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Office glass is heat-shielded to reduce AC costs. That's be my guess - lack of IR.

I think it's more likely that they don't like the warm dry air produced by the radiators that are typically installed below the windows. Chlorophyll doesn't absorb much UV.

I have a lemon tree that is indoors in full sun. Last summer I put it outdoors for the first time and it has 3-5x more foliage than it has ever had.