What could be the problem causing cilantro to have this kind of tip burn / necrotic center? It's grown in a hydroponic shallow raft bed under LED grow light. This always happens after some period of time. Parameters are:
This is an old question however someone may still be interested.
Tip burn is usually caused by poor calcium uptake.
Low PH may cause problems with nutrient uptake. The "5.2" PH is too low, it should be between 5.5 and 6.5. I generally aim for the middle 5.8-6.2.
However, the most common cause of tip burn is poor air circulation. Good Air Flow over the leaves improves transpiration. Calcium is carried up into new growth along with the water during transpiration. If there is not enough calcium available for new growth the cell walls do not form properly. These damaged cells are the wet blackened tissue we call tip burn.
