Figure 4.

Effect of cytokinin preincubation on freezing tolerance of arr1-3, Pro_35S:ARR1, and Pro_35S:GUS plants compared with that in the wild-type plants. A, Freezing tolerance assays with or without cytokinin. arr1-3, Pro_35S:ARR1, Pro_35S:GUS, and wild-type plants were grown for 3 d in the light, transferred to MS plates or MS plates containing 10 µm cytokinin kinetin, and grown for an additional 7 d. These 10-d-old plants preincubated with or without cytokinin were subjected to freezing treatment at −5°C for 4 h. Plants that survived after incubation at 23°C for 2 d for recovery were counted. Experiments were conducted in triplicate, and mean values and se are plotted; n ≥ 30. Statistically significant change at P < 0.05 (Student’s t test) compared with the other samples treated with cytokinin is indicated by the asterisk. B, Example plates showing arr1-3, Pro_35S:ARR1, Pro_35S:GUS, and the wild-type plants preincubated with or without cytokinin for 7 d and subjected to freezing treatment. One plate (approximately 150 mm × 20 mm) contained 10 plants of each type of plant.