WATER-CONTENT DURING ABSCISIC-ACID INDUCED FREEZING TOLERANCE IN BROMEGRASS CELLS

Changes in water content and dry weight were determined in control cells and those induced to cold harden in response to abscisic acid (ABA) treatment (7.5 × 10-5 molar). Bromegrass (Bromus inermis Leyss cv Manchar) cells grown in suspension culture at room temperature (23°C) for 7 days acclimated to -28°C (LT50) when treated with ABA, or to -5°C when untreated. ABA significantly reduced cell growth rates at 5 and 7 days after treatment. Growth reduction was due to a decrease in cell number rather than cell size. When the cell water content was expressed as percent water (percent H2O) or as grams water per gram dry weight (gram H2O/gram dry weight [g DW]), the water content of hardy, ABA-treated cells decreased from 85% to 77% or from 6.4 to 3.3 g H2O/g DW in 7 days. Control cell water content remained static at approximately 87% and 7.5 g H2O/g DW. However, cell water content, expressed as milligrams water per million cells (milligram H2O/106 cells), did not differ in ABA-treated or control cells. The dry matter content of ABA-treated cells, expressed as milligram DW/106 cells increased to 3.3 milligram/106 cells in 7 days, whereas the dry weight of the control cells remained between 1.4 to 2.1 milligrams/106 cells. The osmotic potential of ABA-treated cells decreased by the fifth day while that of control cells increased significantly and then decreased by day 7. Elevated osmotic potentials were not associated with increased ion uptake. In contrast to much published literature, these results suggest that cell water content does not decrease in ABA-treated cells during the induction of freezing tolerance, rather the dry matter mass per cell increased. Cell water content may be more accurately expressed as a function of cell number when accompanying changes to dry cell matter occur.