Plants have sophisticated ways of being informed about their environment. They can detect nearby plants, that one day will become a threat to their light interception, already well before they are casting any shade. They do so by detecting the reflection of far-red light, a wavelength invisible to the human eye but reflected even more than green light by leaves.
Two young cucumber plants exposed to the same light intensity, but the plant on the right in addition receives supplemental far-red light, a natural signal for nearby neighbouring vegetation. Images taken every 2 minutes. See how the plant on the right is growing tall and getting away from the far-red-rich zone.
Most plants hate salt. Salinity of the soil hampers their growth and development. As such it makes sense plants that plants try to get away from salt when their roots encounter it.
These Arabidopsis seedlings (zandraket in Dutch) are growing in the lab on vertical transparent plates so we can see the roots grow. As we have seen before, the roots wave a bit, but in general grow down of course, until the moment they are exposed to salt halfway through the video: When a salt gradient is introduced (salt is added below the diagonal line), we see the roots first stop, then bend and grow away from the salt. We call this salt avoidance response halotropism. Time lapse photograpy and video by Jasper Lamers, WUR Laboratory of Plant Physiology.
In close up: