Phototropism vs. Gravitropism

By Joshua Culberson and Drew Highsmith

Two very important plant processes that are ubiquitous among all plants are the two tropisms: phototropism and gravitropism. A tropism is a response to a stimulus that causes an organism, in this case a plant, to direct itself toward or away from the stimulus depending upon whether or not it is a positive or negative tropism. Positive tropisms respond by moving toward the source of the stimulus as is the case in phototropism. The mechanism of this response is regulated by blue light stimulation and auxin accumulation. Blue light, a higher-energy wavelength contained within white light, stimulates the side of the plant facing the light source. This stimulation causes an accumulation of auxin, a plant hormone, on the shade side of the plant where those cells begin to elongate. This elongation of cells on the side opposite of light stimulation causes the plant to bend toward the light causing a positive tropism. Negative tropisms, on the other hand, respond by moving away from the source of the stimulus as is the case for gravitropism in plant shoots. In a gravitropic response, a specialized group of cells called statocytes found in the endodermic layer of the inflorescence as well as in the root cap are responsible for sensing gravity. Within these cells are tiny, non-pigmented organelles called statoliths that are denser than the surrounding cytoplasm allowing them to sediment in response to gravity within the statocytes. These statoliths fall towards gravity accumulating on the bottom of the statocyte causing auxin levels to rise promoting cell elongation. In plant shoots, auxin stimulates cell elongation, so the shoots grow away from the statoliths, and thus gravity.

The interactions of phototropism and gravitropism were investigated in order to ascertain which of the two tropisms would exhibit a more dominate effect on plants. Phototropism was set against the effect of gravitropism by placing 4 seedlings sideways in a closed box with an LED light bulb set up at the bottom of the box in the center below the seedlings. 5 holes were cut to allow the insertion of the 4 seedlings into the box as well as a camera with just the lens jutting into the box itself. Precautions were made to ensure that no additional sources of light besides that of the light bulb were influencing the seedlings by fully enclosing the box and all insertions. Also, black construction paper lined the inside of the box to mitigate any reflection to keep the light unilateral. A time lapse video was constructed by setting the camera to take a picture once every 10 seconds for a period of 15 hours. We hypothesized that the effects of phototropism would outweigh the effects of gravitropism so that we would observe the plant to grow down towards the light rather than up away from gravity. What we found was that the shoots themselves, the main central stems, grew upward with no apparent hindrances, but the leaves of the seedlings began to orient themselves toward the light source. Also, we observed the process of circumnutation, most notably in the top left plant, as they moved in a circular, almost helical pattern over time. After analyzing these results, it made sense that only the leaves showed a phototropic response as the leaves are the main source of photosynthesis. So, it seems that neither mutually excludes each other but remain independent responses exhibited in different parts of the plant.