Temperature and photocontrol of onoclea spore germination

Germination of Onoclea sensibilis L. spores is controlled by light and temperature. Temperatures of 30 C can induce maximal germination in the dark to a level of 60 to 95% of that induced by a saturating dose of red light (0.38 joules/square meter) providing the spores are placed at the elevated temperature immediately after being sown. Maximum dark germination occurs with a minimum exposure of 16 to 24 hours at 30 C, suggesting that the temperature treatment is required for the induction of germination rather than for the germination process per se. Interaction of temperature and light for induction of germination shows nonadditive behavior. Germination induced by light and temperature applied consecutively never exceeded that which could be induced by a saturating dose of red light alone. Imbibition of the spores at 25 C in the dark for 12 or more hours prior to incubation at 30 C results in a loss of thermosensitivity. Dose response curves for red light induction of germination after varying times of imbibition at 25 C show no concomitant loss of sensitivity of the spores to red irradiation. This suggests that the mechanism and/or pathway of thermoinduction of germination differs from that of photoinduction. The loss of thermosensitivity as a result of presoaking at 25 C can be prevented if the spores are imbibed at 25 C in osmotic agents such as 0.3 molar mannitol or 0.1 gram per liter of polyethylene glycol 400 or in 0.08% dimethylsulfoxide or 10 micrograms per milliliter of herbicide SAN 9789 (4-chloro-5-(methylamino)-2-(α,α,α-trifluoro-m-tolyl-3-(2H)pyridazinone). The latter two substances are hypothesized to act upon membranes. These results suggest that the degree of hydration and possibly changes in membrane properties play a role in the change in sensitivity of Onoclea spores to temperature.