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 CO^sub 2^ enrichment were studied in open-topped field chambers. Plants were raised in 2.4 m^sup 2^ plastic enclosures over three growing seasons from 1996 to 1998. Plots were continuously fertilized with 1, 1.5 and 2 times ambient daytime CO^sub 2^. These were the low (L), medium (M) and high (H) CO^sub 2^ treatments, respectively. Tuber dry matter yields were increased 9 and 40%, respectively, in the M and H treatments compared to the L CO^sub 2^ treatment. Net photosynthesis (P^sub n^) and conductance to water vapor (g^sub s^) of upper canopy leaves were measured at 1 or 2-week intervals at the growth CO^sub 2^ partial pressure and then P^sub n^ of plants in the L treatment was determined at 70 Pa CO^sub 2^ (L^sub 70^). Leaflet P^sub n^ rates averaged over all measurement dates were 28, 49 and 84% greater, respectively, in the M, H and L^sub 70^ CO^sub 2^ treatments, compared to plants in the L treatment. Changes of P^sub n^ in response to the L, M and H CO^sub 2^ treatments were proportional to increases of internal CO^sub 2^ (C^sub i^) and at low leaf-to-air vapor pressure deficits mid-day g^sub s^ was inversely related to growth CO^sub 2^. The ratio of P^sub n^ at H compared to L^sub 70^ was 0.81 when averaged over all measurement dates. Leaf soluble protein, Rubisco protein and chlorophyll (a + b) levels were unaffected by CO^sub 2^ treatment. Total Rubisco activity was decreased by CO^sub 2^ 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 CO^sub 2^ treatment. The above findings indicated that a down regulation of P^sub n^ in response to elevated CO^sub 2^ 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^sub n^ under the elevated CO^sub 2^ growth conditions.[PUBLICATION ABSTRACT]