Up-regulation of sucrose phosphate synthase in rice grown under elevated CO2 and temperature

Rice (Oryza sativa L. cv. IR-30) was grown season-long in outdoor, controll ed-environment chambers at 33 Pa CO2 with day/night/paddy-water temperature s of 28/21/25 degrees C, and at 66 Pa CO2 with five different day/night/pad dy-water temperature regimes (25/18/21, 28/21/25, 31/24/28, 34/27/31 and 37 /30/34 degrees C). Sucrose phosphate synthase (SPS) activities in leaf extr acts at 21, 48 and 81 days after planting (DAP) were assayed under saturati ng and selective (limiting) conditions. Diel SPS activity data indicated th at rice SPS was light regulated; with up to 2.2-fold higher rates during th e day. Throughout the growth season, leaf SPS activities were up-regulated in the CO2-enriched plants, averaging 20 and 12% higher than in ambient-CO2 grown plants in selective and saturating assays, respectively. Similarly, SPS activities increased 2.4% for each 1 degrees C rise in growth temperatu re from 25 to 34 degrees C, but decreased 11.5% at 37 degrees C. Leaf sucro se content was higher, and mirrored SPS activity better, than starch, altho ugh starch was more responsive to CO2 treatment. Leaf sucrose and starch co ntents were significantly higher throughout the season in plants at elevate d CO2, but the N content averaged 6.5% lower. Increasing growth temperature s from 25 to 37 degrees C caused a linear decrease (62%) in leaf starch con tent, but not in sucrose. Consequently, the starch:sucrose ratio declined w ith growth temperature. The data are consistent with the hypothesis that th e up-regulation of leaf SPS may be an acclimation response of rice to optim ize the utilization and export of organic-C with the increased rates of ino rganic-C fixation in elevated CO2 or temperature growth regimes.