Plant Growth and Development

• Plant growth and development focuses on how structures like roots, stems, leaves, flowers, fruits, and seeds arise in an orderly sequence.
• Development is the sum of growth and differentiation
• Plant growth begins with seed germination, which requires favorable environmental conditions.
• If conditions are not suitable, seeds enter a period of dormancy until favorable conditions return and metabolic activities resume.

Growth

• Growth in plants occurs due to meristems, which allow them to grow indefinitely.
• Meristem cells divide and self-perpetuate, with the resulting cells eventually differentiating to form the plant body.
• Primary growth contributes to the elongation of plants along their axes, driven by root and shoot apical meristems.
• Secondary growth increases the girth of organs through lateral meristems like vascular cambium and cork-cambium, common in dicots and gymnosperms.

Measurable Growth

• Growth is measured by parameters such as increase in fresh weight, dry weight, length, area, volume, and cell number
• A maize root apical meristem produce over 17,500 new cells per hour
• Cells in watermelon increase in size up to 350,000 times
• Pollen tube growth is measured by length, and leaf growth by surface area.

Phases of Growth

• Growth occurs in the meristematic phase, where dividing cells are rich in protoplasm and have thin, cellulosic cell walls with plasmodesmatal connections
• Cells then enter elongation, with increased vacuolation, cell enlargement, deposit new cell walls
• Finally maturation, maximal size in thickness and protoplasmic modifications.

Growth Rates

• Growth rate is the increase in growth per unit time, expressed mathematically
• Arithmetic growth involves only one daughter cell continuing to divide after mitosis
• Geometric growth sees both progeny cells retain the ability to divide.
• Geometric growth has an initial slow lag phase which them increases to an exponential phase
• Limited nutrients cause the growth to slow down leading to a stationary phase.
• Exponential growth: W₁ = Woert, W₁ = final size (weight, height, number etc.), W = initial size at the beginning of the period, r = growth rate, t = time of growth, e = base of natural logarithms

Conditions Needed for Growth

• Water, oxygen, nutrients, and an optimal temperature range are all necessary conditions for growth and must be available to the plant
• Environmental signals, light and gravity, also impact growth.

Differentiation, Dedifferentiation and Redifferentiation

• Differentiation leads to maturation specific to the cell
• Cells may undergo major structural changes to cell walls i.e formation of a tracheary element losing protoplasm and developing lignocellulosic walls to carry water over long distances
• Living differentiated cells can regain the capacity to divide under certain conditions, known as dedifferentiation.
• The meristems and tissues divide and produce new cells that lose the capacity to divide - redifferentiation
• Cell/tissue determines its final form
• Example of cells being further away from the root differentiating into root cap cells where as cells pushed to periphery develop as epidermis

Development

• Development includes all changes an organism goes through during its life cycle, from seed germination to senescence.
• Plants exhibiting plasticity that allows them to change their shape by varying the amount of light or availability of water

Plant Growth Regulators

• Plant growth regulators (PGRs) could be indole compounds, adenine derivatives, derivatives of carotenoids, terpenes or gases(ethylene)
• PGRs fall into promoting activities such as cell division, cell enlargement, pattern formation and inhibition such as dormancy
• Auxins, gibberellins, and cytokinins promote cell division, enlargement and tropic growth
• The abscisic acid PGR involves growth dormancy.

Discovery of Plant Growth Regulators

• Auxin was discovered when Charles and Francis Darwin saw canary grass grow towards light
• Auxin was isolated by F.W Went from tips of coleoptiles of oat seedlings
• Gibberellic acid was found by E. Kurosawa from the fungal pathogen Gibberella fujikuroi.
• Miller et al. termed Cytokinins by Skoog that promoted active substance

Plant Growth and Regulator Effects

• Auxins promote flowering of pineapples
• Removal of shoot tips results in growth of lateral buds to assist with tea plantations and hedge making - (apical dominance)
• 2-4-D kills dicot weeds
• Gibberellins increase sugarcane length 20 tonnes per acre
• Ethylene promotes ripening for tomatoes
• Abscisic acid discovery helps regulate dormancy. and protects from stress, therefore is the stress hormone