Hormonal regulation of tomato seed germination at a supraoptimal temperature

Germination of tomato cv. New Yorker seed is inhibited at 35°C. This thermoinhibition was partially counteracted by application of GA4+7 alone, the compound applied in combination with ACC or ethephon markedly enhancing the process. The latter compound alone was not able to induce germination at 35°C. Thermoinhibition of seeds at 35°C was also counteracted by fluridone, an inhibitor of ABA biosynthesis. At 25°C, an optimal temperature, ABA inhibited germination of New Yorker seeds. Although another known growth inhibitor MeJA, when applied at an optimal temperature (25°C), had also a slightly inhibitory effect on germination of those seeds and clearly delayed the process, inhibitors of its biosynthetic pathway (ibuprofen, indoprofen, antypiryne and salicylic acid) did not remove thermoinhibition at 35°C. An increase in endo-β-mannanase activity after 24 hours of incubation at 35°C was observed in the seeds incubated in the presence of gibberellins, ACC, ethephon, fluridone used alone and in combinations, but it was not clearly correlated with the effects of these compounds on alleviation of seed germination. However, fluridone present in the same incubation medium at 35°C with ABA was able to counteract the inhibitory effect of ABA on endo-β-mannanase activity. The results of our study suggest that gibberellins, ethylene (produced from ACC or ethephon) and ABA, but not jasmonates, regulate tomato seed germination at supraoptimal temperatures. Alleviation of thermoinhibition of New Yorker seed germination by plant growth regulators and fluridone is partially associated with their controlling endo-β-mannanase activity.