Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

7. Plant Growth and Development
7.1 Embryonic Stage

Ontogeny is the individual development of an Organism from a zygote (or vegetative propagule) to its natural death. During ontogeny, the hereditary information of the organism (the genotype) is realized under specific environmental conditions, resulting in The formation of the phenotype, which is the sum of all morphological and physiological traits of the individual organism.

Development refers to qualitative Changes in the Structure and Functional activity of PLANTS AND THEIR parts throughout ontogeny. METABOLISM/2.html">THE CONCEPT OF "development" also encompasses age-related changes.

Qualitative differences between Cells, Tissues, and Organs are referred to as differentiation.

Growth is the irreversible increase in the size and mass of a Cell, organ, or entire organism, driven by the formation of new structural elements. The term "growth" reflects the quantitative changes that accompany The Development of an organism or its parts.

Stages of Ontogeny in higher plants. The development of higher plants is divided into four stages: embryonic, juvenile, reproductive, and senescence.

The embryonic stage of seed plant ontogeny encompasses embryo development from a zygote to a mature seed. The zygote is formed by the fusion of a sperm cell from the pollen tube (male gametophyte) with the egg cell of the embryo sac (female gametophyte). The embryo sac is located within the nucellus, which is surrounded by the integument. Embryos undergo a series of sequential developmental phases. For most dicotyledons, these are the proembryo, globular, Heart-shaped, torpedo, and maturation phases.

Following Fertilization, the zygote remains in a latent state for some time (ranging from 2–4 hours to 1–3 days). During this period, RNA Synthesis and zygote volume increase, and the triploid nucleus begins to divide to form the endosperm. Indole-3-acetic acid and Cytokinins, which are necessary for endosperm development, are supplied by the nucellus and Placenta. During the first division of the zygote—which occurs perpendicular to its polarization axis—the daughter cell facing the micropyle becomes significantly larger and, through subsequent divisions, forms a uniseriate cell row known as the suspensor. The suspensor performs the following Functions:

· pushes the embryo deep into the endosperm tissue;

· absorbs nutrients from the nucellus and integument and transfers them to the embryo;

· synthesizes phytohormones.

In some species, the Cells of the suspensor adjacent to the embryo later become part of the apical meristem and the ROOT cap of the embryonic root. Two synergids adjacent to the egg cell and antipodal cells located near the endosperm function as haustoria, absorbing nutrients from the nucellus.

The second cell of the two-celled embryo, oriented toward the chalazal pole, divides twice along the polarization axis to form a quadrant. Subsequently, in most dicotyledons, each of these cells divides transversely, resulting in an octant. An embryo consisting of 1, 2, 4, or 8 cells is referred to as a proembryo. All 8 cells of the proembryo undergo periclinal divisions (parallel to The surface of the octant). This developmental phase is called the globular stage. The outer cells subsequently form the protoderm, while the inner cells give rise to the primary cortex and central cylinder. Cytokinin is particularly essential during this phase; it is supplied by the endosperm, which plays a major role in embryo development.

The next phase is the heart-shaped stage. In the morphologically upper part of the globular embryo, intensive bilateral Cell Division is observed, resulting in the initiation of the primordia of two symmetrically positioned cotyledons. Conversely, cell division sharply slows down in the region between them (the future SHOOT apex). During this period, the supply of indole-3-acetic acid, cytokinin, and adenine is required for normal development.

The torpedo phase (torpedo stage) of embryo development is characterized by cell divisions predominantly transverse to the longitudinal axis, along with more intense cell elongation in the cotyledon primordia and hypocotyl zone. Procambial cells in the hypocotyl are clearly distinguishable by their elongated shape. The root promeristem is formed. In addition to indole-3-acetic acid and cytokinin, gibberellin is required (for hypocotyl elongation). In some dicotyledons, the cotyledons and hypocotyl bend and fold in half during growth. The shoot apex is established between the cotyledons, and the suspensor disintegrates. If the cotyledons function as storage organs, they occupy almost the entire volume of the ripening seed, accumulating Proteins, starch, and fats during the final stages of embryo formation. The influx of nutrients into the ovules and, subsequently, into the ripening seeds and fruits is determined by these regions becoming dominant sinks: their tissues synthesize large amounts of phytohormones, which enhances their attracting capacity.

During the final maturation stage, the seed loses a significant amount of Water and, in most temperate species, enters a state of dormancy. This transition is associated with a decrease in free Auxins, cytokinins, and Gibberellins in the tissues, accompanied by an increase in ABA content.