Planted some popcorn a month ago and now the plants have 4-5 leaves. They are tall and weak - most of them have one leaf that has broken near the stem but is still alive - all those leaves have some white dust like substance in the thing in the middle of the leaf that supports it. Also, all but the newest leaves are going brown at the tips - lower ones the most.

Some pictures:
Plant 1 - http://img408.imageshack.us/img408/4...nt1pic1oz8.jpg
http://img170.imageshack.us/img170/2...nt1pic2gb3.jpg
Plant 2 - http://img186.imageshack.us/img186/2563/plant2zd7.jpg

The window faces south-west. Fertilized it once, with the one you mix in the water, didn't help though. May the problem be the cold air from the window that is open very-very little all the time (0-15 degrees celsius outside), sunburn (about 6 hours of direct sunlight on the plants on cloudless days), saltburn, over-watering or something else?