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

Abstract

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 μ1/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 μ1/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.