Monoecious mechanisms in Castor (Ricinus communis L.) Introduction Plant Morphology Floral morphology Sex expre...
Monoecious mechanisms
in Castor (Ricinus communis L.)
Castor is
a ubiquitous plant in India, which can be found from Northern plains to
Southern tip, Western to Eastern parts and also in Andaman and Nicobar islands
of India. Castor bean contains 40-54% oil content, because of which these
plants were domesticated and are being cultivated primarily in the states of
Gujarat and Rajasthan, Telangana, Andhra Pradesh and to some extent in
Karnataka, Tamil Nadu, Odisha, Maharashtra, Madhya Pradesh and Chhattisgarh.
Castor
oil has commercial value as an industrial oil for manufacturing of lubricants,
polymers, cosmetics, medicines etc. Commercial cultivation of castor is found
in India, African countries (Mozambique, Kenya, Ethiopia, Uganda etc), China
and Brazil accounting for 1.3 million ha with the production of 1.4 million
tonnes. India has lion’s share of 80% area of the world’s castor bean
contributing to 85% of total global production of castor.
Exploitation
of mechanism of sex expression in castor led to development of high yielding
hybrids in castor. Utilizing two line system-pistillate and monoecious
forms for the development of castor hybrids without the use of male sterility
unlike in other crops such as sunflower, maize, pearl millet etc, has played a
major role in harnessing higher productivity in castor. Further understanding
of the different forms of sex expression is required to facilitate
characterization and utilization of castor germplasm across India and the
world.
This
article gives an understanding of the different types of sex expression with
self-explaining photographs.
Botanically,
castor (Ricinus communis L.)
is a monotypic genus and has single species communis, a member of Euphorbiaceae
family but is highly polymorphic in nature. Plants are perennial, branching and
grow upto 6-12 feet height in wild situation, while the cultivated forms are
5-6 feet tall under normal climatic conditions. Stem is geniculate, thick,
branching and terminates with an inflorescence (raceme) with or without
characteristic waxy bloom and the stem colour varies from green to red. Leaves
are large with long petiole (20-50 cm) attached to the stem, lamina of the leaf
is smooth, serrated with a palmate blade which has a width from 10 to 40 cm and
7-11 lobes, which have double-toothed edges. Colour of the leaves is dark green
to red in colour covered with or without waxy bloom.
Every
branch terminates with an inflorescence called “Spike” having both male and
female flowers separately and therefore a monoecious raceme. The upper syncarp
of the fruit is a fragile, spherical, or elongated with a wrinkled or smooth
surface with or without spines. The seed of castor is nut-oval form, covered by
a dense, shiny but fragile hull of two colours. The background is dark, usually
dark-gray, red or brown with mosaic spots of various dimensions and
colours.
The inflorescence
of castor is a receme/spike with the length of 10-100 cm usually bearing
unisexual flowers. Male flowers are usually arranged at the lower part of the
spike, occupying 5-90% of its length; sometimes they are absent or arranged
singly among female ones. The extent of male and female flowers in
racemes/spikes depends on the genetic nature of the plant and the environmental
conditions. There are varieties which bear only female flowers. The flowers are
arranged on spikes in groups called racemes. In every raceme first a central
flower appears and then flowers of first, second and subsequent orders. Opening
of all flowers on the entire spike takes 5-25 days, and their pollination, seed
set and maturation takes further 50-70 days. Spikes differ in shape (conical,
cylindrical and oval), length of their non-productive part and density of
capsules.
Buds of
male flowers are wide-conical with a diameter of 0.4-1 cm, with a five-bladed
simple perianth and with many stamens united into several bundles, each bundle
of which carries twin-chambered golden yellow anterior lobes. There are up to
60-80 pollen grains in each chamber, and 50-60,000 per male flower. The pollen
grains are small, oval and 20-22μm length. The male flower, after blooming,
spreads pollen for 1-2 days, then by forming an abscission layer, parts from
the peduncle of inflorescence along with a part of the pedicel.
Buds of
female flowers are narrow-conical, 0.6-1.2 cm long and 0.2-0.4 cm in diameter;
the perianth is simple, splitting into five segments; its ovary is comprised of
three locules. It is three celled, with growth underneath the short stub and
three bright coloured (pink, red) fringed stigmas. After fertilization (after
7-10 days) the ends of the stigmas dry up.
The
floral morphology of castor is described based on the arrangement of male and
female flowers on the raceme. Normally, castor is monoecious with pistillate
flowers on the upper part of raceme and staminate flowers on the lower part.
This type is referred to as normal monoecious. Several classifications were
given by different authors based on the number and pattern of arrangement of
male and female flowers on the spike (Kulkarni and Ramanamurthy, 1977, Moshkin,
1986). The basic sex forms in castor are monoecious, pistillate and sex
reversion either to monoecious or interspersed staminate flowers. Castor is a
sexually polymorphic species and different forms of sex expression are
described below.
Monoecious (M)
It is the
most natural occurrence of annual and perennial castor. The spike has, either,
only male flowers in the basal 1/3rd to ½ the length. The top portion has
female flowers or male and female flowers arranged in an interspersed fashion
in these few whorls. A line with this type of floral mechanism having lesser
proportion of male flowers can be exploited as high yielding variety.
Pistillate
(P)
Occurs as
a rare recessive mutant with the spike having female flowers throughout the
spike. Pistillateness is prominent in all the early order racemes (4th order).
A recessive mutant TSP-10R is a pistillateness up to 10th order and served as
source for developing pistillate parents in hybrid seed production.
Interspersed staminate
flower (ISF)
A variant
of pistillate form with male flowers interspersed, throughout the female
flowers on the spike. ISF is significant in multiplication of the female parent
in hybrid seeds production.
Sex revertant
It is a
female that turns to monoecious or inter-staminate at later stage.
These
basic sex forms are classified in different ways by different authors as the
expression of sex in different locations is highly influenced by environmental
conditions. Sex reversion in later stage of the plants can be exploited to
maintain pistillate lines.
- Female promoting environment
(F): Winter, low temperature
(<30°C), young plants, early order spikes, high nutrition, less
difference between maximum and minimum temperature promote female flowers
and shift the balance towards femaleness on a spike.
- Male promoting environment (M): Summer, rainy seasons, high temperature
(>32°C), old plants, late order spikes, low nutrition and large
difference between maximum and minimum temperature promote male flowers on
a spike and incline towards maleness.
The
proportion of pistillate and staminate flowers among racemes vary a great deal
both within and among genotypes. It is also influenced by the environment.
Female tendency is highest in winter months while the male tendency
predominates in summer as well as rainy seasons. High day temperature of
>32°C promotes male tendency; low day temperature of <30°C promotes
female tendency. Age of the plant, nutrition also influence sex tendencies.
Femaleness is strongest in younger plants with high level of nutrition.
Maleness is strongest in older plants with a low level of nutrition. This
information is useful in understanding general variation in castor and various
forms of sex expression by new plant breeders/botanists involved in castor
improvement programmes across the world. It also helps people involved in
castor seed production for better understanding and development of new
techniques in hybrid seed production.
References
- Kulkarni, L.G. and
Ramanamurthy, G.V. 1977. Castor. ICAR. New Delhi, p.105.
- Moshkin, V. A.,1986. Castor. Oxonian
Press Pvt. Ltd. New Delhi. pp.47-50
Content
contributors : T Manjunatha* and Ramya K.T
*ICAR-Indian
Institute of Oilseeds Research, Rajendranagar, Hyderabad-500030. Email:
t.manjunatha@icar.gov.in
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