First you should flush with a commercial flushing product using 3 times the volume.

Next question. There are a lot of times when the light will dry out the inside of the leaf. If the temperature if the leaf is to high like over 90 degrees f then the moisture will evaporate as well and dry out the leaf and cause an upturned end and browning too. GK is mostly correct but you should consider temperature too before you take the next step. Flushing is always a good first thing to do at the beginning signs of trouble. It will save you a lot of trouble in the long run. You are growing in dirt so after you flush you need to let the soil dry out before you add back any nutrients.