iliasred, I suspect iron chlorosis LINK Seaside areas, in my experience are associated with saline soils. (High pH and high levels of sodium and other chlorides). This condition can cause iron to be unavailable to the plant; although it may be present in the soil. The easiest way to ascertain the condition (and to correct it) is to provide iron as a foliar spray. You are bypassing the soil. Liquid iron (sometimes with added elements like manganese and magnesium) is comparatively inexpensive and you might want to treat one or two plants to gauge the response. If it is positive, then you know the answer. If it is not, then I am sorry that I misdiagnosed the condition; and all mankind should be quite relieved that I am not an M.D.

Seriously, the only answer might be to grow another species or grow the Photinia in planters in a more suitable soil. It is quite possible that salt air or salt spray plays a role. The very brown leaf tips can indicate that. Another excellent indication is the rusting of iron fixtures exposed to the air.