Sunflowers follow the Sun!

M.V.N. Murthy, The Institute of Mathematical Sciences, Chennai

It is easy to spot a sunflower- it is big and bright in colour compared to many other flowers. Sunflower has got its name because the flowers turn toward the Sun. Even though it is native to North-America, now sunflowers are grown almost everywhere for food, seeds and oil. The sunflower plant (Helianthus annuus) has a large flower at the end of of the stem. The stem of the flower can grow up to 3 meters tall, with a flower head that can grow up to 30 cm in width. The flower head is made of hundreds of tiny flowers called florets. Together they make up a "false flower" also called pseudanthium. It is very easily visible to insects and birds which help pollinate it. A typical sunflower can produce up to 1,000 seeds.

It has been known for a long time that sunflowers, as the name indicates, greet the Sun in the east each morning and follow the movement of the Sun throughout the day until it sets in the west- well almost. But then what happens after the Sun sets? How does the sunflower greet the Sun in the east again next morning? Does it have an internal clock? Scientists are slowly but surely beginning to understand the mystery of the movement helped by some recent research findings from University of California at Davis in the USA. 

Some observations first: It is not that all sunflowers follow the movement of the Sun. This is common misconception, actually a fully grown sunflower does not follow the daily rhythm but faces east all day. It is the young, not fully mature, sunflowers that follow the movement of the Sun. This movement slows down and comes to a stop as as the flower matures to it full size and faces the same direction.

The scientists were interested in the day-night rhythm followed by young sunflowers during their growing phase. Any biological process that follows this rhythm of day-night 24 hour cycle is said display what is known as ``Circadian Rhythm". It is a rhythm that your body also follows and it influences important bodily functions. Sunflower appears to follow this circadian rhythm like humans and many other living beings. 

The next question is then how do they do it: The stem of the sunflower plant has a single sunflower at the top. Scientists have found that during the day the young sunflower follows the Sun's progress and return to the original position in the morning. In fact they have observed that the plant rearranges itself at night to begin same process. It does not matter if the day is longer or shorter than the night. They do this by growing the stem in a peculiar way. During the day eastern edge of the stem grows more than the western edge so that the flower can follow the Sun. And during the night the western edge grows longer than the eastern edge bringing back the flower to the original position to greet the Sun again in the morning. It is some thing like a car wheel close to the curb covers lesser distance than the outer wheel to turn left and the opposite while turning right. The differential growth of the stem on the eastern edge and western edge allows the sunflower to rhythmically move along with the day night cycle. 
Is the growth pattern then determined by an internal circadian clock? In order to find out, scientists grew the sunflower plants inside with artificial light with different light and dark cycles going up to 30 hours instead of 24 hours. However, the sunflower seemed to follow an imaginary path of the Sun maintaining the circadian rhythm showing that indeed they have this incredible internal clock mechanism.
Why do these plants behave in this way? Apparently facing the Sun helps the plants to grow larger maximising the photosynthesis. Furthermore, the early warming of the flower that faced east helps in attracting the bees which help in pollination.
Is this peculiar to sunflower plant alone among plants? Actually the circadian rhythm is widely observed in many variety of plants as in the case of animals. It may not be as dramatic as in the case of sunflower; in fact the research on circadian rhythms began in 1729 when the French astronomer de Mairan noticed the daily leaf movements of a plant (not clear which one). After nearly a century it was accurately measured that these rhythms had a period of 24 hours and are not due to environmental factors, but internal in origin.
The importance of the study on sunflower is because it seems to be the ``first example of a plant's internal clock modulating the growth in a natural environment" as the scientists say.As molecular biologists they had earlier shown the connection between genes responsible for rhythm and the plant hormone responsible for growth. Sunflower plant provides a nice example to understand the relationship between the `clock' and `growth' at the molecular level where the growth is driven by genes that respond to light and the circadian rhythm.