Growth will slow down and even stop. The top layer of soil will be dry and crusty with minimal microbial activity noticeable.  This will lead to a hydrophobic soil that will be difficult to re-hydrate and water will simply drain right out of the bottom of the pot. Under-watering can harm microbes which might require topdressing of worm castings or an organic fertiliser amendment and watered in. This will stimulate bacteria and fungi critical for nutrient cycling in a living organic soil eco-system.