Plant Growth and DevelopmentNEET Botany · Class 11 · NCERT Chapter 15

Plant Growth and Development is a Medium Weightage chapter with 3 to 5 questions in most NEET exams. Questions focus on the five plant growth regulators (auxin, gibberellin, cytokinin, ABA, ethylene), their discoverers, biological effects, agricultural applications, photoperiodism (short-day, long-day, day-neutral plants and the role of phytochrome), vernalisation, and the sigmoid growth curve. Knowing the discoverer-effect-application combinations for each hormone is essential for a strong NEET score.

In arithmetic growth, only one of the daughter cells continues to divide; the other matures and exits the cell cycle. The rate of growth is constant. The graph of growth vs time is a straight line. Formula: Lt = L0 + rt, where L0 is initial length, r is growth rate, and t is time. In geometric (or exponential) growth, both daughter cells continue to divide. The rate of growth is proportional to the current size. The graph is exponential (J-shaped). Formula: W1 = W0 e^(rt). All organisms show geometric growth in the early stages (when nutrients are abundant) and shift to arithmetic-like growth as they hit limits.

The sigmoid growth curve is the S-shaped curve that represents the typical growth pattern of organisms (and individual organs). It has three phases: (1) Lag phase - slow initial growth as cells prepare to divide, (2) Log / exponential phase - rapid growth as both daughter cells divide and resources are abundant, and (3) Stationary phase - growth plateaus as resources run out and cells reach equilibrium. This curve is sometimes also called the typical growth curve. The exponential phase represents geometric growth; the lag and stationary phases represent the natural slowing as conditions become limiting.

Differentiation: meristematic cells (which divide actively) mature into permanent specialised cells. For example, meristematic cells become tracheary elements (vessels, tracheids); they lose their protoplasm to function as efficient water conductors. Dedifferentiation: mature, differentiated, non-dividing cells regain the capacity to divide. Examples: cork cambium forms from cortical cells; interfascicular cambium forms from medullary ray cells; callus forms from any tissue in tissue culture. This shows plant cell totipotency. Redifferentiation: dedifferentiated cells, which can divide, lose this capacity again and mature into specialised cells. Example: secondary xylem and phloem are produced by the dedifferentiated cambium and they redifferentiate into specific cell types.

Long-day plants (LDP) flower when the day length is LONGER than a certain critical value. They flower in the long days of summer. Examples: wheat, barley, oats, radish, spinach. Short-day plants (SDP) flower when the day length is SHORTER than a certain critical value. They flower in the short days of autumn or early winter. Examples: rice, soybean, chrysanthemum, cotton, tobacco, Xanthium. Day-neutral plants (DNP) are not affected by day length. Examples: tomato, cucumber, sunflower, maize. The critical factor is actually the length of the dark period, not the light period. The leaves perceive the photoperiod and produce a flowering signal (florigen, now identified as the FT protein in Arabidopsis) that travels to the shoot apex to induce flowering.

Vernalisation is the requirement of low-temperature exposure (cold treatment) to induce flowering in some plants. Without this cold period, the plant remains vegetative and does not flower. It is found in winter varieties of wheat, barley, rye (sown in autumn, exposed to winter cold, flower in spring), and biennials such as cabbage, carrot, beetroot. The cold treatment can be artificially provided to seeds or seedlings to allow off-season cultivation. Vernalisation is sensed by the shoot apex and the cold response involves epigenetic changes (such as silencing of FLC gene in Arabidopsis). Vernalisation prevents premature flowering before winter; it ensures flowering occurs in optimal seasonal conditions.

Ethylene (also called the ripening hormone) is a gaseous plant hormone that triggers fruit ripening, particularly in climacteric fruits (such as banana, apple, mango, tomato). It causes softening, sweetening, colour change, and aroma development. Commercial use: (1) Ethephon (2-chloroethylphosphonic acid) is sprayed on fruits because it releases ethylene slowly when absorbed; (2) Calcium carbide pieces release acetylene (which mimics ethylene) and are sometimes used to ripen mangoes off the tree (this practice is illegal in India because of the impurities); (3) Ethylene gas is used in ripening chambers to ripen mangoes, bananas, and tomatoes uniformly after harvest; (4) Smoke from burning leaves was traditionally used in Indonesia to ripen mangoes (the smoke contains ethylene). Ethylene also promotes flowering in pineapples, induces abscission of leaves and fruits, and breaks seed and bud dormancy.