Preflowering Levels of Phytohormones in Sorghum III. Photocontrol1

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Indole‐3‐acetic acid (IAA), abscisic acid (ABA) gibberellins (GAs) were determined in two different maturity genotypes of Sorghum bicolor (L.) Moench as function of lighting conditions. Phytohormone levels were determined in bulked samples of preflowering shoot tissue from 28‐day‐old plants grown in a controlled environment and sampled during the day, night and after exposure to far red radiation (FR) (710‐730 nm). Levels of both IAA and ABA were highest in the later flowering genotype (90M) when sampled just prior to the beginning of darkness. The high tissue levels of both phytohormones in 90M declined during the night by 200 ng/g dry weight to levels near the lower limit of detectability. A similar trend was noted for levels of the same phytohormones in an earlier maturing genotype, 60M, but the magnitude of change was substantially less. In contrast to IAA and ABA, there was a net increase in the level of GA‐like activity during the night. Exposure of plants to 5 min of FR, a treatment known to accelerate flower initiation in sorghum (9, 19), had no effect on levels of IAA or ABA. The level of GA‐like compounds, particularly GA3‐like activity, increased in both sorghum genotypes after treatment with FR. The decline in IAA and ABA levels during the night in both genotypes as well as the increase in GA‐like activity, especially in the relatively early genotypes (60M), produce an apparent shift in hormone balance which may favor floral initiation.