Many plants require exposure to low temperature for a certain period during each year. This was determined more than a century ago. Understanding this phenomenon, known as the rest period, is part of a larger problem of understanding the factors which determine the ability of an organism to grow. The property of rest is ultimately a property of the actively growing cells, rather than that of the cells involved with protection and support.

Carrots (Daucus carota L. cv. Royal Chantenay) and sweet corn vi (Zea mays L. cv. Iochief) were grown rm the Utah State University's Greenville farm in 1969 and 1970. The soil was fumigated with Telone (a mixture of 1, 3-dichloropropene and other chlorinated hydrocarbons) at the rates of 10, 20, and 30 gal/acre and Nemagon (1, 2-dibromo-3-chloropropane) at the rates of 1, 2, and 3 gal/acre one week before planting. Samples of uniformly mature carrot roots were taken for the studies of total carotenes, β-carotene, total and reducing sugars, respiration, and ultrastructural changes; and those of sweet corn seeds for the studies of total carotenoids , starch, total sugar, and total nitrogen changes. Compared to the controls, carrots grown on fumigated soil had a significantly higher content of total carotenes, β-carotene, and total sugars, and a lower rate of respiration. The electron microscopic study indicated that the carrots from Telone (30 gal/acre) and Nemagon (3 gal/acre) fumigated soil had larger chromoplasts that contained more globuli and crystals than those of the controls. The soil fumigations with Telone and Nemagon also brought about the increase in total carotenoid content in sweet corn seeds.