ANALYSIS OF THE RELATIONSHIPS AMONG O-3 UPTAKE, CONDUCTANCE, AND PHOTOSYNTHESIS IN NEEDLES OF PINUS-PONDEROSA

We studied.the effects of O3 uptake on conductance (g(wv)) and photosy nthesis (A) in needles of ponderosa pine (Pinus ponderosa Laws.) seedl ings exposed for 70 days to one of three O3 regimes-Low-O3 (0.1 mumol mol-1 daily peak), High-O3 (0.2 mumol mol-1 daily peak), and Low/High- O3 (alternating 2 days Low-O3 and 2 days High-O3). Seedlings exposed t o charcoal-filtered air served as controls. Total O3 exposures, expres sed as ppm-h (the sum of the average hourly concentration in ppm (mumo l mol-1) over die exposure period), were 77, 135, 105 and 4 for the Lo w-O3, High-O3, Low/High-O3 and control treatments, respectively. Condu ctance (g(wv)) 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 seedl ings in the Low/High-O3 treatment, but g(wv) did not decline at all in seedlings in the Low-O3 treatment. At the end of the 70-day experimen t, cumulative O3 uptake, calculated from measured g(wv) values and ass uming an internal O3 concentration of zero, was 12.2, 13.5, and 14.7 m mol m-2 for seedlings in the Low-O3, Low/High-O3, and High-O3 treatmen ts, respectively; however, O3 uptake was reduced by 0, 24, and 36%, re spectively, from that expected if there had been no decline in g(wv) W ith increasing total O3 exposure, A declined, but the reduction was ri ot 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 h ad increased. At the end of the experiment, A at near saturating CO2 ( 1000 mumol mol-1) and saturating photosynthetic photon flux density wa s 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 seed lings. The ratio of internal to external CO2 concentrations, an indica tor of relative change in stomatal limitation of A, did not change ove r time and did not differ among treatments, suggesting that A and g(wv ) decreased in parallel. After 40-60 days without O3, A of seedlings i n all O3 treatments was not significantly different. Our data indicate that O3-induced stomatal closure was a result of reduced A and that d ecreased g(wv) reduced O3 uptake to a rate that needles of ponderosa p ine could tolerate without exhibiting further reductions in gas exchan ge capacity.