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 enrichment were studied in open-topped field chambers. Plants were raised in 2.4 m2 plastic enclosures over three growing seasons from 1996 to 1998. Plots 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, respectively. Tuber dry matter yields were increased 9 and 40%, respectively, in the M and H treatments compared to the L CO2 treatment. Net photosynthesis (Pn) and conductance to water vapor (gs) of upper canopy leaves were measured at 1 or 2-week intervals at the growth CO2 partial pressure and then Pn of plants in the L treatment was determined at 70 Pa CO2 (L70). Leaflet Pn rates averaged over all measurement dates were 28, 49 and 84% greater, respectively, in the M, H and L70 CO2 treatments, compared to plants in the L treatment. Changes of Pn in response to the L, M and H CO2 treatments were proportional to increases of internal CO2 (Ci) and at low leaf-to-air vapor pressure deficits mid-day gs was inversely related to growth CO2. The ratio of Pn at H compared to L70 was 0.81 when averaged over all measurement dates. Leaf soluble protein, Rubisco protein and chlorophyll (a + b) levels were unaffected by CO2 treatment. Total Rubisco activity was decreased 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 Pn in response to elevated CO2 was consistently observed in field-grown potato. This was attributed to a decrease of total Rubisco activity that was potentially due to the presence of inhibitory compounds bound to the active site of the enzyme. The amount of photosynthetic acclimation observed here did not preclude a persistent enhancement of Pn under the elevated CO2 growth conditions.