Assimilation, Distribution, and Root Exudation of C by Ponderosa Pine Seedlings under Induced Water Stress

The effect of specific levels of induced water stress on the root exudation of (14)C from 9-month-old and 12-month-old ponderosa pine (Pinus ponderosa Laws.) seedlings was examined. Polyethylene glycol (PEG-4000) was used to decrease root solution water potentials by 0, -1.9, -2.6, -5.5, -9.6 and -11.9 bars in either aerated 0.25X Hoagland's nutrient solution or aerated distilled water.Assimilation of (14)CO(2) by plants under stress and subsequent translocation of (14)C label to the roots were both inhibited by a decrease in substrate water potential. Six days after (14)CO(2) introduction essentially no (14)C was detected in the roots of plants maintained at solution potentials of -5.5 bars or below. In subsequent studies (14)CO(2) was introduced 4 days prior to induction of stress. This allowed sufficient time for distribution of (14)C label throughout the root system.Root exudation of (14)C-labeled sugars, amino acids, and organic acids from plants in nutrient solution showed an increase from 0 to -1.9 bars, a decline from -1.9 to about -5.5 bars, and then an increase again from -5.5 to -11.9 bars. As substrate potential decreased, sugars as a percentage of total exudate increased, organic acids decreased and amino acids showed a slight decrease. Marked changes in percentages occurred between 0 and -2.6 bars. The exudation of sugars, amino acids, and organic acids from plants in distilled water showed similar trends in response to water stress as those in nutrient solution, but the quantity of total (14)C exuded was greater.