This is likely to be a nutritional stress and you should do a soil test. As you mentioned in your response to Alex it could be potassium. The other thing I would suggest is a Odemea where the cells rupture on the underside of the leaf.

This from the Missouri Botanical Garden
"Oedema is a physiological disorder that develops when roots take up water faster than it can be used by the plant or transpired through the leaves. Water pressure builds up in the internal cells of the leaves causing them to burst, leaving dead cells that are visible as a blister, primarily on the undersides of leaves. Oedema may also be spelled edema"

http://www.missouribotanicalgarden.or...

There is a related issue of Vapor Pressure damage which often happens in greenhouses, see the attached image and here http://www.omafra.gov.on.ca/english/c...