© 1993 Heron Publishing—Victoria, Canada
Analysis of the relationships among O3 uptake, conductance, and photosynthesis in needles of Pinus ponderosa
James A. Weber (1, 2), C. Scott Clark (3) and William E. Hogsett (2)
1. JAW dedicates this paper to honor Prof. Otto L. Lange, in gratitude for his help and instruction / 2. U.S. EPA Environmental Research Laboratory, 200 SW 35th Street, Corvallis, OR 97333, USA / 3. ManTech Environmental Technologies, U.S. EPA Environmental Research Laboratory, 200 SW 35th Street, Corvallis, OR 97333, USA / Received August 14, 1992
Summary
We studied the effects of O3 uptake on conductance (gwv) and photosynthesis (A) in needles of ponderosa pine (Pinus ponderosa Laws.) seedlings exposed for 70 days to one of three O3 regimes—Low-O3 (0.1 µmol mol–1 daily peak), High-O3 (0.2 µmol mol–1 daily peak), and Low/High-O3 (alternating 2 days Low-O3 and 2 days High-O3). Seedlings exposed to charcoal-filtered air served as controls. Total O3 exposures, expressed as ppm-h (the sum of the average hourly concentration in ppm (µmol mol–1) over the exposure period), were 77, 135, 105 and 4 for the Low-O3, High-O3, Low/High-O3 and control treatments, respectively. Conductance (gwv) declined to about 60% of the value in control seedlings by Day 6 in seedlings in the High-O3 treatment and by Day 37 in seedlings in the Low/High-O3 treatment, but gwv did not decline at all in seedlings in the Low-O3 treatment. At the end of the 70-day experiment, cumulative O3 uptake, calculated from measured gwv values and assuming an internal O3 concentration of zero, was 12.2, 13.5, and 14.7 mmol m–2 for seedlings in the Low-O3, Low/High-O3, and High-O3 treatments, respectively; however, O3 uptake was reduced by 0, 24, and 36%, respectively, from that expected if there had been no decline in gwv. With increasing total O3 exposure, A declined, but the reduction was not strictly cumulative, i.e., A measured on Days 49 and 70 was similar for a given treatment even though both total O3 exposure and uptake had increased. At the end of the experiment, A at near saturating CO2 (1000 µmol mol–1) and saturating photosynthetic photon flux density was reduced by about 25, 40 and 50% in seedlings in the Low-O3, Low/High-O3 and High-O3 treatments, respectively, compared to the control seedlings. The ratio of internal to external CO2 concentrations, an indicator of relative change in stomatal limitation of A, did not change over time and did not differ among treatments, suggesting that A and gwv decreased in parallel. After 40–60 days without O3, A of seedlings in all O3 treatments was not significantly different. Our data indicate that O3-induced stomatal closure was a result of reduced A and that decreased gwv reduced O3 uptake to a rate that needles of ponderosa pine could tolerate without exhibiting further reductions in gas exchange capacity.
Keywords:
cumulative uptake, fluctuating ozone concentration, gas exchange, ponderosa pine, stomatal limitation.
