MikeY: One of the ideas I have yet to follow up on is to run a separate mist system during veg, just for the plants. From what I've read, morning and late afternoon are good times.
Google Advanced Foliar Techniques Explored: Herd's an excerpt
Leaves
The exchange of oxygen and carbon dioxide in the leaf (as well as the loss of water vapor in transpiration) occurs through pores called stomata (singular = stoma).
Normally stomata open when the light strikes the leaf in the morning and close during the night.
The immediate cause is a change in the turgor of the guard cells. The inner wall of each guard cell is thick and elastic. When turgor develops within the two guard cells flanking each stoma, the thin outer walls bulge out and force the inner walls into a crescent shape. This opens the stoma. When the guard cells lose turgor, the elastic inner walls regain their original shape and the stoma closes.
Time Osmotic Pressure, lb/in2
7 A.M. 212
11 A.M. 456
5 P.M. 272
12 midnight 191
The table shows the osmotic pressure measured at different times of day in typical guard cells. The osmotic pressure within the other cells of the lower epidermis remained constant at 150 lb/in2. When the osmotic pressure of the guard cells became greater than that of the surrounding cells, the stomata opened. In the evening, when the osmotic pressure of the guard cells dropped to nearly that of the surrounding cells, the stomata closed.
Opening stomata
The increase in osmotic pressure in the guard cells is caused by an uptake of potassium ions (K+). The concentration of K+ in open guard cells far exceeds that in the surrounding cells. This is how it accumulates:
Blue light is absorbed by phototropin which activates
a proton pump (an H+-ATPase) in the plasma membrane of the guard cell.
ATP, generated by the light reactions of photosynthesis, drives the pump.
As protons (H+) are pumped out of the cell, its interior becomes increasingly negative.
This attracts additional potassium ions into the cell, raising its osmotic pressure.
Closing stomata
Although open stomata are essential for photosynthesis, they also expose the plant to the risk of losing water through transpiration. Some 90% of the water taken up by a plant is lost in transpiration.
Abscisic acid (ABA) is the hormone that triggers closing of the stomata when soil water is insufficient to keep up with transpiration (which often occurs around mid-day).
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