4.3 Ideas and Tips
Further experiment ideas involving cress roots
In the gravity-sensing cells in the roots, statoliths indicate the direction of gravity as they move to the lower
cell wall. Due to the contact between the statoliths and the cell wall, a still unknown gravireceptor is
activated. This activation is transferred into a signal that is transmitted to the neighbouring cells to start the
differential flank growth. The result is a bended root, a so-called gravitropic root curvature. In the
clinorotated root, the gravireceptor is, depending on the speed of the clinostat rotation, either intermittently
activated due to many contacts with statoliths or, in the best case, not activated at all. In both cases, the
result is that the cell no longer knows what is up and down.
This raises a question: what is the best clinostat rotational speed to create simulater microgravity conditions?
You may be able to answer this question by changing the speed of the clinostat and observing how the roots
react at different speeds.
We analysed the growth of roots under 1g and simulated microgravity conditions. The direction of the
growth of the roots in both cases looked similar, however. This may be caused by the fact that we had the
same preconditioning: a 1g environment until germination and early growth started. What would have
happened if we had put the seeds on the clinostat before germination started? This would have eliminated
the 1 g preconditioning. The question is: in which direction does a root initially start growing under
simulated microgravity conditions? Such an experiment would need longer hours of observation of the
clinorotated roots, and you would have to prepare a good environment for germination and early growth of
the roots on the clinostat by providing a clinostat chamber.
Tips for experiments
Roots: Shorter roots are better than longer ones, so plants with roots with a length of about 5 to 10 mm
should be used. Longer roots will also react to gravity but, in most cases, much more slowly than shorter
roots.
Shoots: Short cress shoots are good subjects for studying gravitropism. They can be grown very easily on
agar-agar surface in a wet chamber without a Petri dish lid. When you are conducting experiments with
shoots on the clinostat, use a clinostat chamber to prevent dying of the substrate.
Oxygen: Seeds need oxygen for germination. When closing your Petri dishes, leave at least one third of
space between the Petri dish and its lid open.
Wet chamber: Inside a Petri dish that has been covered with a lid, the roots have favourable conditions for
growth. Some wet tissues or a small bowl with water will help to keep the humidity high in the chamber.
Clinostat chamber: To keep humidity high, you can place a small bowl of water under the clinostat. To
achieve the best results, cover all the inner walls with wet tissues.
Light: Most seeds do not need light during the first days of germination. Some, however, like bell pepper
seeds, do. Always remember that light is a strong stimulus for shoot and also for roots. To prevent influence
from light, be sure that light is coming from above and is not too bright. The shoots will grow in the
direction of the light, and the roots will always grow away from phototropism.
Water: For all experiments, tap water is the best choice. The residual minerals in tap water support the first
stages of germination.
