You can see that boron toxicity resulted in the stems breaking off mid-stem. New side shoots have since been produced.
Why would boron toxicity cause stems to break off in the middle? Boron is an immobile nutrient so it should only affect new growth. However, if the roots absorbed the nutrient, then it would need to be translocated to new growth. But if the concentration of the nutrient were toxic, then the boron may have caused damage to the stem as it’s being moved from roots to growing tips so this may explain this phenomenon. Perhaps breaking off mid-stem also functions as a protective mechanism since this allows any excess boron a passageway to escape the stem. Cut off the hand to save the arm. It’s extreme, but probably effective.
Related post: Rotala “Vietnam” boron toxicity
Classical Aquascaping 