Up-regulation of sucrose metabolizing enzymes in Oncidium goldiana grown under elevated carbon dioxide

Abstract

Experiments were conducted in controlled growth chambers to evaluate how increase in CO2 concentration affected sucrose metabolizing enzymes, especially sucrose phosphate synthase (SPS; EC 2.4.1.14) and sucrose synthase (SS; EC 2.4.1.13), as well as carbon metabolism and partitioning in a tropical epiphytic orchid species (Oncidium goldiana). Response of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) to elevated CO2 was determined along with dry mass production, photosynthesis rate, chlorophyll content, total nitrogen and total soluble protein content. After 60 days of growth, there was a 80% and 150% increase in dry mass production in plants grown at 750 and 1100 μl l-1 CO2, respectively, compared with those grown at ambient CO2 (about 370 μl l-1). A similar increase in photosynthesis rate was detected throughout the growth period when measured under growth CO2 conditions. Concomitantly, there was a decline in leaf Rubisco activity in plants in elevated CO2 after 10 days of growth. Over the growth period, leaf SPS and SS activities were up-regulated by an average of 20% and 40% for plants grown at 750 and 1100 μl l-1 CO2, respectively. Leaf sucrose content and starch content were significantly higher throughout the growth period in plants grown at elevated CO2 than those at ambient CO2. The partitioning of photosynthetically fixed carbon between sucrose and starch appeared to be unaffected by the 750 μl l-1 CO2 treatment, but it was favored into starch under the 1100 μl l-1 CO2 condition. The activities of SPS and SS in leaf extracts were closely associated with photosynthetic rates and with partitioning of carbon between starch and sucrose in leaves. The data are consistent with the hypothesis that the up-regulation of leaf SPS and SS might be an acclimation response to optimize the utilization and export of organic carbon with the increased rate of inorganic-carbon fixation in elevated CO2 conditions.