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 (P_n) and conductance to water vapor (g_s) of upper canopy leaves were measured 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 (L₇₀). Leaflet P_n 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 P_n in response to the L, M and H CO₂ treatments were 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. The ratio of P_n 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 P_n 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 P_n under the elevated CO₂ growth conditions.