The theory behind higher humidity for veg and lower humidity for flower is called a vapor pressure deficit or VPD. The theory is low humidiy raises the pressure inside the plant cells causing the stomata to close. Plants signal it as a draught conditions and closes the stomata to conserve water those slowing nutrient exchange and metabolism.
It's more than just humidity leaf temperature is a main factor as well. Grower using hps would likely have higher VPD than an led grower.
Of course strain and genetics plays a role as well some strains may do fine in dry conditions. However MJ's response to dry conditions is to produce resign to seal in water and stop, water loss, so it's a balancing act.
My experience is when I raised temp and humidity I got really good grow lots of yield but not good quality. I ran perpetual system so I did not have much choice so ran 80 degrees and 35% rh and got good results both yield and quality.
The ideal situation is to run higher temps and humidity for part first of flower and crank down rh as you get toward the end of flower. This will get you max yield and max quality.
Under currents Web page had an excellent chart showing temp, nutrient and humidity levels recommended for a grow. Can't find it now but will post it later when I get home.
Nuggly
