Botanical description and reproductive biology
The maca plant is a rosette of frilly leaves with an enlarged fleshy underground organ formed by the taproot and the lower part of the hypocotyl (León 1964; Tello et al.1992). These parts of the plant swell during growth, forming a storage organ resembling a turnip. For simplicity, we will call this organ 'hypocotyl', which is the economic product of maca. The foliage forms a mat, growing in close contact with the ground. The leaves exhibit dimorphism, being larger in the vegetative phase and reduced in the reproductive cycle (Tello et al. 1992). The 'hypocotyls' display a variety of colors from purple to cream and yellow (León 1964). This species is an octoploid with 2n=8x=64 chromosomes (Quirós et al. 1996), considering that the basic genomic number of Lepidieae is x=8. Its meiosis is normal, with the chromosomes associating predominantly as bivalents. This type of association indicates that maca is a disomic polyploid. Polyploidy is a common event among the species in the tribe Lepidieae to which maca belongs (Darlington and Wylie 1945). Most of the pollen collected from the flowers is fertile, as measured by pollen stainability. Consistent with other cruciferous species, pollen grains are trinucleated.
Maca is an annual crop completing its life cycle within a year when climatic conditions are favorable (Quirós et al. 1996). However, often maca is considered a biennial plant (Tello et al. 1992) because it has a vegetative cycle followed by a reproductive phase. Furthermore, in the Junín area maca is grown as a biennial by holding the 'hypocotyls' underground during the dry season. However, during favorable years, when there is enough moisture in the soil and an absence of killing frosts, plants left in the field complete their life cycle within a year. The vegetative phase includes the expansion and growth of the 'hypocotyl' and root. These organs are fully enlarged approximately 7 months after planting. At this time the plants initiate their reproductive phase. Often the first floral buds will appear in a small cluster at the center of the rosette, or as solitary flowers in some of the leaf axils, announcing the initiation of the generative shoots, the main reproductive structures. Only a few of the first flowers will produce fruit. Almost at the same time, at the base of the plant, radially and under the leaves, generative shoots will rapidly grow, producing secondary branches. These will generate most of the seed of the plant. Approximately 20 primary generative branches are produced per plant, and each of these will produce approximately 13 secondary branches (Aliaga Cárdenas 1995). The generative branches will produce profuse flowering racemes for the next 3 months. Each secondary branch will yield racemes with 50-70 flowers each. Therefore, a primary branch will bear close to 1000 flowers. Fruits will set in most of these flowers throughout this period, maturing in approximately 5 weeks. At this time, the fruits will initiate dehiscence and the mature seed will be released. During the long period of flowering, it is possible to observe both fruits and flowers in the generative branches.
Approximately 85% of the fruits will bear seeds. Apparently seeds do not have dormancy, germinating in 5-7 days at 25°C and good moisture conditions. A single plant of maca produces approximately 14 g of seeds. One gram contains approximately 1600 seeds. Seeds are small, measuring 2 mm in length, and are light tan to brown in co lour (Aliaga-Cárdenas 1995).
The flowers of maca are inconspicuous and arranged in axillar racemes. They have four erect, concave sepals, and four small white petals. The ovary is oval and bicarpelar with a short style, which develops into a dehiscent silique of two locules, carrying one seed per locule. Only two stamens, or seldom three, with well-developed anthers are present in the flowers. A variable number of rudimentary stamens consisting only of filaments is also present. The normal number of functional stamens in the family Brassicaceae is six, four larger than the other two. However, androeceum variation reflected in number of complete stamens is a common feature of the genus Lepidium (Thellung 1906). Small nectaries at the base of the stamens are also present. It is unknown, however, whether these are functional. Aliaga-Cárdenas (1995) found that maca is primarily an autogamous species. Pollination is initiated 4-5 days after the flower bud is first visible to the naked eye, and continues for another 3 days. The anthers and petals wither for the next 2 days while the ovary starts to enlarge initiating fruit development. Part of the anthesis takes place while the flower is still closed, thus indicating that the maca flowers are partially cleistogamous. Further evidence of autogamy is provided by spontaneous fruit-setting of flowering plants in growth chambers, where insects were excluded (Quirós et al. 1996). In Junín, the native area of maca production, no insect pollinators working the flowers were observed. Only sporadic visitation by two or three species of Dipterae which landed in the leaves and flowers have been seen (Quirós, unpublished).
In the field at Davis, California, only a few syrphid flies were observed visiting the foliage and seldom the flowers. Plants grown from different accessions are morphologically alike, with a few exceptions. All these observations suggest that maca reproduces predominantly by self-pollination.
