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The activities of gibberellins A1–A15, A17–A27 and A8-glucoside are compared, using original and previously published data, in the barley (Hordeum vulgare L.) aleurone α-amylase, Progress No. 9 dwarf pea (Pisum sativum L.), lettuce (Lactuca sativa L.), 'Tan-ginbozu' dwarf rice (Oryza sativa L.), cucumber (Cucumis sativus L.), and the d-1, d-2, d-3, and d-5 maize (Zea mays L.) mutant bioassays. Gibberellins A3 and A7 show high activity in most bioassays. Gibberellins A9, A10, A23, and A24 are species specific in their activity. Gibberellins A8, A11, A12, A13, A14, A17, A25, A27, and A8-glucoside exhibit low activity. Gibberellin A26 is inactive up to 100 μg in all assays. The cucumber hypocotyl and barley aleurone bioassays respond only to a narrow spectrum of gibberellins while the rice bioassay is sensitive to all the gibberellins except A21, A25, and A26.Activity in the cucumber assay is restricted to gibberellins with a 13-deoxylactonic structure. With the exception of gibberellins A8, A26, and A27, only gibberellins having either an oxygenated C-3 position or a C-2,3 double bond, and a 19,10-lactone function, show more than low activity in the barley aleurone assay. Hydroxylation at the C-2 position may account for the reduced activity of gibberellins A8, A26, and A27. Possible reasons for a lack of correlation between gibberellin structure and biological activity in other bioassays are discussed.

To investigate the possible relationship between gibberellins (GA) and heterosis, a diailel from four elite maize, Zea mays L., inbreds (A632, B73, CH807 [from Mo17], and CO109) was studied under controlled environment conditions. Heterosis for plant growth was substantial at 25/15 °C (day/night), as hybrid seedlings emerged more rapidly, were taller, had heavier shoots, and larger leaf areas than the parental inbreds. Gibberellic acid (GA3) at 0.1, 0.5, or 1.0 mg per plant was pipetted into the leaf whorls of 14‐d‐old seedlings. The exogenous GA3 promoted ligule and total height in all genotypes, but the inbreds were maximally influenced. Consequently, potence ratios, representing the overall degree of dominance, were reduced from an average of 7.7 to 1.7 following GA3 application, indicating decreased heterosis. In an experiment at 15 °C, imbibition of seeds in GA3 accelerated germination and seedling growth, particularly shoot elongation of the parental inbreds. Endogenous concentrations of GA1 and GA19 were determined by gas‐chromatography‐selected ion monitoring using [2H]GAs as quantitive internal standards. Highly significant regressions of height, leaf area, relative growth rate, and height increment versus the log of GA concentration were observed. These results are consistent with the theory that GA are one factor involved in the regulation of heterosis of shoot growth in maize, a theory that suggests that a partial deficiency in endogenous GA in maize inbreds limits their growth, and is a contributing cause of inbreeding depression.

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