Marco-- temp is fine, but what's the RH in there, and is the fan blowing hard over the tops? Any combination of higher temps, low RH, and overly strong air flow can lead to water/transpiration stress, and if that happens, the evaporation cooling from normal transpiration is lost because the plant has closed the stomata to conserve water,...so then heat can build up,.. also, is this a cooled vented hood, or an open bulb type HPS? Sometimes, the closer proximity to an uncooled bulb can make for hot spots near the top, leading to the same symptoms,... And yes, Star asked my next question! HPS light is lousy for diagnostics, and proper color rendering is helpful for doing this,....
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