RESPONSES OF LOBLOLLY-PINE SEEDLINGS TO ELEVATED CO2 AND FLUCTUATING WATER-SUPPLY

Osmotic adjustment of loblolly pine (Pinus taeda L.) seedlings to fluc tuating water supply in elevated CO2 was investigated. Seedlings were grown in controlled-environment chambers in either 350 or 700 mul l-1 CO2 with weekly watering for four months, after which they were either watered weekly (well-watered treatment) or every two weeks (water-str ess treatment) for 59 days. Osmotic adjustment was assessed by pressur e-volume analysis of shoots and by analysis of soluble carbohydrates a nd free amino acids in roots during the last drying cycle. In well-wat ered seedlings, elevated CO2 increased the concentration of soluble su gars in roots by 68%. Water stress reduced the soluble sugar concentra tion in roots of seedling growing in ambient CO2 to 26% of that in roo ts of well-watered seedlings. Elevated CO2 mitigated the water stress- induced decrease in the concentration of soluble sugars in roots. Howe ver, this was probably due, in part, to carbohydrate loading during th e first four months when all seedlings were grown in the presence of a high water supply, rather than to osmotic adjustment to water stress. Water stress caused a doubling in the concentration of free primary a mino acids in roots, whereas elevated CO2 reduced primary amino acid a nd nitrogen concentrations to 32 and 74%, respectively, of those in ro ots of seedlings grown in ambient CO2. There was no indication of larg e-scale osmotic adjustment to water stress or that elevated CO2 enhanc ed osmotic adjustment in loblolly pine.