Use of photosensitive types of maize (Zea mays L.), such as the approximately 250 Latin American races, is severely limited by late flowering and excessive growth under field conditions encountered in latitudes exceeding 30°%. It is known that both shorter day lengths and lower temperatures tend to reduce excessive growth by initiating flowering at an earlier stage of development. The joint effects and possible interactions of such treatments apparently have not been previously studied. Sixteen races of maize, adapted to varying altitudes, growing conditions, and latitudes, were grown in the North Carolina State University Phytotron at day/night temperatures ranging from 10/6 to 34/30 C under short (9-hr) days. The same races were grown under long days at temperatures ranging from 18/14 to 34/30 C. The photoperiod difference was much more effective than temperature in decreasing leaf numbers, and while the interaction of photoperiods and temperatures was significant, it was relatively small. Lowest leaf numbers were generally obtained under the 18/14 C 9-hour day regime. Since these numbers ranged as high as 20 leaves per plant, additional reduction, perhaps by using shorter days with temperatures in the range of 14 to 22 C, would be desirable.

In contrast, temperature rather than photoperiod length is more effective in altering the numbers of tillers per plant. With a single exception, the races that tillered did so more profusely at the lower temperatures.