Humidity plays a major role in plant growth, and it's effects are often underestimated or overlooked. To the right you will see the "tools of the trade" for controlling humidity...the dehumidifier and the humidifier.
Plants breathe through tiny openings on the undersides of their leaves called stomata. Plants can (and do) open and close their stomata under certain conditions, for example if heat becomes excessive and causes a plant to start loosing more water than it can take up, the plant will close it's stomata to slow down the water loss.
The ideal humidity range for healthy plant growth is 50% humidity, plus or minus 10%.
Unfortunately, by closing the stomata and slowing evaporation the plant also has slowed down it's cooling mechanism. This causes heat to build up in the plant tissue, and in temperature too hot the plant actually cooks itself. It is important to understand the opening and closing of the stomata and how it, in turn, controls plant transpiration.
Plant transpiration is how plants breath. Plants do not have lungs, however, so when molecules of gas and water vapor are released from the stomata they tend to just hang there in the absence of any breeze. That is why it is so important to have box fans or oscillating fans in a garden to circulate the air (in addition to exhaust fans). These fans are actually like the plant's lungs, and without them the plants would have no way of moving fresh CO2 molecules into contact with their plant tissue. The plants would slowly choke on their own transpired gasses and water vapor.
Learn more about garden fans here.
As water evaporates from the surfaces of leaves, the surface tension of the water molecules tend to pull the next water molecule along behind it, up through the plant's veins. Water is pulled up through the plant stem, which is pulled from the plant's roots. This creates a negative water pressure in the root zone and allows the roots to suck moisture up out of the root zone like a straw. The process of water absorbing into the plant through the roots is known as osmosis.
Which brings me back to humidity. Water vapor is humidity. As a plant transpires, the humidity immediately surrounding the leaves will become saturated with water vapor. Now, the entire plant transpiration cycle is controlled by evaporation. When gasses surrounding a leaf become saturated with water vapor (100% humidity), there is no place for the next molecule of water vapor to evaporate to.
The end result is that water vapor is not evaporating, so water is not being drawn up from the root zone...and neither are any nutrients. If nutrients are not being taken up, than developing fruits are not getting the food they need to be healthy. This is exactly why high humidity will cause blossom end rot in fruiting tomatoes just like a Calcium deficiency. It is another reason why it is so important to keep box fans and oscillating fans in the garden area to keep the air circulating.
So, evaporation controls plant transpiration. High temperatures and low humidity therefore both cause fast transpiration. Fast transpiration means your plants will be taking up and using lots of water (and nutrients). This is fine, unless you were feeding your plants strong to begin with. Your plants can only handle so much fertilizer within a specific period of time.
So if you now have warm temperatures, low humidity, and fast transpiration rates you may find your plants are using a little too much fertilizer a little too quickly. Leaf tip burn is usually a sign of this. Under these circumstances you can feed your plants with a weaker nutrient solution.
Or, if all other plant growth influencing factors are in their ideal ranges, you can try to maximize plant growth by adding CO2 (in which case you should experience heavy plant growth without showing any signs of stress or damage).