Frost, Freeze and grain fil

​Severity of below freezing temperatures while a corn plant is moving sugars to the grain is dependent on what tissue freezes.  Leaves are most vulnerable because of exposure to the cold air.  Ice crystals form in the leaf cells, killing the individual cells including those with chloroplasts.  Among these chloroplasts are those in guard cells of the stomata. Without photosynthesis the day after freezing, the guard cells do not open to allow transpiration.
 
Water movement from the root tissue to the upper plant is a physical phenomenon in which each molecule evaporating through open leaf guard cells, is replaced by a molecule of water because of water’s molecular structure causes bonding with each other.  Water is essentially pulled from the roots through the xylem structures of the vascular system because of this bonding.  If water is not utilized or transpired, leaves do not receive new supplies of water, further causing wilting of the leaves after frost or a more severe freeze.  
 
Sugars are transported through living phloem cells in vascular tissue.  During grain fill, sugars are drawn from the leaves and the stored reserves in the stem.  Death of leaves eliminates movement from the leaves.  If the low temperatures were not severe enough to kill the phloem and other stem cells, movement from stem to grain continues. 
 
Below freezing temperatures during grain fill will cause some loss of expected grain weight, but the severity will depend upon whether stem phloem is killed and the supply of sugar reserves in the stalk.