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?
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