Photosynthetic responses to elevated CO2 and O-3 in field-grown potato (Solanum tuberosum)

Potato plants (Solanum tuberosum L. cv. Bintje) were grown to maturity in o pen-top chambers under three carbon dioxide (CO2 ambient and 24h d(-1) seas onal mean concentrations of 550 and 680 mu mol mol(-1)) and two ozone level s (O-3; ambient and an 8 h d(-1) seasonal mean of 50 nmol mol(-1)), Chlorop hyll content, photosynthetic characteristics, and stomatal responses were d etermined to test the hypothesis that elevated atmospheric CO2 may alleviat e the damaging influence of O-3 by reducing uptake by the leaves. Elevated O-3 had no delectable effect on photosynthetic characteristics, leaf conduc tance, or chlorophyll content, but did reduce SPAD values for leaf 15, the youngest leaf examined. Elevated CO2 also reduced SPAD values for leaf 15, but not for older leaves; destructive analysis confirmed that chlorophyll c ontent was decreased, leaf conductance was generally reduced by elevated CO 2, and declined with time in the youngest leaves examined, as did assimilat ion rate (A). A generally increased under elevated CO2, particularly in the older leaves during the latter stages of the season, thereby increasing in stantaneous transpiration efficiency. Exposure to elevated CO2 and/or O-3 h ad no detectable effect on dark-adapted fluorescence, although the values d ecreased with time. Analysis of the relationships between assimilation rate and intercellular CO2 concentration and photosynthetically active photon f lux density showed there was initially little treatment effect on CO2-satur ated assimilation rates for leaf 15. However, the values for plants grown u nder 550 mu mol mol(-1) CO2 were subsequently greater than in the ambient a nd 680 pmol mol(-1) treatments, although the beneficial influence of the fo rmer treatment declined sharply towards the end of the season. Light-satura ted assimilation was consistently greater under elevated CO2, but decreased with time in all treatments. The values decreased sharply when leaves grow n under elevated CO2 were measured under ambient CO2, but increased when le aves grown under ambient CO2, were examined under elevated CO2. The results obtained indicate that, although elevated CO2 initially increased assimila tion and growth, these beneficial effects were not necessarily sustained to maturity as a result of photosynthetic acclimation and the induction of ea rlier senescence.