Photosynthetic responses to elevated CO2 and O3 in field-grown potato (Solanum tuberosum)

Potato plants (Solanum tuberosum L. cv. Bintje) were grown to maturity in open-top chambers under three carbon dioxide (CO2; ambient and 24 h d(-1) seasonal mean concentrations of 550 and 680 micromol mol(-1)) and two ozone levels (O3; ambient and an 8 h d(-1) seasonal mean of 50 nmol mol(-1)). Chlorophyll content, photosynthetic characteristics, and stomatal responses were determined to test the hypothesis that elevated atmospheric CO2 may alleviate the damaging influence of O3 by reducing uptake by the leaves. Elevated O3 had no detectable effect on photosynthetic characteristics, leaf conductance, 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 content was decreased. Leaf conductance was generally reduced by elevated CO2, and declined with time in the youngest leaves examined, as did assimilation rate (A). A generally increased under elevated CO2, particularly in the older leaves during the latter stages of the season, thereby increasing instantaneous transpiration efficiency. Exposure to elevated CO2 and/or O3 had no detectable effect on dark-adapted fluorescence, although the values decreased with time. Analysis of the relationships between assimilation rate and intercellular CO2 concentration and photosynthetically active photon flux density showed there was initially little treatment effect on CO2-saturated assimilation rates for leaf 15. However, the values for plants grown under 550 micromol mol(-1) CO2 were subsequently greater than in the ambient and 680 micromol mol(-1) treatments, although the beneficial influence of the former treatment declined sharply towards the end of the season. Light-saturated assimilation was consistently greater under elevated CO2, but decreased with time in all treatments. The values decreased sharply when leaves grown under elevated CO2 were measured under ambient CO2, but increased when leaves grown under ambient CO2 were examined under elevated CO2. The results obtained indicate that, although elevated CO2 initially increased assimilation and growth, these beneficial effects were not necessarily sustained to maturity as a result of photosynthetic acclimation and the induction of earlier senescence.