Yellowing of four o'clock plants

 I've grown four o'clock plants here for years, but this year for some
reason a lot of my plants are yellowing on the top. Here are a couple of
pics:

http://emberts.no-ip.info/stuff/2011/2011_july_garden/Photo-0029.jpg
http://emberts.no-ip.info/stuff/2011/2011_july_garden/Photo-0031.jpg

I have no clue what is causing this - as far as I know nothing has changed,
no change in weather, no change in watering. Plants side by side will have
one do this, and the other just fine.

Anyone recognize this and/or have any suggestions?