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Summary: Endogenous embryo factors, which act mainly in the radicle, prevent germination in Tagetes minuta at high temperatures. These factors act to prevent cell elongation, which is critical for radicle protrusion under optimal conditions. Once the radicle has emerged both cell elongation and cell division are required for post-germination growth. Germination can be induced at high temperatures by fusicoccin, which rapidly stimulates cell elongation. In addition, priming seeds at 25°C on polyethylene glycol (PEG) 6000 and mannitol could also induce germination on water at 36°C, indicating that priming prevents radicle protrusion at a point subsequent to the point of control in thermoinhibited achenes. Flow cytometry studies revealed that DNA synthesis occurs during thermoinhibition and the inhibition of DNA synthesis during this process inhibits subsequent germination on water under optimal conditions, suggesting a protective role for DNA synthesis in thermoinhibited achenes of T. minuta. [Copyright &y& Elsevier]

Summary: Thermoinhibition in Tagetes minuta achenes is tightly and rapidly regulated with regard to its imposition and release, with both processes occurring within 2–3h. Germination at high temperatures is almost exclusively regulated by the embryo, while the pericarp appears to play only a minor role. Thermoinhibition in T. minuta could not be alleviated by any single plant growth regulator application, but a combination of treatments that both reduced ABA levels and increased ethylene levels were able to restore germination at supraoptimal temperatures. This suggests a role for both ethylene and ABA in the imposition of thermoinhibition in this species. [Copyright &y& Elsevier]