Plant Growth And Development – NEET UG BiologyPractice Questions & PYQs

Seed germination begins when tissue rehydration reactivates the dormant embryo, triggering the secretion of specialized hydrolytic enzymes. Among these, alpha-amylase is essential because it catalyzes the breakdown of stored endospermic starch into readily usable glucose. This enzymatic conversion supplies the necessary metabolic energy and carbon skeletons required for the emerging radicle to establish early growth.

Chlorosis and reduced vegetative growth in crops typically stem from a deficiency in essential macronutrients like nitrogen, which serves as a vital structural component of chlorophyll and nucleic acids. Applying a supplementary nitrogenous fertilizer corrects this nutritional shortage and restores healthy leaf pigmentation. Additionally, treating the plants with cytokinins helps delay leaf senescence and promotes overall cellular division and tissue development, ensuring optimal pod formation and maximum seed yield.

Apical dominance is a physiological phenomenon where growth-promoting hormones produced in the terminal shoot tip suppress the development of lower lateral buds. Removing these shoot tips through pruning eliminates the primary source of auxins that enforce this inhibition. Consequently, the previously dormant axillary buds are released from suppression, leading to vigorous lateral branching and a dense, bushy appearance in hedges.

Rapid insect-capturing movements in carnivorous plants rely on mechanically sensitive trigger hairs that generate electrical action potentials upon contact. These electrical signals prompt a swift efflux of protons into the cell walls of the outer leaf layers, dramatically lowering the pH and loosening the structural matrix. Water rushes into these cells osmotically, causing them to expand rapidly while the inner cells lose turgidity, resulting in the instantaneous snapping shut of the trap.

Gibberellins are a class of plant hormones primarily responsible for stimulating both cell division and cell elongation in the internodal regions of stems. During the growth phase known as bolting in rosette plants, these hormones drive dramatic stem elongation prior to flowering. Without adequate levels of this growth regulator, normal internodal stretching and overall plant height fail to develop properly.

Photoperiodic flowering responses depend heavily on the length of uninterrupted darkness rather than the daylight period itself. Certain plant species require a critical continuous dark duration to initiate reproductive development and will fail to flower if that darkness is broken by even a brief flash of light. Since an interrupted night phase prevents flowering, the plant clearly depends on long, continuous periods of darkness to trigger its biological clock.

Trace mineral nutrients like zinc play a vital biochemical role by acting as essential activators for enzymes involved in amino acid metabolism. Specifically, zinc is required for the proper functioning of tryptophan synthetase, an enzyme responsible for producing tryptophan, which serves as the direct biochemical precursor for auxin synthesis. When a plant suffers from a zinc deficiency, its ability to manufacture sufficient amounts of indole-acetic acid becomes severely impaired.

Organogenesis in plant tissue culture is heavily regulated by the relative balance and proportion of two major classes of growth regulators. When an undifferentiated mass of cells is exposed to a growth medium containing a high concentration of cytokinins relative to auxins, it triggers the formation and development of shoots. Conversely, shifting this hormonal balance in favor of auxins promotes root development instead.

According to the biological species concept, distinct species are kept apart in nature due to reproductive barriers such as differing ecological adaptations or timing mechanisms. Although these two tobacco varieties are fully capable of producing fertile offspring when artificially synchronized in a laboratory, their contrasting photoperiod requirements prevent them from ever overlapping in their flowering times outdoors. Consequently, this temporal mismatch isolates them in the wild, ensuring they remain separate natural entities.

Cytokinins are plant hormones that promote cell division and tissue growth in plant cultures. The liquid endosperm of coconuts, commonly known as coconut milk, was discovered to be exceptionally rich in these cell-division-stimulating factors. Specifically, it contains active cytokinins like zeatin, which enable mature plant cells to re-enter and maintain the cell cycle during tissue culture experiments.

Seed dormancy is regulated by endogenous chemical messengers that act as growth inhibitors, preventing germination even when environmental conditions are optimal. Among these, abscisic acid accumulates within the seed tissues and acts as the primary molecular block against precocious germination. Removing or leaching out this inhibitor through heavy rainfall or washing is typically required before the physiological block is lifted and metabolic activity resumes.

Genetically dwarf plant varieties often lack the specific hormonal triggers necessary for internodal elongation, remaining short in stature despite favorable growing conditions. Applying exogenous gibberellic acid bypasses this internal limitation by stimulating dramatic cell elongation and division along the stem axis. This treatment effectively restores normal vertical growth patterns, successfully overcoming the dwarf phenotype.

Leaf aging and the subsequent degradation of vital photosynthetic pigments are heavily regulated by specific stress-associated plant hormones. Abscisic acid acts as the principal promoter of these degenerative metabolic pathways, actively breaking down proteins and chlorophyll within aging tissues. This hormonal action accelerates the transition from active vegetative growth to full tissue senescence.

Photoperiodic classification categorizes plants based on their specific duration requirements of daily light exposure to trigger the transition into the reproductive phase. Long day plants require continuous illumination periods that exceed a specific critical threshold to initiate flowering. Among common crop species, spinach falls squarely into this category, unlike short-day counterparts such as tobacco and soybeans.

The premature dropping of leaves and developing fruits is normally governed by the formation of a specialized band of cells at the base of stalks known as the abscission layer. Synthetic auxins like naphthalene acetic acid suppress the cellular breakdown and enzyme activity responsible for creating this weak zone. Maintaining high local concentrations of this compound effectively anchors the fruit to the stem and delays natural shedding.

Plant buds and specialized storage organs like potato tubers naturally enter a resting phase to survive unfavorable environmental conditions. Gibberellins act as key biochemical triggers that stimulate reserve-mobilizing enzymes and overcome this growth arrest. Applying this specific phytohormone reactivates metabolic pathways, allowing vegetative propagation to resume.

Many dormant seeds possess an extremely tough exterior that physically blocks the water and gas exchange required for germination. Scarification employs mechanical or chemical techniques to artificially rupture or soften this resistant protective layer. Once compromised, moisture can finally penetrate the interior tissues to awaken the embryo.

Gaseous phytohormones play crucial roles in regulating senescence and developmental transitions in plants. Ethylene specifically acts as a powerful signaling molecule that coordinates the complex biochemical pathways of maturation. Its accumulation triggers the pigment changes, cell wall softening, and sugar conversion characteristic of mature fruits.

Phytochrome is a specialized chromoprotein photoreceptor that helps plants detect light quality and day length in their surrounding environment. The pigment exists in two mutually interconvertible forms that absorb distinct wavelengths of the light spectrum. Absorption of red light converts the inactive form into the physiologically active form, which then triggers downstream photomorphogenic responses such as seed germination and flowering.

Apical dominance is regulated by the terminal bud, which suppresses the development of lateral buds through auxin production. Eliminating this dominant source removes the inhibitory effect on nearby nodes. Consequently, the previously suppressed buds activate and develop into active lateral branches.