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Effect of Tropical Photoperiods on the Growth of Sorghum When Grown in 12 Monthly Plantings¹

Effects of day length on number of days to anthesis and plant height were observed on 15 tropical sorghums (Sorghum bicolor (L.) Moench) and compared with 7 U.S. sorghums and a set of 8 maturity genotype testers. Plantings were made for 12 consecutive months in 1964 and 1965. Plantings made as much as 230 days apart flowered within a 17-day period in November while plantings separated by only 30 days reached anthesis as much as 200 days apart. Tropical sorghums appeared to have a lower critical photoperiod than the U.S. sorghums. It was possible, however, to bring all types into flower at about the same time in Puerto Rico by planting in mid-September through mid-November when day lengths were below the critical level for the varieties.

Data indicated that photoperiodic thresholds varied because the sorghums could be separated into five general response classes. Variation in floral induction time was strongly suggested. Among the eight maturity genotype testers, photoperiodic response was observed only in 100- day Milo and 90-day Milo, varieties containing both dominant genes, Ma₁ and Ma₂. None of the other genotypes were significantly affected by the Puerto Rico environment. This indicated that their critical photoperiod must be above 13 hours.

Significant correlation was observed between days from planting to anthesis and plant height in most of the tropical sorghums, but not for the U.S. sorghums which were unaffected by changes in the photoperiod.

Key Words: Photoperiodism • Date-of-planting • Sorghum Bicolor

² Respectively, Research Geneticists and Research Agronomist, Crops Research Division, ARS, USDA, Federal Experiment Station, Mayaguez, P. R. (D. K. B. now at Beltsville, Md.)

Received for publication January 13, 1968.

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