Planting time is one of the important factors which determine growth and productivity of crop plants,because it provides the plant with the best conditions for growth.Accordingly, it is not surprising that you find the planting time had a significant effect on proteins,chlorophyll a and b .It also makes the plant t o avoid stress conditions which influence the activity of oxidative stress enzymes.
The developmental stages in plants incuding corn, which affect yield, protein, and other quality traits as well as stress induced stages are influenced by planting time.
Since I could not figure out the growing conditons f yours I can not be much helpful. Tough, I can provide you one of my articles in wheat.