch flowers (Kessler, 1998). Due to their
small size, milliflora petunias are best suited for use in small containers. The spreading or
trailing type of petunia is a totally new type of petunia that has become quite popular since its
debut in 1992. Initially, trailing petunias were developed in Japan by Suntory Ltd. in
cooperation with Keisei Rose Co. by crossing Petunia hybrida with an unknown South
American species (Rader, 1998). This type is the most vigorous and floriferous of all types of
petunia making it ideal for use as a ground cover in the landscape. The trailing type of petunia
can be divided again into several more groups depending on whether they are produced by seed
or from vegetative cuttings. Additionally, all five types of petunia are available in a double
flowering version.
Floricultural sales of cut flowers, potted flowering plants, and bedding plants in the
United States reached $4.8 billion in 2002. This was a 58% increase from ten years ago. These
estimates correspond to an average purchase of $44 per household. Sales of petunia flats are
expected to total over $138 million in 2002, making petunia the third most important bedding
plant behind geraniums and impatiens (Jerardo, 2002). Petunias are planted in containers,
window boxes, hanging baskets, and in mass plantings in the landscape. They are sold either by
seed or by vegetative cuttings in all fifty states, and are produced all over the world. Currently
there are at least 10 large, privately owned companies that have petunia breeding programs
including: PanAmerican Seed, Goldsmith, American Takii, Bodger, Sakata Seeds, Flower
Fields, and Ball Flora Plant.
Taxonomy
Kingdom
Subkingdom
Superdivision
Division
Class
Subclass
Order
Family
Genus
species
Plantae
Tracheobionta (vascular plants)
Spermatophyta (seed plants)
Magnoliophyta (flowering plants)
Magnoliopsida (dicotyledons)
Asteridae
Solanales
Solanaceae
Petunia
hybrida
Biogeography
The geographic origin of Petunia is South America, where various species have been
found in Argentina, Brazil, Bolivia, Paraguay, and Uruguay. The hot spots for species diversity
are mostly limited to the three Brazilian provinces of Parana, Santa Catarina, and Rio Grande do
Sul particularly along river banks and isolated areas (Sink, 1984).
Early Taxonomic Work
Jussieu first established the genus Petunia in 1803. Since that time, the genus of Petunia
has undergone constant restructuring and is still somewhat unsettled today. Fries wrote the first
Petunia monograph in 1911 where he proposed the division of the genus Petunia into 2 distinct
subgenera, Pseudonicotiana and Eupetunia. Species in the subgenera of Pseudonicotiana had
long, narrow corolla tubes, while species in the subgenera of Eupetunia had short, wide corolla
tubes (Sink, 1984). In 1985, Wijsman and de Jong recognized two distinct groups of genetically
isolated species within the genus, Petunia, and separated them into two different genera, Petunia
and Calibrachoa. Wijsman and de Jong distinguished the two different groups based on the
following characters: Calibrachoa – 2n = 18, small shrubs with woody stems, flower limbs were
white or purple with yellow or pale corolla tubes; and Petunia – 2n = 14, large herbaceous
plants, with flowers which were either entirely white or purple (Wijsman and de Jong, 1985;
Stehmann and Semir, 1997). Today the two genera of Calibrachoa and Petunia each contain 25
and 15 species respectively (Watanabe et al., 1999). Although species of these two distinct
genera are often found growing in the same regions in Argentina, Brazil, and Uruguay, no
naturally occurring intergeneric hybrids have ever been reported (Ando et al., 2001).
Furthermore, artificial crosses between selected species of these two genera have never been
successful (Ando et al., 2001). This evidence strongly suggests that genetic isolation exists
between Calibrachoa and Petunia species. Because these two genera are not sexually
compatible, other non-conventional techniques must be developed in order to be able to transfer
genetic material between the two.
Parents of Petunia hybrida
The cultivated garden petunia, Petunia hybrida is not a true species but actually a
complex interspecific hybrid of two or more Petunia species. In earlier literature, many
taxonomists and scientists suggested that as many as 5 different species including P. axillaris, P.
integrifolia, P. parodii, P. inflata, and P. violacea all contributed to the origin of P. hybrida.
These theories were made increasingly complicated as the genus of Petunia continued to
undergo reclassification. Even today there is still disagreement over whether many species of
Petunia like P. inflata, P. occidentalis, P. parodii are actually true species or are they subspecies
of either P. integrifolia or P. axillaris (Wijsman, 1982; Griesbach and Beck, 2000; Mishiba et al.,
2000).
Although there is still controversy surrounding this issue, most believe the broad
interpretation that P. integrifolia and P. axillaris are the two predominant species, which
contributed to the origin of Petunia hybrida (Sink, 1984; Ando et al., 2001). In a study by
(Ando et al., 2001) artificial crosses of P. axillaris by P. integrifolia were successful in
producing fertile interspecific hybrids especially when P. axillaris was used as the female parent.
This work suggests that there are no genetic barriers between these two species. Interestingly,
wild populations of these two species exist together in close proximity, but interspecific
hybridization does not occur naturally because of differences in insect pollinators. P. axillaris is
pollinated exclusively by nocturnal hawkmoths, and P. integrifolia is pollinated by diurnally
active bees (Ando et al., 2001).
Closely Related Species
Petunia hybrida is a member of the Solanaceae family, which includes agronomically
important crops like tomato (Lycopersicon), potato (Solanum), and tobacco (Nicotiana). Other
important ornamentals like Browallia, Brumansia, Capsicum, Nicotiana, Salpiglossis, and
Schizanthus are also members of the Solanaceae family (Dole and Wilkins, 1999).
All 15 species of Petunia have 14 chromosomes and are freely crossable with Petunia
hybrida (Watanabe et al., 1996). These species include: P. altiplana, P. axillaris, P. bajeensis,
P. bonjardinensis, P. exserta, P. guarapuavensis, P. integrifolia, P. interior, P. littoralis, P.
mantiqueirensis, P. reitzii, P. riograndensis, P. saxicola, and P. scheideana. Some promising
taxa include P. mantiqueirensis which may contribute increased shade tolerance (Ando and
Hashimoto, 1994), P. interior which may contribute increased branching (Ando and Hashimoto,
1996), P. bajeensis which may contribute increased vigor (Ando and Hashimoto, 1998), P.
riograndensis which may contribute darker corolla tube pigmentation (Ando and Hashimoto,
1998), P. bonjardinensis which may contribute larger flower size with exerted pistil position
(Ando and Hashimoto, 1993), P. altiplana which may contribute improved prostrate growth and
significant adventitious rooting ability (Ando and Hashimoto, 1993), and P. guarapuavensis
which may contribute glabrous leaves (Ando and Hashimoto, 1995).
Currently the USDA is working with one promising ecotype, P. integrifolia subsp.
integrifolia var. depauperata, which is both drought tolerant and has glabrous leaves which are
not covered with the sticky glandular trichomes traditionally found on P. hybrida leaves. This
ecotype has been successfully hybridized with a standard P. hybrida cultivars and evaluated for
trichome density and insect resistance in the F2 generation. According to the results, leaf
trichome density appeared to be quantitatively inherited with a heritability of 0.88 indicating that
it would be quite easy to introgress this glabrous trait into a breeding program (Griesbach et al.,
2002). Unfortunately spider mite resistance was significantly lowered in F2 plants with fewer
trichomes, which could become a problem both in the greenhouse and in a homeowner’s yard.
The USDA is also working with Petunia exserta - a bright red (RHS 45A) flowering
species recently discovered in Brazil. So far researchers have determined that this species has
great potential for improving red-flowering inbred lines of Petunia hybrida that are often
weaker than other inbred lines with different flower colors. Although P. exserta does not contain
novel anthocyanins or pH levels, it does have a totally different genetic background from all
other inbred lines which could be used to create new red flowering breeding lines and thus
expand the gene pool (Griesbach et al., 1999). Other ecotypes identified by the USDA were
fragrant, shrub-like, and even cold tolerant in the snow – all traits that could greatly improve the
horticultural value of Petunia.
Calibrachoa variabilis is a wild species of Calibrachoa most noted for its highly
branched, prostrate growth habit that could be useful in a petunia breeding program for spreading
types. However, successful hybridization is not possible using conventional techniques because
C. variabilis (2n=18) has a different number of chromosomes. Fortunately, in 1992 Japanese
researchers were able to create 3 somatic hybrids between P. hybrida and C. variabilis using
electrofusion (Taguchi et al., 1993). These hybrids were fully fertile and could be used as a
means of transferring interesting traits from the genus of Calibrachoa.
Botany
Petunia seed is 0.6-0.7 mm in length, 0.5-0.6 mm in diameter, spheroidal-angular, light to
dark brown in color, reticulate, thin walled with a conspicuous hilum that is the same color as the
seedcoat (Sink, 1985). There are approximately 250,000 petunia seeds per ounce (Sink, 1985).
Petunia seeds should be sown directly onto well drained, sterilized media with a pH of 5.5 to 6.0.
Seeds should not be covered. Germination will occur after 10-12 days when held at constant
temperature between 75 to 78 F. Supplemental lighting during germination is not normally
required, but will help with uniform germination (Corr, 1998). Many freshly harvested petunia
seeds possess endogenous inhibitors, which block immediate germination (Cathey, 1984).
However, most seeds stored for 3 months either at room temperature or in cooler 10 C no longer
exhibit any dormancy (Cathey, 1984). The fruit of a petunia is a capsule which varies in size,
shape, and seed count depending on the species (Sink, 1984). The flowers of petunia are perfect
with five epipetalous stamens that terminate at various heights below the stigma (Sink, 1984).
The petunia leaves are alternate or opposite, broad-ovate to cordate shaped, 1.5-3.0 inches long,
simple, entire margins, with stems and leaves covered with viscid pubescence (Still, 1984).
Petunia pollen is binucleate and can be stored for a short period of time at 4 C in sealed
containers. Pollen that is first air-dried can be stored for at least a year if stored between –10 C
and –35 C (Frankle and Galun, 1977).
Self-incompatibility
In the 1930’s, two separate groups (Harland and Atteck, and Wergin) independently
confirmed that the genus of Petunia possesses a gametophytic system of self-incompatibility
(Ascher, 1984). In this gametophytic SI system, “a diploid plant heterozygous for S alleles
expresses both S specificities in the pistil but meiosis results in a 1:1 segregation of S alleles in
the pollen and, therefore, 2-pollen phenotypes” (Ascher, 1984). Later work identified that the
self-incompatibility of Petunia is caused by a single multiallelic S-locus with as many as 10
different S-alleles (Robbins et al., 2000).
Since petunia breeding programs are based solely on creating F1 hybrid seed from inbred
lines, self-incompatibility can be a major roadblock. Fortunately, Yasuda discovered in the
1930’s that the SI reaction in petunia cultivars develops over the course of flower development
and could be circumvented by pollinating immature flowers. Generally, bud-pollination 2-5
days before anthesis will result in selfed seed in a self-incompatible plant (SI) (Ascher, 1984;
Shivanna and Rangaswamy, 1969). Treatment of a SI stigma with extract from a compatible
pistil also overcomes self-incompatibility in petunia if done immediately prior to pollination
(Sharma and Shivanna 1986). Also, applying nucleic acid and protein metabolism inhibitors like
olivomycin, actinomycin D, cycloheximide, and chloramphenicol to unopened buds also helps,
but is not widely used due to the additional cost of the chemicals (Kovleva et al., 1978).
To further complicate matters, pseudo-self compatible (PSC) individuals exist making it
difficult to select against self-incompatibility in a breeding program. Pseudo-self incompatibility
exists in situations where a self-incompatible system can be disrupted by certain genetic or
environmental conditions allowing for selfed seed set in self-incompatible plants (Ascher, 1984).
The progeny of these “unexpected” selfing events often segregate for both (SI) and (PSC)
making it extremely difficult to maintain fertility in inbred lines. Since (PSC) in petunia is
affected by both temperature and light intensity it is extremely important to test for (PSC) in
multiple environments i.e. winter and summer (Flaschenriem and Ascher, 1980).
Fortunately, although there are a number of (SI) petunia cultivars, a majority of the plants
used today in breeding programs are (SC) due to the existence of a single fertility allele. This
fertility allele, labeled either Sf or S0, was first discovered by East in 1929 and later studied indepth by Takahashi in 1973 (Ascher, 1984). Interestingly, studies have predicted that this single
S0 allele is present in over 80% of existing cultivars of petunia (Robbins et al., 2000). Currently
there are two theories that suggest that this single S0 allele was either contributed by one of the
parents of the original cross between P. axillaris or P. integrifolia, or that the hybridization itself
caused the formation of this S0 allele (Robbins et al., 2000).
Male Sterility
There are three types of male sterility in petunia. The first type of male sterility occurs
spontaneously in advanced inbred lines and can lead to partial or complete male sterility caused
by a physiological breakdown in microsporogenesis. Because this occurrence of sterility occurs
in highly homozygous lines it is presumably associated with the accumulation of recessive
deleterious alleles. This type of male sterility is not useful for hybrid production because it is
unpredictable and often associated with female infertility and a decline in overall plant vigor
(Izhar, 1984).
The second type of male sterility is genically controlled by male sterile (ms) and male
restorer alleles (mfr or Rf). In petunia, there are two systems of restorer alleles – one governed
by a single dominant allele and one governed by a series of multiple quantitative genes (Izhar,
1984).
The third type of male sterility is controlled by the interaction of sterile cytoplasm (S)
and the proper male restorer alleles (mfr) mentioned above. A comprehensive study by Izhar and
Frankel in 1976 analyzed various petunia taxa with cytoplasmic male sterility and determined
that there is only one source of sterile cytoplasm (S) in petunia. This single sterile cytoplasm (S)
was probably contributed by P. axillaris during an early cross to a cultivar of P hybrida made
by Everett and Gabelman (Izhar, 1984). Unfortunately, the parents of this original cross were
not recorded so it is impossible to determine whether the (S) cytoplasm arose as a substitution of
P. axillaris’s cytoplasm or a spontaneous rearrangement of the mitochondrial genome in the
hybrid (Hanson et al., 1995).
Today a number of breeding companies use cytoplasmic male sterility and restorer alleles
to produce F1 hybrid seed (Hanson et al., 1995). In this system a cytoplasmic male sterile inbred
line (S) is used as the female parent, which is crossed to a complementative male fertile, inbred
line with normal cytoplasm. Since the F1 hybrid does not need to be male fertile, restorer genes
are not necessary.
Elite Inbred A
(Srr)
(cytoplasmic male sterile)
X
Elite Inbred B
(Nrr)
(normal cytoplasm)
F1 hybrid
(cytoplasmic male sterile)
male sterile source
(Srr)
X
Elite inbred A
(Nrr)
(Srr) that is 50% A
X Elite inbred A
(Srr) that is 75% A
X
Elite inbred A
(Srr) that is 88% A
X
Elite inbred A
Several generations
(Srr) that is 99% A
The Inheritance of Flower Color
Today, cultivars of petunia are found in an astounding range of colors including whites,
yellows, pinks, mauves, reds, magentas, lavenders, and purples with one notable exception of
blue. Additionally, all of these colors are found in different combinations together in bicolors,
tricolors, picottees, flecked phenotypes, dark throats with light face, light throats with dark face,
etc.. Thus, one could correctly assume that the genetics controlling flower color in petunia is
quite complicated.
Flower color in petunia is primarily due to the presence or absence of the six different
anthocyanidins: cyanidin, peonidin, delphinidin, petunidin, malvidin, and pelargonidin (Wiering
and de Vlaming, 1984). In petunia these different anthocyanidins occur individually or together
in many different combinations. Furthermore, the way in which each anthocyanidin is
hydroxylated, glycosilated, methylated, and acylated to form anthocyanins is also very critical in
determining the actual flower color. In addition to the type of anthocyanin, there are many
compounding factors including the presence or absence of copigments in the vacuole, the
vacuolar pH, and the shape of the cell that also play an important role in determining flower
color. The patterning and venation of a petunia corolla is also quite complicated and is
controlled by a totally different set of genes. As you can see, the genetics of flower color is
entirely too difficult to summarize without breaking these different factors into 10 different
groups of genes based on what factors they control. These ten groups control anthocyanin
expression, hydroxylation, glycosilation, methylation, acylation, the type and content of
copigments like flavonols, venation pattern, flower color intensity, pH, and pollen color (Wiering
and de Vlaming, 1984). Within these 10 groups there a least 40 different genes that all have
different patterns of inheritance. Some genes only have two alleles that are either dominant or
recessive, some are multiallelic and codominant, some loci are epistatic to others, and some loci
have lethal alleles (Wiering and de Vlaming, 1984). Because there are so many different genes
associated with flower color, linkage between these genes is a problem that prevents the
development of blue flowered petunias. For example, although there are anthocyanins present in
some cultivars that are typically blue they are present at vacuolar pH’s that make them appear
red.
The Inheritance of Plant Height, Flower Size, and Time to Flower
Flowering time, plant height, and flower size are all quantitative traits in petunia. Diallel
analysis revealed that these three characters are controlled by an additive-dominance polygenic
system with 3, 3, and 5 major genes controlling flowering time, plant height, and flower size
respectively (Hussain and Misiha, 1979). For flowering time, additive variance was greater than
the dominance variance component indicating that selection of inbred lines with a greater
diversity in flowering times may be possible. For plant height and flower size the variance due
to dominance was greater than the additive component of variance. Heritability estimates were
0.84, 0.88, and 0.89 for broad-sense heritability and 0.40, 0.49, and 0.37 for narrow sense
heritability for flowering time, plant height, and flower size respectively. Since the narrow sense
values are below 0.50 for flowering time and flower size, selection should be based on the
performance of multiple plants in replicated locations, years, or seasons. The heritability
estimate for plant height is “border-line” indicating that single plant selection may be useful.
Heterosis was determined as a percent increase over the higher plant for flowering time (+ 9.7%
to + 13.3%) and for flower size (+2.5% to +16.0%) For plant height, heterosis was determined
as a percent decrease below the lower parent (-13.6% to –20.3%) (Hussain and Misiha, 1979).
The Inheritance of Floral Longevity
The genetics of floral longevity were examined by Krahl and Randle who concluded that
significant variation exists for floral longevity in different petunia cultivars (Krahl and Randle,
1999). Furthermore they were able to ascertain that there are significant additive gene effects for
floral longevity in these cultivars that can be selected for successfully.
The Inheritance of Variegated Leaves
In petunia, leaf variegation is quite rare and is usually propagated asexually. In general,
variegated leaves are the result of abnormal achlorotic plastids or a chimeral situation where
different histogenic layers have different phenotypes. Recently a study in Japan revealed that
leaf variegation in petunia was caused by abnormal plastid formation that was almost totally
maternally inherited (Aoki et al., 1995). Thus any breeding work done with this trait must use
the variegated parent as the female.
The Inheritance of an Apetalous Flower Type
In the early 1970’s Claude Hope discovered a mutant form of petunia where the normal
corolla tube was replaced with a second set of sepals. Analysis of F1, F2, F3, and BC1
generations with this mutant, indicated that this apetalous condition is controlled by a single
recessive gene independent of the two genes D and G which control flower doubleness and
grandiflora vs. multiflora (Sink, 1973). This apetalous mutant was seriously considered for use
in hybrid seed production because it eliminated the need for removing the corolla prior to
emasculation. However, this mutant never became important because parent identification was
difficult without the corolla, and the sepals of this apetalous mutant often died back and
surrounded the developing seed pod where Botrytis infection became a problem (Ewart, 1984).
Insect Resistance
In general, petunia cultivars are naturally resistant to most greenhouse pests. This natural
insect resistance is most likely attributed to the dense layer of glandular hairs, which covers
almost the entire surface of the plant excluding the roots and the flowers. Additionally, petunia
species contain over 3 dozen steroid derivatives that are involved in insect resistance (Elliger and
Waiss, 1991). However even with these defense compounds, white flies, thrips, aphids, and
fungus gnats can become a problem if left uncontrolled (Dole and Wilkins, 1999).
Disease Resistance
Unfortunately, petunia cultivars are often affected by diseases caused by Pythium,
Rhizoctonia, powdery mildew, Sclerotinia, and Botrytis either during greenhouse production or
in the landscape. Both Pythium and Rhizoctonia infection usually results in seedling death, soft
brown mushy roots in older plants, basal stem canker and lower leaf collapse and yellowing.
Powdery mildew rarely kills a petunia but the unsightly appearance of the symptoms greatly
reduces its value. Sclerotinia infection in petunia has just recently been discovered and is
usually associated with tan to brown stem lesions, wilt, and branch death in the presence of
white, cottony mycelia and black sclertoia (Holcomb, 2001). Prostrate growing cultivars are
most prone to Sclerotinia infection.
Botrytis blight is the most common fungal disease of petunia cultivars that is usually
characterized by sunken, pale brown lesions on the leaves and flowers. Since the warm, humid
environment of a greenhouse favors Botrytis blight development, problems with this pathogen
often occur during production. Additionally, petunia cultivars are also at risk of developing
Botrytis blight in the landscape following heavy rains in the spring, summer, or fall. In 1978,
researchers at Michigan State University tested two different methods of screening for Botrytis
resistance in petunia, and determined that screening whole plants in the greenhouse correlated
better with actual field performance relative to screening detached flowers in a mist chamber. In
this experiment ‘Resisto Blue’ was the only cultivar that exhibited some resistance to Botrytis
infection (Laemmlen and Sink, 1978). Recently, faculty at the University of Georgia expanded
on this research and developed a quick and easy screening procedure for Botrytis resistance in
petunia. This method involves artificial wounding and inoculation of petunia leaves either with
DI water or DI water plus Botrytis spores followed by a subsequent assessment of infection. The
48 cultivars that were tested showed a continuous range of variability for mean percentage of
infection suggesting that resistance to Botrytis infection in petunia is probably quantitatively
controlled (Krahl and Randle, 1999). Overall, ‘Pink Sensation Improved’ was the only cultivar
that showed promising resistance and may prove to be an important source of germplasm for
future breeding programs for Botrytis resistance.
Virus Resistance
Since the 1992 introduction of vegetatively propagated petunia cultivars, virus infection
has become a serious problem experienced in every country where vegetative propagation of
petunia occurs. As of today, 11 viruses have already been detected in petunia cultivars including
tobacco mosaic (TMV), tomato mosaic (ToMV), alfalfa mosaic (AMV), cucumber mosaic
(CMV), petunia vein-clearing (PVCV), potato Y (PVY), tomato yellow leaf curl (TYLC), and
broad bean wilt 1 viruses (Lesemann, 1991; Sikron et al., 1995; Bellardi et al., 1991). Petunia
vein banding virus (PetVBV) and petunia flower mottle virus (PetFMV) are two additional
viruses that were only recently discovered in petunia cultivars. These two viruses have a very
narrow host range within the Solanaceae family and appear to be specifically adapted for
infecting petunia cultivars (Alexandre et al., 2000; Feldhoff et al., 1998). All of these viruses are
mechanically transmitted by plant to plant contact, handling by workers, or during the harvest of
cuttings by hand or with infected tools (Spence et al., 2001). The symptoms of virus infection of
petunia are quite variable and include mosaic or chlorotic mottling of the leaves, leaf
deformation, vein clearing, color breaking of the petals, flower mottling, and an overall stunted
appearance (Spence et al., 2001). In most cases, these symptoms are usually caused by the
presence of either tobacco mosaic virus (TMV) or potato virus Y (PVY) that can occur
individually or in a synergistic capacity (Spence et al., 2001).
Studies by Cohen and Sikron looked at the susceptibility of 18 different petunia cultivars
to TMV and ToMV infection. They found that all 18 cultivars were easily infected with TMV
and ToMV simply by using a contaminated knife to remove cuttings for propagation. Although
all 18 cultivars were infected with virus there was a tremendous variability in symptom
expression among cultivars. ‘BlueSpark’, ‘BrightDream’, ‘Cascadias Red’, ‘Charisma’,
‘Chipper’, ‘Happy Dream’, and ‘Sweet Dream’ were all symptomless for ToMV, and ‘Bright
Dream’ and ‘Charme’ were symptomless for TMV (Cohen and Sikron, 1999). This study
indicates that some petunia taxa exhibit different degrees of viral symptomology ranging from no
symptoms to extreme symptoms. Thus it could be possible to identify and select individuals that
do not express symptoms in response to TMV and ToMV, and then use them in a breeding
program to produce a line of cultivars which do not visually appear to suffer from viral infection
even though they are carrying the virus. Furthermore, since it is fairly simple to screen for viral
resistance using artificial inoculation and ELISA identification it may be possible to test
populations or petunia for TMV and ToMV resistance. If individuals are found with resistance
to virus infection, a line of petunia cultivars could be developed that are resistant to viral
infection. Nonetheless, until virus resistance is identified and integrated into cultivars of petunia,
maintenance of clean stock plants, proper sanitation, and disinfection of tools either with heat or
2.8 g/L of sodium troclosene are all extremely important in preventing the spread of all viruses
(Cohen and Sikron, 1999).
General Breeding Procedures
The pollination procedure for petunia is not elaborate, but it is highly labor-intensive.
Under controlled conditions in a greenhouse, self-pollination of selected plants is achieved by
lightly brushing an open flower either with an artist’s paintbrush or a pipe cleaner in a way that
distributes self-pollen to the stigma. These flowers are then tagged with the symbol X . When
making cross-pollinations, unopened flower buds must first be emasculated to avoid selfpollination. This can be done by simply grabbing hold of half of the corolla about 1 cm above
the sepals with tweezers and tearing away the top half of the corolla and the unopened anthers.
Pollen from the selected male parent that has properly matured can then be transferred to the
stigma either using an artist’s paintbrush, a pipe cleaner, or by picking the entire anther and
lightly dusting the stigma with pollen. During the warm summer months, seed maturation takes
about 4 weeks. Seed pods should be monitored and collected just as the top 1/3 begins to turn
brown and the pod begins to split. Small coin envelops make excellent containers for the
harvested seed pods. Soon after harvest the seeds should be removed by hand from the
senescing seed pods and allowed to air-dry before they are repackaged into glycine envelops
which should then be filed into new coin envelops for storage at 18 C with 20% relative
humidity. Petunia seeds stored under these conditions will remain viable up to 5 years (Ewart,
1984).
Since petunia cultivars exhibit accelerated floral senescence in response to pollination, it
is possible to monitor the success of hybridization in artificial crosses based on how quickly the
flower senescences following pollination. In general both compatible and incompatible crosspollinations begin to wilt in 2-3 days, however compatible crosses wilt and totally senesce faster
than incompatible crosses. Self-pollinations wilt in 3-5 days, and unpollinated flowers wilt after
8-9 days (Ascher, 1984).
The History of Petunia Breeding
In the early 1920’s and 1930’s as with most crops, petunia seed was sold as an inbred
line. In 1953, petunia breeding dramatically changed when Claude Hope released the first F1
hybrid cultivar, ‘Comanche’, which was a cross between two superior inbred lines (Ewart, 1984).
The immediate success and superiority of ‘Comanche’ over other inbred cultivars proved that the
production of expensive hybrid seed was worth the extra effort. From that time on till the
present, petunia cultivars have been sold almost exclusively as F1 hybrids.
The Genetics of Multiflora, Grandiflora, and Double Petunias
As discussed earlier, cultivars of petunia are grouped into distinct groups. Multiflora,
grandiflora, and double flowering versions of each (either multiflora or grandiflora) will now be
discussed in detail. These 3 groups are dictated by the independent action of 2 loci, with one
gene controlling flower size (G) and one gene controlling single vs. double flowering (D). The
grandiflora type is dominant (GG) or (Gg) to the multiflora type which is homozygous recessive
(gg) (Ewart, 1984). However, it is very important to note that the homozygous dominant (GG)
genotype is very weak and often produces less pollen and less seed than expected. These factors
result in much lower than expected ratios of grandiflora plants in segregating populations of
multiflora and grandiflora plants. There are several theories suggesting that linkage exists
between the G allele and sub-lethal or lethal genes, however, there are no definitive explanations
as of yet.
Breeding for Multiflora Single Flowering Hybrids
The following procedure is described in detail in (Ewart, 1984).
In general, most petunia breeding programs use a modified pedigree system for creating
inbred lines to be used in F1 hybrid seed production. First, a population must be developed
which includes experimental F1 crosses, elite inbred lines, elite cultivars, exotic germplasm, and
anything else you would like to include. This population should then be planted out in the field
and open pollinated F2 seed should be collected and sown in the greenhouse. The F2 plants and
the subsequent F3’s should be evaluated and selected in the transplant pack stage for traits like
pack performance, earliness to bloom, branching and height in the pack, flower color, leaf color,
foliage color, and disease resistance. Selection in the F4 generation should also be done in the
pack stage but the remaining plants of each selected family should then be transplanted to the
field in order to begin evaluating field performance for bloom production, plant habit, vigor, and
“weatherability”. After a couple more generations of selection both in the greenhouse and in the
field, experimental crosses should be set up between top performing lines which are almost
totally homozygous in the F8 (remember that multifloras are homozygous recessive (gg)). The
progeny of the experimental hybrid crosses should then be critically evaluated both in the pack
and in the field using proven elite cultivars as comparisons. This entire process usually takes 1012 years depending on how quickly generations can be reproduced and evaluated.
Breeding for Grandiflora Single Flowering Hybrids
The following procedure is described in detail in (Ewart, 1984).
Because of the weakened condition of the (GG) genotype, grandiflora production is
completely dependent on hybrid production of (Gg) individuals from F1 crosses involving a (gg)
multiflora inbred as the female parent and a (GG) grandiflora inbred as the male parent. The
multiflora inbred line is developed as previously described with emphasis on selecting for larger
flower size and field performance. The development of the grandiflora inbred line requires
special attention due to its weak phenotype. The best source of (GG) plants for inbred line
development comes either from an experimental cross of two (Gg) grandiflora plants or from an
F2 generation of F1 grandiflora hybrids. The progeny of these two sources will segregate for
(GG), (Gg), and (gg) individuals. The (GG) progeny are usually much weaker and can be
selected after about the fourth week and transplanted. As the selected (GG) individuals develop
and begin to flower, self pollinations should be made and grown out. If segregation for flower
size (grandiflora and multiflora) occurs a (Gg) individual was accidentally selected - this familiy
should be discarded. In each generation following here after, lines should be selected for
maximum vigor and fertility. Since (GG) grandiflora plants are generally weak, field testing
would be a disaster. Therefore, all selection of the (GG) inbred line must be done in the
greenhouse. Once both the (GG) inbred lines and their complementary (gg) inbred lines have
been developed, experimental crosses should be set up. The (Gg) grandiflora experimental
hybrids should then be critically evaluated both in the pack and in the field using proven elite
cultivars as comparisons.
Breeding for Double Flowering Hybrids (multiflora and grandiflora types)
The following procedure is described in detail in (Ewart, 1984).
In petunia, a single gene (D) controls the phenotype for double versus single flowering.
The double flowering allele (D) is dominant to the single flowering allele (d). Homozygous
dominant (DD) flowers are double and totally female sterile, where as, heterozygous (Dd)
flowers are also double but form some functional pistils, which can be used in making crosses.
In general, double flowering cultivars are heterozygous (Dd) for flowering and are created using
single multiflora or grandiflora inbred lines as the female and double flowering multiflora inbred
lines as the male.
ddGG
X
DDgg
(single, grandiflora)
(double, multiflora)
ddgg
X
DDgg
(single, multiflora)
(double, multiflora)
DdGg
(double, grandiflora)
Ddgg
(double, multiflora)
The double, multiflora (DDgg) inbred line is usually developed either by selfing an elite double
flowering cultivar (DdGg) and selecting out the Ddgg progeny with the best vigor which can
later be selfed to recover the DDgg inbred lines, or by crossing two related DdGg heterozygotes
and selecting the Ddgg progeny with the best vigor which can later be selfed to recover the
DDgg inbred lines. Once the DDgg lines are developed they must be asexually reproduced since
they are female sterile. The (single, grandiflora) and (single, multiflora) inbred lines used as the
female inbred line are developed as previously discussed.
Inbreeding Depression
Because petunia is a natural outcrosser, inbreeding depression is often a problem in
breeding programs if generations are repeatedly selfed every generation. Usually around the F5
or F6 generation, mass selection or sibling mating should be used to maintain an adequate level
of heterogeneity within a line (Ewart, 1984).
Field Performance and Selection
In general, most cultivars of petunia perform best in warm climates when the moisture
supply is adequate, but not in excess. Periods of high humidity or heavy rain often cause petunia
flowers to discolor, spot, and disintegrate. Unfortunately most petunia breeding programs are
usually limited to only a few outdoor trialling sites due to the high cost of maintaining several
different locations. As a result, many programs release material that has not been properly
trialled in multiple environments where high humidity or heavy rains can be a problem. One
option available to private breeders that is not often utilized is the All America Selections
system. This system is a non-profit organization that trials seed and vegetatively propagated
bedding plants, flowers, and vegetables. Currently there are 42 All America Selections testing
sites in 22 U.S. States and 5 Canadian provinces that represent a diversity of different
environments. At each site a judge evaluates entries for desirable flower forms, flower colors,
fragrance, length of flowering season, plant habit, and disease or pest tolerance. These sites
provide a great opportunity for a breeder to properly assess the limitations of currently available
cultivars and develop future breeding goals appropriately.
The overall process of selection in petunia has become more complicated in the past 10
years due to the recent and dramatic evolution of so many different “types” of petunia including
grandiflora, multiflora, floribunda, milliflora, and spreading types. Of these types some are best
suited for use in containers and window boxes, where others are more suitable for hanging
baskets or as a mass planting in the landscape. These different types of petunia are usually
physiologically different from each other and thus perform quite differently depending on the
type of environment. A breeder must be conscious of this fact and try to make selections
appropriately. One approach would develop and release cultivars for a single, narrow use like
hanging baskets or landscape plantings. For this approach, a breeder would select genotypes
with superior performance in one type of environment. A second approach would develop
multiple-use cultivars with higher stability across all environments. This second approach would
require testing in containers, hanging baskets, and in the landscape over many different locations.
Although this would require more investment initially it would ultimately produce a better
cultivar that was adapted for many different environments. One possible short-cut, involves
determining with an experiment whether performance in a container versus performance in a
hanging basket or in the landscape correlates well over a broad range of genotypes. If adequate
correlation does exist among these environments, selection in one environment could substitute
for selection in all.
Conclusion
In the last ten years the entire bedding plant industry has changed dramatically. The
homeowner is no longer satisfied with a pink petunia or a red geranium. They want something
different, something exotic, and they want it now. The industry has responded by producing
more and more vegetatively propagated bedding plants with weird variegation, interesting flower
morphologies, etc. To respond to these changes, petunia breeding must evolve. Breeders must
incorporate new sources of germplasm with novel traits and they must do it quickly. Moreover,
consumers are also becoming more demanding. They want a plant that will perform well in all
environments, for the entire season, without any inputs. Consumers in the Southeast are tired of
their petunias looking haggard in the humidity. On the flip-side, drought and water restrictions
are on the rise in many communities. Thus, breeders must begin to evaluate landscape
performance more critically in more replicated trials representing all environments.
Furthermore, breeders need to find disease and insect resistance to incorporate into their breeding
programs. Wide-spread disease and virus screening would be fairly simple even with a huge
sample size because the protocols have already been developed. Overall, petunia breeding has
advanced considerably over the last 100 years. There are 5 distinct types of petunia in almost
every color, pattern, and shape. Now advancements must be made to refine all of these types and
make them more suitable for their changing environment.
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