The plants grow towards the light and the roots grow away from the light. Sometimes we see roots start to grow up out of the foam, but they will turn back into the foam to avoid the light and reach the nutrient solution. If the light is reflected from shiny surfaces inside the PGF, the plants may not grow straight up towards the PGF lights because they are receiving light from many directions. Both light and gravity tell a plant which direction to grow on earth. In space there is almost no gravity so the plants only grow towards the light.
Gravity does play a role in the strength of developing stems.
However, plants which are grown from seeds in space may have weak stems. CO2 inside the PGCs is controlled to about 1500 ppm which is about 3 times the CO2 on earth. The PGF removes extra CO2 from the air in the PGCs. CO2 in the space shuttle can be much higher. However, we try very hard to contain any loose material that might be produced by the plants.