Plants Responsive to Environmental Stimuli, Biology tutorial

Introduction:

Plants have evolved intricate and detailed set of complex and dramatic responses to environment. These responses permit plants not only to stay alive in unfavorable conditions which would kill most animals, but also to coordinate the growth and development with suitable environmental situations.

Responses of plants to ecological stimuli like light, gravity and touch happen in several ways and comprise such varied events as flowering and growth of stems towards light. Behaviors like flowering takes longer and are generally related with changing seasons. Environmental signals activate all the apparently unrelated responses. The main element in the responses is development.

Tropism:

Plant growth toward or away from stimulus like, light or gravity is known as tropism. There are many types of tropisms, each of which is termed for stimulus which causes response. For instance, phototropism is growth of stem or coleoptile toward light, and gravitropism is growth towards or away from gravity. Tropisms result from differential growth. It means that one side of responding organ lengthens faster than other side. This causes curative of organ toward or away from stimulus. Growth of organ toward stimulus is known as positive tropism. Therefore, stems which grow toward light are positively phototropic. On the other hand, growth of organ away from stimulus is known as negative tropism. Roots that grow away from light, are negatively phototropic.

Phototropism:

Phototropisim is tropic response to uni-directional light (light coming from one direction, Frits Went, He found that in phototropism, rapid elongation of cells along shaded side of coleoptiles is handled by IAA coming from apex. Went formulated the theory together with Russian plant physiologist Nicolai Cholodny, and work became called as Cholodny-Went hypothesis. Though hypothesis was major step in the understanding of phototropism, mechanism by which IAA controlled phototropism remained mystery. Either of two hypotheses could have accounted for increased activity of IAA on shade side of coleoptile.

1) Hypothesis I:

Light destroys IAA along lighted side of coleoptile. Such mechanism could result in more IAA along shaded side of coleoptile, that would account for phototropic curvative. Evidence: In 1950s Winelow Briggs and his colleagues found that amount of IAA produced by coleoptiles grown in light is same as which made by coleoptiles grown in dark. Therefore, light doesn't destroy IAA.

2) Hypothesis 2:

Light causes IAA to move shaded side of coleoptile. According to hypothesis, difference in IAA concentration between lighted and shaded sides of coleoptile would result from movement of IA rather than its destruction. Evidence: Briggs and colleagues accumulated more IAA from shaded side of coleoptiles than from lighted sides, recommending that light causes IAA to move to shaded side of stem. This conclusion was verified by inserting impermeable barriers between split tips of coleoptiles. The barriers blocked movement of IAA to shaded side of coleoptiles. Consequently, coleoptiles didn't curve towards light. More recent experiments utilizing IAA labeled with radioactive carbon (14C) have verified that unidirectional light causes IAA to move to shaded side of coleoptiles, where its increased concentration causes cells there to lengthen more rapidly than cells on lighted side of coleoptiles. Consequently, coleoptiles curves toward light.

Though Cholodny-Went hypothesis sufficiently describes phototropism in coleoptiles, it frequently fails to account for phototropism by stems that contain more chlorophyll than do coleoptiles. In stems, unidirectional light triggers production of the inhibitor which illustrates cellular elongation on illuminated side of stem. As cellular elongation doesn't change on shaded side of stem the stem curves toward light.

Gravistropism:

This type of tropism causes stems to rise upward and roots down-wards. Both of the responses are of adaptive connotation. Stems which grow upward are appropriate to receive more light than those that don't roots which grows down-ward are more probable to encounter the favorable environment than those roots that don't.

A phenomenon is known as gravitropism as it is clearly response to gravity and not to earth. In shoots which are placed horizontally, differences in auxin concentration soon grow between upper and lower sides with greater concentrations on lower side. Auxin is powerful inhibitor of root growth, and extremely small concentration induces root to curve toward side where auxin concentration is greater. These differences cause growth responses which are liable for shoots growing upwards against force gravity - negative gravitropism. One of the initial experiments finished to learn gravitropism was done by Charles Darwine. Experiment involved responses of roots when caps had been surgically removed. The roots continued to grow but didn't react to gravity. Experiments illustrated that root cap is essential for root gravitropism.

Other researchers made exciting discovery cells in centre of cap have several starchladen amyloplasts that, under influence of gravity, sediment to lower side of the cells. Many subsequent experiments recommended that this was true, that further intensified study of amyloplasts is gravitysensors in plant roots.

More recent research by Randy Wayne and his colleague propose that roots react to gravity by sensing gravitational pressures exerted by photoplast, not sedimentation of amyloplasts. When roots are oriented horizontally, development slows along lower side of elongating zone, thus causing root to curve downward. One of the first events which eventually cause differential growth is accumulation, not of hormones, but of calcium ions (CA2+). Ca2+ moves to lower side of cap and elongating zone of horizontally oriented roots. This accumulation of Ca2+ along lower side of root triggers accumulation of IAA along lower side of root tip. Such IAA inhibits cellular elongation in roots, lower side of roots grows lower than upper side of root and roots curve down-ward. When root reaches vertical position, lateral asymmetries of Ca2+ and IAA disappear, and straight development resumes. IAA and Ca2+ also direct negative gravitropism of shoots. IAA collects along lower side and Ca2+ along upper side of horizontally oriented stems. Auxin induced MRNA disappear from cortex and epidermis of upper (i.e., more slowly growing) side and accumulate on lower (i.e. more rapidly growing) side of horizontally oriented hypocotyls. Such mRNAs, or encoded proteins, stimulate cellular elongation along lower side of stem, thus producing upward curvative.

Hydrotropism:

Development of roots toward soil moisture is known as hydrotropism. Roots whose caps removed are not receptive to moisture gradients that suggest that root cap is site of moisture perception by roots. Interactions between gravity, light and soil moisture could thus account for occasional mending growth of roots through soil.

Thigmotropism:

This is growth response of plants to touch. Most common example is coiling of tendrils or the entire stem of plants like morning glory. Before touching the object, tendrils and twining stems frequently grow in the spiral pattern known as Circumulation which increases possibilities of contacting object to which it can cling. Contact with the object is recognized by specialized epidermal cells that induce differential growth in tendril. Such growth can be very rapid; the tendril can encircle object with 5 - 10 minutes. In addition, thigmotropism is frequently long lasting. Stroking the tendril of garden pea for only the couple of minutes can induce curling response which lasts for many days. Thigmotropism is possibly influenced by IAA and ethylene, such hormones induce thigmotropic-like curvature of tendril even in absence of touch tendrils can also store memory of touch. Tendrils which are touched while growing in dark don't react until they are illuminated. Therefore though tendrils can store sensory information received in dark, light is needed for growth response to proceed. This light-induced expression of thigmotropism may be because requirement for ATP, as ATP will substitute for light inducing thighmotropism of desk-stimulated tendrils.

Nastic Movements:

These are movements which also happen in response to environmental stimuli. Unlike tropisms, nastic movements are independent of direction of stimulus. They happen in atomically predetermined direction, rather than toward or away from stimulus. Nastic movements comprise some of most unusual plant kingdom.

Seismonasty:

This is nastic movement resulting from contact or mechanical disturbances like shaking. Seismonastic movements are based on plant's skill to rapidly transmit stimulus from touch-sensitive cells in one part of plant to reacting cells located elsewhere. Among most dramatic of the replies are those shown by sensitive plant (Mimosa pindica) touching leaf causes leaflets to fold and petiole to drop.

The response happens in the given ways:

i. Touching the leaf generates the electrical signal which moves along petiols

ii. This electrical signal is translated into chemical signal which causes cell membranes to turn out to be more permeable to K+ and other ions. Cell which are affected are known as motor cells, that are large, thin- walled parenchyma cells situated in joint-like structure known as pulvinus. In sensitive plant, pulvinus is situated at base of every leaflet petiols.

iii. Movement of ions out of motor-cells decreases water potential in surrounding extra cellular space, that causes water to move out of motor cells by means of osmosis. Loss of water causes motor cells to shrink, thus producing seismonastic movement.

Unfolding of leaves take 15-30 minutes and is accompanied by reversing process. Motor cells take up K = and other ions, causing water to enter cells via osmosis. This influx of water inflates cells to the original size, thus unfolding leaves to the original position.

Nyctinasty:

The most common nyctinactic responses happen in prayer plant (Maranto species), and ornamental houseplant. In day, leaves of prayer plant are horizontal, thus maximizing the interception of light. At night, leaves fold vertically into shape resembling pair of praying hands. This movement of leaves in reply to light and dark results from changes in tugor of motor cells in pulvinus situated at base of every leaf. In dark, K+ ions are transported from cells of upper side of pulvinus to cells along its lower side. This movement of ions causes water to move by means of osmosis in cells along lower side of pulvinus. Therefore, in turn causes cells along lower side of pulvinus to lose water and shrink as cells along lower side gain water and expand. At sunrise, process is reversed and leaf again presumes its horizontal position.

Photoperiodism:

All eulcaryotic organisms are affected by cycle of night and day and several cahracteristics of plant growth are keyed to changes in proportions of light and dark in daily 24-hour cycle. These responses comprise photoperiodism, the mechanism whereby organisms estimates seasonal changes in relative day and night length. One of the most obvious of the photoperiodic reactions concerns production of flowers by angiosperms.

Flowering Responses:

Day length changes with seasons; the farther from the equator one is, the greater is the variation. The flowering responses of plants fall in 2 fundamental categories in relations today length. Short-day plants being to form flowers when days become shorter than critical length. Long-day plants on other hand, initiate when days become longer then critical length. In both types of plants, it is really length of darkness (night) which is important, and not length of day. Additionally to long-day and short-day plant, number of plants are explained as day neutral.

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