Once a plant gets totally root bound it will start to turn yellow starting at the bottom,they will take in little to no nutrients and growth will stop. A tree can only grow as big as it's root system. They do look a bit small to be root bound in those pots but that seems to be the case. You could trim the root ball down to a smaller size but this is usually only done on mothers to control growth.repotting is your best bet and make sure to loose up the root ball before you transplant so they get a wider more flattened out root base to start from.