Strange yellowing on leaves

[showerstall]

I am growing a gorilla glue #4 plant--it's now at about day 40+ in flower. I have recently noticed a leaf condition that doesn't seem to appear in any of the excellent growweedeasy plant problems pix...a thin yellowing occurs along the main leaf vein, then slowly spreads to other veins. Area between main veins stays green, although eventually the leaves affected dry up and crumble at the touch. Not all leaves are affected--since the canopy is trellised, all leaves are pretty much at the same level. Happens with "old" leaves and new. Flowering seems to be going quite well...

Any thoughts.

And thanks for this wonderful web site!

300 watt LED
smart pot (7 gal)
Foxfarm soil; top covered with gnatnix (glass)
Foxfarm bloom nutrient

 

[Masi808]

From my experience this happend to me early into flower with just one of my 5 plants i flushed and went up a little on the nutes and the yellowing stopped