resistance (6, 9, 15). ABA may aid plants in preserving better water balance (4, 7) and in tolerating more chilling stress (8,10). The following reports on water relations and the metabolism of the ABA in corn leaves after mefluidide treatment before, during, and after chilling.
MATERIALS AND METHODSSeeds of 'A632 x C103YU', a single cross corn hybrid (Zea mays L.), were sown into a soil/sand/peat (3:2:2, v/v) medium in 1 5-cm diameter pots. Seedlings were thinned to two per pot. Pots were kept in a growth chamber with a regime of 12-h photoperiod, 400-450 E m-2 s-' PAR, and 26°C D/N3 temperature. When plants had 3 to 4 true leaves, they were sprayed with analytic grade mefluidide at concentrations of 10 and 20 ppm (mg/L). The mefluidide was first dissolved in 2 to 3 ml of acetone, and then quickly diluted with distilled H20 to an appropriate volume and a few drops of Tween 20 surfactant added. Foliar spray was stopped when leaf tips showed droplets.Twenty-four h after treatment, plants were transferred from Water Relations. Leaf water potentials (WP), using the first true leaf, were measured with a pressure chamber. Seven measurements from each of 6 plants for each treatment were made at each sampling date. Leaf osmotic potentials were measured with a vapor pressure osmometer. Discs were punched from the 3rd true leaf. Leaf sap was pressed from the disc and collected with a filter-paper disc which was then placed in an osmometer. OP was calculated according to OP = iCRT, where i = isosmotic coefficient, C = leaf sap concentration, R = gas coefficient, and T = absolute temperature. Seven OP measurements from each of 6 plants were made for each treatment at each sampling date. RWC was calculated according to RWC = (fresh weight -dry weight/(turgid weight -dry weight). Turgid weight was obtained by placing the cut end of the first true leaf in a beaker containing water. The beaker was enclosed in a humid-atmosphere chamber for 12 h at either 26C or 4C ambient temperatures, corresponding to the plant-grown temperatures. Submerged ends of leaves were blotted rapidly before weighing. Leaves were dried in an oven at 1 20°C for about 48 h and dry weights determined. RESULTS ABA Content. Endogenous free ABA in controls and mefluidide-treated plants before, during and after chilling is shown in Figure 1. During the first 24 h period after mefluidide treatment at 26°C, control plants had a constant ABA content of 28, ng/g 699 www.plantphysiol.org on May 11, 2018 -Published by Downloaded from
