Effects of 2,5-norbornadiene on cocklebur seed germination and rice coleoptile elongation in response to CO2 and C2H4

To examine in more detail the mechanisms of cocklebur (Xanthium pennsylvanicum Wallr.) seed germination and rice (Oryza sativa L. cv. Sasanishiki) coleoptile elongation that were responsive to both C₂H₄ and CO₂, the effects of NBD (2,5-norbornadiene), a cyclic olefin known as a competitive inhibitor of C₂H₄, on those phenomena were tested under various conditions. NBD strongly inhibited germination of cocklebur seeds and their axial and cotyledonary growth. The NBD effects were significantly negated by endogenously evolved and exogenously applied CO₂ regardless of incubation temperature. Similarly, the inhibitory NBD effect was negated by C₂H₄ at 23°C, but at 33°C a low concentration (3 1/L) of C₂H₄ rather enhanced the inhibitory NBD effect. This phenomenon reflected the growth responses of the tip zone of axial tissues in cocklebur seeds to NBD and C₂H₄, in which both gases were antagonistic in regulating the axial growth at 23°C but additive in inhibiting it at 33°C. Maximal negation of these inhibitory NBD effects was brought about by simultaneous application of CO₂ and C₂H₄. Similarly, elongation of rice coleoptiles was suppressed by NBD, and when they were immature, its inhibitory action was counteracted by both C₂H₄ and CO₂, especially during simultaneous application. However, the inhibitory NBD effect was completely negated by C₂H₄ applied alone at concentrations above 500 1/L regardless of the physiological age of coleoptiles. These inhibitory NBD effects are additional evidence suggesting that C₂H₄ acts as a growth regulator in both cocklebur seed germination and rice coleoptile elongation. That NBD was capable of counteracting CO₂ action in some cases but was incapable of negating inhibitory C₂H₄ action, such as that observed in cocklebur seeds, suggests that NBD acts with some side effects besides being a competitive inhibitor of C₂H₄ actions.