Photosynthetic enhancement and conductance to water vapor of field-grown Solanum tuberosum (L.) in response to CO2 enrichment

The photosynthetic responses of potato [Solanum tuberosum (L.)] to CO2 enri chment were studied in open-topped field chambers. Plants were raised in 2. 4 m(2) plastic enclosures over three growing seasons from 1996 to 1998. Plo ts were continuously fertilized with 1, 1.5 and 2 times ambient daytime CO2 . These were the low (L), medium (M) and high (H) CO2 treatments, respectiv ely. Tuber dry matter yields were increased 9 and 40%, respectively, in the M and H treatments compared to the L CO2 treatment. Net photosynthesis (P- n) and conductance to water vapor (g(s)) of upper canopy leaves were measur ed at 1 or 2-week intervals at the growth CO2 partial pressure and then P-n of plants in the L treatment was determined at 70 Pa CO2 (L-70). Leaflet P -n rates averaged over all measurement dates were 28, 49 and 84% greater, r espectively, in the M, H and L-70 CO2 treatments, compared to plants in the L treatment. Changes of P-n in response to the L, M and H CO2 treatments w ere proportional to increases of internal CO2 (C-i) and at low leaf-to-air vapor pressure deficits mid-day g(s) was inversely related to growth CO2. T he ratio of P-n at H compared to L-70 was 0.81 when averaged over all measu rement dates. Leaf soluble protein, Rubisco protein and chlorophyll (a + b) levels were unaffected by CO2 treatment. Total Rubisco activity was decrea sed by CO2 enrichment in 1998, but percent activation was similar in the L, M and H plots. Leaf starch was increased but sucrose, glucose and fructose were unaffected by CO2 treatment. The above findings indicated that a down regulation of P-n in response to elevated CO2 was consistently observed in field-grown potato. This was attributed to a decrease of total Rubisco act ivity that was potentially due to the presence of inhibitory compounds boun d to the active site of the enzyme. The amount of photosynthetic acclimatio n observed here did not preclude a persistent enhancement of P-n under the elevated CO2 growth conditions.