Effects of Photoperiod on Sex Expression in Ambrosia trifida

Ambrosia trifida, a short-day plant, after starting growth on long day, was subjected to two, ten, and twenty photoinductive cycles at each of three photoperiods-15, 12, and 6 hours. Following this treatment, the plants were returned to a long-day bench to complete growth. 2. Rate of maturity was most rapid and yield of fruit was greatest in the plants exposed to twenty photoinductive cycles at the 12-hour photoperiod. 3. Sex expression as measured by development of pistillate flowers in the position usually occupied by staminate increased with increasing numbers of photoinductive cycles and decreasing lengths of photoperiod, being greatest in the plants exposed to twenty photoinductive cycles at the 6-hour photoperiod.