lettuce

Lettuce (Lactuca sativa L.) is a member of the family Asteraceae (Compositae), a successful and diverse
group of plants with a global distribution (Funk et al.
2005). The Asteraceae are thought to be the largest
family of plants comprising some 23,000 to 30,000
species (Bremer 1994; Bayer and Starr 1998; Funk
et al. 2005). Although the phylogeny of the dandelion tribe Cichorieae (Lactuceae) has been problematic (Funk et al. 2005), Lactuca species have been
placed in subfamily Cichorioideae, tribe Cichorieae
(Lactuceae) by Bentham (1873), Bremer (1994), and
Cronquist (1955) on the basis of morphometric characteristics. More recent molecular analyses have supported this position and have helped clarify tribal
placements (Bayer and Starr 1998; Funk et al. 2004,
2005).
Cultivated lettuce is believed to have been domesticated in the Mediterranean region from the wild
species Lactuca serriola L. (Durst 1930; Harlan 1992)
with the center of origin apparently southwest Asia
(De Vries 1997). Lettuce is known to have been cultivated at least 4,500 years BP as long-leaved Costype lettuce was depicted on the walls of Egyptian
tombs (Lindqvist 1960). Formally, cultivated lettuce
is grouped into seven different types: Cos (a.k.a. Romaine), Cutting (a.k.a. Leaf), Stalk (or Asparagus),
Butterhead, Crisphead (a.k.a. Iceberg or Cabbage),
Latin, and Oilseed (Trehane 1995; De Vries 1997).
All groups, except the Oilseed group, are selections
within Lactuca sativa, while the Oilseed group are
derived from either L. serriola or L. sativa or may
be a hybrid between these two taxa.

The Cos group, named after the Greek island, is
typified by plants with oblong upright rigid leaves
and a prominent midrib with dark green leaves (De
Vries 1997). According to Ryder (1986) this is the most
common type in the Mediterranean region. The Cutting group forms no firm heads but instead produces
a dense mass of leaves in the center of the plant. The
leaf may vary from smooth margins to deeply lobed,
and may include frilled, curled, or fringed leaves. This
group is usually referred to as either greenleaf or
redleaf based on leaf color, which ranges from green
to yellowish-green and includes various shades of red
depending on anthocyanin content and light intensity
during growth. The Stalk lettuce group does not form
heads and has prominent thickened fleshy stems and
upright ovate leaves; both the leaves and stem may
be eaten. The Butterhead group forms a small head
of nearly spherical (orbicular) leaves surrounded by
wrapper leaves. The leaf texture is distinct; leaf color
varies from green to yellowish-green and cultivars
with anthocyanin have been developed. The ButterheadgroupiswidelyusedinEuropeandwassecondin
commercial importance in the early twentieth century
in the US (Durst 1930). The Crisphead group forms
tight, dense heads which are comprised of spherical leaves folded upon each other. The leaf is crispy
and the veins are prominent. Leaf color varies from
deep green to light green, with some genotypes containing anthocyanin. Crisphead lettuce is the most
popular lettuce type in the US, although the market share of Cos and Cutting types have markedly
increased in recent years (USDA 2005a). The Latin
group forms a poorly organized rosette that is similar
in appearance to the Butterhead group. The orbicular
leaves of the Latin group are thick, have entire margins, and are green. This lettuce type is cultivated in
the Mediterranean region including North Africa and
in South America (see De Vries 1997). The Oilseed
group is typified by a multistemmed upright growth
habit with green leaves that are oblong to oblanceo

Economic Importance
Lettuce is the second most valuable vegetable produced in the United States with farm-market receipts
of over $1.98 billion in 2005 (USDA 2005a). About
75% of all lettuce produced in the US is grown in
California, and combined with Arizona these two
states account for 96% of the total lettuce production (USDA 2005a). China produces almost one-half
of the world’s lettuce and at more than 11 million Mt
their production is more than double that of the US
(Table1).Since1980worldwidelettuceproductionhas
increased ∼2.7-fold with 22.4 million Mt produced in
2005 (FAO 2006). The increased production was primarily due to an increase in acreage planted as opposed to an increase in yield. From 1980 to 2005,
lettuce yield (Hg/ha) increased by 118% compared
to an increase of 149%, 135%, 146% and 128% for
maize, potato, paddy rice and tomato, respectively
(data not presented, FAO 2006). This might reflect the
greater genetic resources developed for these crops
or alternatively, the greater yield gain of these other
species might be an effect of selecting for the harvested organ (seed, fruit or tuber) whereas the vegetative growth in lettuce cannot be further increased.
In terms of biomass harvested per hectare, of all major
agronomic and vegetable crops, only the tomato has
a greater yield than lettuce (FAO 2006). If vegetative
growth has been maximized in lettuce, any increase

in yield can only be derived from improving disease
and insect resistance.

Nutritional Value
The nutritional content of lettuce varies considerably
with type (Table 2; Mou 2005; USDA 2005b). In general, Crisphead types are depauperate of nutritional
value whereas the Cos and Cutting types have appreciable amounts of ascorbic acid, vitamin A, vitamin K, folate, and the carotenoids β-carotene and
lutein+zeaxanthin (Table 2). The Cutting types are
especially enriched with vitamin A and β-carotene,
having about 15-fold higher amounts than Crisphead
types. Cos types are relatively rich in vitamin C and
folate compared to the other lettuce types.
Whereas the role of vitamins and minerals in
human health are clearly established, other natural compounds such as flavonoids, tocopherols, and
carotenoids, etc., are linked to human health only
through epidemiological studies. Establishing a clear
link between naturally occurring compounds and human health is an active area of research and efforts
are underway in lettuce to increase the concentrations
of these complex phytochemicals. Although lettuce is
not the single richest source for many nutrients, it has
the advantage that it is almost universally eaten raw
thus preserving nutritional value that would otherwise be lost upon cooking.
Significant genetic variation in nutritional and
phytochemical content exists within lettuce types.
Mou (2005) screened 52 genotypes of Crisphead, Cutting, Cos, Butterhead, Latin, Stalk and the closely related taxa of L. serriola L., L. saligna L. and L. virosa L.
for β-carotene and lutein content. In Crisphead lettuce
types β-carotene concentration varied over fivefold
(μ = 328 ug/100 g FW) while Butterhead types varied
over fourfold, Cos types by twofold and Cutting type
(greenleaf) by approximately twofold. Similar ranges
inconcentrationwithinlettucetypeswereobservedin
lutein concentrations and a high correlation between
β-carotene and lutein content was noted (Mou 2005).
Mainly due to their antioxidant activity
carotenoids have been linked to human health, thus
providing the impetus for increasing the carotenoid
content in lettuce and other food groups (for review
see Cooper 2004). Because per capita consumption
of lettuce is high, lettuce may be a significant source
of dietary antioxidants. Carotenoids are tetraterpene
molecules (C40) synthesized in the nonmevalonate

pathway that serve as minor accessory lightharvesting pigments absorbing and transferring light
energy to chlorophyll molecules (Malkin and Niyogi
2000). They also play a major role in protecting
the photosynthetic apparatus from photo-oxidative
damage by preventing oxidation by singlet oxygen.
Although not experimentally validated, lettuce plants
bred for higher carotenoid concentrations may have
a higher tolerance for environmental stress, which
could be particularly beneficial in the high light
environments of the low desert production areas of

the US. Because carotenoids function to ameliorate
oxidative stress under high light conditions, their
concentration might be expected to be higher in
plants grown under high light conditions and this has
been observed. Carotenoid concentration was higher
in lettuce plants grown in fields versus glasshouses,
and in those maturing in the summer versus fall
(Kimura and Rodgriguez-Amaya 2003). Because
carotenoid synthesis is light responsive, it may not
be possible to increase carotenoid concentration in
Crisphead lettuce types via breeding because the

tight compact head must be retained; instead, the
carotenoid pathway may have to be stimulated via
a constitutive promoter.
Other antioxidants, such as flavonols, are found
in appreciable amounts in lettuce (Hohl et al. 2001).
In Butterhead types, although present in the inner
leaves, the highest concentrations of phenolic compounds (i.e., quercetin, chlorogenic acid, and chicoric
acid) are found in the outer leaves. Concentrations of
phenolic compounds increased in inner leaves by 10-
to 20-fold once illuminated (Hohl et al. 2001). Fourfold differences in quercetin concentrations were observedamongcultivarswithinButterheadtypes(Hohl
et al. 2001) and between lettuce types (DuPont et al.
2000). Although other fruits and vegetables may have
higher concentrations of phenolic compounds, the
peroxyl radical scavenging activity of lettuce (especially redleaf types) is higher when expressed on the
basis of total phenolic concentration (Caldwell 2003).