SOLAR TRACKING OF SUN FLOWER PLANTS

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 SUN FLOWERS TRACKING THE SUN: 


               Sun flowers are fascinating, little     specimens of nature, which allow us to know that, the plants are not static beings. Sunflowers have usually Bright yellow petals surrounding large Flower heads. Scientific name of sun flower is  Helianthus annus. Many people are in misconception of sunflowers, that their flower heads actually faces the sun, as it rises in the East and follows it across the sky, until the sun sets in the west. But all Sunflowers won't track the sun. This process of tracking sun rays is observed only in immature flower buds of sunflower plants, and they only turns in the direction of the sun. How ever, if the Flower buds mature and blossoms, the mature flowers stops tracking the sun.

Matured Flowers Facing East Direction while the sun sets in the West
       
           When immatured sunflowers track sun rays, they start to develop seeds and therefore, droop by the weight of the seeds. Then the stem stiffens and the flower becomes fixed facing the east ward direction after maturation. So in matured sun flower plants, leaves exhibit solar tracking, but not the flowers.This fascinating phenomenon of flowers tracking the sun rays across the sky is called  HELIOTROPISM or SOLAR TRACKING.

 WHY YOUNG FLOWERS TURN TOWARDS THE SUN :
Young Flower Facing Sun


          Process of  young sun  flowers tracking sun is called Heliotropism. It can be explained by Circadian rhythms. The behavioural changes tied to an internal clock as like humans also have, which roughly  follows 24 hour cycle. In this circadian cycle, young plants faces east, when sun rises and then to west in the evening, and in night again to east waiting for sun to arrive and to start the cycle again.
             In this cycle, why young plats turn towards sun is that, the sun flower plant contains hormones called Auxins. These hormones are sensitive to sun light and want to seek shade. When the stem tip receives unilateral light, Auxins migrate from the part of the plant that is exposed to sun to the shaded region in the stem. Then the concentration of Auxins increases in the shaded side. As Auxins are the growth hormones, this causes the stem to become bulkier in the shaded region. As the sun moves across the sky, the auxins also continue to migrate to the part of the stem that is shaded continuously, making the stem constantly bending away from the bulkier part of the stem. So flowers ends up in bending continuosly through out the day in the opposite direction to the bulk of auxins means in the direction of sun.

Immature Flowers having Bracts

                 Mainly young flower heads exhibit this property of Heliotropism. This is because of younger flowers has Green Bracts. The obvious reason for the flower tracking the sun at this stage is to maximise photosynthesis. Each sunflower has only one flower at its stem. There fore, in pollination, it is essential that the plant's only means of reproduction gets by Pollination mainly by Insects. Continuous facing towards the sun also helps flowers to heat up Quickly. So,the main reason that sunflowers turn towards sun is for the warmth that attract insects and helps in pollination. This warmth gives them advantage in photosynthesis, as warm flowers attracts insects. There fore, its the plant's best interest that, the flowers always faces the sun. so, its always highly visible to these pollinating insects.

 HOW PLANTS ARE ABLE TO TRACK THE SUN RAYS :

           There is a collection of specialised cells at the base of the flower bud or leaf called PULVINUS, that carry motor cells that enable the plant leaf or flower to track the sun. These cells enlarge or shrink according to the turgor pressure from the water inside them. In response to blue wave length light, Pottassium Ion concentration increases in the Motor cells of  the shadow side of the Pulvinus. With the increase of Potassium Ions, the Osmotic Potential in the cells become more negative and cells absorb more water and elongate turning the face of the flower towards the sun.

 HOW HELIOTROPISM IS ADVANTAGEOUS TO PLANTS : 
24 hours Experiment done from 6:00 AM on wards till night. Frequent changes leaves responding  for Heliotropism was shown.

           It is very easy to understand, why Heliotropism is advantageous for leaves. Because, turning the leaf, perpendicular to the sun, provides maximum sun light that helps in Photosynthesis. At mid day, to avoid over heating, the same leaves may change their orientation to parallel, rather than the perpendicular. So that, the edge of the leaves faces the sun and not the surface of the leaves. In order to avoid over heating in after noons, this Heliotropism will be very useful. It is thought that, why buds of the sunflower and other flowers track the sun is that, insects are attracted to warmth from the sun and their presence is especially necessary for pollination.
            so, heliotropism/ sun tracking is useful for pollination in sunflower alike in other flowers. Heliotropism in sunflower is only observed in immature flowers but not in blossoms.

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