Root growth in dry soils

​Corn seedlings respond to water stress by reduced leaf growth but increased root growth.  The physiology involved in that root growth as it ‘searches’ for water has a complex interaction with plant hormones, cell development, osmosis dynamics and genetics. 
 
Primary root in a corn seedling was initiated with germination at the point where the scutellum is at node connecting the radical and the shoot parts of the seedling.  It is the main root structure until about 2 leaves are expanded above ground and the secondary nodal roots are initiated about 1 inch below soil surface.
 
Root growth occurs with cell division at the root tip and cell expansion a short distance from the tip.  Whereas leaf expansion of water stressed seedlings appears to be reduced, primary root growth increases under these conditions.  The plant hormone abscisic acid (ABA) increases in the tip.  This is associated with an increase in osmotic pressure, and thus more water, into the newly formed root cells.  Also increasing ABA is linked to an increase in the amino acid proline, perhaps delaying the finishing of cell walls.  As a consequence, the cells near the root tip become longer than in roots not facing water deficient conditions.  ABA also appears to interfere with production of ethylene in the cells, a compound associated with inhibiting cell growth.  
 
Four genes are associated with the ABA interaction in water-stressed primary root growth.  There is evidence that as many as 1779 genes are expressed in root cells of water stressed primary roots as opposed to 1297 genes expressed in roots with no water stress.  Corn plants react to environments in complex ways that we usually are not aware.  We select genetics for the preferred gross reaction, but a lot is happening within the plant.
 
Detailed summary of this complex interaction can be found on internet at https://www.ncbi.nlm.nih.gov/pubmed/15448181.