Plant Growth and Development

The One Thing

Plant growth and development are continuous, coordinated processes in which meristematic activity produces new cells and tissues, while differentiation, hormones, genes, and environmental signals determine their size, form, function, and developmental progression.

Reactions, Processes and Experiments

Key Terms

• Growth: A permanent and irreversible increase in the size, dry mass, volume, length, or number of cells in an organism.
• Development: The sum of all changes that occur in a plant during its life cycle, including growth, differentiation, maturation, flowering, and ageing.
• Open form of growth: A characteristic of plants in which new cells, tissues, and organs can continue to be produced throughout life because of the activity of meristems.
• Meristem: A region of actively dividing, undifferentiated plant cells that produces new tissues.
• Apical meristem: A meristem at the tips of roots and shoots that increases plant length and causes primary growth.
• Lateral meristem: A meristem, such as vascular cambium or cork cambium, that increases the girth of stems and roots and causes secondary growth.
• Intercalary meristem: A meristem near nodes or at the bases of leaves, especially in grasses, that helps regenerate and elongate parts after cutting or grazing.
• Cell division phase: The first phase of growth, in which meristematic cells undergo repeated mitotic divisions.
• Cell enlargement phase: The phase in which newly formed cells increase in size through water uptake, vacuole formation, and synthesis of cellular materials.
• Cell maturation phase: The phase in which cells attain their final size, shape, structure, and specialized function.
• Growth curve: A graph showing growth in relation to time; growth of many plant organs follows a sigmoid or S-shaped curve.
• Lag phase: The initial period of slow growth when cells prepare for active division and expansion.
• Log or exponential phase: The period of rapid growth when the rate of increase is at its maximum.
• Stationary phase: The phase in which growth slows and approaches a constant value because resources or space become limiting.
• Arithmetic growth: Growth in which one daughter cell continues to divide while the other differentiates, producing a constant increase in size or length per unit time.
• Geometric growth: Growth in which both daughter cells continue to divide, causing the growth rate to increase rapidly under ideal conditions.
• Absolute growth rate: The total growth increase per unit time: absolute growth rate = (final size - initial size) / time.
• Relative growth rate: The growth increase per unit initial size per unit time: relative growth rate = (final size - initial size) / (initial size × time).
• Differentiation: The process by which cells become structurally and functionally specialized.
• Dedifferentiation: The process by which mature living cells regain the ability to divide, as seen in the formation of interfascicular cambium.
• Redifferentiation: The process by which dedifferentiated cells again become specialized and mature.
• Plant growth regulators: Small organic substances produced naturally by plants or used artificially that control growth and developmental responses.
• Auxins: Growth regulators that promote cell elongation, apical dominance, adventitious rooting, fruit development, and vascular differentiation.
• Gibberellins: Growth regulators that promote stem elongation, bolting, breaking of seed dormancy, enzyme production during germination, and fruit growth.
• Cytokinins: Growth regulators that promote cell division, delay leaf senescence, support shoot formation, and help overcome apical dominance.
• Abscisic acid: A growth-inhibiting regulator that promotes seed and bud dormancy, closes stomata during water stress, and supports stress responses.
• Ethylene: A gaseous growth regulator that promotes fruit ripening, leaf and fruit abscission, senescence, and responses to mechanical stress.
• Apical dominance: The inhibition of lateral bud growth by auxin produced in the shoot apical meristem.
• Photoperiodism: The response of plants to the relative duration of light and darkness, especially in relation to flowering.
• Vernalisation: The promotion of flowering by exposing a plant or imbibed seed to a period of low temperature.
• Short-day plant: A plant that flowers when the photoperiod is shorter than a critical length, provided the dark period is sufficiently long.
• Long-day plant: A plant that flowers when the photoperiod is longer than a critical length.
• Day-neutral plant: A plant whose flowering is not controlled mainly by the relative length of day and night.
• Senescence: The natural ageing process that leads to the decline and eventual death of plant cells, organs, or the whole plant.
• Abscission: The controlled shedding of leaves, flowers, fruits, or other plant organs from the parent plant.
• Plasticity: The ability of a plant to modify its growth and form in response to environmental conditions or developmental stage.

Easily Confused

• Growth and development: Growth is an irreversible increase in size, mass, volume, length, or cell number; development includes all life-cycle changes, including growth, differentiation, maturation, flowering, and ageing.
• Primary and secondary growth: Primary growth increases root and shoot length through apical meristems; secondary growth increases stem and root girth through lateral meristems.
• Arithmetic and geometric growth: Arithmetic growth produces a constant increase because one daughter cell continues dividing; geometric growth accelerates because both daughter cells continue dividing.
• Absolute and relative growth rate: Absolute growth rate measures total increase per unit time; relative growth rate measures increase relative to initial size and time.
• Differentiation and dedifferentiation: Differentiation produces specialized cells; dedifferentiation restores the capacity of mature living cells to divide.
• Dedifferentiation and redifferentiation: Dedifferentiation forms a dividing state, whereas redifferentiation returns dedifferentiated cells to a specialized mature state.
• Auxins and cytokinins: Auxins are strongly associated with elongation, apical dominance, rooting, and parthenocarpy; cytokinins are associated with cell division, delayed senescence, shoot formation, and overcoming apical dominance.
• Abscisic acid and ethylene: Abscisic acid generally promotes dormancy and stress responses; ethylene is associated particularly with fruit ripening, abscission, senescence, and mechanical-stress responses.
• Photoperiodism and vernalisation: Photoperiodism depends on the relative duration of light and darkness; vernalisation depends on exposure to low temperature.
• Short-day and long-day plants: Short-day plants require a photoperiod shorter than a critical length and a sufficiently long dark period; long-day plants flower when the photoperiod exceeds a critical length.
• Abscission and senescence: Senescence is ageing and functional decline; abscission is the controlled shedding of an organ.

What Gets Asked

• Definitions and distinctions: Questions may ask for the meanings of growth, development, open form of growth, meristems, differentiation, dedifferentiation, redifferentiation, senescence, abscission, or plasticity. Marks are lost by defining growth as any change rather than as a permanent and irreversible increase.
• Growth phases and curves: Questions may require the three major phases of growth or identification of the lag, log or exponential, and stationary phases on a sigmoid curve. Marks are lost by omitting cell maturation or confusing the stationary phase with continued rapid growth.
• Growth calculations: Questions may require use of Lt = L0 + rt, the absolute growth rate formula, or the relative growth rate formula. Marks are lost by failing to divide by initial size when calculating relative growth rate.
• Meristems and types of growth: Questions may ask which meristem causes primary or secondary growth, or describe the role of an intercalary meristem. Marks are lost by assigning increased girth to apical meristems instead of lateral meristems.
• Plant growth regulators: Questions may match auxins, gibberellins, cytokinins, abscisic acid, and ethylene with their effects. Marks are lost by treating one regulator as acting alone, since responses depend on concentration, organ, developmental stage, environmental conditions, and interactions among regulators.
• Flowering responses: Questions may compare photoperiodism, short-day, long-day, and day-neutral plants, or explain vernalisation. Marks are lost by considering only total light duration and ignoring the importance of the uninterrupted dark period.