It looks like a combination of both to me. A watering schedule the plants can't keep up with (for whatever reason), coupled with dry, hot air, or intense light- hence the zippering of certain leaves. Some strains are more susceptible to differing turgor pressures and needed growing conditions than others. For instance, my WW x OG even while being grown in the same environment, medium, and nutrient dosages/waterings exhibits slightly more zippering of the top most leaves than the other two strains being grown in the exact same situation. Find the ideal for what you're growing, and try your best to accomodate it. Transpiration rates are something to take note of, as well as nutrient levels. From my own experience, in order to stave off the downward leaf curl, up the nutrient solution a bit so that while underneath the intense lighting, the plants uptake less water and acheive a little bit more nutrient/water balance. This is however, very subjective, and ideally, IMO something that can only truly be evaluated by someone there in your own growroom. My blackberry kush exhibited signs of overwatering even though I knew it wasn't the case. The answer lied in the fact that I didn't trust the nutrient company's recommendations, and was, in fact, not feeding them enough. This issue never surfaced in any sort of typical nutrient deficiency, but in an ionic salt/transpiration battle. In my case, I saw leaf stems extended far beyond the plant's needs, and slightly more lanky growth. I've since learned from these experiences WITH THIS STRAIN, and have modified my dealings with it. More nutes, less frequent waterings, and alls been good. Maybe this will help? *shrug*