The phenomenon of Heterosis has been most commercially exploited first in ........

Explanation:

Heterosis is used to increase yields, uniformity, and vigor. Exploitation of the phenomenon of heterosis for crop improvement was first proposed by G.H. Shull in 1914. Because of the economic importance of heterosis, maize breeding has evolved over the past century from a reliance on openpollinated varieties to the use of hybrids.