plant tip turning dark

i just saw today tip of my plant leaf is turning black for some reason, i dont have much experience growing marijuana so can anyone tell by seeing in the picture what can be wrong with them?

this is auto sour diesel growing outdoor in 34 c temprature

Whatever medium ( soil ) that is , is not agreeing with that seedling. It looks compacted and nutrient deficient.

this is basicaly potting soil made with coir peat, neem cake , karanj cake, vermi compost, cow dung etc it was doing good but this happened after i transplanted from cup to the pot, like even leaf went little droopy there is enough npk nutrient in the medium/soil so i was only giving ph balanced water