*2.1.1.3 Contractile growth and crown development*

The contractile growth occurs in a process where hypocotyl shortens and has a thicker growth due to carbohydrate storage within plant. This stage pulls the

#### **Figure 1.**

*Growing stages of alfalfa (source: https://nutrien-ekonomics.com/latest-fertilizer-research/alfalfadevelopment-and-growth-staging)/.*

cotyledonary and the unifoliate node underneath and crown develops. At this point of growth, critical crown buds develop. This stage commences from first or second week of emergence and lasts for 2 weeks.

## *2.1.2 Flowering in alfalfa*

The inflorescence of alfalfa is a dense raceme, which is compact around 2 inches long holding up to 50 short-stalked flowers emerging from the leaf axils [30]. The florets commence to open from the base to the tip of the raceme, which takes around a week. A floret may open any hour of the day and remain open for a week if not pollinated, but withers within few hours of pollination (**Figure 2**) [31]. Flowers have five petals, with a larger standard petal upright to curved backward with deep colored veins. The corolla consists of a standard petal, which is the landing platform for pollinators, two smaller wing petals, called lateral petals that project forward and two fused petals called keel. The keel encloses the reproductive parts, two groups of stamen and a style from the green ovary, enclosing anthers and stigma [32]. The color of corolla varies from purple or violet through shades of blue, green, yellow, cream or white [33]. The calyx is short, green with five long pointed lobes with or without fine hair. Each flower has a small pointed bract (**Figure 3**). The sexual organs are non-functional unless released from keel; once released (tripped), fertilization happens if successful pollination occurs. The ovules in the ovary begin to develop, resulting in curly spiral pods twisting in 2 or 3 spirals, the number depends on the number of ovules that progress as complete seed. The number of seeds in a pod depends on degree of pollen compatibility. The pod matures and is equipped for harvest after a month of pollination [34]. A characteristic pod has fine hairs that turns black as matures and release kidney shaped seeds.

### *2.1.2.1 Tripping*

The release of reproductive parts is a phenomenon that is mandatory in alfalfa fertilization and profitable seed production [35, 36]. Tripping is defined as the release of sexual column which includes style, stigma, part of ovary and 10 stamens, by an explosive force as a string under tension is released. Pollinator inserts it proboscis into the flower throat and exerts pressure among the keel petals to access pollen and nectar [37]. When a flower visitor trips the flower, the pistil and stamens hit the pollinators head, where the pollen get deposited and is dislodged by future tripping's of other flowers [38]. Tripping also breaks a membrane on the stigma, which would prevent fertilization of ovule [39]. Natural tripping agents are pollinators such as honey bees and other types of bees and wind [40]. Alfalfa is partially incompatible at fluctuating degrees. If cross-pollinated pollen is not deposited when the flower is tripped, alfalfa may still fertilize by self-pollen (**Figure 4**).

## *2.1.2.2 Pollination: honey yield, nectar secretion and pollen production*

A typical alfalfa plant produces racemes of small pale to dark purple flowers, which require insect visitation for pollination. When a bee visits for pollination, it opens the keel petals and the stamens and pistil move forward, striking the bee with a force. The usual visitors grasp a technique to avoid this mechanism by approaching the flower from the side and inserting their proboscis in between the petals from the base of flower to reach the nectar. Alfalfa secretes a large quantity of nectar, that lures a variety of bee species, of which honey bees produce excellent crop of high-quality honey. The estimated yield of nectar is 416–1933 pounds of nectar per acre [41]. Several terpenoid compounds are identified in alfalfa crops that attracts

**Figure 2.** *Alfalfa flowers.*

**Figure 3.** *Inflorescence of alfalfa.*

the pollinators towards alfalfa, of which, the aromatic compound ocimene has great role [42, 43]. Alfalfa excretes tremendous quantity of nectar, which lures variety of bees, and from which honey bees produce excellent crops of high-quality honey [41]. Approximately 400–1900 pounds of nectar is secreted by alfalfa [42]. Honey bees are reluctant pollinator of alfalfa as they have to make effort to struck forcefully in the head for the flower to be tripped to enter into forage for nectar. The leaf cutter bee is efficient in pollinating alfalfa than honey bees [44]. High densities

*Reproductive Ecology of Forage Alfalfa (*Medicago sativa *L.): Recent Advances DOI: http://dx.doi.org/10.5772/intechopen.100640*

#### **Figure 4.** *Tripping.*

of bees can lead to adequate pollination to produce desirable honey quantity. A number of 12–15 colonies per acre are recommended for efficient pollination and better honey production. A strong honey bee colony per acre of seed alfalfa could produce 50–100 pounds of honey. Several wild bees also extend their pollination services in alfalfa [45]. Breeding alfalfa cultivars more friendly to honey bees with traits such as plants with stamen protruding beyond keel, plants that could be more easily tripped, plants with high nectar production etc. will aid in greater pollination services, thereby more seed set [46]. Bee colonies in alfalfa crop, yields honey potentially as they are the chief alfalfa pollinators. If the crop is cut for hay before flowering, the bee keeper will get very little quantity of honey [47]. If the crop is for seed production, a better yield of honey can be harvested, which depends on the plant density, competition from other pollinators and other environmental as well as agronomic factors. Alfalfa is a poor source of pollen for honey bees, but relished by other pollinators such as *Bombus*, *Megachile*, *Halictus* and *Nomia* [48].

#### *2.1.2.3 Seed set*

A miniature portion of the total ovules develop into seed. A flower should be visited at least once by a pollinator, to get tripped before fertilization. Once tripped, the racemes hold the developing seed pods on the lower part, few open florets in the middle and unopen buds at the top [49]. The cross-pollinated pods produce comparatively more seeds than self-pollinated plants. Moreover, the plants regenerated from self-pollinated plants are less competitive with low survival rates. It is estimated that 2 billion flowers per acre of alfalfa were produced, of which 200 million flowers have the ability to set pods. In a probability of 5 seeds per pod, 2,200,000 seeds per pound, a potential seed production of 50,000 pounds of seed per acre is estimated [50].

#### *2.1.2.4 Seed dispersal*

Alfalfa seeds are stood in coiled leguminous pod and it is nonshattered. The local mode of dispersal is natural and non-mechanized. Wind dispersal of seeds is very rare as the seeds are very dense and smooth. Seed dispersal by animals is a possible mode, in situations where animals forage on them [51].

#### *2.1.2.5 Seed quality*

Seed quality is the critical aspect in agriculture for offspring generation as well as their conservation, which is measured by germination percentage. The factors influencing seed quality includes proper irrigation and nutrient supply, climatic conditions during seed formation, maturation, harvesting, and drying techniques. Seeding in suitable field condition at proper timing, smearing good crop management aspects, adopting suitable harvesting and drying techniques, minimizing mechanical injuries and ensuring minimum deterioration in storage will favor in obtaining alfalfa seeds of prior quality [52].

#### **2.2 Taxonomy and genetics**

*Medicago sativa* L. belongs to the order Fabales, family Fabaceae, tribe Trifolieae, genus *Medicago*. The genus *Medicago*, is wide, comprises more than 50 species; mostly annuals and few species are perennials [53]. *M. sativa* is a perennial herbaceous legume, that is a tetraploid (2n = 4x = 16) [54]. The allogamy and autotetraploid nature of this species contribute to genetic variations within the population. The commercially cultivated species for forage is *M. sativa* belongs to a *M. sativa* complex, a group of closely related sub species sharing similar karyotypes, the collection of chromosomes and interfertile, capable of interbreeding. The diverse subspecies in the complex comprises subsp. *glutinosa*, subsp. *coerulea* etc. [55]. The most commonly cultivated alfalfa in the globe is *M. sativa* and *M. falcata* is cultivated to some extend around the globe. *M. sativa* is a tetraploid, characterized by purple flowers and coiled pods, whereas *M. falcata* occurs both as tetraploid and diploid and characterized as yellow flowers and sickle shaped. Two other closely related species *M. prostrata* and *M. glomerata* is utilized in natural hybridization with alfalfa [56]. All other members of *M. sativa* species readily cross pollinate with the cultivated species. Several alfalfa species exhibit genetic self-incompatibility or self-sterility and will not successfully self-pollinate [57]. Alfalfa is severely affected by inbreeding depression i.e. reduced forage and seed yield due to self-fertilization attributed to accumulation of recessive alleles due to self-pollination or pollination among close relatives. Alfalfa plays a great role in any genetic engineering context in their improvement as it is an allogamous tetraploid, highly sensitive to inbreeding depression, inability to survive and perpetuate its genetic material [58], highly variable among cultivars and most are synthetic cultivars produced by recurrent phenotypic selection [59]. Therefore, advancement in alfalfa readily lies in other species or sub species of economic importance.

#### **2.3 Ecology of alfalfa**

Alfalfa is well adapted to most environments as demonstrated by its wide global distribution. The perennial growth habit of alfalfa, ability to survive extreme temperature, nitrogen fixing symbiosis through their roots and their hard seed coat facilitate their survival in extended ecological regions [60]. High summer temperature, drought, competition from other grasses, water logging and root and crown diseases leads to stand loss [61].

**Hard seed:** Hard seed is a post-harvest dormancy attributed by alfalfa seeds. A hard water impervious seed coat temporarily prevents water uptake, thereby delays germination. Majority of alfalfa seeds imbibe water and germinate readily and not exhibiting true physiological dormancy [62]. **Auto toxicity:** Alfalfa plants and plant residues produce compounds that provoke an autotoxic reaction that hinder seed germination [63].
