External Factors and Plant Growth

Plant Growth Regulators

• The word hormone comes from the Greek word “To excite” and hormones stimulate biochemical activities that cause observable physiological responses by organisms

• Plant hormones or plant growth regulators are rather different from animal hormones in chemical structure, mode of synthesis, and function. Higher animals possess glands that are part of their endocrine system that are specialized organs for production of hormones

• Plant hormones on the other hand are synthesized in the cells of general organs – the stems, leaves, roots, and flowers

Tropisms

• A tropism is a growth response involving bending or curving of a plant part toward or away from an external stimulus that determines the direction of movement

• Response toward the stimulus is positive, and response away from the stimulus is negative

Phototropism

• Phototropism is the growth of stems of plants toward light - it is probably the best known of the plant tropisms - phototropism is caused by elongation of the cells on the shaded part of the plant - so that entire plant bends or curves toward the light

• This growth pattern is caused by the hormone auxin - auxin migrates to the shaded part of the plant and stimulates increased cell growth and elongation on the shaded part of the plant

a, Grass seedlings have a sheath called the coleoptile that surrounds the first set of leaves. Growth of the coleoptile depends on the tip, and removal of the tip stops growth. Adding the tip back asymmetrically illustrates that the growth-promoting effect travels downward and not laterally, causing the seedling to bend because one side is growing faster than the other. Auxin can replace the tip for this effect. b, Stem cuttings can be induced by auxin to produce roots. c, Strawberries depend on auxin produced by their developing seeds for expansion and maturation. If the seeds are removed, little growth occurs. Normal growth can be restored with auxin.

Gravitropism

• Gravitropism is a response to gravity - if a seedling is placed on its side, the root will curve to grow downward and the shoot will curve to grow upward

• Auxin may play a role in the response of shoots - auxin-inducible genes exist and auxin induces increased growth in cells in which the gene is activated - auxin activates the transcription of certain genes on the side of the shoot showing increased growth - it is still not clear whether this transcriptional activation is due to an increase in auxin concentration or to an increase in sensitivity to auxin already present in the shoot

Gravitropism in plants

More Gravitropism

• Calcium also plays a role in gravitropism - calcium movements are mediated by the calcium binding protein calmodulin - calcium moves to upper surfaces of shoot cells before the shoot actually curves upward and calcium moves to bottom surfaces of root cells before the root actually curves downward

More Gravitropism

• The perception of gravity is correlated with sedimentation of amyloplasts (starch containing plastids) within certain cells of the shoot and root - such cells are found near the vascular bundles in shoots and in the root cap of roots, particularly the central column of the root cap

• Amyloplasts are sedimented to the transverse walls (perpendicular to root surface) in the root cells - if the root is placed in a horizontal position, these amyloplasts slide downward and come to rest on what were once vertically oriented walls - within hours the root curves downward through growth of upper cells and the amyloplasts return to their previous position

Thigmotropism

• Thigmotropism is response to touch or contact with a solid object - this is most commonly seen in tendrils, modified leaves or stems depending on the species - the tendrils wrap around any object they come in contact with and so enable the plant to cling and climb - the response can be rapid, a tendril can wrap around a support one or more times in less than an hour

Circadian Rhythms

• Many plant activities seem to occur on a daily schedule - some plants open their leaves at dawn and shut them at dusk - some plants may open and shut flowers as the day changes from dawn to dusk

• Photosynthesis, auxin production, and the rate of cell division all have regular daily rhythms and these rhythms continue even when environmental conditions are kept constant

More Circadian Rhythms

• Entrainment occurs when a periodic repetition of light and dark (or some other external cycle) causes a biological clock to remain synchronized with the same cycle as the entraining factor

• Light-dark cycles and temperature cycles are the principle sources of entrainment

• The main advantage of a biological clock is that it allows the plant to respond to the changes in seasons by accurately measuring changing day length - thus changes in the environment trigger responses that result in adjustments of growth, reproduction, and other activities of the organism

Photoperiodism

• Photoperiodism is a biological response to a change in the proportions of light and dark in a 24-hour daily cycle - photoperiodism was first discribed in plants and has since been found in insects, worms, fish, birds and mammals

Plants have three basic patterns of flowering

• Short-day plants will only flower if the light period is shorter than a critical length - they typically flower in spring or fall

• Long-day plants will only flower if the light period is longer than a critical length - they typically flower in the summer

• Day-neutral plants flower without respect to day length

Temperature Control

• Often photoperiodism works in concert with temperature to control plant growth and development

• For millennia, variations in day length with each passing month have been as regular as the sun’s shifting position in the sky and in temperate climates, an annual winter’s chill can be reasonably expected to occur.

• The consistent occurrence of such events makes them conditions to which the physiology of plants has become adapted, using them as external cues to turn on internal processes

California redbud in bloom

Aging and Ripening

• When fruits ripen and when leaves are prepared for abscission (separation from stems) they undergo a process of aging or senescence – a process directed by hormones, often in response to external cues

Plant Hormones and Senescence

• Once senescence is begun it is irreversible so plants must be able to tightly control when senescence begins. Some plant hormones inhibit senescence – cytokinin (mainly promotes cell division), auxin, and gibberellin all function to maintain normal function and cell structure in plants.

• Two other plant growth regulators act to promote senescence – ethylene a gas and abscisic acid – named because it was thought to promote leaf abscission in all plants. But now it appears that ethylene is more commonly used for that.

Fall Foliage

Dormancy

• Dormancy is a special condition of arrested growth• Plants do not grow at the same rate all the time -

during unfavorable seasons, they limit their growth or cease to grow altogether - this allows plants to survive water scarcity or low temperatures

• Normally, after a period of rest (no growth), growth resumes when the temperature becomes milder or water becomes available

• In contrast, a dormant seed or bud can be activated only by a precise environmental cue or set of cues

Dormancy