*4.1.1 Lighting*

*Coffee - Production and Research*

embryogenic structures (**Figure 5C**).

**3.3 Somatic embryo of** *Coffea*

indirect callus.

competent cells [86].

than in the spring-summer season.

on average, on one or two sides of the rectangular leaf explant. Normally, these structures range in size from 2 to 4 mm (**Figure 5B**) and remain this way until the end of cultivation. About 50 days after the start of cultivation the structures start to oxidize (**Figure 5C**) and by 150 they are completely oxidized [75]. The formation of somatic embryos usually starts from 90 days of cultivation but in low quantity and around 120 days this number tends to increase. Somatic embryos in addition to being formed at the edges of explants (**Figure 5D**) also develop over the surface of

The main product of somatic embryogenesis is the somatic embryo. Somatic embryos are structures that go through different stages of development until they reach that of a plant [18, 28]. These stages of development are perfectly organized, with all the morphological characteristics corresponding to the same stages of development of zygotic embryos [18, 84, 85] being globular, heart, and seedling. Somatic embryos are bipolar morphological structures, presenting radicle, hypocotyl and cotyledons. The somatic embryo does not exhibit endosperm differentiation and is independent of explant tissue after initiation and development. Somatic embryos do not go through the maturation or desiccation phase as in zygotic embryos. This system is not connected to the vascular tissue of the mother or explant during its initiation and development [86]. Moreover, in the direct pathway, it is possible to find somatic embryos at different developmental stages in the same explant [16]. This response pattern can also be found in the

Somatic embryos formed by the direct and indirect pathways are transferred to the germination stage and generally use MS/2 culture medium added 20 g/L sucrose and without plant growth regulators. This same medium can also be used for embryo growth and development to the seedling stage. These observations suggest that *Coffea* somatic embryos may have a hormonal balance that favors differentiation of developmental stages, requiring only nutrients from the culture medium

without phytoregulators for germination and growth and development.

**4. Factors that influence the occurrence of somatic embryogenesis**

The control of the occurrence of somatic embryogenesis in *Coffea* is not yet completely identified. Some authors relate the genetic pattern of the species with the absence or low responsiveness [87]. Knowing the factors that control the occurrence of somatic embryogenesis in *C. arabica* will allow to optimize its application and especially the direct pathway. The high or low capacity of somatic embryogenesis of a species is related to the presence of competent cells or not in the explant, inherent to their totipotency [88]. The maintenance of somatic embryogenesis capacity requires the use of conditions that maintain the proliferation of determined and

Somatic embryogenesis regeneration capacity is also associated with other factors such as explant donor plant developmental stage, explant donor plant physiological conditions, explant position relative to the plant [89], *in vitro* culture conditions and mainly of plant growth regulators. The seasons influenced the indirect somatic embryogenesis response of plant explants to eight *C. arabica* genotypes in the field [56]. Explants formed more somatic embryos in the fall-winter season

**10**

In indirect somatic embryogenesis of *C. arabica*, calli are induced and initiated in the absence or presence of light, but they reach a larger size only if maintained in the absence of light [45, 86]. The size of these callus increases gradually each month, and can reach sizes up to 30 mm.

On the direct pathway, *C. arabica* explants also have difficulty responding in the presence of light. In this way, at the edge of the explants, small structures are formed, which are called embryogenic structures, which remain without change in size and shape in the presence or absence of light.
