Heliotropism

Heliotropism, a form oftropism, is thediurnal motionor seasonal motion of plant parts (flowers or leaves) in response to the direction of the sun. The habit of some plants to move in the direction of the sun was already known by the Ancient Greeks. They named one of those plants after that propertyHeliotropium, meaningsun turn. The Greeks assumed it to be a passive effect, presumably the loss of fluid on the illuminated side, that did not need further study.[1]Aristotle's logic that plants are passive and immobile organisms prevailed. In the 19th century, however, botanists discovered that growth processes in the plant were involved, and conducted increasingly ingenious experiments.A. P. de Candollecalled this phenomenon inanyplantheliotropism(1832).[2]It was renamedphototropismin 1892, because it is a response to light rather than to the sun, and because the phototropism of algae in lab studies at that time strongly depended on the brightness (positive phototropic for weak light, and negative phototropic for bright light, like sunlight).[3][4]A botanist studying this subject in the lab, at the cellular and subcellular level, or using artificial light, is more likely to employ the more abstract word phototropism. The French scientistJean-Jacques d'Ortous de Mairanwas one of the first to study heliotropism when he experimented with theMimosa pudicaplant.Contents[hide] 1Floral heliotropism 2Leaf heliotropism 3References 4External linksFloral heliotropism[edit]Heliotropic flowers track the sun's motion across the sky from east to west. During the night, the flowers may assume a random orientation, while at dawn they turn again toward the east where the sun rises. The motion is performed bymotor cellsin a flexible segment just below the flower, called apulvinus. The motor cells are specialized in pumpingpotassiumionsinto nearby tissues, changing theirturgor pressure. The segment flexes because the motor cells at the shadow side elongate due to aturgor rise. Heliotropism is a response to light from the sun.Several hypotheses have been proposed for the occurrence of heliotropism in flowers: The pollinator attraction hypothesis holds that the warmth associated with fullinsolationof the flower is a direct reward forpollinators.[5] The growth promotion hypothesis assumes that effectiveabsorptionof solar energy and the consequent rise in temperature has a favourable effect onpollen germination, growth of thepollen tubeandseed production.[6] The cooling hypothesis, appropriate to flowers in hot climates, assumes that the position of flowers is adjusted to avoid overheating.[7]In general, flower heliotropism could increase reproductive success by increasingpollination,fertilizationsuccess, and/or seed development,[8]especially in the spring flowers.Some solar tracking plants are not purely heliotropic: in those plants the change of orientation is an innate circadian motion triggered by light, which continues for one or more periods if the light cycle is interrupted.Tropicalconvolvulaceousflowers show a preferred orientation, pointing in the general direction of the sun but not exactly tracking the sun. They demonstrated nodiurnalheliotropism but strongseasonalheliotropism. If solar tracking is exact, the suns rays would always enter thecorolla tubeand warm thegynoecium, a process which could be dangerous in atropical climate. However, by adopting a certain angle away from the solar angle, this is prevented. The trumpet shape of these flowers thus acts as aparasolshading the gynoecium at times of maximumsolar radiation, and not allowing the rays to impinge on thegynoecium.[9]In case ofsunflower, a commonmisconceptionis that sunflower heads track the Sun across the sky. The uniform alignment of the flowers does result from heliotropism in an earlier development stage, thebudstage, before the appearance of flower heads. The buds are heliotropic until the end of the bud stage, and finally face east. The flower of the sunflower preserves the final orientation of the bud, thus keeping the mature flower facing east.Leaf heliotropism[edit]Leaf heliotropism is the solar tracking behavior of plant leaves. Some plant species have leaves that orient themselves perpendicularly to the sun's rays in the morning (diaheliotropism), and others have those that orient themselves parallel to these rays at midday (paraheliotropism).[10]Floral heliotropism is not necessarily exhibited by the same plants that exhibit leaf heliotropism.References[edit]1. Jump up^Whippo, Craig W. (2006)."Phototropism: Bending towards Enlightenment".The Plant Cell18(5): 11101119.doi:10.1105/tpc.105.039669.PMC1456868.PMID16670442. Retrieved2012-08-08.2. Jump up^Hart, J.W. (1990).Plant Tropisms: And other Growth Movements. Springer. p.36. Retrieved2012-08-08.3. Jump up^"Phototropism and photomorphogenesis of Vaucheria".4. Jump up^Donat-Peter Hder,Michael Lebert (2001).Photomovement. Elsevier. p.676. Retrieved2012-08-08.5. Jump up^Hocking B., Sharplin D. (1965)."Flower basking by arctic insects"(PDF).Nature206(4980): 206215.doi:10.1038/206215b0.6. Jump up^Kevan, P.G. (1975). "Sun-tracking solar furnaces in high arctic flowers: significance for pollination andinsects.".Science189(4204): 723726.doi:10.1126/science.189.4204.723.7. Jump up^Lang A.R.G., Begg J.E. (1979). "Movements of Helianthus annuus leaves and heads".J Appl Ecol16: 299305.doi:10.2307/2402749.8. Jump up^Kudo, G. (1995). "Ecological Significance of Flower Heliotropism in the Spring Ephemeral Adonis ramosa (Ranunculaceae)".Oikos72(1): 1420.doi:10.2307/3546032.9. Jump up^Patio, S.; Jeffree, C.; Grace, J. (2002)."The ecological role of orientation in tropical convolvulaceous flowers"(PDF).Oecologia130: 373379.doi:10.1007/s00442-001-0824-1.10. Jump up^officially replaced by diaphototropism and paraphototropism