Photosynthetic responses to elevated CO 2and O 3 in field-grown potato( Solanum tuberosum)

Abstract

Summary Potato plants ( Solanum tuberosum L. cv. Bintje) were grown to maturity in open-top chambers under three carbon dioxide (CO 2 ; ambient and 24 h d −1 seasonal mean concentrations of 550 and 680 μmol mol −1 ) and two ozone levels (O 3 ; 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 CO 2 may alleviate the damaging influence of O 3 by reducing uptake by the leaves. Elevated O 3 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 CO 2 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 CO 2 , and declined with time in the youngest leaves examined, as did assimilation rate ( A ). A generally increased under elevated CO 2 , particularly in the older leaves during the latter stages of the season, thereby increasing instantaneous transpiration efficiency. Exposure to elevated CO 2 and/or O 3 had no detectable effect on dark-adapted fluorescence, although the values decreased with time. Analysis of the relationships between assimilation rate and intercellular CO 2 concentration and photosynthetically active photon flux density showed there was initially little treatment effect on CO 2 -saturated assimilation rates for leaf 15. However, the values for plants grown under 550 μmol mol −1 CO 2 were subsequently greater than in the ambient and 680 μmol 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 CO 2 , but decreased with time in all treatments. The values decreased sharply when leaves grown under elevated CO 2 were measured under ambient CO 2 , but increased when leaves grown under ambient CO 2 were examined under elevated CO 2 . The results obtained indicate that, although elevated CO 2 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.