Chapter 13: Plant Growth and Development (Plant Physiology)

Growth: an irreversible permanent increase in size of an organ, its parts or a cell, accompanied by metabolism. Differentiation: the maturation of cells from meristems to perform specific functions, with structural changes in walls and protoplasm. Development: all the changes an organism goes through in its life cycle from germination to senescence (growth + differentiation). Dedifferentiation: the regaining of the capacity to divide by living differentiated cells (e.g. cork cambium from parenchyma). Redifferentiation: the maturation, for specific functions, of cells produced by dedifferentiated tissue, which again lose the capacity to divide. Determinate growth: growth that stops after a certain size is reached (leaves, flowers, fruits). Meristem: a group of cells that can divide and self-perpetuate (apical, lateral, intercalary). Growth rate: increased growth per unit time.

Growth is an increase in protoplasm, which cannot be measured directly; and different organs grow in different ways at different stages. In a meristem growth is mainly an increase in cell number; in fruits such as watermelon an increase in cell size; a pollen tube grows in length; a leaf in area; a tree trunk in girth and dry weight. Fresh weight changes with water content. So no one parameter (length, area, weight, volume or cell number) can represent growth over the whole life of the plant.

(a) Arithmetic growth: after mitosis only one daughter cell continues to divide while the other differentiates; growth is at a constant rate, e.g. a root elongating steadily; length vs. time is a straight line: Lₜ = L₀ + rt. (b) Geometric growth: both daughter cells keep dividing; growth is slow at first (lag), then exponential (log), W₁ = W₀eʳᵗ, then slows as nutrients become limited (stationary). (c) Sigmoid curve: the S-shaped curve obtained by plotting growth against time in geometric growth, with lag, log and stationary phases; typical of organisms, organs and cells in nature. (d) Absolute growth rate: total growth per unit time; relative growth rate: growth per unit time per unit initial size; two leaves growing by 5 cm² have equal absolute rates, but the smaller leaf has the higher relative rate.

Auxins, gibberellins, cytokinins, abscisic acid and ethylene.Gibberellins: Discovery: Japanese farmers noticed "bakanae" (foolish seedling) disease of rice, in which seedlings grew abnormally tall; it was caused by the fungus Gibberella fujikuroi. E. Kurosawa (1926) produced the same symptoms with sterile filtrates of the fungus; the active substance was identified as gibberellic acid. Over 100 GAs are known (GA₁, GA₂, GA₃ …), all acidic. Functions: increase length of the axis (stem elongation); cause bolting in rosette plants (beet, cabbage); delay senescence; break seed dormancy (germination). Uses: longer grape stalks; elongated, better-shaped apples kept longer on trees; faster malting in brewing; sugarcane stems lengthen, raising yield by up to 20 tonnes per acre; early seed production in juvenile conifers.

ABA helps plants tolerate stresses: during water stress it causes rapid closure of stomata, reducing water loss; it increases tolerance to drought, cold, salinity and other stresses; it induces dormancy in seeds and buds so they survive desiccation and unfavourable seasons; and it inhibits growth and metabolism under stress. Its level rises under stress, hence the name stress hormone.

Growth is open because meristems keep adding new cells throughout the plant's life (indeterminate growth). Differentiation is open because cells from the same meristem can mature into different structures depending on their position in the organ and the conditions (e.g. root apical meristem cells become root-cap cells or epidermis by position); differentiated cells can even dedifferentiate and redifferentiate. So the final structure is not fixed in advance: plant development is flexible (plastic, e.g. heterophylly).

Flowering depends on whether the day length exceeds or falls short of a critical photoperiod, which differs from species to species; it is not an absolute number of hours. A long-day plant flowers when days are longer than its critical length; a short-day plant flowers when days are shorter than its critical length (really, nights longer than a critical dark period). If, for example, the LDP needs more than 12 hours and the SDP needs less than 14 hours, both will flower when the day is 13 hours long. So both can flower at the same place and time.

(a) Auxin (IBA, NAA). (b) Ethylene (ethephon). (c) Cytokinin. (d) Cytokinin (overcomes apical dominance; or remove the apex). (e) Gibberellin (GA₃). (f) Abscisic acid.

No. The photoperiodic stimulus is perceived by the leaves, which then produce a flowering signal (hormone, "florigen") that moves to the shoot apices. A plant without leaves cannot perceive the day length, so it does not respond to the photoperiodic cycle by flowering.

(a) The seedlings would show excessive stem elongation, becoming thin and tall (like bakanae, "foolish seedling" disease) and may fall over. (b) The cells would keep dividing into an undifferentiated mass (callus or a tumour-like growth); no specialised tissues or organs would form. (c) The rotten fruit releases much ethylene, which would make the unripe fruits ripen quickly (and then rot). (d) Cells would not divide properly: callus would not proliferate and no shoots would form (with auxin alone, mainly cell enlargement and roots).