Ozone (O 3 ) and water deficit can suppress net carbon exchange rate (NCER) of plant tissues, but little is known concerning their combined effects on NCER of field‐grown plants. Soybean [ Glycine max (L.) Merr. ‘Young’] plants were exposed to two soil moisture regimes (providing well‐watered and periodically water‐deficient conditions) and three levels of O 3 in open‐top field chambers throughout the growing season. Seasonal (12 h d −1 ) mean O 3 concentrations were 0.018, 0.059, or 0.085 µL L −1 . Measurements of NCER, transpiration (Tn), and stomatal resistance (Rs) were conducted periodically on center leaflet of the fifth or sixth trifoliolate from the apex. During vegetative and early reproductive growth, less visible foliar injury due to O 3 occurred on water‐deficient plants. Ozone usually suppressed leaflet NCER and leaflet WUE (NCER/Tn). Leaflet NCER was usually suppressed and WUE increased in water‐deficient treatments during water deficit episodes. Diurnal measurements illustrated that leaflet NCER was limited in water‐deficient plants when leaf water potential fell to about −1.1 MPa. On two occasions under low photosynthetically active radiation, NCER was greater for leaflets in water‐deficient treatments than in well‐watered treatments. Significant O 3 × H 2 O interactions for NCER were infrequent on individual days, although water deficit suppressed response to O 3 when data were analyzed over the season.
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Vozzo et al. (1995) studied this question.