Answer the following questions in about 150 words each: (a) What are the consequences of domestication of plants? Explain…

(a) Consequences of Plant Domestication

Domestication is a process of artificial selection that fundamentally alters plant genetics, morphology, and physiology, leading to specific consequences. Genetically, it causes severe erosion of allelic diversity; wild populations like Oryza rufipogon possess vast genetic variation, whereas cultivated rice (Oryza sativa) retains only a fraction, increasing vulnerability to pests and climate change. Morphologically, selection favors traits beneficial to humans, such as increased seed size, synchronous ripening to facilitate harvesting, and the loss of shattering mechanisms, which prevents seed dispersal and ensures harvest efficiency. Physiologically, domesticated plants often exhibit reduced seed dormancy to allow immediate germination upon sowing and become less sensitive to photoperiod, enabling cultivation across diverse latitudes. However, these adaptations come at a cost: reduced fitness in wild conditions, as domesticated plants lose competitive abilities and defense mechanisms against natural predators. In the Indian context, the transition from wild to cultivated wheat in the Fertile Crescent and subsequent adoption in Mehrgarh illustrates how these changes enabled the shift from foraging to settled agriculture, forming the basis of the Indus Valley Civilization’s agrarian economy.

(b) Successive Cambia and Stem Thickening

In some flowering plants, secondary thickening is achieved through the formation of successive cambia rather than a single continuous vascular cambium. Initially, a primary vascular cambium forms between the xylem and phloem. As this cambium matures, it differentiates into xylem and phloem. Crucially, the innermost layer of the newly formed phloem (or sometimes the pericycle) retains meristematic activity and differentiates into a new, secondary vascular cambium. This new cambium produces another set of xylem and phloem, and the innermost layer of this new phloem again becomes a cambium. This cycle repeats, resulting in concentric rings of vascular bundles, each surrounded by its own cambium. This phenomenon is known as anomalous secondary growth. Two families that display this phenomenon are Chenopodiaceae (e.g., Chenopodium album, where stem successive cambia are prominent) and Nyctaginaceae (e.g., Boerhaavia diffusa). This structural adaptation allows for significant stem thickening without the formation of a single, continuous cylindrical cambium, providing mechanical support in certain herbaceous or shrubby species.

(c) Types of Agamospermy and Importance of Apomixis

Agamospermy is the formation of seeds without fertilization (syngamy), though meiosis may or may not occur. It is distinguished into three main types. First, Adventitious Embryony (Sporophytic apomixis), where embryos arise from nucellar or integumentary cells, often resulting in polyembryony; meiosis is typically absent. Second, Gametophytic Apomixis, where the embryo develops from an unreduced gamete. This is further divided into Apospory (embryo from a megaspore mother cell or its daughter cell) and Diplospory (embryo from a haploid egg cell that undergoes mitotic doubling). Third, Apogamy, a rare type where the embryo develops from a synergid or antipodal cell. Apomixis is now regarded as a tool of practical importance because it allows for the fixation of heterosis (hybrid vigor) in seeds. Unlike sexual reproduction, which leads to segregation and loss of hybrid traits, apomixis produces clonal seeds that are genetically identical to the parent. This enables the mass production of hybrid crops, such as in Citrus, mango, and guava, ensuring uniformity, high yield, and the preservation of desirable traits without the need for repeated cross-breeding, thereby bypassing incompatibility barriers and reducing breeding time.

(d) Fibre-Yielding Plants

Five important fibre-yielding plants are:

1. Cotton (Gossypium hirsutum): Family Malvaceae; Part used: Seed hairs (lint).
2. Jute (Corchorus capsularis): Family Malvaceae; Part used: Phloem fibre (bast).
3. Flax (Linum usitatissimum): Family Linaceae; Part used: Phloem fibre (bast).
4. Sunn Hemp (Crotalaria juncea): Family Fabaceae; Part used: Phloem fibre (bast).
5. Sisal (Agave sisalana): Family Asparagaceae; Part used: Leaf fibre.

(e) Zygotic vs. Somatic Embryos

Zygotic and somatic embryos differ in origin, genetics, and structure. A zygotic embryo originates sexually from the fusion of male and female gametes (zygote). It is genetically variable due to recombination and fertilization. Structurally, it typically possesses a suspensor and is associated with the formation of endosperm (usually triploid) which provides nutrition. In contrast, a somatic embryo originates asexually from somatic cells, such as those from callus cultures or nucellar tissue (in vivo). It is genetically uniform, being a clone of the parent plant. Somatic embryos generally lack a suspensor or have a reduced one and do not form endosperm; instead, they rely on maternal tissues (in vivo) or culture media (in vitro) for nutrition. While zygotic embryos ensure genetic diversity essential for evolution, somatic embryogenesis is crucial in plant tissue culture for mass propagation and genetic transformation, allowing the rapid cloning of elite genotypes without the variability introduced by sexual reproduction.